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Cell (biology)
Cell (biology)
Cell (biology)
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Cell
A eukaryotic cell as in animals (left) and a prokaryotic cell as in bacteria (right)
Identifiers
MeSHD002477
THH1.00.01.0.00001
FMA686465
Anatomical terminology

The cell is the basic structural and functional unit of all forms of life or organisms. The term comes from the Latin word cellula meaning 'small room'. A biological cell basically consists of a semipermeable cell membrane enclosing cytoplasm that contains genetic material. Most cells are only visible under a microscope. Except for highly-differentiated cell types (examples include red blood cells and gametes) most cells are capable of replication, and protein synthesis. Some types of cell are motile. Cells emerged on Earth about four billion years ago.

All organisms are grouped into prokaryotes, and eukaryotes. Prokaryotes are single-celled, and include archaea, and bacteria. Eukaryotes can be single-celled or multicellular, and include protists, plants, animals, most types of fungi, and some species of algae. All multicellular organisms are made up of many different types of cell. The diploid cells that make up the body of a plant or animal are known as somatic cells, and in animals excludes the haploid gametes.

Prokaryotic cells lack the membrane-bound nucleus present in eukaryotic cells, and instead have a nucleoid region. In eukaryotic cells the nucleus is enclosed in the nuclear membrane. Eukaryotic cells contain other membrane-bound organelles such as mitochondria, which provide energy for cell functions, and chloroplasts, in plants that create sugars by photosynthesis. Other non-membrane-bound organelles may be proteinaceous such as the ribosomes present (though different) in both groups. A unique membrane-bound prokaryotic organelle the magnetosome has been discovered in magnetotactic bacteria.

Cells were discovered by Robert Hooke in 1665, who named them after their resemblance to cells in a monastery. Cell theory, developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all organisms, and that all cells come from pre-existing cells.

Types

[edit]
Phylogenetic tree of life

Organisms are broadly grouped into eukaryotes, and prokaryotes. Eukaryotic cells possess a membrane-bound nucleus, and prokaryotic cells lack a nucleus but have a nucleoid region.[1] Prokaryotes are single-celled organisms, whereas eukaryotes can be either single-celled or multicellular. Single-celled eukaryotes include microalgae such as diatoms. Multicellular eukaryotes include all animals, and plants, most fungi, and some species of algae.[2][3][4]

Prokaryotes

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Main article: Prokaryote
Structure of a typical bacterial cell, generally similar with the archaeal cell structure. The bacterial flagellum shown, differs from the archaellum in archaea

All prokaryotes are single-celled and include bacteria and archaea, two of the three domains of life.[5] Prokaryotic cells were likely the first form of life on Earth,[6][7] characterized by having vital biological processes including cell signaling. They are simpler and smaller than eukaryotic cells, lack a nucleus, and the other usually present membrane-bound organelles.[8] Prokaryotic organelles are simple structures typically non-membrane-bound.[9]

Bacteria and archaea divide by binary fission

Bacteria

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Main article: Bacterial cell structure

Bacteria are enclosed in a cell envelope, that protects the interior from the exterior.[10] It generally consists of a plasma membrane covered by a cell wall which, for some bacteria, is covered by a third gelatinous layer called a bacterial capsule. The capsule may be polysaccharide as in pneumococci, meningococci or polypeptide as Bacillus anthracis or hyaluronic acid as in streptococci. Mycoplasma only possess the cell membrane.[11] The cell envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective mechanical and chemical filter.[12] The cell wall consists of peptidoglycan and acts as an additional barrier against exterior forces.[13][12] The cell wall acts to protect the cell mechanically and chemically from its environment, and is an additional layer of protection to the cell membrane. It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to a hypotonic environment.[14]

The DNA of a bacterium typically consists of a single circular chromosome that is in direct contact with the cytoplasm in a region called the nucleoid. Some bacteria contain multiple circular or even linear chromosomes.[15][16][17] The cytoplasm also contains ribosomes and various inclusions where transcription takes place alongside translation.[18][19] Extrachromosomal DNA as plasmids, are usually circular and encode additional genes, such as those of antibiotic resistance.[20] Linear bacterial plasmids have been identified in several species of spirochete bacteria, including species of Borrelia which causes Lyme disease.[21] The prokaryotic cytoskeleton in bacteria is involved in the maintenance of cell shape, polarity and cytokinesis.[22]

Compartmentalization is a feature of eukaryotic cells but some species of bacteria, have protein-based organelle-like microcompartments such as gas vesicles, and carboxysomes, and encapsulin nanocompartments.[23][24][25][26] Certain membrane-bound prokaryotic organelles have also been discovered. They include the magnetosome of magnetotactic bacteria,[24] and the anammoxosome of anammox bacteria.[27][28]

Cell-surface appendages can include flagella, and pili, protein structures that facilitate movement and communication between cells.[29] The flagellum stretches from the cytoplasm through the cell membrane and extrudes through the cell wall.[30] Fimbriae are short attachment pili, the other type of pilus is the longer conjugative type.[31] Fimbriae are formed of an antigenic protein called pilin, and are responsible for the attachment of bacteria to specific receptors on host cells. [32]

Most prokaryotes are the smallest of all organisms, ranging from 0.5 to 2.0 μm in diameter.[33] The largest bacterium known, Thiomargarita magnifica, is visible to the naked eye with an average length of 1 cm, but can be as much as 2 cm[34] [35]

Archaea

[edit]
Main article: Archaea

Archaea are enclosed in a cell envelope consisting of a plasma membrane and a cell wall. An exception to this is the Thermoplasma that only has the cell membrane.[11] The cell membranes of archaea are unique, consisting of ether-linked lipids. The prokaryotic cytoskeleton has homologues of eukaryotic actin and tubulin.[22] A unique form of metabolism in the archaean is methanogenesis. Their cell-surface appendage equivalent of the flagella is the differently structured and unique archaellum.[36][31] The DNA is contained in a circular chromosome in direct contact with the cytoplasm, in a region known as the nucleoid. Ribosomes are also found freely in the cytoplasm, or attached to the cell membrane where DNA processing takes place.[18][37]

The archaea are noted for their extremophile species, and many are selectively evolved to thrive in extreme heat, cold, acidic, alkaline, or high salt conditions.[38] There are no known archaean pathogens.[39]

Eukaryotes

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Main article: Eukaryote

Eukaryotes can be single-celled, as in diatoms (microscopic algae), or multicellular. Animals, plants, most fungi, and some algae are multicellular.[40] Eukaryotes are distinguished by the presence of a membrane-bound nucleus.[41] The nucleus gives the eukaryote its name, which means "true nut" or "true kernel", where "nut" means the nucleus.[42] Eukaryotic cells can be 2 to 1000 times larger in diameter than a typical prokaryote.[43]

Property Archaea Bacteria Eukaryota
Cell membrane Ether-linked lipids Ester-linked lipids Ester-linked lipids
Cell wall Glycoprotein, or S-layer; rarely pseudopeptidoglycan Peptidoglycan, S-layer, or no cell wall Various structures
Gene structure Circular chromosomes, similar translation and transcription to Eukaryota Circular chromosomes, unique translation and transcription Multiple, linear chromosomes, but translation and transcription similar to Archaea
Internal cell structure No membrane-bound organelles (?[44]) or nucleus No membrane-bound organelles or nucleus Membrane-bound organelles and nucleus
Metabolism[45] Various, including diazotrophy, with methanogenesis unique to Archaea Various, including photosynthesis, aerobic and anaerobic respiration, fermentation, diazotrophy, and autotrophy Photosynthesis, cellular respiration, and fermentation; no diazotrophy
Reproduction Asexual reproduction, horizontal gene transfer Asexual reproduction, horizontal gene transfer Sexual and asexual reproduction
Protein synthesis initiation Methionine Formylmethionine Methionine
RNA polymerase One One Many
EF-2/EF-G Sensitive to diphtheria toxin Resistant to diphtheria toxin Sensitive to diphtheria toxin

Multicellular organisms are made up of many different types of cell known overall as somatic cells.[46] Typical plant cells include parenchyma cells including transfer cells, and collenchyma cells. Animal cells include all those that make up the four main tissue types of epithelium – a number of different epithelial cells; connective tissue such as osteoblasts in bone, and chondrocytes in cartilage; nervous tissue including different brain cells and nerves, and muscle tissue having different muscle cells.[1] The number of cells in these tissues vary with species. Studies on the human have estimated a total cell count at around 30 trillion cells (~36 trillion cells in the male, and ~28 trillion in the female).[47][48]

Eukaryotic cells

[edit]

The cells of eukaryotes have a cell membrane that surrounds a gel-like cytoplasm; it contains the cytoskeleton, the cell nucleus, the endomembrane system, and organelles including mitochondria, and the Golgi apparatus.

Cell membrane

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Main article: Cell membrane
Colored illustration showing the membrane of an animal cell surrounded by tissue. The nucleus, mitochondria, gogli, ER, and lysosomes are labelled.
Diagram of cell membrane detailing the lipid bilayer
Colored illustration showing the cell membrane and membrane proteins.
Diagram of cell membrane detailing membrane proteins

The cell membrane, or plasma membrane, is a selectively permeable membrane as an outer boundary of the cell that encloses the cytoplasm.[49] The membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a lipid bilayer of phospholipids, which are amphiphilic (partly hydrophobic and partly hydrophilic), and is sometimes referred to as a fluid mosaic membrane.[50] Embedded within the cell membrane is a macromolecular structure called the porosome the universal secretory portal in cells and a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell.[18] The membrane is semi-permeable, and selectively permeable, in that it can either let a substance (molecule or ion) pass through freely, to a limited extent or not at all.[51] Cell surface receptors embedded in the membrane allow cells to detect external signaling molecules such as hormones.[52]

Underlying, and attached to the cell membrane is the cell cortex, the outermost part of the actin cytoskeleton.[53] Its thickness varies with cell type and physiology.

Cytoplasm

[edit]

The membrane encloses the cytoplasm of the cell. It is made up of two main components, the cytosol, and the protein filaments that make up the cytoskeleton.[54][55] The cytosol is a gel-like substance made up of water, ions, and non-essential biomolecules. The network of filaments and microtubules of the cytoskeleton gives shape and support to the cell, and has a part in organising the cell components. The cytoplasm surrounds all the organelles of the cell.[54][55]

Cytoskeleton

[edit]

The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells after cell division; and moves parts of the cell in processes of growth and mobility. The cytoskeleton is composed of microtubules, intermediate filaments and microfilaments. In a neuron the intermediate filaments are known as neurofilaments. There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. The outermost part of the cytoskeleton is the cell cortex, or actin cortex, a thin layer of cross-linked actomyosins.[53]

The centrosome is the cytoskeleton organizer in the animal cell that produces the microtubules of a cell—a key component of the cytoskeleton.[56] It directs the transport through the ER and the Golgi apparatus.[57] Centrosomes are composed of two centrioles which lie perpendicular to each other in which each has an organization like a cartwheel, which separate during cell division and help in the formation of the mitotic spindle.[56]

Organelles

[edit]
Main article: Organelle

Organelles are parts of the cell that are specialized for carrying out one or more vital functions.[18] There are several types of organelles in a cell held in the gelatinous cytosol of the cytoplasm that fills the cell and surrounds the organelles, forming 30%–50% of a cell volume.[58] Most organelles vary in size and/or number based on the growth of the host cell.[59]

Nucleus

[edit]
Deoxyribonucleic acid (DNA)

The cell nucleus is the largest organelle in the animal cell.[60] It houses the cell's chromosomes, and is the place where almost all DNA replication and RNA synthesis (transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the nuclear envelope, space between these two membrane is called perinuclear space. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or copied into a special RNA, called messenger RNA (mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The nucleolus is a specialized region within the nucleus where ribosome subunits are assembled.[18] Cells use DNA for their long-term information storage that is encoded in its DNA sequence.[18] RNA is used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA). Transfer RNA (tRNA) molecules are used to add amino acids during protein translation.[61] Mitochondria have their own DNA (mitochondrial DNA).[62] The mitochondrial genome is a circular DNA molecule distinct from nuclear DNA. Although the mitochondrial DNA is very small compared to nuclear chromosomes,[18] it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs.[63]

The DNA of each cell is its genetic material, and is organized in multiple linear molecules, called chromosomes, that are coiled around histone proteins and housed in the cell nucleus.[41][64] In humans, the nuclear genome is divided into 46 linear chromosomes, including 22 homologous chromosome pairs and a pair of sex chromosomes. The nucleus is a membrane-bound organelle. Other organelles in the cell have specific functions such as mitochondria which provide the cell's energy.[65]

Golgi apparatus

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The Golgi apparatus processes and packages the macromolecules, such as proteins and lipids, that are synthesized by the cell. It is organized as a stack of plate-like structures known as cisternae.[66]

Mitochondria

[edit]

Mitochondria generate energy for the cell. Mitochondria are self-replicating double membrane-bound organelles that occur in various numbers, shapes, and sizes in the cytoplasm of the cell.[18] Respiration occurs in the cell mitochondria, which generate the cell's energy by oxidative phosphorylation, using oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP (aerobic respiration).[67] Mitochondria multiply by binary fission.[68]

Lysosomes

[edit]

Lysosomes contain enzymes (acid hydrolases). They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Lysosomes are optimally active in an acidic environment. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system.[18][69]

Peroxisomes

[edit]

Peroxisomes have enzymes that rid the cell of toxic peroxides,

Vacuoles

[edit]

Vacuoles sequester waste products. Some cells, most notably Amoeba, have contractile vacuoles, which can pump water out of the cell if there is too much water.[70]

Endomembrane system

[edit]
Diagram of the endomembrane system

The endomembrane system consists of all the different internal membranes of the cell. These membranes are held in the cell's cytoplasm and divide the various organelles.

Endoplasmic reticulum

[edit]

The endoplasmic reticulum (ER) is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough endoplasmic reticulum (RER), which has ribosomes on its surface that secrete proteins into the ER, and the smooth endoplasmic reticulum (SER), which lacks ribosomes.[18]

A ribosome is a large complex of RNA and protein molecules.[18] They each consist of two subunits, one larger than the other, and act as an assembly line where RNA from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane of the rough endoplasmatic reticulum.[71]

The smooth ER plays a role in calcium sequestration and release, and helps in synthesis of lipid.[72]

Cells of major eukaryote groups

[edit]

Animal cells

[edit]
Further information: Animal embryonic development and Cell types
Structure of an animal cell

All the cells in an animal body develop from one totipotent diploid cell called a zygote. During the development of an animal, the cells differentiate into the specialised tissues and organs of the organism. (An exception is the simple sponge). The estimated cell count in a typical adult human body is around 30 trillion cells. Different groups of cells differentiate from the three germ layers. Differentiation results in structural or functional changes to the typical eukaryotic cell.

Some types of specialised cell are localised to a particular animal group. Vertebrates for example have specialised, structurally changed cells including muscle cells. The cell membrane of a skeletal muscle cell or of a cardiac muscle cell is termed the sarcolemma.[73] And the cytoplasm is termed the sarcoplasm. Skeletal muscle cells also become multinucleated. Populations of animal groups evolve to become distinct species, where sexual reproduction is isolated. The many species of vertebrates for example have other unique characteristics by way of additional specialised cells. In some species of electric fish for example modified muscle cells or nerve cells have specialised to become electerocytes capable of creating and storing electrical energy for future release, as in stunning prey, or use in electrolocation.[74] These are large flat cells in the electric eel, and electric ray in which thousands are stacked into an electric organ comparable to a voltaic pile.[75]

Organelles are parts of the cell that are specialized for carrying out one or more vital functions, analogous to the organs of the human body (such as the heart, lung, and kidney, with each organ performing a different function).[18] In addition to the organelles shared by all eukaryotes, animal cells often have cilia or flagella.[76][77] Many animal cells are ciliated and most cells except red blood cells have primary cilia. Primary cilia play important roles in chemosensation and mechanosensation.[78] Each cilium may be "viewed as a sensory cellular antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."[79] Ciliated cells in the respiratory epithelium play an important role in the movement of mucus. Some animal cells have flagella such as flagellated protists and sperm cells, that enable movement.[76] Invertebrate planarians have ciliated excretory flame cells.[80] Other excretory cells also found in planarians are solenocytes that are long and flagellated.

Plant cells

[edit]
Main article: Plant cell
Structure of a typical plant cell
Onion (Allium cepa) root cells in different phases of the cell cycle (drawn by E.B. Wilson, 1900)

Other types of organelle specific to plant cells, are pigment-containing plastids, especially chloroplasts that contain chlorophyll, and large water-storing vacuoles.

Chloroplasts capture the sun's energy to make carbohydrates through photosynthesis.[81]Chromoplasts contain fat-soluble carotenoid pigments such as orange carotene and yellow xanthophylls which helps in synthesis and storage. Leucoplasts are non-pigmented plastids and helps in storage of nutrients.[82] Plastids divide by binary fission. Vacuoles in plant cells store water. They are liquid filled spaces surrounded by a membrane.[83]The vacuoles of plant cells are usually larger than those of animal cells.They are described as liquid filled spaces and are surrounded by a membrane.[83] Vacuoles of plant cells are surrounded by a membrane which transports ions against concentration gradients.[84]

Algal cells

[edit]

Algae members are photoautotrophs able to use photosynthesis to produce energy. Photosynthesis is made possible by the use of plastids, organelles in the cytoplasm known as chloroplasts. Algal photoautotrophs include red algae.[85]

Alginate is a polysaccharide found in the matrix of the cell walls of brown algae, and have many important uses in the food industry, and in pharmacology.[86]

Fungal cells

[edit]
Main article: Fungus

The cells of fungi have in addition to the shared eukaryotic organelles a spitzenkörper in their endomembrane system, associated with hyphal tip growth. It is a phase-dark body that is composed of an aggregation of membrane-bound vesicles containing cell wall components, serving as a point of assemblage and release of such components intermediate between the Golgi and the cell membrane. The spitzenkörper is motile and generates new hyphal tip growth as it moves forward.[87]

Protist cells

[edit]

The cells of protists may be bounded only by a cell membrane, or may in addition have a cell wall, or may be covered by a pellicle (in ciliates), a test (in testate amoebae), or a frustule (in diatoms).

Some protists such as amoebae may feed on other organisms and ingest food by phagocytosis. Vacuoles known as phagosomes in the cytoplasm may be used to draw in and incorporate the captured particles. Other types of protists are photoautotrophs, providing themselves with energy by photosynthesis.[88] Most protists are motile and generate movement with cilia, flagella, or pseudopodia.[88]

Physiology

[edit]
See also: Cell cycle and Cell physiology
Prokaryotes divide by binary fission, while eukaryotes divide by mitosis or meiosis.

Replication

[edit]
Main article: Cell division
Human cancer cells, specifically HeLa cells, with DNA stained blue. The central and rightmost cell are in interphase, so their DNA is diffuse and the entire nuclei are labelled. The cell on the left is going through mitosis and its chromosomes have condensed.

During cell division, part of the cell cycle, a single cell, the mother cell divides into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation (vegetative reproduction) in unicellular organisms. Prokaryotic cells divide by binary fission, while eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. A diploid cell may undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells.[89]

DNA replication, or the process of duplicating a cell's genome,[18] always happens when a cell divides through mitosis or binary fission.[89] This occurs during the S (synthesis) phase of the cell cycle.

In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before meiosis I. DNA replication does not occur when the cells divide the second time, in meiosis II.[90] Replication, like all cellular activities, requires specialized proteins.[18]

DNA repair

[edit]
Main article: DNA repair

All cells contain enzyme systems that scan for DNA damage and carry out repair. Diverse repair processes have evolved in all organisms. Repair is vital to maintain DNA integrity, avoid cell death and errors of replication that could lead to mutation. Repair processes include nucleotide excision repair, DNA mismatch repair, non-homologous end joining of double-strand breaks, recombinational repair and light-dependent repair (photoreactivation).[91]

Growth and metabolism

[edit]
Main articles: Cell growth, Metabolism, and Photosynthesis

Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions.[92]

Complex sugars can be broken down into simpler sugar molecules called monosaccharides such as glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP),[18] a molecule that possesses readily available energy, through two different pathways. In plant cells, chloroplasts create sugars by photosynthesis, using the energy of light to join molecules of water and carbon dioxide.[93]

Protein synthesis

[edit]
Main article: Protein biosynthesis

Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from amino acid building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: transcription and translation.[61]

Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give messenger RNA (mRNA), which is free to migrate into the cytoplasm. mRNA molecules bind to protein-RNA complexes called ribosomes located in the cytosol, where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to transfer RNA (tRNA) adapter molecules in binding pockets within the ribosome.[61] The new polypeptide then folds into a functional three-dimensional protein molecule.

Motility

[edit]
Main article: Motility

Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include flagella and cilia.[31]

In multicellular organisms, cells can move during processes such as wound healing, the immune response and cancer metastasis. For example, in wound healing in animals, white blood cells move to the wound site to kill the microorganisms that cause infection. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins.[94] The process is divided into three steps: protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each step is driven by physical forces generated by unique segments of the cytoskeleton.[95][94]

[edit]
See also: Cybernetics § In biology

In August 2020, scientists described one way cells—in particular cells of a slime mold and mouse pancreatic cancer-derived cells—are able to navigate efficiently through a body and identify the best routes through complex mazes: generating gradients after breaking down diffused chemoattractants which enable them to sense upcoming maze junctions before reaching them, including around corners.[96][97][98]

Death

[edit]
Main article: Cell death

Cell death is the event of a biological cell ceasing to carry out its functions. This may be the result of the natural process of old cells dying and being replaced by new ones, as in programmed cell death, or may result from factors such as diseases, localized injury, exposure to a toxic substance, or the death of the host organism. Apoptosis or Type I cell-death, and autophagy or Type II cell-death are both forms of programmed cell death, while necrosis is a non-physiological process that occurs as a result of infection or injury.[99][100]

Cell ancestry traces back in an unbroken lineage for over 3 billion years. The immortality of a cell lineage depends on the maintenance of cell division potential,[101] which may be lost because of cell damage, terminal differentiation as occurs in nerve cells, or programmed cell death during development. Maintenance of division potential over successive generations depends on the avoidance and the accurate repair of cellular damage, particularly DNA damage. Sexual processes provide an opportunity for effective repair of DNA damage in the germ line by homologous recombination.[101][102]

Multicellularity

[edit]
Main article: Multicellular organism

Cell differentiation

[edit]
Main article: Cellular differentiation
Staining of a nematode Caenorhabditis elegans highlights the nuclei of its cells.

Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms.[103] Microorganisms cloned from a single cell can form visible microbial colonies. A microbial consortium of two or more species of microorganisms can form a biofilm community,[104] such as dental plaque. The cell-to-cell adhesion found in microbial colonies may have been the first evolutionary step toward more complex multicellular organisms.[105]

In complex multicellular organisms, cells specialize into different cell types that are adapted to particular functions.[106] In animals, major cell types include skin cells, muscle cells, neurons, blood cells, fibroblasts, stem cells, and others. Cell types differ both in appearance and function, yet are genetically identical. Cells are able to be of the same genotype but of different cell type due to the differential expression of the genes they contain.[107]

Most distinct cell types arise from a single totipotent cell, called a zygote, that differentiates into hundreds of different cell types during the course of development. Differentiation of cells is driven by different environmental cues (such as cell–cell interaction) and intrinsic differences (such as those caused by the uneven distribution of molecules during division).[108]

Signaling

[edit]
Main article: Cell signaling

Cell signaling is the process by which a cell interacts with itself, other cells, and the environment. Typically, the signaling process involves three components: the first messenger (the ligand), the receptor, and the signal itself.[109] Most cell signaling is chemical in nature, and can occur with neighboring cells or more distant targets. Signal receptors are complex proteins or tightly bound multimer of proteins, located in the plasma membrane or within the interior.[110]

Each cell is programmed to respond to specific extracellular signal molecules, and this process is the basis of development, tissue repair, immunity, and homeostasis. Individual cells are able to manage receptor sensitivity including turning them off, and receptors can become less sensitive when they are occupied for long durations.[110] Errors in signaling interactions may cause diseases such as cancer, autoimmunity, and diabetes.[111]

Origin of multicellularity

[edit]

Multicellularity has evolved independently at least 25 times,[112] including in some prokaryotes, like cyanobacteria, myxobacteria, actinomycetes, or Methanosarcina. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and plants.[113] It evolved repeatedly for plants (Chloroplastida), once or twice for animals, once for brown algae, and perhaps several times for fungi, slime molds, and red algae.[114] Multicellularity may have evolved from colonies of interdependent organisms, from cellularization, or from organisms in symbiotic relationships.[115]

The first evidence of multicellularity is from cyanobacteria-like organisms that lived between 3 and 3.5 billion years ago.[112] Other early fossils of multicellular organisms include the contested Grypania spiralis and the fossils of the black shales of the Palaeoproterozoic Francevillian Group Fossil B Formation in Gabon.[116]

The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in evolution experiments using predation as the selective pressure.[112]

Origins

[edit]
Main article: History of life

The origin of cells has to do with the origin of life, which began the history of life on Earth.

Origin of life

[edit]
Further information: Abiogenesis and Evolution of cells
Stromatolites are left behind by cyanobacteria, known as blue-green algae. They are among the oldest fossils of life on Earth. This one-billion-year-old fossil is from Glacier National Park in the United States.

Small molecules needed for life may have been carried to Earth on meteorites, created at deep-sea vents, or synthesized by lightning in a reducing atmosphere. There is little experimental data defining what the first self-replicating forms were. RNA may have been the earliest self-replicating molecule, as it can both store genetic information and catalyze chemical reactions.[117] This process required an enzyme to catalyze the RNA reactions, which may have been the early peptides that formed in hydrothermal vents.[118]

Cells emerged around 4 billion years ago.[119][120] The first cells were most likely heterotrophs. The early cell membranes were probably simpler and more permeable than modern ones, with only a single fatty acid chain per lipid. Lipids spontaneously form bilayered vesicles in water, and could have preceded RNA.[121][122]

First eukaryotes

[edit]
Main article: Eukaryogenesis
In the theory of symbiogenesis, a merger of an archaean and an aerobic bacterium created the eukaryotes, with aerobic mitochondria, some 2.2 billion years ago. A second merger, 1.6 billion years ago, added chloroplasts, creating the green plants.[123]

Eukaryotic cells were created some 2.2 billion years ago in a process called eukaryogenesis. This is widely agreed to have involved symbiogenesis, in which archaea and bacteria came together to create the first eukaryotic common ancestor.[123] However, the sequence of the steps involved has been disputed.[citation needed] It evolved into a population of single-celled organisms that included the last eukaryotic common ancestor, gaining capabilities along the way.[124][125]

This cell had a new level of complexity and capability, with a nucleus[126][124] and facultatively aerobic mitochondria.[123] It featured at least one centriole and cilium, sex (meiosis and syngamy), peroxisomes, and a dormant cyst with a cell wall of chitin and/or cellulose.[127][125] The last eukaryotic common ancestor gave rise to the eukaryotes' crown group, containing the ancestors of animals, fungi, plants, and a diverse range of single-celled organisms.[128][129] The plants were created around 1.6 billion years ago with a second episode of symbiogenesis that added chloroplasts, derived from cyanobacteria.[123]

History of research

[edit]
Main article: Cell theory § Discovery of cells
Robert Hooke's drawing of cells in cork, 1665

In 1665, Robert Hooke examined a thin slice of cork under his microscope, and saw a structure of small enclosures. He wrote "I could exceeding plainly perceive it to be all perforated and porous, much like a honeycomb, but that the pores of it were not regular".[130] To further support his theory, Matthias Schleiden and Theodor Schwann studied cells of both animal and plants. What they discovered were significant differences between the two types of cells. This put forth the idea that cells were fundamental to both plants and animals.[131]

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Cell (biology)

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In biology, the cell is the fundamental structural, functional, and biological unit of all known living organisms, often called the "building block of life." It represents the smallest entity capable of independent existence, performing essential processes like , growth, and while forming the basis of all tissues and organisms. According to , established in the , all living things are composed of one or more cells, the cell is the basic unit of structure and function in organisms, and all cells arise from the division of preexisting cells. Cells are broadly categorized into two types: prokaryotic and eukaryotic. Prokaryotic cells, such as those in bacteria, lack a true nucleus and membrane-bound organelles, with their genetic material dispersed in the cytoplasm; they are typically unicellular and simpler in organization. In contrast, eukaryotic cells, found in plants, animals, fungi, and protists, contain a membrane-enclosed nucleus housing DNA and various specialized organelles, enabling more complex functions; multicellular eukaryotes like humans consist of trillions of these cells working together. This distinction arose evolutionarily, with prokaryotes appearing around 3.5 billion years ago and eukaryotes later developing through endosymbiosis. The structure of a typical cell includes a plasma membrane that acts as a selective barrier, controlling the exchange of materials with the environment. Inside, the —a gel-like matrix—houses the for support and movement, ribosomes for protein synthesis, mitochondria for energy production, the and Golgi apparatus for processing and transport of molecules, and lysosomes for waste degradation. In eukaryotic cells, the nucleus serves as the control center, directing cellular activities via genetic instructions. Prokaryotic cells, while lacking these compartments, still carry out analogous functions through simpler mechanisms. Cells perform vital functions that sustain life, including nutrient uptake, energy conversion, , and response to environmental signals. They reproduce via division—binary fission in prokaryotes and or in eukaryotes—ensuring continuity and diversity. In multicellular organisms, cells differentiate into specialized types, such as neurons or muscle cells, to form tissues and organs, while also communicating through signaling pathways. The study of cells, known as , reveals their role in , , and , with ongoing research mapping cellular diversity in projects like the Human Cell Atlas.

Overview

Definition and Key Characteristics

In biology, a cell is defined as the smallest structural and functional unit of life capable of independent reproduction and metabolism. All living organisms are composed of one or more cells, which serve as the fundamental building blocks for tissues, organs, and entire multicellular bodies. Cells exhibit several universal key characteristics that distinguish them as the basis of life: compartmentalization through a plasma membrane that separates the internal environment from the external one, enabling controlled exchanges; metabolism, involving enzymatic chemical reactions to process nutrients and generate energy, often stored as ATP; growth and responsiveness to environmental stimuli via receptors and signaling pathways; reproduction through cell division, ensuring propagation; and heredity encoded by DNA, which directs cellular structure, function, and inheritance. These properties allow cells to maintain homeostasis, adapt to changes, and sustain life processes autonomously or in coordination within multicellular organisms. Cells manifest in diverse forms, including unicellular organisms that exist as single, self-sufficient entities, such as bacteria (prokaryotes) and amoebae (eukaryotic protists), which perform all life functions independently. In multicellular organisms like humans or plants, specialized cells collaborate, with examples including neurons for signal transmission or leaf cells for photosynthesis, yet each retains the core ability to divide and differentiate when needed. Notably, viruses are distinguished from cells as non-cellular entities; they lack metabolism, independent reproduction, and cellular structures, instead relying on host cells to replicate their genetic material. Cell sizes vary significantly by type, providing insight into their complexity and function. Prokaryotic cells, such as , typically range from 0.1 to 5 μm in diameter, reflecting their simpler organization. Eukaryotic cells, found in , , and fungi, are generally larger, measuring 10 to 100 μm, which accommodates membrane-bound organelles and greater internal compartmentalization. These scale differences the evolutionary between cell types while highlighting the cell's as life's minimal viable unit.

Cell Theory

Cell theory, a cornerstone of modern , was formulated in the through the pioneering work of several key scientists. In 1838, German botanist observed under the that all tissues are aggregates of individual cells, proposing that the cell is the fundamental unit of structure and development; this insight was published in his seminal paper "Beiträge zur Phytogenesis" (Contributions to Phytogenesis). Building on Schleiden's findings, German physiologist extended the concept to animals in 1839, demonstrating through detailed microscopic examinations that animal tissues similarly consist of cells and exhibit comparable growth patterns to ; his comprehensive , "Mikroskopische Untersuchungen über die Übereinstimmung in der Struktur und dem Wachstum der Tiere und Pflanzen" (Microscopical Researches into the Accordance in the and Growth of Animals and ), laid the groundwork for the theory's initial principles. These contributions by Schleiden and Schwann established the unifying idea that cells form the basic building blocks of all living organisms. The theory was completed in 1855 by German pathologist , who, through his studies of tissue and , asserted that all cells originate from pre-existing cells—a principle encapsulated in his famous axiom "omnis cellula e cellula" (every cell from a cell); this was articulated in his lectures and subsequent publication "Die Cellularpathologie in ihrer Begründung auf physiologische und pathologische Gewebelehre" (). Virchow's addition refuted earlier notions of and emphasized cellular continuity in health and disease. The resulting classical comprises three core tenets: all living organisms are composed of one or more cells; the cell is the basic unit of structure, function, and organization in organisms; and all cells arise from pre-existing cells via division. Cell theory profoundly impacts biology by providing a unified framework that connects the structure and function of across scales, from unicellular microbes to complex multicellular organisms; it underpins disciplines such as cytology, which examines cellular structures and processes, and , which investigates cellular at the microscopic level. In the , the theory has been extended to incorporate advances in and biochemistry: cells carry hereditary information through , which is replicated and transmitted during division; moreover, cells serve as the primary sites for metabolic activities, including energy flow via processes like and information processing through molecular signaling. Apparent exceptions, such as syncytia—multinucleated cytoplasmic masses in structures like or certain fungi—do not contradict the theory, as they form through fusion of individual cells and function as single, albeit complex, cellular units with coordinated nuclear activity.

History of Cell Research

Early Discoveries

The invention of the compound in the late 16th and early 17th centuries, building on earlier simple lenses, enabled the first glimpses into the microscopic world, though initial instruments suffered from poor and image distortion. In 1665, English scientist published , detailing his observations of thin slices of cork viewed through a with up to 50x ; he described the honeycomb-like compartments as "cells," likening them to small rooms, and noted their porous composed of rigid walls. These were actually the empty lignified cell walls of dead plant tissue from cork oak (), marking the first documented use of the term "cell" in . Concurrently in the 1660s, Italian physician Marcello Malpighi employed early microscopes to examine and animal tissues, revealing intricate structures such as the vascular bundles in and the networks connecting arteries and veins in frog lungs, which he described in works like De pulmonibus (1661). His studies of insect , including the tubular structures now known as Malpighian tubules, demonstrated that tissues in both and animals consisted of organized, minute components, laying groundwork for comparative . A decade later, in the 1670s, Dutch microscopist crafted superior single-lens microscopes achieving 270x magnification and over 1 μm resolution; in letters to the Royal Society, he reported observing "animalcules"—tiny, motile organisms—in samples of pond water, rainwater, and , which included like Paramecium and the first sightings of such as those in the mouth. These findings, published in Philosophical Transactions starting in 1677, expanded the known diversity of life and confirmed the ubiquity of microscopic entities. Early microscopy's limitations, including chromatic and spherical aberrations that blurred images and restricted resolution to about 1-2 μm, fueled debates on cellular continuity; observers like 18th-century microscopists often misinterpreted tissue as a continuous fibrous network rather than discrete units, as finer details such as cell boundaries in living animal tissues remained elusive. In 1831, Scottish botanist Robert Brown advanced these observations by identifying a dark, opaque structure—the nucleus—within the cells of orchid (Orchidaceae) epidermal tissue during studies of fertilization, naming it in a presentation to the Linnean Society and emphasizing its consistent presence across plant cells. This discovery, detailed in his 1833 publication, highlighted internal cellular organization previously overlooked. Building on these advances, the 1830s saw the formulation of . In 1838, German proposed that all plant tissues are composed of cells and that cells are the fundamental units of plant life. The following year, extended this idea to animals, stating that all living organisms are made up of cells. In 1855, added the crucial insight that all cells arise from preexisting cells, completing the classical . These principles unified the understanding of life at the cellular level. The transition to the 19th century brought pivotal improvements, such as achromatic lenses developed by Joseph Jackson Lister in the 1830s, which corrected color fringing and spherical distortion to achieve resolutions below 1 μm, enabling clearer views of dynamic cellular processes and paving the way for detailed investigations of cell division and morphology.

Development of Modern Understanding

The advent of electron microscopy in the 1930s marked a pivotal advancement in visualizing cellular ultrastructure, surpassing the resolution limits of light microscopy. Invented by Ernst Ruska and Max Knoll in 1931, the transmission electron microscope (TEM) achieved magnifications up to 100,000 times, enabling the first detailed images of subcellular components. By the 1940s, biologists like Keith Porter and Ernest Ruska applied TEM to fixed tissue sections, revealing organelles such as the endoplasmic reticulum in 1945 and mitochondria's internal cristae, which transformed understanding of cellular compartmentalization. In 1953, James Watson and Francis Crick proposed the double-helix structure of DNA, providing a molecular framework for genetic information storage and replication within cells. This discovery, built on X-ray diffraction data from Rosalind Franklin and Maurice Wilkins, elucidated how DNA's base-pairing mechanism underpins cellular heredity and protein synthesis, bridging cytology with molecular genetics. The model implied that genetic mutations could alter cellular function, influencing subsequent research on gene expression in prokaryotes and eukaryotes. Lynn Margulis advanced cellular evolution in 1967 with her endosymbiotic theory, positing that eukaryotic organelles like mitochondria and chloroplasts originated from engulfed prokaryotic symbionts. Published in the Journal of Theoretical Biology, her hypothesis integrated ultrastructural evidence from electron microscopy—such as organelles' double membranes and independent replication—with biochemical data on their prokaryote-like ribosomes and DNA. Initially controversial, the theory gained acceptance through genetic sequencing confirming shared ancestry, reshaping paradigms of cellular complexity and symbiosis. The 1970s ushered in the era with technology, allowing direct manipulation of cellular genomes. Pioneered by Stanley Cohen and in 1973, they inserted foreign DNA into using restriction enzymes and plasmids, creating the first recombinant organisms. This breakthrough enabled gene cloning and expression in host cells, facilitating studies of cellular processes like insulin production and accelerating applications. A major leap in cellular editing occurred in 2012 with the development of -Cas9, a precise, programmable tool derived from bacterial immune systems. , , and colleagues demonstrated that -Cas9 uses to target and cleave specific DNA sequences, enabling efficient modifications in eukaryotic cells. This versatility revolutionized cell engineering, from knocking out genes to study function to creating disease models. Post-2020, advanced cellular therapies; in 2023, the FDA approved Casgevy (exagamglogene autotemcel), the first -edited therapy for , involving editing of hematopoietic stem cells to boost production. By 2025, over 50 -based clinical trials targeted cellular therapies for cancers and genetic disorders, demonstrating durable engraftment and reduced off-target effects. Recent decades have seen explosive growth in technologies, resolving cellular heterogeneity unattainable by bulk methods. Emerging in the early 2010s with protocols like Smart-seq (2012), the field advanced through high-throughput platforms such as Drop-seq (2015), which encapsulated cells in droplets for parallel profiling of thousands. By 2025, innovations like long-read single-cell sequencing integrated with revealed dynamic in tissues, aiding discoveries in tumor microenvironments and developmental trajectories. These tools have supported thousands of studies. Optogenetics, introduced in 2005 by and , enabled light-mediated control of cellular activity via genetically encoded ion channels like channelrhodopsin-2. Expressed in target cells, these proteins allow millisecond-precision modulation of neuronal firing or other cellular signaling, transforming studies of circuits and muscle function. Advances through 2025 include expanded toolkits for non-neuronal cells, such as , and two-photon excitation for deeper tissue penetration, with thousands of publications elucidating cellular networks in health and disease. Synthetic biology has progressed toward engineering minimal and artificial cells, exemplified by J. Craig Venter's 2010 creation of mycoides JCVI-syn1.0, the first self-replicating cell with a chemically synthesized . This 1.08-megabase , transplanted into a recipient cell, demonstrated bootable cellular life from digital code, identifying essential genes for viability. Updates by 2025 include artificial s, such as enzyme-loaded vesicles mimicking mitochondria for targeted ATP production, and nucleated synthetic cells with intercompartment communication via . These constructs, often lipid-based protocells, replicate division and metabolism, paving the way for designer therapeutics. AI-assisted cell imaging has complemented these efforts; models since 2020 analyze data to predict organelle dynamics and protein localization, enhancing synthetic design.

Cell Classification

Prokaryotic Cells

Prokaryotic cells are the structural and functional units of prokaryotes, which are organisms that lack a membrane-bound nucleus and other membrane-bound organelles, distinguishing them from eukaryotic cells. Prokaryotes encompass the two domains of life, and , which together represent the majority of known cellular diversity on . These cells typically measure between 0.1 and 5 micrometers in , with archaeal cells often ranging from 0.7 to 4 micrometers. Prokaryotes exhibit diverse morphologies, including cocci (spherical), (rod-shaped), and spirilla (spiral), which influence their motility and environmental interactions. Prokaryotes inhabit virtually every conceivable environment, from and to the gut, with particularly noted for thriving in extreme conditions such as hypersaline lakes, acidic hot springs, and deep-sea hydrothermal vents. Their genomes are typically organized as a single circular housed in a region, often supplemented by smaller plasmids that carry accessory genes for adaptation, such as resistance. Their diversity is vast, with over 26,000 validly described species as of November 2025, predominantly bacteria.

Eukaryotic Cells

Eukaryotic cells are characterized by the presence of a membrane-bound nucleus that houses their genetic material, setting them apart from prokaryotic cells, which lack this enclosure. They also feature an array of membrane-bound organelles that facilitate specialized cellular processes, contributing to their compartmentalized organization. These cells form the basis of life in the domain Eukarya, encompassing diverse kingdoms including protists, fungi, plants, and animals. In terms of size and complexity, eukaryotic cells are typically much larger than prokaryotes, with diameters often ranging from 10 to 100 micrometers, which supports intricate internal structures and functions. Their genomes are substantially larger, comprising multiple linear chromosomes packaged with proteins within the nucleus, enabling sophisticated regulation. Unlike the binary fission of prokaryotes, eukaryotic cells divide through (for somatic cells) or (for gametes), precise mechanisms that distribute replicated chromosomes to daughter cells. Eukaryotes display vast diversity, spanning unicellular forms such as the yeast , a model fungus used in genetic studies, and species, ciliated protists that exemplify free-living single-celled eukaryotes, to highly organized multicellular entities.

Common Structural Features

Plasma Membrane

The plasma membrane, also known as the , forms a selectively permeable barrier that encloses the cell and separates its internal environment from the external medium. It is primarily structured as a bilayer, where hydrophilic heads face the aqueous environments on both sides and hydrophobic tails form the inner core, with various proteins embedded or associated within this matrix. This organization is encapsulated in the , which depicts the membrane as a dynamic, two-dimensional fluid where lipids and proteins can diffuse laterally, allowing for flexibility and functionality. In terms of composition, the plasma membrane typically consists of about 50% and 50% proteins by weight, along with smaller amounts of carbohydrates attached to (glycolipids) and proteins (glycoproteins). The are predominantly phospholipids, such as and , which provide the basic bilayer framework. In eukaryotic cells, molecules are interspersed within the bilayer, modulating by restricting lipid movement at higher temperatures while preventing solidification at lower ones. Carbohydrates, often in the form of short chains, contribute to cell surface diversity but comprise less than 10% of the membrane mass. The plasma membrane performs essential functions that maintain cellular and enable interaction with the environment. It exhibits selective permeability, allowing small nonpolar molecules like oxygen and to pass freely via simple while restricting larger or charged substances. Transport across the membrane occurs through passive mechanisms, such as facilitated via channel proteins, and active processes, including ATP-driven pumps like the sodium-potassium , which establish ion gradients. Additionally, the membrane facilitates cell recognition through glycoproteins and glycolipids that serve as identifiers in processes like , and it supports signaling by hosting receptors that detect extracellular signals and transduce them into intracellular responses, such as via G-protein coupled receptors. A key aspect of membrane function involves maintaining an , which generates a typically ranging from -20 to -90 mV in living cells. This potential arises from unequal distributions across the , balanced by the lipid bilayer's low permeability to s. The quantifies the equilibrium potential EE for a specific , below which the concentration gradient drives influx and above which electrical forces drive efflux: E=RTzFln⁡([\ion\out][\ion∈])E = \frac{RT}{zF} \ln \left( \frac{[\ion_{\out}]}{[\ion_{\in}]} \right) Here, RR is the , TT is the absolute temperature, zz is the 's valence, FF is Faraday's constant, and [\ion\out][\ion_{\out}] and [\ion∈][\ion_{\in}] are the extracellular and intracellular concentrations, respectively. This illustrates how concentration differences create an electrochemical driving force, essential for processes like nerve impulse propagation and nutrient uptake; for example, the potassium Nernst potential in neurons is around -90 mV, contributing to the . While the phospholipid-based is universal, variations exist across cell types, notably in , which inhabit extreme environments. Archaeal plasma membranes incorporate ether-linked , where isoprenoid chains are connected via bonds to glycerol, unlike the ester bonds in bacterial and eukaryotic phospholipids. These enhance stability against high temperatures, acidity, and salinity, enabling adaptations; for instance, in thermophilic like , they prevent lipid and maintain integrity up to 90°C.

Cytoplasm and Cytoskeleton

The , also known as the when referring specifically to its fluid component, is the aqueous gel-like substance that fills the interior of the cell, excluding the nucleus and membrane-bound organelles. It consists primarily of , along with dissolved ions, salts, small organic molecules, and proteins, forming a viscous medium that constitutes the majority of the cell's volume. This gel-like consistency arises from its high concentration of macromolecules, which imparts a thickness greater than pure , enabling dynamic changes in that facilitate intracellular and of materials. The serves as the primary site for numerous metabolic reactions, including and other soluble enzyme-catalyzed processes, while also acting as a storage for essential molecules such as , sugars, and ions. It provides a supportive platform for the operation and positioning of non-membrane-bound cellular components, allowing for efficient diffusion-based transport within the cell. These functions are conserved across prokaryotic and eukaryotic cells, underscoring the cytoplasm's role in maintaining cellular and enabling rapid responses to environmental changes. Embedded within the cytoplasm is the cytoskeleton, a dynamic network of protein filaments that provides and enables mechanical functions in all cells. In eukaryotic cells, it comprises three main types of filaments: , assembled from α- and β-tubulin dimers with a diameter of approximately 25 nm; microfilaments, or actin filaments, formed by globular (G-actin) monomers polymerizing into double-helical strands about 7 nm in diameter; and intermediate filaments, diverse fibrous proteins such as keratins that assemble into rope-like structures 8–12 nm thick. Prokaryotes lack these exact structures but possess homologs, including MreB proteins that form actin-like helical filaments for shape maintenance. These components undergo continuous assembly () and disassembly (), driven by hydrolysis, allowing the cytoskeleton to adapt to cellular needs. For instance, in actin , on incorporated G-actin subunits promotes net filament flux, with the treadmilling rate given by j=kon[G-actin]−koffj = k_{\text{on}} [\text{G-actin}] - k_{\text{off}}, where konk_{\text{on}} and koffk_{\text{off}} are the association and dissociation rate constants at the filament ends. The plays critical roles in maintaining cell shape by resisting mechanical stresses—microtubules and microfilaments counter compression and tension, respectively, while intermediate filaments offer tensile strength—and in facilitating intracellular , where serve as tracks for motor proteins carrying vesicles and organelles. It is also essential for , with forming the mitotic spindle to segregate chromosomes and filaments contracting to cleave the during . These functions highlight the cytoskeleton's integration with the to ensure structural integrity and dynamic processes across cell types.

Ribosomes and Protein Synthesis Machinery

Ribosomes are ribonucleoprotein complexes that serve as the primary sites for protein synthesis in all cells. In prokaryotes, the ribosome is a 70S particle composed of a small 30S subunit and a large 50S subunit; the 30S subunit contains one 16S rRNA and 21 proteins, while the 50S subunit includes one 23S rRNA, one 5S rRNA, and 34 proteins. In eukaryotes, the ribosome forms an complex with a small subunit and a 60S large subunit; the subunit has one 18S rRNA and 33 proteins, whereas the 60S subunit contains one 28S rRNA (or 25S in some organisms), one 5.8S rRNA, one 5S rRNA, and 47 proteins, resulting in approximately 80 proteins total. These structures enable the to decode (mRNA) and assemble into polypeptide chains. Ribosomes are located either free in the , where they synthesize soluble and nuclear proteins, or bound to the cytosolic face of the rough (ER) in eukaryotic cells, facilitating the co-translational insertion of proteins into the ER lumen for secretion or membrane integration. In prokaryotes, ribosomes are predominantly free in the or associated with the plasma membrane, reflecting the absence of membrane-bound organelles. The binding of ribosomes to the rough ER is mediated by signal recognition particles that recognize specific mRNA sequences, directing the machinery to appropriate locations based on protein destiny. Protein synthesis on ribosomes, known as , proceeds in three principal stages: , elongation, and termination, ensuring the accurate transfer of genetic information from mRNA to proteins. During , the small ribosomal subunit binds the mRNA and the initiator tRNA, followed by large subunit joining to form the complete . Elongation involves sequential addition of , while termination occurs upon recognition of stop codons, releasing the completed polypeptide. The , which assigns nearly universal triplet codons on mRNA to specific via transfer RNAs (tRNAs), underpins this process across prokaryotes and eukaryotes, with minor variations in organelles or certain microbes. Central to elongation is the peptidyl transferase activity within the large ribosomal subunit, where the rRNA catalyzes formation between the growing polypeptide on the peptidyl-tRNA in the and the incoming in the A site, achieving rates up to 20 bonds per second in bacteria.00185-2) Elongation factors, such as EF-Tu in prokaryotes and eEF1A in eukaryotes, deliver to the A site in a GTP-dependent manner, with kinetic studies showing that GTP accelerates tRNA accommodation and translocation, limiting overall synthesis speed to about 15-20 per second under optimal conditions.00293-1) These factors ensure fidelity and efficiency, preventing errors in codon-anticodon matching. Structural variations in ribosomes provide targets for antibiotics, such as , which binds the subunit in prokaryotes to block entry into the A site, selectively inhibiting bacterial protein synthesis without affecting eukaryotic 40S subunits. Recent cryo-electron (cryo-EM) structures from the 2020s have illuminated dynamics, capturing intermediate states during translocation and revealing conformational changes driven by elongation factors like , which rotate the subunits to advance mRNA and tRNAs.00908-0) These advances highlight the ribosome's mechanistic flexibility, essential for adapting to cellular demands.

Prokaryotic-Specific Structures

Cell Wall and Envelope

The cell wall in prokaryotes serves as a rigid protective layer external to the plasma membrane, providing structural integrity and resistance to environmental stresses. In , this wall is primarily composed of , also known as murein, a complex consisting of alternating units of (NAG) and N-acetylmuramic acid (NAM) linked by β-1,4 glycosidic bonds, with short chains attached to the NAM residues that enable cross-linking. This mesh-like structure forms a sacculus that encases the cell, determining its shape—such as rods, spheres, or spirals—and counteracting internal to prevent osmotic . In , the cell wall analog is , found in certain methanogens, which features β-1,3 glycosidic linkages between N-acetyltalosaminuronic acid and instead of the bacterial β-1,4 bonds, along with cross-links using different like L-lysine or , conferring similar rigidity but distinct chemical properties. Bacterial cell walls are classified into Gram-positive and Gram-negative types based on their response to Gram staining, reflecting differences in thickness and architecture. possess a thick layer, often 20–80 nm, comprising up to 90% of the wall and interspersed with teichoic acids that anchor the wall to the membrane and contribute to ion regulation. In contrast, have a thin layer, about 2–7 nm thick, sandwiched between the inner cytoplasmic membrane and an outer membrane composed of lipopolysaccharides (LPS), which adds an additional barrier against hydrophobic compounds and host defenses. The includes a periplasmic space between the two membranes, a gel-like compartment enriched with proteins that facilitate modification, degradation, and of enzymes such as β-lactamases for resistance. The primary functions of the prokaryotic cell wall include maintaining cell shape under mechanical stress and providing osmotic protection, as the network withstands turgor pressures up to 20 atm in some . It also plays a critical role in susceptibility; for instance, β-lactam antibiotics like penicillin target synthesis by acylating transpeptidase enzymes, preventing cross-linking and leading to cell during growth. of involves sequential steps: UDP-N-acetylglucosamine and UDP-N-acetylmuramyl-pentapeptide precursors are synthesized in the , lipid II carriers transport them across the , and glycosyltransferases polymerize the glycan chains, followed by transpeptidation to form peptide cross-links between D-alanine and another , typically L-lysine in Gram-positives or in Gram-negatives, creating a robust lattice. While the cell wall is a defining prokaryotic feature, some eukaryotes exhibit analogous structures with different compositions. Fungal cell walls are primarily built from , a β-1,4-linked of that provides tensile strength and is cross-linked with β-glucans for rigidity. In plants and algae, the wall consists mainly of , a β-1,4-linked glucose forming microfibrils embedded in a matrix of hemicelluloses and pectins, enabling cell expansion and mechanical support. These eukaryotic variations underscore the of protective barriers, though they differ fundamentally from prokaryotic in and enzymatic vulnerabilities.

Nucleoid and Genetic Organization

In prokaryotic cells, the nucleoid serves as the non-membrane-bound region housing the genomic DNA, forming an irregular complex of DNA and proteins that occupies a significant portion of the cytoplasm. This structure centers on a supercoiled circular chromosome, typically 1–4 megabases (Mb) in length, which in Escherichia coli measures approximately 4.6 Mb and adopts a compact, helical ellipsoid shape for spatial confinement within the cell. The DNA's supercoiling, facilitated by topoisomerases, enables the chromosome to fit within the cell dimensions while maintaining accessibility for cellular processes. The organization of the nucleoid relies on nucleoid-associated proteins (NAPs) that compact and structure the DNA into higher-order domains. Key NAPs include HU, which binds non-specifically to DNA bends and junctions to promote compaction—E. coli cells contain about 55,000 HU molecules during —and others such as H-NS, Fis, and IHF that help form plectonemic loops, macrodomains (e.g., ~1 Mb regions like Ori and Ter in E. coli), and smaller topological domains of 10–400 kb separated by diffusion barriers. Polyamines, including and spermidine, further enhance stability by neutralizing DNA's negative charge, reducing electrostatic repulsion and aiding compaction without sequence specificity. This architecture contrasts sharply with the eukaryotic nucleus, which is enclosed by a double that isolates from the , whereas the nucleoid allows immediate interaction with cytosolic components like ribosomes. Functionally, the nucleoid supports coupled transcription and translation, as nascent mRNA is directly accessible to ribosomes in the cytoplasm, enabling rapid gene expression tied to the chromosome's three-dimensional organization. In bacteria like Vibrio cholerae, which possess two chromosomes, each harbors a distinct replication origin (oriC I on the large chromosome and oriC II on the small one), facilitating temporally coordinated replication initiation and contributing to nucleoid dynamics. During cell division, the nucleoid undergoes partitioning to ensure equal distribution to daughter cells via binary fission; the nucleoid occlusion model prevents septum formation over unsegregated DNA by inhibiting FtsZ ring assembly through proteins like SlmA in E. coli, which binds specific DNA sequences and blocks division-site maturation. This dynamic reorganization, including increased long-range DNA contacts in stationary phase, maintains nucleoid integrity across growth conditions.

Surface Appendages

Surface appendages in prokaryotes are dynamic extracellular structures that extend from the cell envelope, enabling functions such as , , and . These appendages, including flagella, pili (also known as fimbriae), and capsules, are primarily found in and , where they interact with the surrounding milieu to facilitate survival and adaptation. Unlike the static , these structures are often assembled and disassembled in response to environmental cues, contributing to processes like colonization and . Bacterial flagella are long, helical filaments composed of a basal body embedded in the and wall, a flexible hook, and an extracellular filament made primarily of proteins. The acts as a rotary motor, with stator units (MotA/MotB complexes) harnessing the proton motive across the to generate , powering counterclockwise rotation for smooth swimming. This ion-driven mechanism, rather than , allows flagella to achieve speeds up to 100 body lengths per second in some bacterial species, such as . In , the analogous structure is the archaellum, which shares a superficial rotary function but differs fundamentally in assembly: archaella are type IV pilus-like, polymerizing from the base with distinct subunits (e.g., FlaA/B) and powered by ATPases, lacking homology to bacterial flagellar components. Flagella enable bacterial motility through chemotaxis, where cells exhibit a "run-and-tumble" pattern: prolonged counterclockwise rotation produces straight runs toward attractants, while clockwise switches cause filament bundling and random tumbles for reorientation. This biased random walk optimizes navigation in gradients, as seen in E. coli, where tumbling frequency decreases in favorable conditions. Pili, shorter and more numerous than flagella, include type IV variants that extend and retract via ATP-powered assembly and disassembly at the pilin tip. Retraction, driven by dedicated motors like PilT, generates forces exceeding 100 pN, facilitating twitching motility and adhesion to surfaces. Common pili (type 1 fimbriae) mediate host cell attachment in pathogens like uropathogenic E. coli, while sex pili (e.g., F-pili in conjugative plasmids) form bridges for DNA transfer during bacterial conjugation, stabilizing donor-recipient pairs under shear stress. Capsules and the broader are polysaccharide-based layers enveloping the cell, often loosely associated with the to form a protective sheath. These structures trap water and nutrients, shielding from , , and antibiotics; for instance, the capsule of inhibits complement activation for immune evasion. S-layers, crystalline protein arrays on many prokaryotes, further contribute to protection by masking surface antigens and promoting formation, where appendages like pili and capsules anchor communities. In biofilms, glycocalyx matrices enhance and resistance, as exemplified by exopolysaccharides that withstand host defenses. Recent advances in biofilm engineering leverage these appendages for biotechnological applications, such as designing synthetic microbial consortia for remediation and medical implants. For example, engineered E. coli capsules have been modified since 2020 to form tunable that deliver therapeutics in the gut, improving efficacy and reducing colonization. As of 2025, engineered continue to advance, with studies demonstrating precise modulation of the intestinal microenvironment for treating through targeted payload delivery.

Eukaryotic-Specific Structures

Nucleus

The nucleus serves as the control center of eukaryotic cells, housing the genetic material and orchestrating key processes such as and . Enclosed by the , it separates the genomic DNA from the , enabling precise regulation of transcription and RNA processing that is characteristic of eukaryotic complexity. This is essential for maintaining cellular identity and responding to environmental cues through controlled activity. The nuclear envelope consists of a double lipid bilayer—the inner and outer nuclear membranes—perforated by nuclear pore complexes (NPCs) that facilitate selective transport between the nucleus and cytoplasm. The inner nuclear membrane is lined by the nuclear lamina, a meshwork of type V intermediate filaments primarily composed of lamin proteins, which provides structural support and anchors chromatin to the envelope. Chromatin, the complex of DNA and histone proteins, is organized into euchromatin, which is loosely packed and transcriptionally active, and heterochromatin, which is densely condensed and generally inactive; this packaging modulates access to genetic information. Within the nucleus, the nucleolus forms a prominent substructure dedicated to ribosomal RNA (rRNA) synthesis and ribosome assembly, containing high concentrations of RNA polymerase I and ribosomal proteins. The primary functions of the nucleus include the storage and protection of DNA, as well as the initiation of transcription where DNA is copied into messenger RNA (mRNA) by RNA polymerases. This process underlies the central dogma of molecular biology, in which genetic information flows from DNA to RNA and subsequently to proteins, with transcription occurring exclusively in the nucleus. Mature mRNA is then exported to the cytoplasm through NPCs via the Ran GTPase cycle, a GTP-binding protein system that generates a nuclear-cytoplasmic gradient to drive directional transport; Ran-GTP promotes export complex assembly in the nucleus, while its hydrolysis in the cytoplasm disassembles them. The nucleolus supports this by producing rRNA components essential for translation machinery. During the , the nucleus exhibits dynamic changes, particularly in , where the breaks down to allow segregation. of proteins by cyclin-dependent kinases causes the lamina to depolymerize into dimers, leading to envelope disassembly and condensation. Post-mitosis, enables lamina reassembly and envelope reformation around daughter nuclei. These dynamics ensure equitable distribution of genetic material while temporarily suspending transcription. Variations in nuclear organization occur in certain cell types, such as multinucleated syncytia like fibers, where hundreds of nuclei share a common to support high metabolic demands and rapid protein synthesis. In these cells, nuclei maintain individual identities but coordinate to optimize tissue function, illustrating adaptations in nuclear for specialized roles.

Membrane-Bound Organelles

Membrane-bound organelles in eukaryotic cells enable compartmentalization, allowing specialized biochemical reactions to occur in isolated environments while facilitating communication through vesicular transport. These include the components such as the , Golgi apparatus, and lysosomes, as well as other organelles like peroxisomes, mitochondria, and chloroplasts (in photosynthetic eukaryotes), each with distinct roles in protein processing, , degradation, detoxification, production, and . The (ER) is a continuous network of tubules and sacs that spans the , divided into rough and smooth domains based on ribosomal association and function. The rough ER, studded with ribosomes on its cytoplasmic surface, serves as the primary site for co-translational folding and modification of secretory and proteins, where chaperones like BiP assist in attaining native conformations to prevent aggregation. In contrast, the smooth ER lacks ribosomes and specializes in synthesis, including phospholipids and steroids essential for membrane biogenesis, as well as calcium ion storage in its lumen via pumps like , which buffers cytosolic Ca²⁺ levels for signaling. The Golgi apparatus consists of stacked cisternae organized into cis, medial, and trans compartments, exhibiting polarity with the cis face receiving vesicles from the ER and the trans face directing cargo to destinations like the plasma membrane or lysosomes. It functions in , particularly , where N-linked and O-linked sugars are added to proteins by resident glycosyltransferases, influencing protein stability, trafficking, and cell-cell recognition. Sorting occurs here via mannose-6-phosphate tags for lysosomal enzymes or clathrin adaptors for endocytic pathways, ensuring precise delivery. Lysosomes are acidic vesicles (pH ≈5) containing over 50 hydrolytic enzymes, including acid hydrolases like cathepsins and nucleases, that degrade macromolecules from , , or . The low pH, maintained by proton pumps, optimizes activity for breaking down proteins, , and nucleic acids into reusable monomers. In , lysosomes fuse with autophagosomes to digest damaged organelles or cytosolic components, recycling nutrients during stress. Plant vacuoles serve analogous roles, combining degradative functions with turgor maintenance. Mitochondria are double-membrane-bound organelles present in nearly all eukaryotic cells, serving as the primary sites of aerobic respiration and ATP production via . The outer is freely permeable to small molecules, while the highly folded inner forms cristae that house the complexes and . Mitochondria possess their own circular genome (mtDNA) and ribosomes, enabling semi-autonomous protein synthesis, and originated from endosymbiotic alpha-proteobacteria. Beyond energy production, they regulate cellular metabolism, calcium homeostasis, through cytochrome c release, and generate (ROS) as signaling molecules. Dysfunctional mitochondria are implicated in aging and diseases like Parkinson's. Chloroplasts are specialized membrane-bound organelles found in the cells of plants and algae, responsible for , which converts light energy into stored in glucose. Enclosed by a double , they contain internal thylakoid membranes stacked into grana, where I and II capture light to drive electron transport and generate ATP and NADPH. The fluid stroma surrounding the thylakoids hosts the Calvin-Benson cycle for carbon fixation, along with chloroplast DNA (cpDNA) and ribosomes, reflecting their endosymbiotic origin from . Chloroplasts also synthesize fatty acids, , and hormones, and coordinate with the nucleus via . Peroxisomes are single-membrane organelles that perform β-oxidation of very long-chain fatty acids, shortening them for mitochondrial processing, and detoxify (ROS) like using and peroxidases. This dual role positions peroxisomes as key regulators of lipid and oxidative balance, with impaired function linked to disorders like . Recent studies highlight peroxisome-mitochondria crosstalk at contact sites, where shared metabolites influence ROS signaling in aging. The organelles integrate via vesicular transport, where COPII coats mediate anterograde ER-to-Golgi trafficking, COPI handles retrograde Golgi-to-ER recycling, and coats facilitate sorting from the trans-Golgi to lysosomes or the plasma membrane. This dynamic network maintains cellular by coordinating synthesis, modification, and degradation.

Extracellular Matrix

The (ECM) in eukaryotic cells is a of secreted macromolecules that surrounds and supports cells in tissues, providing both structural and biochemical cues essential for cellular behavior. Primarily found in animal tissues, the ECM differs fundamentally from the rigid cellulose-based cell walls of , which offer mechanical protection but lack the dynamic flexibility and signaling capabilities of the animal ECM. In animals, the ECM enables tissue resilience and adaptability, allowing for processes like and development. The composition of the ECM includes fibrous proteins such as collagens, which form the structural scaffold; , which imparts elasticity; and glycoproteins like , which facilitate . Additionally, proteoglycans—core proteins decorated with long () chains—contribute to the hydrated that fills the matrix, providing a gel-like medium for and lubrication. Collagens, the most abundant proteins, assemble into that vary by type (e.g., type I in tendons for tensile strength), while GAGs like attract water to maintain tissue hydration and compressive resistance. The ECM serves multiple functions, including mechanical support to withstand physiological stresses, mediation of cell adhesion through integrins that link the matrix to the cytoskeleton, and regulation of signaling pathways via sequestered growth factors such as transforming growth factor-beta (TGF-β). Integrins, transmembrane receptors, bind ECM components like fibronectin and collagen, transmitting signals that influence cell proliferation, migration, and survival. Growth factors embedded in the matrix are released upon remodeling, modulating cellular responses in development and repair. ECM types include the , a thin (approximately 50-100 nm) sheet-like structure underlying epithelial and endothelial cells, composed mainly of , , and proteoglycans to provide a barrier and function. In contrast, the interstitial matrix is a looser, fibrillar network in connective tissues, rich in type I and III collagens and , filling spaces between cells to enable flexibility and nutrient exchange. These distinct architectures support specialized tissue roles, such as the basal lamina's role in organ compartmentalization. ECM dynamics involve continuous remodeling, primarily driven by matrix metalloproteinases (MMPs), a of enzymes that degrade ECM components to facilitate tissue , , and repair. Dysregulated MMP activity can lead to excessive matrix deposition and , a pathological scarring process seen in diseases like liver , where persistent TGF-β signaling promotes activation and overproduction. This remodeling balance is crucial for maintaining tissue .

Cellular Processes

Metabolism and Energy Production

Cellular metabolism consists of catabolic pathways that degrade complex molecules to release and anabolic pathways that utilize this to synthesize cellular components. Catabolism provides the free energy needed for and other energy-demanding processes, while builds essential biomolecules such as proteins, nucleic acids, and . Central to these processes is (ATP), the primary energy currency of the cell, which stores and transfers through the hydrolysis of its phosphoanhydride bonds, releasing approximately 30.5 kJ/mol under standard conditions. Glycolysis represents the foundational catabolic pathway for glucose breakdown, occurring in the of both prokaryotic and eukaryotic cells and comprising ten sequential enzymatic reactions divided into an energy-investment phase and an energy-payoff phase. In the investment phase, two ATP molecules are consumed to phosphorylate glucose and its derivatives, while the payoff phase generates four ATP through and two NADH by oxidizing glyceraldehyde-3-phosphate. The net yield per glucose is two ATP and two NADH, with the overall reaction summarized as: Glucose+2NAD++2ADP+2Pi→2pyruvate+2NADH+2ATP+2H2O+2H+\text{Glucose} + 2\text{NAD}^{+} + 2\text{ADP} + 2\text{P}_{i} \rightarrow 2\text{pyruvate} + 2\text{NADH} + 2\text{ATP} + 2\text{H}_{2}\text{O} + 2\text{H}^{+} This pathway is universal and evolutionarily ancient, enabling rapid ATP production even in the absence of oxygen. Under aerobic conditions, pyruvate from glycolysis is transported into mitochondria (in eukaryotes) or the cytoplasm (in prokaryotes with analogous systems), where it is decarboxylated to acetyl-CoA and enters the citric acid cycle, producing additional NADH and FADH₂. These electron carriers donate electrons to the electron transport chain (ETC), a series of four transmembrane protein complexes (I–IV) embedded in the inner mitochondrial membrane or plasma membrane. Complex I (NADH dehydrogenase) and complex II (succinate dehydrogenase) feed electrons from NADH and FADH₂, respectively, which are then transferred through complexes III (cytochrome bc₁) and IV (cytochrome c oxidase) to oxygen, the terminal acceptor, forming water. This electron flow drives proton pumping across the membrane, establishing an electrochemical gradient (proton-motive force). ATP synthesis occurs via chemiosmosis, where protons flow back through ATP synthase (complex V), rotating its rotor to catalyze ADP phosphorylation, yielding up to 30–32 ATP per glucose molecule when coupled with glycolysis and the citric acid cycle. In anaerobic environments, common among prokaryotes, oxidative phosphorylation is unavailable, so cells rely on to regenerate NAD⁺ for continued . Fermentation pathways, such as in some or alcoholic fermentation in , convert pyruvate to byproducts like lactate or , yielding no net ATP beyond the two from glycolysis but allowing ATP production at rates up to 100 times faster than aerobic respiration under low-oxygen conditions. In photosynthetic organisms like cells, chloroplasts perform that generate ATP and NADPH via a proton gradient across the membrane, powering carbon fixation in the [Calvin cycle](/page/Calvin cycle) to produce glucose as an form. Metabolic flux through these pathways is precisely regulated to balance , primarily via allosteric modulation of enzymes. For example, phosphofructokinase-1 (PFK-1), which catalyzes the committed step of (fructose-6-phosphate to fructose-1,6-bisphosphate), is allosterically activated by AMP and fructose-2,6-bisphosphate when is low, but inhibited by high ATP and citrate levels to prevent unnecessary glucose consumption. Similar feedback mechanisms control the and ETC, ensuring efficient resource allocation; for instance, ATP utilization in protein synthesis on ribosomes consumes a significant portion of cellular ATP, linking to biosynthetic demands.

Growth, Division, and Reproduction

Cell growth refers to the increase in cellular mass and volume, primarily occurring during the interphase of the cell cycle, which consists of three main phases: G1, S, and G2. In the G1 phase, the cell synthesizes proteins and organelles to prepare for DNA replication, ensuring sufficient resources for subsequent division. The S phase involves DNA synthesis, where the genetic material is duplicated to produce identical sister chromatids. The G2 phase allows for further growth and checks for DNA replication errors before entering mitosis. These phases are tightly regulated by checkpoints to prevent errors; for instance, the G1/S checkpoint assesses DNA damage, with the tumor suppressor protein p53 activating repair mechanisms or halting progression if damage is irreparable. Prokaryotic cells primarily reproduce through binary fission, an asexual process where the cell elongates, replicates its circular , and divides into two genetically identical daughter cells. Central to this mechanism is the Z-ring, formed by of the tubulin-like protein , which assembles at the midcell division site and constricts the membrane inward, guiding septum formation. 's activity drives of protofilaments, enabling dynamic ring contraction and coordination with other divisome proteins for efficient . This process typically takes 20-60 minutes in like Escherichia coli, allowing rapid under favorable conditions. In eukaryotes, cell division occurs via for somatic cells or for gametes, both preceded by growth. ensures equal distribution of replicated chromosomes to two daughter cells and comprises four phases: , where chromosomes condense and the mitotic spindle—a -based apparatus—begins forming from centrosomes; , in which chromosomes align at the equatorial plate via attachments to spindle fibers; , where separate and migrate to opposite poles driven by shortening; and , marked by reformation and decondensation of chromosomes. follows, dividing the via an actin-myosin contractile ring in animal cells or a in . , essential for , involves two divisions: I reduces chromosome number by separating homologous pairs (with crossing over in I for ), followed by II, which resembles to yield four haploid cells. Cell cycle progression is regulated by cyclin-dependent kinases (CDKs), serine/threonine kinases activated by binding to phase-specific s, which fluctuate in concentration to drive transitions—such as D-CDK4/6 for G1, E-CDK2 for G1/S, A-CDK2 for S/G2, and B-CDK1 for G2/M. Checkpoints monitor fidelity; dysregulation, like unchecked proliferation, can trigger , a pathway involving activation to eliminate damaged cells and maintain tissue . For example, induces via Bax/Bak if DNA damage persists beyond repair thresholds. At the cellular level, asexual reproduction predominates in prokaryotes via binary fission and in unicellular eukaryotes via mitosis, producing clonal offspring for efficient propagation in stable environments. Sexual reproduction, conversely, relies on meiosis to generate haploid gametes that fuse during fertilization, introducing genetic variation through recombination and independent assortment, which enhances adaptability in eukaryotes. This distinction underscores mitosis and binary fission's role in growth and maintenance versus meiosis's contribution to diversity.

DNA Replication and Repair

DNA replication is a fundamental process in which genetic information is duplicated to ensure accurate transmission to daughter cells during . In both prokaryotes and eukaryotes, replication proceeds semi-conservatively, meaning each new DNA double helix consists of one parental strand and one newly synthesized strand, as demonstrated by the classic density-labeling experiments using in . This mechanism was confirmed through centrifugation analysis showing hybrid density DNA after one generation and a mix of hybrid and light DNA after subsequent generations. Replication initiates at specific origins of replication, where unwinds , forming a replication fork that moves bidirectionally. In prokaryotes, such as E. coli, the primary replicative polymerase is (Pol III), a holoenzyme complex with high processivity that synthesizes both leading and lagging strands. Pol III incorporates nucleotides at a rate of approximately 1000 nucleotides per second per fork, enabling rapid genome duplication. In eukaryotes, replication is more complex due to larger genomes and multiple origins; (Pol δ) primarily synthesizes the lagging strand, while (Pol ε) handles the leading strand, both operating at about 50 nucleotides per second. The lagging strand is synthesized discontinuously as short , each primed by and later joined by after primer removal; this was first observed in E. coli studies revealing nascent DNA segments of 1000–2000 nucleotides. by the 3'→5' activity of these polymerases reduces incorporation errors to approximately 10−710^{-7} per , enhancing fidelity. Despite these safeguards, DNA is susceptible to damage from endogenous and environmental sources, necessitating repair mechanisms to maintain genomic integrity. (BER) addresses small, non-helix-distorting lesions like oxidative damage or ; recognize and excise the damaged base, creating an apurinic/apyrimidinic ( that is processed by AP endonuclease and repaired using the intact strand as a template. (NER) removes bulky, helix-distorting adducts, such as those induced by (UV) light forming cyclobutane ; in eukaryotes, this involves damage recognition by XPC or stalling, excision of a 24–32 oligonucleotide by XPF-ERCC1 and XPG, and resynthesis. Mismatch repair (MMR) corrects base-pairing errors or small insertion/deletion loops from replication slippage; MutS homologs (MSH proteins, e.g., MSH2-MSH6) recognize mismatches, followed by excision and resynthesis directed by strand discrimination. Double-strand breaks (DSBs), the most severe lesions, are repaired by homologous recombination (HR) or non-homologous end joining (NHEJ). HR, predominant in S/G2 phases, uses a sister chromatid template for error-free repair via strand invasion and synthesis, as elucidated in early yeast models. NHEJ ligates broken ends directly, often introducing small insertions/deletions, and is active throughout the cell cycle via Ku proteins and DNA-PKcs. Telomeres, the repetitive ends of linear eukaryotic chromosomes, pose a replication challenge due to the end-replication problem; telomerase, a reverse transcriptase with an integral RNA component serving as a template (e.g., 3'-CAAUCCCAAUC-5' in humans), extends 3' overhangs by adding TTAGGG repeats, preventing progressive shortening and senescence. Errors persisting through replication and repair can lead to , including point , insertions, deletions, or chromosomal rearrangements, which accumulate and drive oncogenesis. Defects in replication fidelity or repair pathways, such as deficiencies or MMR inactivation, elevate rates and are implicated in ~10–15% of cancers, including colorectal tumors with .

Motility and Movement

Cells exhibit through diverse mechanisms that enable locomotion across surfaces or through fluids, essential for processes such as acquisition, predator avoidance, and . In prokaryotes, flagellar rotation drives via a rotary motor embedded in the , powered by proton motive force, achieving rotation speeds of 100-300 Hz under optimal conditions. This helical flagellar filament propels bacteria like at velocities up to 20-30 body lengths per second. , observed in bacteria such as , relies on type IV pili, which extend and retract to pull the cell forward across solid surfaces at speeds of 2-5 μm/min, without requiring flagella. Eukaryotic cells employ more varied strategies for extracellular movement. Cilia and flagella feature a characteristic 9+2 arrangement of , where outer arms generate ATP-powered sliding between doublets, resulting in bending waves that propel cells like at 100-200 μm/s. Amoeboid locomotion, typical of leukocytes and amoebae, involves at the leading edge to form , enabling irregular crawling through tissues at rates of 0.1-1 μm/min. Similarly, many adherent cells crawl using lamellipodia, broad actin-rich protrusions driven by Arp2/3-mediated branching , facilitating migration at 0.5-10 μm/min on substrates. Intracellular motility transports vesicles and organelles along cytoskeletal tracks, primarily , using motor proteins and . moves toward the microtubule plus end with an 8 nm step size per ATP hydrolyzed, achieving velocities of 0.2-0.8 μm/s, while travels to the minus end with variable steps of 8-32 nm and speeds up to 1 μm/s. The vv of these motors is given by v=dτv = \frac{d}{\tau}, where dd is the step size and τ\tau is the dwell time between steps. All these mechanisms are fueled by , which provides the for conformational changes in motor proteins. Cellular motility often responds to environmental cues, such as phototaxis in photosynthetic protists like Euglena, where light gradients direct flagellar beating toward optimal illumination, and geotaxis (or gravitaxis) in dinoflagellates, enabling upward or downward migration in response to gravity via reorientation of swimming direction. Emerging biohybrid nanomotors, integrating synthetic components with cellular elements, show promise for enhanced motility control, though applications remain in early development as of 2025.

Cell Communication and Signaling

Intracellular Signaling

Intracellular signaling, also known as , refers to the processes by which cells convert external or internal stimuli into specific cellular responses through a series of molecular interactions within the and nucleus. These pathways enable cells to sense changes in their environment and regulate functions such as , growth, and . Key components include receptors that detect signals, second messengers that amplify them, and cascades that propagate the response, often culminating in transcriptional changes. Dysregulation of these pathways is implicated in diseases like cancer.00665-3) Receptors such as G-protein-coupled receptors (GPCRs), which feature seven transmembrane domains, initiate signaling by activating heterotrimeric G proteins upon binding, leading to the dissociation of Gα and Gβγ subunits that modulate downstream effectors. Enzyme-linked receptors, particularly receptor tyrosine kinases (RTKs), dimerize and autophosphorylate upon activation, creating docking sites for adaptor proteins and initiating relays. These receptor types transduce signals through distinct but sometimes overlapping intracellular mechanisms.00665-3) Second messengers play a central role in amplifying signals. Cyclic adenosine monophosphate (cAMP), produced by adenylyl cyclase upon GPCR activation, activates protein kinase A (PKA), which phosphorylates targets to influence gene expression and metabolism. Inositol 1,4,5-trisphosphate (IP3) and calcium ions (Ca²⁺) are released from the endoplasmic reticulum following phospholipase C activation, where IP3 binds IP3 receptors to trigger Ca²⁺ efflux, forming oscillatory waves that regulate processes like muscle contraction and secretion. These messengers enable rapid, diffusible propagation of signals within the cell. Cascade pathways, such as the (MAPK) pathway, involve sequential events. In the ERK branch, receptor activation leads to Ras GTP loading, which recruits ; then and activates MEK, which in turn ERK, promoting translocation to the nucleus for activation and . This modular cascade allows integration of multiple inputs and fine-tuned outputs. A prominent example is insulin signaling, where the , an RTK, phosphorylates insulin receptor substrates (IRS), recruiting phosphatidylinositol 3-kinase (PI3K) to generate PIP3, which activates . Akt promotes by inhibiting GSK3 and translocating transporters to the membrane, ensuring metabolic homeostasis. This pathway exemplifies how signaling integrates nutrient sensing with cellular responses. Negative feedback mechanisms maintain signaling fidelity and prevent overstimulation. Phosphatases, such as protein phosphatases (PTPs), dephosphorylate activated kinases in RTK and MAPK pathways, terminating the signal; for instance, MAPK phosphatases (MKPs) specifically inactivate ERK to reset the cascade. These loops ensure transient, proportional responses to stimuli.30197-0) Dysregulation of intracellular signaling often drives oncogenesis, with in oncogenes like RAS leading to constitutive MAPK activation and uncontrolled proliferation in cancers such as pancreatic . Similarly, PI3K/Akt hyperactivation from PTEN loss promotes survival and . Targeted therapies, like MEK inhibitors, exploit these vulnerabilities.

Intercellular Communication

Intercellular communication enables cells within multicellular organisms to coordinate behaviors essential for development, , and response to environmental cues. This process involves the transmission of signals between cells, often through specific molecules that bind to receptors on target cells, triggering responses that maintain tissue integrity and organismal function. In contrast to intracellular signaling, which handles internal processing, intercellular signaling focuses on external exchanges that integrate collective cellular activities. Cells employ diverse mechanisms for intercellular communication, categorized by the distance and mode of signal transmission. occurs when a cell releases a that binds to receptors on its own surface, influencing its own activity; for example, immune cells like T lymphocytes use autocrine signaling with to amplify their responses. Direct communication occurs via physical connections, such as gap junctions in animal cells, which form channels composed of proteins allowing the passage of ions, second messengers, and small metabolites between adjacent cytoplasms. In , plasmodesmata serve a similar role, creating symplastic pathways through cell walls for the transport of nutrients and signaling molecules like sugars and hormones. involves local diffusion of signaling molecules to nearby cells; for instance, released by mast cells induces and in adjacent bronchial cells via H1 receptors. Endocrine signaling transmits hormones, such as insulin, through the bloodstream to distant target cells, regulating systemic processes like glucose by binding to specific receptors with high affinity. Synaptic signaling, a specialized paracrine form, occurs at neuronal junctions where neurotransmitters like are released into the synaptic cleft to rapidly propagate electrical impulses across short distances. Key signaling molecules include cytokines and growth factors, which mediate ligand-receptor interactions with precise binding affinities. Cytokines, such as interleukins, act as paracrine or endocrine signals to orchestrate immune responses; excessive release can lead to cytokine storms, where hyperactivation of immune cells causes widespread inflammation and tissue damage, as observed in severe infections like COVID-19. Growth factors like epidermal growth factor (EGF) bind to the EGF receptor (EGFR) with a dissociation constant (Kd) of approximately 0.1-1 nM, promoting cell proliferation and differentiation through dimerization and autophosphorylation of the receptor. In development, the Notch-Delta pathway exemplifies juxtacrine signaling, where membrane-bound Delta ligands on one cell interact directly with Notch receptors on adjacent cells, cleaving the intracellular Notch domain to regulate cell fate decisions, such as neurogenesis and somitogenesis. The evolutionary origins of intercellular communication trace back to unicellular organisms, where bacterial using autoinducer molecules like acyl-homoserine lactones enables population-level coordination of behaviors such as formation. This primitive signaling evolved into sophisticated multicellular systems, adapting mechanisms like diffusible signals into contact-dependent pathways to support tissue organization in eukaryotes. Recent advances highlight exosome-mediated communication, where extracellular vesicles carrying proteins, , and RNAs transfer information between cells, contributing to disease progression; for example, tumor-derived exosomes promote by altering behavior in cancer, and dysregulated exosomal cargo exacerbates in neurodegenerative disorders like .

Multicellularity

Cell Differentiation and Specialization

Cell differentiation is the process by which unspecialized cells in multicellular organisms acquire distinct structures and functions, enabling the formation of diverse tissues and organs essential for complex body plans.00282-7) In animals, this begins with totipotent zygotes that can give rise to all cell types, including extraembryonic tissues; these transition to pluripotent embryonic stem cells capable of forming any body cell lineage, followed by multipotent stem cells restricted to specific tissues, and ultimately terminally differentiated cells like neurons.00039-0) For instance, neural stem cells differentiate into mature neurons through sequential stages involving progenitor proliferation and lineage commitment, driven by environmental cues and intrinsic factors. Epigenetic modifications play a central role in locking in these differentiated states by altering gene accessibility without changing the DNA sequence. DNA typically represses by adding methyl groups to bases in promoter regions, silencing pluripotency genes as cells specialize, while modifications—such as promoting open for active transcription or variants enabling repression—fine-tune lineage-specific programs. These changes ensure stable inheritance of cellular identity during division, preventing reversion to less committed states under normal conditions. Gene regulation during differentiation is orchestrated by master transcription factors that activate or repress cascades of target genes. , a family of homeobox-containing transcription factors, pattern the anterior-posterior axis in animal embryos by specifying segmental identities; their clustered and collinear expression along the body axis, first elucidated in , ensure precise spatial control of development. Similarly, tissue-specific factors like , a basic helix-loop-helix protein discovered in 1987, drive differentiation by binding enhancers to induce myogenic genes, converting non-muscle cells into myoblasts and myotubes. A prominent example of differentiation is hematopoiesis, where hematopoietic stem cells in the give rise to all types through a hierarchical process: multipotent progenitors commit to myeloid or lymphoid lineages, yielding erythrocytes, platelets, and various leukocytes, regulated by cytokines and transcription factors to meet physiological demands.00125-6) In , totipotency is more widespread, allowing differentiated somatic cells—such as leaf protoplasts—to dedifferentiate and regenerate entire via , a plasticity exploited in for propagation. Cellular plasticity challenges the irreversibility of differentiation, as demonstrated by induced pluripotent stem cells (iPSCs). In a seminal 2006 study, introduction of four transcription factors—Oct4, Sox2, Klf4, and c-Myc (Yamanaka factors)—reprogrammed mouse fibroblasts to a pluripotent state resembling embryonic stem cells, enabling differentiation into any cell type and opening avenues for regenerative medicine.00976-7) Conversely, pathological dedifferentiation occurs in cancer, where tumor cells revert to a stem-like state, reacquiring proliferative and migratory capacities; this process, often triggered by epigenetic dysregulation or signaling aberrations, underlies tumor heterogeneity and resistance to therapy.

Evolution of Multicellular Organisms

The transition from unicellular to multicellular represents a major evolutionary milestone, with evidence indicating that multicellular eukaryotes emerged around 1.6 billion years ago. Fossil discoveries from formations in , , and reveal microscopic, algal-like organisms such as Qingshania magnifica, consisting of strings of up to 20 cells with distinct walls and spores, pushing back the timeline of multicellularity by nearly a billion years from previous estimates. These early forms likely arose from eukaryotic ancestors that had already developed complex body plans, including cylindrical and spherical structures, as seen in 1.642-billion-year-old fossils from . Morphological and chemical analyses of 1.6-billion-year-old fossils from further support this timeline by revealing preserved cellular structures and eukaryotic-specific chemical signatures that distinguish these multicellular clusters from prokaryotic mats. Mechanisms facilitating this transition involved the of and , progressing from loose colonial aggregates to integrated multicellular organisms with division of labor. In animals, cadherins—calcium-dependent proteins—played a pivotal role, with classical cadherins emerging in the metazoan lineage to form adherens junctions via binding to β-catenin, enabling stable cell-cell contacts essential for tissue formation. Premetazoan precursors like choanoflagellates, the closest unicellular relatives to animals, exhibit cadherin-like proteins and form multicellular colonies through direct microvilli contacts and actomyosin-mediated contractility, foreshadowing animal . Similarly, in the green algal lineage leading to , volvocine algae such as evolved multicellularity through embedding and coordinated , with genomic evidence showing co-option of regulators for specialization around 234 million years ago during the . This shift from colonial to true multicellularity imposed a germline-soma distinction, where somatic cells support by cells, resolving intracellular conflicts but requiring mechanisms to suppress selfish mutations. Multicellularity conferred significant advantages, including increased body size for enhanced resource acquisition and resistance to predation, while allowing cellular specialization for efficient division of labor. Experimental evolution in unicellular algae like demonstrates that multicellular clusters reduce predation rates by up to 2.5-fold against predators such as rotifers, as larger sizes exceed filtration thresholds. In animals, this enabled the development of specialized functions, such as nutrient uptake in choanoflagellate-like collar cells, paving the way for complex tissues. However, costs include elevated bioenergetic demands for maintaining intercellular connections and the evolutionary trade-off of reduced individual motility, as seen in volvocine algae where multicellular forms sacrifice flagellar propulsion for colonial integrity. The -soma separation further incurs repair costs, prioritizing integrity over somatic maintenance to ensure reproductive fidelity across generations. Representative examples illustrate varying degrees of multicellular complexity. Sponges (Porifera), emerging over 600 million years ago, represent simple multicellularity without true tissues, relying on choanoflagellate-derived adhesion for larval settlement and cell aggregation into functional modules like choanocytes for feeding. In contrast, cnidarians, diverging around 580 million years ago, achieved tissue-level organization with epithelial layers and nerve nets, utilizing Wnt signaling for polarity and minicollagens for connective tissues that support contractility and predation. These lineages highlight how incremental adhesion and signaling innovations built upon unicellular precursors to foster multicellular advantages in diverse environments.

Evolutionary Origins

Origin of Life and First Cells

The origin of life on Earth is thought to have arisen through a process of chemical evolution, where simple inorganic molecules gradually formed complex organic compounds under prebiotic conditions. This likely occurred in a or aquatic environments rich in energy sources, leading to the synthesis of life's building blocks such as , , and . Key experiments and hypotheses have illuminated these early stages, emphasizing the transition from non-living chemistry to self-sustaining systems capable of replication and . A landmark demonstration of chemical evolution came from the Miller-Urey experiment conducted in 1953, which simulated conditions by mixing gases like , , , and in a and subjecting them to electrical sparks to mimic lightning. After one week, the reaction produced several , including and , at yields up to 2-5% of the initial carbon input, showing that organic monomers essential for proteins could form abiotically from inorganic precursors. Subsequent analyses and variations of the experiment, using revised atmospheric compositions, have confirmed the synthesis of over 20 under diverse conditions, underscoring the plausibility of prebiotic . Central to many theories of life's emergence is the debate between "replication first" and "metabolism first" scenarios, which address the primacy of genetic information or energy-processing networks in proto-life. The replication-first view, embodied by the RNA world hypothesis, posits that RNA molecules served dual roles as both genetic material and catalysts in early evolution, predating DNA and proteins. Proposed in 1986, this idea gained support from the discovery of ribozymes—RNA enzymes capable of self-splicing and peptide bond formation—and laboratory demonstrations of RNA polymerization from prebiotic nucleotides, suggesting RNA could have enabled the first self-replicating systems around 4 billion years ago. In contrast, the metabolism-first hypothesis argues that autocatalytic cycles of chemical reactions, such as those involving iron-sulfur minerals, established energy gradients and metabolic pathways before replicators evolved, as evidenced by simulations showing sustained geochemical cycles in hydrothermal settings that produce organic intermediates without templated replication. The formation of s—primitive, membrane-bound compartments—represents a critical step toward cellular , encapsulating reactive molecules to concentrate chemistry and enable division. vesicles, formed spontaneously from amphiphilic fatty acids in aqueous environments, can grow by incorporating new and divide through shear forces or , mimicking basic cellular behaviors; experiments show these vesicles encapsulate and maintain internal gradients for up to hours. Coacervates, liquid droplets arising from of polyelectrolytes like peptides and nucleic acids, provide an , as they concentrate biomolecules without and exhibit selective permeability, with recent studies demonstrating their stability in saline conditions relevant to early oceans. Hydrothermal vents, particularly alkaline ones on the seafloor, are favored sites for assembly due to their thermal gradients and mineral-rich fluids, where membranes form at the interface between vent alkaline solutions and acidic , facilitating proton gradients akin to those in modern cells. Geological places the of the first cells between 4.1 and 3.8 billion years ago, shortly after Earth's oceans formed around 4.4 billion years ago, with the oldest undisputed signs of microbial life in 3.5-billion-year-old from . These layered structures, formed by cyanobacterial mats trapping sediments, indicate photosynthetic prokaryotes thrived by 3.48 billion years ago, as confirmed by isotopic analysis showing carbon fractionation consistent with biological activity. Microfossils and chemical biomarkers from 3.7-billion-year-old rocks in further support this timeline, though debates persist over abiotic mimics. In November 2025, AI analysis revealed chemical biosignatures of life in 3.3-billion-year-old rocks and of oxygenic in rocks up to 2.5 billion years old, further supporting early microbial activity. Recent advances in prebiotic chemistry, accelerated by machine learning simulations as of 2025, have refined models of these early processes by predicting reaction pathways and molecular stabilities with high accuracy. For instance, -based simulations have explored prebiotic reactions like nucleotide phosphoester bond formation, enabling microsecond-scale studies with near-quantum accuracy at reduced computational cost. These computational tools bridge gaps in experimental data, supporting hybrid metabolism-replication models and highlighting the role of surfaces in catalyzing both pathways.

Emergence of Eukaryotic Cells

The emergence of eukaryotic cells represents a pivotal evolutionary transition from prokaryotic ancestors, primarily driven by endosymbiotic events that integrated bacterial symbionts into a host cell, leading to the development of complex organelles and cellular structures. The foundational endosymbiosis involved the engulfment of an alpha-proteobacterium by an archaeal-like host cell, giving rise to mitochondria approximately 1.8 to 2.2 billion years ago (Ga). This event provided the host with enhanced energy production through , fundamentally altering cellular metabolism and enabling larger cell sizes and increased complexity. Subsequently, in the lineage leading to photosynthetic eukaryotes, a primary endosymbiosis occurred around 1 to 1.5 Ga, when a cyanobacterium was incorporated as the progenitor of chloroplasts in the ancestor of . Secondary endosymbioses, involving the engulfment of primary plastid-bearing by non-photosynthetic eukaryotes, further diversified types in groups such as chromalveolates, occurring as recently as 60 million years ago in some cases. Compelling evidence for these endosymbiotic origins includes the retention of distinct genetic and structural features in mitochondria and chloroplasts that mirror their bacterial ancestors. Mitochondria possess their own circular genome (mtDNA), encoding 13 to 37 genes primarily involved in respiration and protein synthesis, a reduced set compared to free-living bacteria but indicative of an independent prokaryotic heritage. Both organelles are bounded by double membranes, with the inner membrane derived from the symbiont's plasma membrane and the outer from the host's phagocytic vesicle. Additionally, their ribosomes are of the 70S type, characteristic of bacteria, contrasting with the 80S ribosomes of the eukaryotic cytosol. Recent models, supported by phylogenetic reconstructions updated around 2023, propose that peroxisomes arose from the endoplasmic reticulum (ER), possibly linked metabolically to the mitochondrial lineage, enhancing compartmentalization of oxidative processes. Beyond endosymbiosis, key innovations distinguished early eukaryotes, including the of the , likely formed by invaginations and reshaping of the ER to enclose the genome, facilitating regulated gene expression around 1.5 Ga. The emerged through the co-option and diversification of prokaryotic filament systems, incorporating , , and intermediate filaments for dynamic intracellular transport, , and shape maintenance, with roots in the archaeal host. also arose as an early eukaryotic trait, enabling genetic recombination via and cell fusion, likely evolving once in the last eukaryotic common ancestor (LECA) to repair damage and promote diversity. The LECA, estimated to have existed around 1.8 Ga, encompassed these features and marked the diversification of eukaryotic lineages. Fossil evidence supports this timeline, with Grypania spiralis, interpreted as an early eukaryotic alga, dating to approximately 1.87 Ga in strata. These developments collectively enabled eukaryotes to exploit new ecological niches, setting the stage for multicellularity.

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