Biochemistry and Molecular Biology - Belyasova, N. A. 2002

Structure and Functions of Cellular Components
Biomembranes
Transport of Substances Across Membranes

Cellular activity is inextricably linked to the continuous exchange of contents between The Cell and its surrounding environment. Similarly, within the cell itself, molecules and structures constantly move between various Organelles and compartments. All of these processes require overcoming a primary barrier—the membrane that encloses either the organelle or the entire cell. It is important to bear in mind that the fundamental function of Introduction/36.html">Biological Membranes is to selectively regulate The transport of various substances and ions. The mechanisms of membrane transport can be broadly categorized into four main types: passive diffusion, Facilitated Diffusion, Active Transport, and cytosis.

Passive diffusion is the Transmembrane Transport of substances from a region of higher concentration to a region of lower concentration (along a chemical gradient) that does not involve transport Proteins or require the input of energy. Small, uncharged molecules—such as gases, certain anesthetics, and Water—cross the membrane via this mechanism. To traverse the bilayer, a molecule must overcome surface tension at the membrane boundary, enter The Lipid Bilayer, diffuse across it, and emerge on the opposite side, once again surmounting the energy barrier at the phase boundary. This explains why the lipid bilayer exhibits selective permeability to small non-electrolyte molecules. Remarkably, water permeates membranes with exceptional ease and speed: studies have shown that a single water molecule takes a mere 1 µs to cross the bilayer. To account for this phenomenon, recent experimental evidence has led to the hypothesis that membranes may contain specialized proteinaceous water channels, or alternatively, that water molecules exploit transient local defects within the bilayer Structure.

The movement of pure water molecules across a semipermeable membrane can be viewed as a specific case of diffusion known as osmosis. Osmosis refers to the net Movement of water molecules from a region of high water potential and low solute concentration to a region of low water potential and high solute concentration (Fig. 4.9). In this scenario, water molecules flow from a hypotonic solution into a hypertonic solution until dynamic equilibrium is reached and both solutions become isotonic relative to one another.

To quantify the decrease in water potential caused by the presence of solutes, the term "osmotic pressure" is used. Osmotic pressure is defined as the external pressure that must be applied to a solution to halt the osmotic influx of water through a semipermeable membrane. Increasing the solute concentration raises the osmotic pressure while simultaneously lowering The water potential of the solution.

The movement of water across cellular Plasma Membranes in accordance with the laws of osmosis poses significant physiological challenges, particularly for aquatic organisms. Consequently, osmoregulation—the homeostatic maintenance of cellular water potential—is a vital physiological function for the vast majority of organisms, frequently consuming a substantial portion of the cell's metabolic energy reserves.

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Fig. 4.9. Movement of water molecules in accordance with the principles of osmosis. Solution A has a higher glucose concentration than Solution B, making it hypertonic relative to B. Conversely, Solution B is hypotonic with respect to Solution A. The osmotic movement of water results in the equalization of both water and glucose concentrations, rendering the solutions isotonic to each other.

The rate of molecular diffusion across membranes depends on numerous factors, including the solubility of the substance in the membrane, its diffusion coefficient within the lipid phase, and the concentration gradient between the extracellular and intracellular environments (Fig. 4.10).

Facilitated diffusion. This transport mechanism is mediated by specific transport proteins operating along an electrochemical gradient (the combined difference in electrical potential and solute concentration) without Energy Expenditure. It is a selective process: a substance will only be transported across the membrane if a functional transport protein specific to it is present. Because facilitated diffusion relies on proteins, the process—unlike simple passive diffusion—can reach a state of saturation. The saturation plateau (Fig. 4.10) corresponds to the condition where all available transport proteins for a given substrate are fully occupied, and the transport rate reaches its maximum velocity.

Facilitated diffusion facilitates the transmembrane transport of numerous essential substances, including hydrophilic molecules such as CARBOHYDRATES, Amino Acids, NUCLEOTIDES, and various ions. Furthermore, the rate of facilitated diffusion vastly exceeds that of passive diffusion (Fig. 4.10). In principle, substances and ions can cross the membrane via two distinct pathways: carriers and channels. Since Transmembrane Movement of carrier proteins themselves has not been observed in biological membranes, the "ping-pong" mechanism has been proposed to describe carrier-mediated transport. According to this model, solute translocation is coupled to Conformational Changes in the carrier Protein Structure induced by the binding of the transported molecule (Fig. 4.11).

Fig. 4.10. Dependence of passive and facilitated diffusion rates on the concentration gradient of the transported substance.

Fig. 4.11. The "ping-pong" model illustrating the operation of integral transport proteins.

The functioning of Ion Channels can be exemplified by the Acetylcholine Receptor. This integral membrane protein is predominantly localized in the neuromuscular junctions of Skeletal Muscle. It is composed of five subunits of four distinct types and opens in response to the binding of the neurotransmitter acetylcholine. Upon binding acetylcholine, the channel undergoes a conformational shift in its subunits, opening to permit the selective passage of specific ions (such as Na⁺, K⁺, Ca⁺⁺, and others) for approximately 1 ms before closing again. This selective influx and efflux of cations alters the transmembrane potential, triggering the electrical excitation of the muscle cell and ultimately leading to Muscle contraction. Structural Analysis of the acetylcholine receptor reveals that its five protein subunits are arranged within the lipid bilayer around a central pore with a diameter of 3 nm, through which the cations pass. The channel's impermeability to anions and its threefold higher permeability to cations compared to uncharged molecules can be attributed to Electrostatic Interactions generated by bipolar or negatively charged amino acid residues lining the channel gate.

A specialized mode of facilitated transport involves The Use of ionophores, whose properties have been extensively studied using artificial lipid membranes. Ionophores are low-molecular-weight peptidic compounds that selectively transport ions across membranes. They are generally divided into channel-forming ionophores (such as gramicidin A and amphotericin B) and mobile carrier ionophores (such as valinomycin, enniatins, and beauvericin).

Valinomycin is a depsipeptide antibiotic that Functions as an ionic "trap". In non-polar Solvents, the conformation of valinomycin resembles a bracelet, with a central cavity precisely tailored to accommodate potassium ions. Because its outer surface is hydrophobic, valinomycin can diffuse freely within the lipid bilayer, thereby ferrying ions across it.

A well-characterized example of a channel-forming ionophore is gramicidin A, a peptide antibiotic consisting of 15 amino acids. Two gramicidin molecules can span the membrane either as a double helix or by forming a HEAD-to-head dimer. In these Conformations, gramicidin A molecules assemble a hollow transmembrane cylinder through which Metal Ions can diffuse.

To date, ionophore-mediated transport has not been detected in natural biological membranes.

Active transport is the energy-dependent translocation of molecules or ions across a membrane against an electrochemical gradient, mediated by specific transport proteins. Active transport enables living Cells to maintain a transmembrane potential difference—an electrical charge where, in the vast majority of studied cells, the intracellular milieu is negatively charged relative to the extracellular environment.

Active transport entails significant metabolic costs: certain cells expend more than one-third of their total stored energy merely to establish and maintain ionic gradients across their membranes. This expenditure is essential for critical physiological processes such as osmoregulation, the GENERATION AND PROPAGATION of nerve impulses, and the cellular uptake of nutrients (including sugars and amino acids).

Diverse active transport systems are primarily distinguished by their energy source: ATP, ion gradients, phosphoenolpyruvate, or light. The most thoroughly studied active transport system is the sodium-potassium pump ((Na⁺/K⁺)-ATPase), which operates in the plasma membranes of animal cells. This integral protein consists of two subunits: a larger catalytic polypeptide bearing binding sites for sodium ions and ATP on its cytoplasmic surface, and potassium-binding sites on its extracellular surface; and a smaller glycoprotein subunit. The (Na⁺/K⁺)-ATPase functions by hydrolyzing a single ATP molecule to extrude 3 Na⁺ ions from the cell while importing 2 K⁺ ions from the extracellular space. Because more positive charges are pumped out than are brought in, an excess negative charge accumulates on the inner leaflet of the membrane, rendering the cell electrogenic. Although membranes typically contain leakage pathways that allow passive Na⁺ and K⁺ diffusion down their electrochemical gradients, a living cell prevents the collapse of these gradients through the continuous activity of primary active transporters like the (Na⁺/K⁺)-ATPase. Thus, ion pumps involved in primary active transport generate Membrane Potential and establish the electrochemical gradients that store cellular energy.

Secondary active transporters harness pre-existing electrochemical gradients as the driving force for solute transport. This mechanism is clearly observable in intestinal epithelial cells. The fundamental Building Blocks of digested food (such as Amino Acids and glucose) are absorbed into the bloodstream via diffusion across capillary membranes, a process driven by symport (co-transport) with sodium ions. Sodium ions tend to flow back into the cell down their facilitated diffusion gradient, effectively dragging nutrient molecules along with them. Specific sugar and amino acid cotransporters located in The cell membrane accumulate these nutrients intracellularly by scavenging them even from highly dilute solutions—that is, working against a chemical gradient. These same transport systems can also facilitate passive diffusion when substrates move down their chemical gradient. In addition to the aforementioned symport of nutrients driven by returning sodium ions, cells also utilize antiport (counter-transport). For instance, the anion exchanger (Band 3 protein) of erythrocytes mediates the coupled, reciprocal transport of Cl^- and HCO₃^- ions in opposite directions across THE RED Blood cell membrane.

In aerobic Bacteria, nutrient uptake is driven by symport with protons rather than Na⁺ ions. The best-characterized paradigm of such a carrier is the lactose permease of *Escherichia coli*. This integral membrane protein utilizes the proton electrochemical gradient established across the membrane by Oxidative Phosphorylation to drive lactose symport: for every proton allowed to re-enter the cell, exactly one lactose molecule is cotransported.

It is worth noting that ATPases are Enzymes capable of catalyzing reversible reactions: during ATP Hydrolysis, ions are pumped against their electrochemical gradient, whereas the downhill movement of ions through ATPase channels can drive the synthesis of ATP.

Endocytosis and Exocytosis. These mechanisms of transmembrane transport involve The formation of membrane invaginations and specialized membrane vesicles that facilitate the passage of large macromolecules and particles across the membrane. Endocytosis mediates the uptake of substances into the cell, whereas exocytosis drives their release. Cytoses are traditionally subdivided into two types: pinocytosis and phagocytosis. Pinocytosis is the mechanism by which proteins and other fluid-phase macromolecules are transported across the membrane. Phagocytosis involves the internalization of large particles, such as bacteria and Viruses, by the cell. These transport pathways are predominantly characteristic of Eukaryotic cells; in animals, however, phagocytosis is restricted to specialized cells, such as macrophages. For many protists, including amoebas, phagocytosis serves as the primary mode of Nutrition.

A crucial feature of cytoses is the sequential formation and fusion of vesicles enclosing the transported substance. The secreted or internalized molecules remain sequestered within these vesicles, avoiding admixture with other cellular macromolecules or organelles. Through a mechanism not yet fully understood, each vesicle fuses exclusively with specific membrane structures (Fig. 4.12).

Cytosis is underpinned by another hallmark property of biomembrane lipid layers: the capacity for aggregation, whereby small vesicles coalesce into larger ones or merge with The Plasma Membrane of the cell. This mechanism relies on the structural universality of Biomembranes involved in the formation of cellular organelles and protoplasts. A comparable phenomenon can be observed in foams, where soap bubbles—similarly composed of amphiphilic molecules (soaps, which are salts of Fatty acids)—tend to coalesce to form larger structures. The aggregation capacity of membranes forms The basis of protoplast fusion, a widely used method for transferring Genetic information.

The rate of cytosis is remarkably high. Studies have shown that Liver cells take up a volume of fluid via endocytosis within 1 hour that equals at least 20% of their total volume, along with an amount of membrane material whose surface area exceeds that of their plasma membrane fivefold. The similarity between cytoses and other modes of transmembrane transport lies in the requirement for the transported substance to be "recognized" by membrane components; in other words, this process also exhibits selective membrane permeability for various compounds.

Fig. 4.12. Schematic representation of cytosis. Stages: 1-1 — a secretory vesicle pinches off from the Golgi apparatus; 1-2 — the vesicle diffuses toward the plasma membrane and adheres to it; 1-3 — fusion of the vesicle with the membrane and release of the vesicular contents into the extracellular environment; 2-1 — a particle is engulfed by the membrane; 2-2 — a phagosome pinches off from the membrane; 2-3 — the phagosome diffuses toward a lysosome; 2-4 — the phagosome adheres to and fuses with the lysosome.