MODERN BOTANY - P. RAVEN - 1990

SECTION I. THE PLANT CELL

CHAPTER 4. MOVEMENT OF SUBSTANCES INTO AND OUT OF CELLS

Structure of Cell Membranes

Currently, the most widely accepted model of the membrane is the fluid-mosaic model, according to which the membrane consists of a lipid bilayer in which Globular Proteins are embedded (Fig. 4-7). These proteins, known as integral proteins, frequently protrude from the bilayer on one or both sides of the membrane. The segment of the protein molecule embedded in the bilayer is hydrophobic, while the portion outside it is hydrophilic. It is hypothesized, though not definitively proven, that some integral proteins contain hydrophilic channels. This structural pattern is characteristic of all cellular membranes, including Cell/30.html">The Plasma Membrane and internal membranes.

Class="center">Fig. 4-7. The fluid-mosaic model of the membrane. The membrane consists of a lipid bilayer (double layer) of lipid molecules with their hydrophobic tails oriented inward, alongside large protein molecules. Proteins embedded within the bilayer are termed integral proteins. Peripheral proteins are attached to certain integral proteins on the inner surface of the membrane. The portion of the protein molecule embedded in The Lipid Bilayer is hydrophobic, whereas the projecting part is hydrophilic. Short carbohydrate chains are attached to the proteins on the outer surface of the plasma membrane. Overall, the Structure is fluid, and proteins are thought to drift within a lipid "sea."

The two surfaces of the membrane differ in their chemical composition. For instance, the two primary types of Lipids in the plasma membrane of plant Cells—Phospholipids (which are more abundant) and sterols—are present in unequal concentrations across the two halves of the bilayer. Furthermore, integral proteins possess a specific orientation within the bilayer, meaning that the portions of their molecules extending outward on opposite sides differ in Amino Acid Composition and tertiary structure. Additional proteins, known as peripheral proteins, are attached to integral proteins on the inner surface of the membrane, while short-chain CARBOHYDRATES are found on the outer surface. These carbohydrates on the exterior surface of certain Eukaryotic cells are believed to play a crucial role in intercellular recognition and surface modification.

While the lipid bilayer establishes the fundamental structure of cell membranes, proteins are responsible for the majority of Membrane Functions. Most membranes are composed of 40 to 50% lipids by weight and 50 to 60% proteins, with the quantity and specific types of Membrane Proteins reflecting their particular functions. Some act as Enzymes regulating membrane-associated reactions, others function as carriers involved in transporting specific molecules into or out of The Cell, and still others serve as receptors for receiving and transducing chemical signals from the environment. Although certain integral proteins appear to be "anchored" in place, the lipid bilayer is highly dynamic. Some proteins float more or less freely within it and, much like lipid molecules, can move laterally to form various shifting assemblages, or mosaics, whose structure is constantly changing.

Transport Across Membranes

Molecules cross membranes through three distinct processes: simple diffusion, Facilitated Diffusion, and Active Transport. Nonpolar (hydrophobic) substances, including lipid-soluble O₂, typically cross the membrane via simple diffusion. (The observation that hydrophobic molecules readily diffuse through Plasma Membranes provided the initial Evidence for the lipid Nature of the membrane.)

At first glance, one might expect the lipid bilayer of the membrane to be impermeable to Water and other polar (hydrophilic) molecules; nevertheless, hydrophilic molecules and ions do pass through it. How does this occur? In the case of water and certain other polar molecules, such as СО₂, diffusion across the membrane is possible partly because these molecules are small and uncharged.

The diffusion of nonpolar or small uncharged molecules through the membrane exemplifies passive transport. Its direction is governed solely by the concentration difference of the substance across the membrane (the concentration gradient). However, if a molecule is charged, its transport is influenced by both the concentration gradient and the overall electrical gradient across the membrane (the Membrane Potential). Together, these two gradients constitute the electrochemical gradient. In plant cells, electrical gradients typically exist across both the plasma membrane and the tonoplast. The ground substance carries a negative charge relative to both the aqueous environment surrounding the cells and the contents (cell sap) of the vacuole.

Most substances required by cells are polar and are transported across the membrane via embedded transport proteins (carrier proteins). Some transport proteins carry a single solute across the membrane (uniport). Others operate as cotransport systems, where the translocation of one solute depends on the simultaneous or sequential movement of a second substance. This second substance may move in the same direction (symport) or in the opposite direction (antiport) (Fig. 4-8). All transport proteins appear to form a continuous protein passageway through the membrane, ensuring that the substances they transport do not come into direct contact with the hydrophobic interior of the lipid bilayer.

Carrier-mediated transport operates via two principal mechanisms: facilitated diffusion and active transport. Facilitated diffusion is driven by a concentration gradient, with molecules moving down this gradient. Neither simple nor facilitated diffusion—both forms of passive transport—can operate against a concentration gradient (Fig. 4-9) or an electrochemical gradient. Moving solutes against a concentration or electrochemical gradient requires energy and is referred to as active transport (Fig. 4-10). In plant and fungal cells, active transport is powered by a proton pump driven by ATP energy, which is hydrolyzed by an H⁺-ATPase localized in the membrane. This enzyme generates a substantial electrical potential and pH gradient, providing the driving force for the uptake of substances cotransported with H⁺.

Fig. 4-8. Diagram of transport protein mechanisms. Uniport involves the simple transfer of a single solute across the membrane. In cotransport systems, the movement of one substance relies on the simultaneous or sequential transport of another, either in the same direction (symport) or in the opposite direction (antiport).

Carrier-mediated transport, whether facilitated diffusion or active transport, is highly selective. A transport protein may interact with a specific molecule while ignoring another that is nearly identical. Crucially, the transport protein itself undergoes no permanent chemical change during the transport process. In this respect, transport proteins resemble enzymes, leading early researchers to term them permeases. Unlike enzymes, however, transport proteins typically do not catalyze chemical alterations in the substances to which they temporarily bind.

Fig. 4-9. Schematic comparison of passive transport down an electrochemical gradient and active transport against an electrochemical gradient. Simple diffusion and facilitated diffusion are passive processes, whereas active transport requires an input of energy.

The Sodium-Potassium Pump

One of the most vital and thoroughly investigated active transport systems in animal cells is the sodium-potassium pump. (Mechanisms that mediate active transport are commonly referred to as pumps.) Most animal cells maintain distinct concentration gradients of sodium (Na⁺) and potassium (K⁺) ions across the plasma membrane, preserving a low internal concentration of Na⁺ and a high concentration of K⁺. Animal cells utilize the Na⁺ gradient (much as plant and fungal cells use an H⁺ gradient) to drive the uptake of substances cotransported with Na⁺. The energy required to run the sodium-potassium pump is supplied by ATP molecules generated during cellular Respiration. The profound biological importance of this system is underscored by the fact that in a resting animal, over a third of the body's ATP expenditure is devoted to powering the sodium-potassium pump.

The transport of Na⁺ and K⁺ is carried out by a specialized protein that, according to some researchers, alternates between two conformational states. One state features a cavity open to the cell interior, which can bind a Na⁺ ion; the other features a cavity open to the exterior, which binds a K⁺ ion. As illustrated in Fig. 4-10, a Na⁺ ion first binds to the transport protein. ATP is then hydrolyzed to ADP, and the released phosphate group attaches to the protein, resulting in its phosphorylation. This induces a conformational shift in the protein, carrying the Na⁺ ion to the extracellular side of the membrane and releasing it. Subsequently, the transport protein binds a K⁺ ion, triggering dephosphorylating of the protein, its return to its original conformation, and the release of the K⁺ ion into the cell interior. This cyclical process generates and maintains the transmembrane Na⁺ and K⁺ gradients.

Fig. 4-10. Operational model of the sodium-potassium pump. A. A Na⁺ ion in the Cytoplasm binds to the transport protein molecule. B. A reaction involving ATP results in The transfer of a phosphate group (P) to the protein, releasing ADP (adenosine diphosphate). C. Phosphorylation induces a conformational change in the protein, resulting in the release of Na⁺ outside the cell. D. An extracellular K⁺ ion binds to the transport protein (E), which in this conformation has a higher affinity for K⁺ than for Na⁺. F. The phosphate group is cleaved from the protein, restoring its original conformation and releasing the K⁺ ion into the cytoplasm. The transport protein is now primed to export another Na⁺ ion from the cell.

The plant and fungal proton pump is similar to the animal sodium pump. The plant protein is phosphorylated in a reaction pathway similar to the one described above. Despite the similarities in chemical structure and enzyme mechanism, these two pumps utilize different cations: H+ in plants and Fungi, and Na+ in animals.