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Cell physiology
Cell physiology
Cell physiology
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Cell physiology is the biological study of the activities that take place in a cell to keep it alive. The term physiology refers to normal functions in a living organism.[1] Animal cells, plant cells and microorganism cells show similarities in their functions even though they vary in structure.[2][page needed]

General characteristics

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There are two types of cells: prokaryotes and eukaryotes. Prokaryotes were the first of the two to develop and do not have a self-contained nucleus. Their mechanisms are simpler than later-evolved eukaryotes, which contain a nucleus that envelops the cell's DNA and some organelles.[3]

Prokaryotes

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Typical prokaryotic cell structure

Prokaryotes have DNA located in an area called the nucleoid, which is not separated from other parts of the cell by a membrane. There are two domains of prokaryotes: bacteria and archaea. Prokaryotes have fewer organelles than eukaryotes. Both have plasma membranes and ribosomes (structures that synthesize proteins[clarification needed] and float free in cytoplasm). Two unique characteristics of prokaryotes are fimbriae (finger-like projections on the surface of a cell) and flagella (threadlike structures that aid movement).[2]

Eukaryotes

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Typical eukaryotic animal cell structure

Eukaryotes have a nucleus where DNA is contained. They are usually larger than prokaryotes and contain many more organelles. The nucleus, the feature of a eukaryote that distinguishes it from a prokaryote, contains a nuclear envelope, nucleolus and chromatin. In cytoplasm, endoplasmic reticulum (ER) synthesizes[clarification needed] membranes and performs other metabolic activities. There are two types, rough ER (containing ribosomes) and smooth ER (lacking ribosomes). The Golgi apparatus consists of multiple membranous sacs, responsible for manufacturing and shipping out materials such as proteins. Lysosomes are structures that use enzymes to break down substances through phagocytosis, a process that comprises endocytosis and exocytosis. In the mitochondria, metabolic processes such as cellular respiration occur. The cytoskeleton is made of fibers that support the structure of the cell and help the cell move.[2]

Physiological processes

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There are different ways through which cells can transport substances across the cell membrane. The two main pathways are passive transport and active transport. Passive transport is more direct and does not require the use of the cell's energy. It relies on an area that maintains a high-to-low concentration gradient. Active transport uses adenosine triphosphate (ATP) to transport a substance that moves against its concentration gradient.[4][page needed]

Movement of proteins

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The pathway for proteins to move in cells starts at the ER. Lipids and proteins are synthesized[clarification needed] in the ER, and carbohydrates are added to make glycoproteins. Glycoproteins undergo further synthesis[clarification needed] in the Golgi apparatus, becoming glycolipids. Both glycoproteins and glycolipids are transported into vesicles to the plasma membrane. The cell releases secretory proteins known as exocytosis.[2]

Transport of ions

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Ion Transport: Direction of Na/K flow

Ions travel across cell membranes through channels, pumps or transporters. In channels, they move down an electrochemical gradient to produce electrical signals. Pumps maintain electrochemical gradients. The main type of pump is the Na/K pump. It moves 3 sodium ions out of a cell and 2 potassium ions into a cell. The process converts one ATP molecule to adenosine diphosphate (ADP) and Phosphate.[clarification needed] In a transporter, ions use more than one gradient to produce electrical signals.[3]

Endocytosis in Animal Cells

Endocytosis in animal cells

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Endocytosis is a form of active transport where a cell takes in molecules, using the plasma membrane, and packages them into vesicles.[2]: 139–140 

Phagocytosis

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In phagocytosis, a cell surrounds particles including food particles through an extension of the pseudopods, which are located on the plasma membrane. The pseudopods then package the particles in a food vacuole. The lysosome, which contains hydrolytic enzymes, then fuses with the food vacuole. Hydrolytic enzymes, also known as digestive enzymes, then digest the particles within the food vacuole.[2]: 139–140 

Pinocytosis

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In pinocytosis, a cell takes in ("gulps") extracellular fluid into vesicles, which are formed when plasma membrane surrounds the fluid. The cell can take in any molecule or solute through this process.[2]: 139–140 

Receptor-mediated endocytosis

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Receptor-mediated endocytosis is a form of pinocytosis where a cell takes in specific molecules or solutes. Proteins with receptor sites are located on the plasma membrane, binding to specific solutes. The receptor proteins that are attached to the specific solutes go inside coated pits, forming a vesicle. The vesicles then surround the receptors that are attached to the specific solutes, releasing their molecules. Receptor proteins are recycled back to the plasma membrane by the same vesicle.[2]: 139–140 

References

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Cell physiology

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Cell physiology is the scientific discipline that examines the physical, chemical, and biological functions of cells, the fundamental units of life, encompassing processes such as , energy metabolism, , and . Cells include both prokaryotic and eukaryotic types; in the , which consists of eukaryotic cells, there are approximately 37 trillion cells (as of 2023). Eukaryotic cells consist of three primary components: the plasma membrane, which regulates the entry and exit of substances; the , a gel-like medium housing organelles for metabolic activities; and the nucleus, which directs cellular operations through genetic material. Key physiological processes in cells include passive and across the selectively permeable membrane, where passive mechanisms like and move molecules down concentration gradients without energy expenditure, while , such as the sodium-potassium pump, utilizes ATP to counteract gradients and maintain ionic balance essential for cellular . Cellular involves catabolic pathways that generate ATP, including in the and in mitochondria, powering all cellular activities, alongside anabolic processes for synthesizing proteins and in organelles like the and ribosomes. enables communication via receptors on the plasma membrane that detect extracellular signals, such as hormones or neurotransmitters, triggering intracellular cascades like second messenger systems to elicit responses ranging from to movement. The , regulated by checkpoints in (G1, S, G2 phases) and , ensures accurate and division, with disruptions potentially leading to uncontrolled proliferation as seen in cancer. These interconnected functions allow cells to adapt to environmental changes, interact in tissues, and sustain organismal life.

Cellular Organization

Prokaryotic Cells

Prokaryotic cells, characteristic of and , exhibit a streamlined physiological organization without membrane-bound organelles, allowing for direct integration of metabolic processes within the and plasma membrane. Unlike eukaryotic cells, which compartmentalize functions into specialized organelles, prokaryotes perform essential activities such as respiration and directly on the plasma membrane or its infoldings. For instance, in certain , the plasma membrane forms specialized invaginations, such as chromatophores in photosynthetic species or intracytoplasmic membranes in some respiring , which increase surface area for electron transport chains and ATP synthesis, serving as functional analogs to mitochondrial cristae. This arrangement enables efficient energy production in a compact cellular volume, typically 1-10 μm in diameter. The genetic material in prokaryotes consists of a single, circular located in a region, often accompanied by smaller, extrachromosomal plasmids that enhance physiological adaptability. This circular topology facilitates rapid , with replication forks progressing at speeds up to 1000 per second, allowing completion of genome duplication in under an hour for organisms like . Plasmids, which can carry genes for resistance or metabolic capabilities, replicate independently and contribute to quick evolutionary responses to environmental stresses, such as nutrient scarcity or toxins. This genetic simplicity supports high replication fidelity and minimal regulatory overhead, optimizing prokaryotic cells for fast adaptation in dynamic habitats. Cell division in prokaryotes primarily occurs via binary fission, a process that partitions the replicated and into two genetically identical daughter cells, enabling exponential under favorable conditions. In optimal environments, such as nutrient-rich media at 37°C, E. coli can double every 20-30 minutes, leading to billions of cells from a single progenitor in hours. This rapid division relies on a simple cytoskeletal apparatus involving proteins like , which forms a contractile ring at the division site, ensuring precise septum formation without the complex checkpoints of eukaryotic . Binary fission thus underpins the prokaryotic strategy of overwhelming competitors through sheer numerical proliferation. Nutrient acquisition in prokaryotes occurs mainly through passive across the plasma membrane for small, uncharged molecules, supplemented by porin channels in the outer membrane of to facilitate uptake of ions and hydrophilic solutes. In nutrient-limited settings, specific porins like OprD in enhance rates for essential compounds, preventing starvation. Many motile prokaryotes employ , a directed movement toward higher concentrations of attractants such as sugars or , mediated by flagellar rotation and sensory transduction pathways; for example, E. coli biases random tumbling to prolong "runs" up gradients, increasing encounter rates with scarce resources by up to 100-fold. This behavioral physiology integrates with for survival in heterogeneous environments. A classic example of prokaryotic physiological regulation is the lactose (lac) operon in E. coli, which coordinates for in response to carbon source availability. When is present and glucose is low, the dissociates from the operator, allowing transcription of , permease, and transacetylase genes, thereby inducing utilization only when energetically favorable. This inducible system exemplifies , where cyclic AMP levels signal glucose scarcity to activate the operon via the , linking sensing directly to metabolic efficiency and preventing wasteful production.

Eukaryotic Cells

Eukaryotic cells are characterized by their complex internal organization, featuring membrane-bound compartments that enable specialized physiological functions distinct from the simpler, non-compartmentalized structure of prokaryotic cells. This compartmentalization allows for efficient separation of cellular processes, such as DNA management, energy production, and protein processing, which enhance regulatory control and metabolic efficiency. The nucleus stands as the central organelle, housing the cell's genetic material in a double-membrane envelope that safeguards DNA from cytoplasmic activities and regulates gene expression through controlled transcription. By sequestering DNA, the nucleus ensures that transcription occurs in a dedicated environment, while translation takes place in the cytoplasm on ribosomes, preventing interference and allowing for post-transcriptional modifications. Mitochondria serve as the primary sites for ATP synthesis in eukaryotic cells via , a process where electrons from breakdown drive proton gradients across the to power . This organelle's cristae structure maximizes surface area for the , yielding up to 30-32 ATP molecules per glucose molecule oxidized, far surpassing alone. The (ER), particularly the rough ER studded with ribosomes, facilitates initial , , and , ensuring nascent polypeptides achieve proper three-dimensional structures before export. Subsequently, the Golgi apparatus receives these proteins for further modifications, such as additional or proteolytic cleavage, and sorts them into vesicles destined for lysosomes, , or plasma integration, maintaining cellular . The cell cycle in eukaryotes is meticulously regulated through phases—G1 (growth and checkpoint for DNA integrity), S (DNA replication), G2 (preparation and damage assessment), and M (mitosis and cytokinesis)—with checkpoints ensuring progression only upon successful completion of prior steps, such as verifying DNA replication fidelity at G2/M. Mitosis involves prophase chromosome condensation, metaphase alignment via spindle fibers, anaphase separation, and telophase decondensation, culminating in cytokinesis where an actin-myosin contractile ring divides the cytoplasm, producing two genetically identical daughter cells. Eukaryotic cells, often 10-100 μm in diameter, rely on the cytoskeleton for structural integrity and dynamics; microtubules composed of tubulin dimers provide tracks for motor proteins like kinesin and dynein, facilitating vesicle and organelle transport, while actin filaments support cell shape, motility, and division. Saccharomyces cerevisiae, or yeast, exemplifies as a due to its conserved cellular machinery and ease of genetic manipulation. In this unicellular , division occurs via , where a small protrusion forms on the mother cell, expands through polarized growth involving actin cytoskeleton, and separates after nuclear migration and , allowing study of processes like control and inheritance.

Membrane Structure and Function

Composition and Properties

The , or plasma membrane, is primarily composed of a bilayer, which forms a fundamental semi-permeable barrier enclosing the cell's contents. are amphipathic molecules consisting of a hydrophilic (polar) head group, typically containing a moiety attached to a backbone, and two hydrophobic (nonpolar) tails. In an aqueous environment, these molecules spontaneously self-assemble into a bilayer structure, with the hydrophilic heads oriented toward the extracellular and intracellular aqueous phases, while the hydrophobic tails cluster together in the interior, shielded from water. This arrangement, first quantitatively supported by experiments on membranes, creates a stable yet dynamic barrier that restricts the free passage of polar solutes. Embedded within this phospholipid bilayer are proteins that contribute to the membrane's functional diversity, as described by the . This model posits the membrane as a two-dimensional fluid where lipids and proteins are interspersed in a mosaic-like pattern, allowing lateral diffusion and dynamic interactions. Integral membrane proteins, such as ion channels and receptors, span the bilayer with hydrophobic domains anchoring them in the lipid core, while peripheral proteins associate loosely with the membrane surface via electrostatic or hydrophobic interactions. Examples include transmembrane receptors like the , which facilitate , and channels such as voltage-gated sodium channels that span the membrane. The , proposed based on thermodynamic principles and electron microscopy observations, underscores how protein mobility enables adaptive cellular responses. In animal cells, is a key component that integrates into the bilayer, comprising up to 50% of the content in some , and plays a critical role in modulating fluidity. By intercalating between phospholipid tails, cholesterol disrupts tight packing of saturated fatty acids at physiological temperatures, preventing gel-phase transitions and maintaining a liquid-ordered state that balances rigidity and flexibility. This modulation is essential for integrity, as excessive cholesterol can rigidify the bilayer, while depletion increases permeability to small molecules. Studies on model bilayers and intact cells demonstrate that cholesterol's planar ring structure and hydroxyl group enable it to order acyl chains without immobilizing them, thus optimizing the membrane for protein function. Overlaying the outer leaflet of the plasma is the , a carbohydrate-rich layer composed of glycoproteins, glycolipids, and proteoglycans that varies in thickness and composition across cell types, from the dense on intestinal epithelial cells to the thinner coat on neurons. This extracellular matrix-like structure provides mechanical protection against and pathogens, while also mediating cell-cell recognition through specific glycan motifs that interact with or antibodies. The glycocalyx's anionic nature, due to sulfated and sialylated residues, contributes to charge-based repulsion and stabilization of curvature during processes like . Research on endothelial cells highlights its role in shielding proteins and influencing local environments. The biophysical properties of the cell membrane arise directly from its composition, conferring selective permeability, lipid asymmetry, and adaptability to . Selective permeability stems from the hydrophobic core of the bilayer, which impedes polar molecules larger than while allowing passive diffusion of nonpolar substances like oxygen; this property is fine-tuned by and protein content. Membrane asymmetry is maintained by -dependent lipid flippases and scramblases, resulting in the outer leaflet being enriched in and , while the inner leaflet favors and — a distribution critical for signaling and . , influenced by asymmetric lipid packing and tension, facilitates membrane fusion events, such as vesicle , by lowering the barrier for ; biophysical models show that high increases permeability to ions by orders of magnitude compared to flat membranes. These properties collectively ensure the membrane's role as a dynamic interface, though their impact on specific processes is addressed elsewhere.

Permeability Barriers

The plasma membrane serves as a selective permeability barrier that restricts the free diffusion of polar molecules and ions, thereby enabling cells to maintain distinct internal environments from the . This selectivity arises primarily from the lipid bilayer's hydrophobic core, which impedes the passage of hydrophilic substances while allowing small nonpolar molecules like oxygen and to diffuse readily. As a result, concentration gradients of essential ions such as (high inside) and sodium (high outside) are preserved, which is crucial for cellular and functions like osmotic balance. In multicellular organisms, additional paracellular barriers enhance this selectivity, particularly in epithelial tissues where tight junctions form continuous seals between adjacent cells to prevent unregulated leakage through intercellular spaces. These junctions, composed of proteins like claudins and occludins, create a highly restrictive barrier that limits paracellular transport of ions and solutes, ensuring vectorial transport across epithelia. A prominent example is the blood-brain barrier, where endothelial tight junctions in capillaries exhibit exceptional tightness, restricting the passage of polar molecules and pathogens to protect neuronal function while permitting essential nutrient exchange. For large polar molecules that cannot cross the directly, provides a limited exception to the by engulfing extracellular material into vesicles, thus bypassing the . The also sustains electrochemical gradients, including a typical of approximately 7.2 compared to 7.4 extracellularly, which supports enzymatic activities and proton-coupled transport. Electrical gradients manifest as the resting , around -70 mV in neurons, generated by unequal ion distributions and selective permeability, primarily to ions. Pathological disruptions of these barriers can compromise cellular integrity, as seen with detergents like (SDS), which solubilize and increase permeability to ions and metabolites, leading to loss of , , and cell . Such disruptions mimic effects of certain bacterial toxins that target components, highlighting the barrier's vulnerability and the importance of its maintenance for cellular survival.

Transport Mechanisms

Passive Diffusion and Facilitated Transport

Passive diffusion, also known as simple diffusion, is the unassisted movement of small, nonpolar molecules across the of cell membranes down their concentration gradient, without the requirement of energy input. This process is driven by the random thermal motion of molecules and follows Fick's first law of diffusion, which quantifies the (J) as proportional to the negative concentration gradient: J=−DdcdxJ = -D \frac{dc}{dx} where DD is the diffusion coefficient, cc is the concentration, and xx is the position across the . The diffusion coefficient DD depends on factors such as size, , and properties, with smaller nonpolar s like oxygen (O₂) and (CO₂) exhibiting high permeability due to their in the hydrophobic core of the bilayer. For instance, O₂ and CO₂ readily diffuse across erythrocyte membranes to facilitate in the lungs and tissues. Facilitated transport enhances the passive movement of polar or larger solutes that cannot easily cross the unaided, utilizing specific membrane proteins such as channels or carriers, still driven solely by electrochemical gradients. Channel proteins form hydrophilic pores that allow rapid of or small molecules; a prominent example is aquaporins, which selectively facilitate water across membranes in response to osmotic gradients. Discovered by in 1992, aquaporin-1 (AQP1) was the first identified water channel protein, enabling high-volume water flux while excluding protons and other to maintain cellular integrity. Carrier proteins, in contrast, undergo conformational changes to bind and translocate substrates like glucose; the glucose transporter 1 (), first purified from human erythrocytes in 1977, exemplifies this by mediating glucose uptake into red blood cells via a rocking-bundle mechanism. Ion channels also illustrate facilitated transport, such as leak channels (e.g., K₂P family members), which remain open at rest and permit K⁺ efflux, contributing to the negative resting of approximately -70 mV in neurons and muscle cells by countering Na⁺ influx. Osmosis represents a specialized form of passive for water molecules across semi-permeable membranes, moving from regions of higher to lower, often mediated by aquaporins to accelerate the process. In plant cells, this influx generates , the hydrostatic force that presses the plasma membrane against the rigid , providing structural support and driving cell expansion for growth. typically ranges from 0.4 to 0.8 MPa in hydrated plant cells, balancing osmotic influx and preventing in hypotonic environments. Unlike simple diffusion, which shows linear kinetics with substrate concentration, facilitated transport exhibits saturation due to limited protein availability, following Michaelis-Menten-like kinetics: V=Vmax⁡[S]Km+[S]V = \frac{V_{\max} [S]}{K_m + [S]} where VV is the transport rate, Vmax⁡V_{\max} is the maximum rate, [S][S] is substrate concentration, and KmK_m is the concentration at half Vmax⁡V_{\max}. For , KmK_m is approximately 1-2 mM for glucose, ensuring efficient uptake under physiological conditions but limiting flux at high concentrations. This saturation distinguishes facilitated mechanisms from , which requires energy to move solutes against gradients.

Active Transport and Ion Pumps

Active transport mechanisms enable cells to move ions and molecules against their concentration gradients, utilizing energy derived from or pre-existing electrochemical gradients established by passive processes. Primary active transport directly couples to the translocation of substrates, with the sodium-potassium (Na⁺/K⁺-) serving as the paradigmatic example. This ubiquitous P-type , first identified in crab nerve membranes, hydrolyzes one ATP molecule to extrude three sodium ions (Na⁺) from the and import two ions (K⁺), thereby maintaining essential ionic gradients for cellular . The reaction can be represented as: ATP+H2O+3Nain++2Kout+→ADP+Pi+3Naout++2Kin+\text{ATP} + \text{H}_2\text{O} + 3\text{Na}^+_{\text{in}} + 2\text{K}^+_{\text{out}} \rightarrow \text{ADP} + \text{P}_\text{i} + 3\text{Na}^+_{\text{out}} + 2\text{K}^+_{\text{in}} This 3:2 stoichiometry generates a net outward positive charge movement, contributing to the membrane potential. Secondary active transport, in contrast, harnesses the energy stored in ion gradients—typically Na⁺ or H⁺—created by primary pumps to drive uphill transport of other solutes. These transporters, often symporters or antiporters, couple the downhill flux of the driving ion to the accumulation of a substrate. A key example is the sodium-glucose linked transporter (SGLT1) in intestinal epithelial cells, which co-transports one glucose molecule with two Na⁺ ions into the cell, facilitating nutrient absorption against a glucose gradient. This process relies on the low intracellular Na⁺ concentration maintained by the Na⁺/K⁺-ATPase. Structural studies reveal alternating access mechanisms where Na⁺ binding induces conformational changes that enable glucose uptake. Calcium pumps, such as the sarco/endoplasmic reticulum Ca²⁺-ATPase (), exemplify primary in intracellular compartments. isoforms, embedded in the sarcoplasmic or membrane, use ATP to pump two Ca²⁺ ions from the into the lumen per ATP hydrolyzed, countering the steep concentration gradient (cytosolic [Ca²⁺] ~100 nM vs. luminal ~1 mM). This action is crucial for terminating Ca²⁺ signaling events, such as muscle relaxation following contraction, by rapidly lowering cytosolic Ca²⁺ levels. Regulation involves phospholamban, which inhibits under basal conditions but relieves inhibition upon during sympathetic stimulation. Proton pumps, particularly the vacuolar H⁺-ATPase (), perform primary to acidify organelles like lysosomes in animal cells and vacuoles in . , a rotary motor complex, hydrolyzes ATP to translocate H⁺ into the lumen, establishing gradients (lysosomal pH ~4.5–5.0) essential for degradative enzyme activity and vesicular trafficking. In plant vacuoles, this acidification drives secondary transport of nutrients and maintains . The enzyme's V₁ domain handles , while the V₀ domain conducts protons across the membrane. Many ion pumps exhibit electrogenic properties, directly influencing the through unequal . The Na⁺/K⁺-ATPase's 3:2 produces a hyperpolarizing current, contributing ~5–10 mV to the neuronal of -70 mV, which stabilizes excitability and aids after action potentials. In neurons, this electrogenic activity modulates firing rates, particularly during high-frequency activity when Na⁺ influx increases pump demand. Similarly, V-ATPases in certain plasma membranes contribute to potential differences in specialized cells. The physiological significance of these mechanisms is evident in osmoregulation, where the Na⁺/K⁺/2Cl⁻ cotransporter (NKCC2) in thick ascending limb cells uses the Na⁺ to drive paracellular Na⁺ and Cl⁻ uptake, generating the medullary osmotic for concentration. NKCC2-mediated transport accounts for ~25% of filtered Na⁺ , and its dysfunction underlies , highlighting its role in fluid and balance.

Intracellular Movement

Protein Synthesis and Trafficking

Protein synthesis in cells begins with transcription of mRNA in the nucleus, followed by on ribosomes in the . Ribosomes can be free-floating in the , synthesizing proteins destined for intracellular locations, or bound to the rough (ER), where they produce proteins for , insertion, or lysosomal targeting. ER-bound occurs co-translationally, allowing nascent polypeptides to be threaded directly into the ER lumen through the Sec61 translocon channel. Targeting of mRNAs to the ER relies on the (SRP), a ribonucleoprotein complex that recognizes an N-terminal sequence—typically a stretch of 15-30 hydrophobic —emerging from the during initiation. Upon binding, SRP pauses and docks the -nascent chain complex to the SRP receptor on the ER membrane, resuming and facilitating translocation. This mechanism, first proposed in the signal hypothesis by , ensures efficient sorting of approximately 30% of the eukaryotic proteome to the secretory pathway. In the ER lumen, proteins undergo essential post-translational modifications, including N-linked , where oligosaccharides are covalently attached to residues in the Asn-X-Ser/Thr (X ≠ Pro). This modification, initiated by the oligosaccharyltransferase complex shortly after translocation, aids in , stability, and by serving as tags for chaperones and inspectors. Additional modifications, such as disulfide bond formation and initial trimming of the by signal peptidase, occur concurrently to achieve proper three-dimensional structure. Chaperone proteins, such as family members, play a critical role in assisting folding by binding hydrophobic regions of nascent or misfolded polypeptides, preventing aggregation and promoting correct domain assembly through ATP-dependent cycles. , in cooperation with co-chaperones like Hsp40, captures unfolded substrates and facilitates their release upon , enabling iterative folding attempts. These molecular chaperones are particularly vital in the crowded ER environment, where they buffer against proteotoxic stress. Quality control mechanisms ensure only properly folded proteins proceed, with misfolded ones targeted for degradation via ER-associated degradation (ERAD). In ERAD, ubiquitin ligases like Hrd1 recognize aberrant proteins, often marked by unglucosylated glycans or exposed hydrophobic patches, leading to retrotranslocation to the through the Sec61 channel for proteasomal degradation. This process maintains ER homeostasis and prevents accumulation of toxic aggregates. A representative example is insulin synthesis in pancreatic s, where preproinsulin mRNA is translated on ER-bound ribosomes, and the directs translocation into the ER lumen. There, proinsulin folds with chaperone assistance, forming three disulfide bonds, and undergoes cleavage by prohormone convertases PC1/3 and PC2 in the trans-Golgi and immature secretory granules to yield mature insulin. Defective proinsulin folding can trigger ER stress and ERAD, contributing to dysfunction in .

Cytoskeletal Involvement in Movement

The plays a pivotal role in facilitating intracellular movement by providing structural tracks along which motor proteins transport proteins, vesicles, and , ensuring efficient distribution within the cell. Composed of filaments, , and intermediate filaments, the enables both short- and long-range , with motor proteins like myosins, kinesins, and dyneins converting chemical energy from into mechanical work. This dynamic network supports essential processes such as positioning and , adapting to cellular needs through and . Actin filaments, or microfilaments, primarily mediate short-range transport and cell crawling by forming dynamic networks near the cell periphery, where motors generate force for movement. Myosin II and myosin V, for instance, walk along tracks to propel vesicles over distances of a few micrometers or drive lamellipodia extension during migration, with velocities reaching up to 0.5 μm/s in non-muscle cells. This actomyosin system powers processes like in cells and phagocytic engulfment, where polymerization at the leading edge pushes the membrane forward while contraction retracts the rear. Microtubules serve as tracks for long-distance , particularly in elongated cells like neurons, where motors drive anterograde movement from the cell body to synapses, and motors enable retrograde transport back to the soma. Kinesin-1, a processive motor, moves cargos at speeds of 0.5–1 μm/s along microtubule plus ends, while cytoplasmic achieves similar velocities in the opposite direction, coordinating bidirectional flow to maintain axonal integrity over centimeters. In neurons, this microtubule-based system transports vesicles containing neurotransmitters and mitochondria, preventing synaptic failure if disrupted. Intermediate filaments contribute to structural stability during movement by resisting mechanical stress and anchoring other cytoskeletal elements, preventing deformation under shear forces generated by motor activity. and filaments, for example, form a resilient that maintains cell shape during migration, with significantly higher tensile strength and extensibility than filaments or , enabling them to resist mechanical stress and large deformations without breaking. Their integration with and via cross-linkers like plectin ensures coordinated force transmission. A key example of cytoskeletal involvement is vesicle transport in neurons, where and motors ferry synaptic vesicles along , with pauses and direction reversals regulated by cargo adaptors to avoid collisions. Another is , where an actin-myosin II contractile ring assembles at the cell equator and constricts via myosin-powered sliding of antiparallel actin filaments, reducing the cell diameter by up to 50% to divide the . Microtubule dynamics, crucial for track remodeling during , are regulated by GTP in β-tubulin subunits, which promotes rapid when GTP-bound and into unstable GDP-tubulin protofilaments upon . This GTP cap at growing ends stabilizes , enabling dynamic instability with growth rates of 1–2 μm/min and catastrophe frequencies tuned by microtubule-associated proteins (MAPs). Such cycles allow to explore cellular space and adapt to demands.

Vesicular Trafficking

Endocytosis Pathways

Endocytosis encompasses several distinct pathways that enable cells to internalize extracellular materials, such as nutrients, signaling molecules, and pathogens, through the formation of membrane-bound vesicles. These mechanisms are essential for maintaining cellular , regulating receptor levels on the plasma membrane, and facilitating intracellular trafficking. The primary pathways include clathrin-mediated , caveolae-mediated endocytosis, macropinocytosis, and , each tailored to specific types of cargo and cellular contexts. Clathrin-mediated endocytosis is the most well-characterized pathway, involving the assembly of -coated pits at the plasma to selectively uptake receptor-ligand complexes. Adaptor proteins, such as AP-2, recruit clathrin triskelions to form a lattice that curves the inward, concentrating like transferrin receptors or particles. The assembles into collars around the neck of the invaginating pit, hydrolyzing GTP to drive membrane fission and release the coated vesicle. Caveolae-mediated endocytosis relies on flask-shaped invaginations stabilized by caveolin proteins, which interact with and in lipid rafts to form specialized membrane domains. This pathway internalizes raft-associated cargo, including signaling receptors and GPI-anchored proteins, often in a non-selective manner compared to clathrin-dependent uptake. Caveolin-1, the principal isoform, oligomerizes to scaffold the structure, with similarly implicated in vesicle scission, though the process is less dependent on . Macropinocytosis facilitates the bulk uptake of and solutes through large, irregular macropinosomes formed by actin-driven membrane ruffles, particularly prominent in immune cells like macrophages and dendritic cells. Unlike receptor-mediated pathways, it is non-selective and driven by extracellular stimuli such as growth factors, leading to the enclosure of volumes up to 5 μm in diameter. In macrophages, this process supports rapid sampling of the extracellular environment without specific ligands. Phagocytosis is a specialized form of primarily in professional such as macrophages, neutrophils, and dendritic cells, involving the engulfment of large particles greater than 0.5 μm, including microorganisms, dead cells, and debris, via actin-driven pseudopod extensions that form phagosomes. This receptor-mediated process, often triggered by opsonins like antibodies or complement proteins binding to Fc or complement receptors, enables clearance and . Phagosomes mature by fusing with lysosomes for degradation of internalized material. Physiologically, endocytosis pathways play critical roles in nutrient acquisition and immune functions; for instance, in macrophages, - and macropinocytosis-mediated uptake enable the internalization of iron-bound for metabolic support and soluble antigens for processing and presentation to T cells via molecules, while allows engulfment of and apoptotic cells for immune defense. These mechanisms ensure efficient resource scavenging in nutrient-poor environments and contribute to pathogen surveillance. These pathways are energy-dependent, with - and caveolae-mediated primarily relying on GTP by and other for vesicle budding and fission, as well as ATP-fueled remodeling to propel protrusions and invaginations in macropinocytosis and . Following internalization, uncoated vesicles fuse with early endosomes, where cargo sorting occurs: receptors often recycle back to the plasma via recycling endosomes, while other materials proceed to lysosomes for degradation, maintaining a dynamic balance in cellular uptake and turnover.

Exocytosis and Secretion

Exocytosis is the process by which cells release molecules into the through the fusion of intracellular vesicles with the plasma membrane, enabling essential functions such as hormone and neurotransmitter release. This outward-directed vesicular trafficking contrasts with by facilitating the export of cellular contents, including proteins, , and signaling molecules, while incorporating vesicle membranes into the plasma membrane. Exocytosis occurs via two primary modes: constitutive and regulated, each tailored to specific cellular needs. Constitutive exocytosis involves the continuous, unregulated fusion of vesicles with the plasma , primarily for the steady delivery of (ECM) components and proteins in non-specialized cells. In this pathway, vesicles derived from the trans-Golgi network fuse immediately upon formation, supporting basal without external stimuli, as seen in fibroblasts producing and other ECM constituents. This mode ensures ongoing maintenance of cell surface architecture and extracellular environment. Regulated exocytosis, in contrast, is triggered by specific signals, such as calcium (Ca²⁺) influx, allowing rapid and controlled release in response to physiological demands. A classic example occurs at synapses, where neurotransmitter-filled vesicles fuse upon Ca²⁺ elevation, mediated by SNARE proteins including syntaxin, SNAP-25 on the target membrane, and VAMP on the vesicle. These SNAREs form a stable trans-complex that drives bilayer fusion, with syntaxin and SNAP-25 anchoring the vesicle via VAMP to zipper-like interactions along their coiled-coil domains. The fusion mechanism is orchestrated by Rab GTPases, which ensure vesicle targeting and tethering to the correct plasma membrane sites in their GTP-bound active state. Rabs recruit effector proteins to dock vesicles prior to SNARE engagement, preventing off-target fusions. Post-fusion, the SNARE complex is disassembled by the NSF (N-ethylmaleimide-sensitive factor), which uses to unwind the cis-SNARE bundle, recycling SNARE components for subsequent rounds of . This disassembly, facilitated by α-SNAP, is essential to avoid SNARE exhaustion and maintain secretory capacity. In pancreatic beta cells, regulated of insulin granules exemplifies Ca²⁺-triggered release: glucose stimulation depolarizes the cell, opening voltage-gated Ca²⁺ channels, which promotes SNARE-mediated fusion of mature granules containing insulin, , and zinc. Similarly, lysosomal secretion involves Ca²⁺-dependent of lysosomes, releasing hydrolases like β-hexosaminidase to degrade extracellular substrates or repair plasma membrane damage. Following , membrane components from fused vesicles are recycled back to the Golgi apparatus via endocytic pathways, preserving cellular and preventing excessive plasma membrane expansion. This retrieval, often involving clathrin-coated pits, returns lipids and proteins to the trans-Golgi network for repackaging into new secretory vesicles. Such is particularly critical in high-secretion cells like neurons and endocrine cells to sustain repeated exocytic events.

Cell Signaling and Communication

Receptor Activation and Signal Transduction

Cell surface receptors play a central role in transducing extracellular signals into intracellular responses, enabling cells to respond to environmental cues such as hormones, neurotransmitters, and growth factors. The primary types of receptors involved in this process include G-protein-coupled receptors (GPCRs), ionotropic receptors (also known as -gated channels), and enzyme-linked receptors, such as receptor tyrosine kinases (RTKs). GPCRs, which constitute the largest family of cell surface receptors, feature seven transmembrane helices and couple to heterotrimeric G proteins upon activation. Ionotropic receptors form channels that open directly in response to ligand binding, allowing rapid flux across the . Enzyme-linked receptors, exemplified by RTKs, possess intrinsic enzymatic activity in their cytoplasmic domains, typically catalyzing events. Ligand binding to these receptors induces specific conformational changes that initiate . For GPCRs, binding stabilizes an active conformation, characterized by an outward tilt of transmembrane helix 6, which facilitates recruitment and exchange on the Gα subunit. In RTKs, such as the , binding promotes receptor dimerization, bringing the domains into proximity and enabling trans-autophosphorylation on residues in the activation loop, which fully activates the . Ionotropic receptors, like the , undergo a simpler conformational shift upon binding, directly opening the pore for cation influx. These initial changes ensure precise and rapid signal initiation, with the conformational dynamics often amplified by the receptor's oligomeric state. Signal amplification occurs as a single activated receptor can engage multiple downstream effectors, enhancing sensitivity to low concentrations. A classic example is in phototransduction, where one activated molecule—a GPCR in rod cells—catalyzes the activation of up to several hundred molecules through sequential GDP-GTP exchange, leading to a cascade that closes hundreds of cyclic nucleotide-gated channels per absorbed. This multistep amplification allows detection of single quanta of light. In RTKs, autophosphorylated tyrosines serve as docking sites for adaptor proteins, propagating signals to multiple pathways from one dimer. Such mechanisms ensure robust physiological responses without requiring high ligand levels. To prevent overstimulation and maintain cellular , receptors undergo desensitization through and binding. G-protein-coupled receptor kinases (GRKs) activated GPCRs on / residues in the C-terminal tail and intracellular loops, recruiting β-arrestins that sterically hinder G interaction and promote receptor internalization. For instance, in the β2-adrenergic receptor, GRK-mediated following binding enables β-arrestin association, uncoupling the receptor from G within seconds to minutes. Similar mechanisms apply to RTKs, where activity or deactivates signaling. These regulatory steps are crucial for terminating signals and allowing resensitization. A prominent example of receptor activation is the binding of adrenaline (epinephrine) to the β-adrenergic GPCR in cardiac and cells, triggering the . Adrenaline binds the orthosteric site, inducing a conformational change that activates Gs proteins, ultimately leading to downstream second messenger production and increased or bronchodilation. This pathway exemplifies how receptor activation translates hormonal signals into coordinated physiological effects.

Second Messengers and Response Cascades

Second messengers are intracellular signaling molecules that relay and amplify signals from cell surface receptors, initiating cascades that lead to diverse physiological responses such as enzyme activation, , and cytoskeletal reorganization. These molecules, including cyclic nucleotides and lipid-derived products, enable rapid and specific communication within the cell, often through events or release. In cell physiology, second messenger systems integrate extracellular cues to regulate processes like , contraction, and proliferation. Cyclic adenosine monophosphate (cAMP) serves as a prototypical second messenger, first identified in 1958 by Earl Sutherland in the context of hormonal regulation of glycogenolysis. Upon activation of G protein-coupled receptors (GPCRs) by ligands such as epinephrine, stimulatory G proteins (Gs) interact with adenylyl cyclase, an enzyme embedded in the plasma membrane that catalyzes the conversion of ATP to cAMP. Elevated cAMP levels bind to and activate protein kinase A (PKA), a tetrameric holoenzyme that dissociates into regulatory and catalytic subunits, allowing the catalytic subunits to phosphorylate target proteins. This phosphorylation cascade modulates downstream effectors, including transcription factors and ion channels, thereby influencing cellular responses like glycogen breakdown and ion transport. Another key second messenger system involves the hydrolysis of (PIP2) by (PLC), activated via Gq-coupled receptors or receptor tyrosine kinases. This enzymatic cleavage produces inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), both of which propagate signaling. IP3 diffuses to the , where it binds to IP3 receptors, triggering the release of stored calcium ions (Ca²⁺) into the . Concurrently, DAG remains in the and recruits and activates (PKC), which phosphorylates substrates involved in , differentiation, and . This dual-messenger mechanism, discovered through studies on calcium mobilization in response to hormones, amplifies signals for rapid cellular adjustments. Calcium ions (Ca²⁺) function as a ubiquitous second messenger, coordinating a wide array of physiological events through binding to the calmodulin (). Upon elevation of cytosolic Ca²⁺—often from IP3-mediated release or influx through plasma membrane channels—Ca²⁺ binds to , inducing a conformational change that exposes binding sites for target enzymes. The Ca²⁺/ complex activates calmodulin-dependent kinases, such as (MLCK), which phosphorylates myosin light chains to promote actin-myosin interactions essential for and cytoskeletal dynamics. This regulation extends to other processes, including release and transcription, underscoring Ca²⁺'s role in integrating multiple signaling inputs. Signaling pathways mediated by second messengers often exhibit crosstalk, allowing integration of diverse stimuli for nuanced cellular responses. For instance, in signaling, receptor tyrosine kinases activate the (MAPK) cascade, which intersects with cAMP and Ca²⁺ pathways to modulate proliferation and . The MAPK/ERK pathway, involving sequential of Raf, MEK, and ERK kinases, can be influenced by PKA of Raf or Ca²⁺-dependent activation of upstream Ras, thereby fine-tuning and cell fate decisions. Such interactions prevent isolated pathway activation and enable context-specific outcomes, as seen in developmental and stress responses. To ensure signal fidelity and prevent overstimulation, second messenger levels are tightly controlled, particularly through enzymatic degradation. Phosphodiesterases (PDEs) hydrolyze cAMP to its inactive 5'-AMP form, rapidly terminating PKA activation and compartmentalizing signaling spatially and temporally. Multiple PDE isoforms, such as PDE4, exhibit tissue-specific expression and regulation, allowing precise modulation of gradients within subcellular domains. Similarly, phosphatases dephosphorylate activated kinases, while Ca²⁺ pumps and buffers restore basal levels, collectively damping cascades to maintain cellular .

Bioenergetics and Metabolism

ATP Generation Processes

Cells generate (), their primary energy currency, through several interconnected processes that vary by oxygen availability and cellular demands. The fundamental mechanisms include during and in mitochondria, with anaerobic fermentation serving as an alternative under low-oxygen conditions. These pathways ensure ATP production supports essential physiological functions such as ion transport, , and . Glycolysis occurs in the and represents the initial stage of glucose , a 10-step enzymatic pathway that converts one of glucose into two molecules of pyruvate, yielding a net gain of 2 ATP and 2 NADH per glucose . This process begins with the of glucose by and proceeds through energy-investment and energy-payoff phases, where key regulatory enzymes like phosphofructokinase-1 (PFK-1) catalyze the committed step of fructose-6-phosphate to fructose-1,6-bisphosphate, allosterically inhibited by high ATP levels to prevent unnecessary flux. Substrate-level occurs at two sites: and , directly transferring phosphate groups from high-energy intermediates to ADP without requiring a . In the presence of oxygen, pyruvate enters the mitochondria for further oxidation, culminating in oxidative phosphorylation, which accounts for the majority of ATP production. The electron transport chain (ETC), embedded in the inner mitochondrial membrane, consists of four protein complexes (I-IV) that transfer electrons from NADH and FADH₂ to oxygen, pumping protons into the intermembrane space to establish an electrochemical gradient (proton motive force). This gradient drives ATP synthesis via ATP synthase, a rotary enzyme complex comprising the membrane-embedded F₀ subunit (which conducts protons) and the peripheral F₁ subunit (which catalyzes ATP formation from ADP and inorganic phosphate, Pᵢ). The chemiosmotic theory, proposed by Peter Mitchell, explains this coupling: the energy from proton translocation down the proton motive force (Δμ_H+ = F Δψ - 2.303 RT ΔpH) balances the phosphorylation potential ΔG_p = ΔG° + RT ln([ATP]/([ADP][P_i])) at equilibrium, where n protons (typically ~3-4 per ATP) provide the necessary free energy for synthesis. Under anaerobic conditions, cells rely on to regenerate NAD⁺ for continued , bypassing . In , alcoholic fermentation reduces pyruvate to via pyruvate decarboxylase and , yielding no additional ATP beyond the 2 from but allowing NADH oxidation. In mammalian muscle cells during intense exercise, converts pyruvate to lactate through , similarly producing a net 2 ATP per glucose while buffering the cytosolic state. Substrate-level phosphorylation via alone yields only 2 ATP per glucose, whereas complete aerobic oxidation through , the tricarboxylic acid cycle, and generates approximately 30 ATP per glucose, highlighting the efficiency of mitochondrial processes. In specialized tissues like , uncoupling protein 1 () in the dissipates the proton gradient as heat rather than ATP synthesis, enabling non-shivering for without net ATP production.

Metabolic Regulation in Cells

Cells maintain metabolic homeostasis by dynamically regulating fluxes through biochemical pathways, ensuring energy production aligns with physiological demands such as growth, stress, or availability. This regulation occurs at multiple levels, including enzymatic modulation, hormonal signaling, , and sensing, preventing wasteful or harmful imbalances. For instance, when energy is abundant, inhibitory mechanisms slow catabolic processes like , while anabolic pathways like are favored under nutrient-rich conditions. Allosteric regulation provides rapid, reversible control of key enzymes without altering , allowing cells to fine-tune in response to immediate levels. A classic example is the feedback inhibition of phosphofructokinase-1 (PFK-1) by ATP in ; high ATP concentrations bind to an allosteric site on PFK-1, reducing its affinity for fructose-6-phosphate and slowing glycolytic flux to conserve glucose when energy is plentiful. This mechanism, first elucidated in studies of rat heart and tissues, exemplifies how product inhibition prevents overproduction of ATP and maintains energetic balance. Hormonal signals integrate systemic cues to orchestrate cellular across tissues. Insulin, secreted by pancreatic β-cells in response to elevated blood glucose, promotes in muscle and adipose cells via translocation of transporters to the plasma membrane and activates by dephosphorylating through the PI3K-Akt pathway. Conversely, from α-cells counters low glucose by binding hepatic receptors, elevating cAMP levels to activate , which phosphorylates enzymes favoring and while inhibiting . These opposing actions ensure blood glucose stability, with insulin dominating in fed states and in . Compartmentalization spatially segregates metabolic reactions, enabling independent regulation of cytosolic and mitochondrial processes to optimize efficiency and avoid interference. and initial occur in the , where high NADH levels can drive lactate production under anaerobic conditions, whereas the tricarboxylic acid cycle and are confined to mitochondria, relying on shuttles like the malate-aspartate system to transfer reducing equivalents across the inner . This separation allows cells to prioritize cytosolic pathways for rapid ATP needs during bursts of activity, while mitochondrial oxidation supports sustained production; disruptions, such as in mitochondrial disorders, shift reliance to cytosolic . The NAD+/NADH serves as a critical sensor, dictating pathway directionality by influencing activities and overall metabolic flux. In the , a high NAD+/NADH favors by enabling the oxidation of lactate to pyruvate, whereas a low under hypoxia promotes via reduction of pyruvate to lactate, regenerating NAD+ for continued ATP production. Mitochondrially, this modulates the ; elevated NADH inhibits , slowing the TCA cycle when reducing power is abundant, thus linking state to energy demand and preventing oxidative overload. In cancer cells, the Warburg effect illustrates maladaptive regulation favoring proliferation over efficiency, where aerobic is upregulated despite oxygen availability, producing lactate to support synthesis. This shift, observed in tumor tissues, involves altered allosteric control of PFK-1 and increased expression of glycolytic enzymes, diverting glucose from mitochondrial oxidation to provide intermediates for and production, thereby accelerating at the cost of energetic yield.

References

  1. https://nces.ed.gov/ipeds/cipcode/cipdetail.aspx?y=55&cipid=87459
  2. https://pressbooks-dev.oer.hawaii.edu/anatomyandphysiology2021/chapter/3-cells/
  3. https://www.pnas.org/doi/10.1073/pnas.2303077120
  4. https://www.unco.edu/nhs/biology/about-us/labs/pullen-nicholas/documents/BIO_550-009_Pullen_F24.pdf
  5. https://cell.byu.edu/Undergraduate/Cell-Biology-and-Physiology
  6. https://pmc.ncbi.nlm.nih.gov/articles/PMC8894267/
  7. https://www.ncbi.nlm.nih.gov/books/NBK26826/
  8. http://www.nature.com/scitable/topicpage/genome-packaging-in-prokaryotes-the-circular-chromosome-9113
  9. https://pmc.ncbi.nlm.nih.gov/articles/PMC98921/
  10. https://cals.cornell.edu/microbiology/research/active-research-labs/angert-lab/epulopiscium/binary-fission-and-other-forms-of-reproduction-bacteria
  11. https://pressbooks.umn.edu/introbio/chapter/mitosiscelldivision/
  12. https://pmc.ncbi.nlm.nih.gov/articles/PMC1317591/
  13. https://pmc.ncbi.nlm.nih.gov/articles/PMC6883170/
  14. https://www.nature.com/articles/s41467-023-43143-z
  15. https://www.ncbi.nlm.nih.gov/books/NBK564361/
  16. https://pressbooks-dev.oer.hawaii.edu/biology/chapter/prokaryotic-gene-regulation/
  17. https://www.ncbi.nlm.nih.gov/books/NBK9845/
  18. https://pmc.ncbi.nlm.nih.gov/articles/PMC7767752/
  19. https://www.sciencedirect.com/science/article/pii/S009286740100215X
  20. https://www.nature.com/scitable/topicpage/how-do-proteins-move-through-the-golgi-14397318/
  21. https://www.ncbi.nlm.nih.gov/books/NBK9893/
  22. https://pmc.ncbi.nlm.nih.gov/articles/PMC4012490/
  23. https://pmc.ncbi.nlm.nih.gov/articles/PMC5514200/
  24. https://www.ncbi.nlm.nih.gov/books/NBK26871/
  25. https://www.science.org/doi/10.1126/science.175.4023.720
  26. https://www.ncbi.nlm.nih.gov/books/NBK9898/
  27. https://pmc.ncbi.nlm.nih.gov/articles/PMC8529036/
  28. https://www.nature.com/articles/s41598-019-53952-2
  29. https://www.ncbi.nlm.nih.gov/books/NBK9928/
  30. https://www.ncbi.nlm.nih.gov/books/NBK9847/
  31. https://fluidsbarrierscns.biomedcentral.com/articles/10.1186/2045-8118-9-23
  32. https://pmc.ncbi.nlm.nih.gov/articles/PMC11537529/
  33. https://journals.physiology.org/doi/full/10.1152/physiologyonline.2002.17.1.1
  34. https://www.ncbi.nlm.nih.gov/books/NBK538338/
  35. https://pubs.acs.org/doi/10.1021/jp4124315
  36. https://www.nature.com/articles/s41421-020-00233-2
  37. https://pmc.ncbi.nlm.nih.gov/articles/PMC7182109/
  38. https://pubmed.ncbi.nlm.nih.gov/33633837/
  39. https://www.nobelprize.org/uploads/2018/06/agre-lecture.pdf
  40. https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.18683
  41. https://www.ncbi.nlm.nih.gov/books/NBK537088/
  42. https://pmc.ncbi.nlm.nih.gov/articles/PMC3299169/
  43. https://www.annualreviews.org/content/journals/10.1146/annurev-biophys-083012-130429
  44. https://journals.physiology.org/doi/abs/10.1152/physrev.00055.2009
  45. https://pmc.ncbi.nlm.nih.gov/articles/PMC8588740/
  46. https://journals.physiology.org/doi/abs/10.1152/physrev.00032.2008
  47. https://pmc.ncbi.nlm.nih.gov/articles/PMC6751294/
  48. https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2022.931777/full
  49. https://link.springer.com/article/10.1007/s10158-018-0221-7
  50. https://www.ncbi.nlm.nih.gov/books/NBK26910/
  51. https://journals.physiology.org/doi/full/10.1152/ajprenal.00432.2014
  52. https://pmc.ncbi.nlm.nih.gov/articles/PMC4494666/
  53. https://www.nature.com/articles/s41586-022-05638-5
  54. https://www.nobelprize.org/uploads/2018/06/blobel-lecture.pdf
  55. https://pmc.ncbi.nlm.nih.gov/articles/PMC3805129/
  56. https://pmc.ncbi.nlm.nih.gov/articles/PMC5881517/
  57. https://www.nature.com/articles/s41392-024-01886-1
  58. https://pmc.ncbi.nlm.nih.gov/articles/PMC4442082/
  59. https://pubmed.ncbi.nlm.nih.gov/26947578/
  60. https://pmc.ncbi.nlm.nih.gov/articles/PMC5026485/
  61. https://www.pnas.org/doi/10.1073/pnas.96.10.5452
  62. https://royalsocietypublishing.org/doi/10.1098/rsob.200089
  63. https://www.nature.com/articles/nrm2546
  64. https://pubmed.ncbi.nlm.nih.gov/35940909/
  65. https://rupress.org/jcb/article/204/6/869/54556/ER-associated-degradation-Protein-quality-control
  66. https://pmc.ncbi.nlm.nih.gov/articles/PMC3934755/
  67. https://pmc.ncbi.nlm.nih.gov/articles/PMC6463291/
  68. https://link.springer.com/article/10.1007/s00125-020-05192-7
  69. https://www.annualreviews.org/doi/10.1146/annurev.neuro.23.1.39
  70. https://pmc.ncbi.nlm.nih.gov/articles/PMC5666633/
  71. https://www.annualreviews.org/doi/10.1146/annurev-biochem-062917-012530
  72. https://www.nature.com/articles/s41565-021-00858-8
  73. https://www.nature.com/articles/s41467-019-12434-9
  74. https://rupress.org/jcb/article/161/4/673/44016/Caveolae-raft-dependent-endocytosis
  75. https://journals.biologists.com/jcs/article/116/23/4707/27370/Caveosomes-and-endocytosis-of-lipid-rafts
  76. https://pmc.ncbi.nlm.nih.gov/articles/PMC6176211/
  77. https://royalsocietypublishing.org/doi/10.1098/rstb.2018.0147
  78. https://www.ncbi.nlm.nih.gov/books/NBK26870/
  79. https://pmc.ncbi.nlm.nih.gov/articles/PMC7280488/
  80. https://journals.physiology.org/doi/abs/10.1152/physrev.1997.77.3.759
  81. https://pmc.ncbi.nlm.nih.gov/articles/PMC7463864/
  82. https://pmc.ncbi.nlm.nih.gov/articles/PMC9886713/
  83. https://pmc.ncbi.nlm.nih.gov/articles/PMC519036/
  84. https://journals.biologists.com/jcs/article/136/5/jcs260749/292583/Endosomal-sorting-sorted-motors-adaptors-and
  85. https://pmc.ncbi.nlm.nih.gov/articles/PMC4880020/
  86. https://www.cell.com/neuron/fulltext/S0896-6273%2800%2980238-X
  87. https://www.annualreviews.org/doi/pdf/10.1146/annurev.cb.03.110187.001331
  88. https://pmc.ncbi.nlm.nih.gov/articles/PMC2394575/
  89. https://www.frontiersin.org/journals/molecular-neuroscience/articles/10.3389/fnmol.2017.00109/full
  90. https://diabetesjournals.org/diabetes/article/51/suppl_1/S3/11819/Molecular-Determinants-of-Regulated-Exocytosis
  91. https://www.pnas.org/doi/10.1073/pnas.94.3.769
  92. https://www.jbc.org/article/S0021-9258%2818%2948702-9/fulltext
  93. https://pmc.ncbi.nlm.nih.gov/articles/PMC3710122/
  94. https://pmc.ncbi.nlm.nih.gov/articles/PMC2139964/
  95. https://www.pnas.org/doi/10.1073/pnas.221450198
  96. https://febs.onlinelibrary.wiley.com/doi/full/10.1046/j.1432-1327.1999.00043.x
  97. https://www.mdpi.com/1422-0067/21/7/2576
  98. https://www.pnas.org/doi/10.1073/pnas.74.11.5073
  99. https://pmc.ncbi.nlm.nih.gov/articles/PMC432101/
  100. https://www.science.org/doi/10.1126/science.181.4099.561
  101. https://www.nature.com/articles/s41392-024-01803-6
  102. https://pmc.ncbi.nlm.nih.gov/articles/PMC3949355/
  103. https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Principles_of_Biology/01%253A_Chapter_1/07%253A_Cell_Communication/7.02%253A_Types_of_Receptors
  104. https://www.pnas.org/doi/10.1073/pnas.2110085119
  105. https://www.pnas.org/doi/10.1073/pnas.0914157107
  106. https://elifesciences.org/articles/50279
  107. https://www.sciencedirect.com/science/article/pii/S0960982202007546
  108. https://www.pnas.org/doi/10.1073/pnas.262789099
  109. https://www.frontiersin.org/journals/pharmacology/articles/10.3389/fphar.2019.00125/full
  110. https://www.ahajournals.org/doi/10.1161/CIRCRESAHA.124.323067
  111. https://www.ccjm.org/content/ccjom/57/5/481.full.pdf
  112. https://pmc.ncbi.nlm.nih.gov/articles/PMC4968160/
  113. https://pmc.ncbi.nlm.nih.gov/articles/PMC3504441/
  114. https://www.ahajournals.org/doi/10.1161/01.res.0000247760.34779.f5
  115. https://www.nobelprize.org/prizes/medicine/1971/sutherland/facts/
  116. https://www.spandidos-publications.com/10.3892/mmr.2016.5005
  117. https://www.annualreviews.org/doi/pdf/10.1146/annurev.biophys.31.082901.134259
  118. https://www.nature.com/articles/312315a0
  119. https://pmc.ncbi.nlm.nih.gov/articles/PMC2075238/
  120. https://pubmed.ncbi.nlm.nih.gov/7803766/
  121. https://pmc.ncbi.nlm.nih.gov/articles/PMC8830543/
  122. https://www.ahajournals.org/doi/10.1161/01.RES.0000225860.41648.63
  123. https://pmc.ncbi.nlm.nih.gov/articles/PMC2043147/
  124. https://www.sciencedirect.com/science/article/pii/S0167488910003228
  125. https://pmc.ncbi.nlm.nih.gov/articles/PMC3063353/
  126. https://pmc.ncbi.nlm.nih.gov/articles/PMC4275405/
  127. https://journals.physiology.org/doi/10.1152/ajpheart.00766.2011
  128. https://www.ncbi.nlm.nih.gov/books/NBK482303/
  129. https://www.sciencedirect.com/science/article/pii/S0021925824024086
  130. https://employees.csbsju.edu/hjakubowski/classes/ch331/oxphos/OP_8C9_ATP_synthase.html
  131. https://pmc.ncbi.nlm.nih.gov/articles/PMC4262006/
  132. https://pmc.ncbi.nlm.nih.gov/articles/PMC7983055/
  133. https://www.khanacademy.org/science/ap-biology/cellular-energetics/cellular-respiration-ap/a/oxidative-phosphorylation-etc
  134. https://pmc.ncbi.nlm.nih.gov/articles/PMC3355862/
  135. https://pmc.ncbi.nlm.nih.gov/articles/PMC7973386/
  136. https://pubmed.ncbi.nlm.nih.gov/14484231/
  137. https://pubmed.ncbi.nlm.nih.gov/4222062/
  138. https://pmc.ncbi.nlm.nih.gov/articles/PMC6170977/
  139. https://pmc.ncbi.nlm.nih.gov/articles/PMC3563428/
  140. https://pmc.ncbi.nlm.nih.gov/articles/PMC10155461/
  141. https://pmc.ncbi.nlm.nih.gov/articles/PMC6541229/
  142. https://pmc.ncbi.nlm.nih.gov/articles/PMC5737637/
  143. https://rupress.org/jgp/article/8/6/519/12408/THE-METABOLISM-OF-TUMORS-IN-THE-BODY
  144. https://pmc.ncbi.nlm.nih.gov/articles/PMC4783224/
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