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Transport Across Membranes Solutes Cross Membranes by Simple Diffusion,Facilitated Diffusion, and Active Transport The Movement of a Solute Across a Membrane Is Determined by Its Concentration Gradient or Its Electrochemical Potential Simple Diffusion Simple Diffusion: Unassisted Movement Down the Gradient Diffusion Always Moves Solutes Toward Equilibrium Osmosis Is the Diffusion ofWater Across a Selectively Permeable Membrane Simple Diffusion Is Limited to Small, Nonpolar Molecules The Rate of Simple Diffusion Is Directly Proportional to the Concentration Gradient Facilitated Diffusion Protein-Mediated Movement Down the Gradient Carrier Proteins and Channel Proteins Carrier Proteins Alternate Between Two Conformational States Carrier Proteins Are Analogous to Enzymes in Their Specificity and Kinetics Carrier Proteins Transport Either One or Two Solutes The Erythrocyte Glucose Transporter and Anion Exchange Protein Are Examples of Carrier Proteins Facilitated Transport Channel Proteins Facilitate Diffusion by Forming Hydrophilic Transmembrane Channels Ion Channels:Transmembrane Proteins That Allow Rapid Passage of Specific Ions Porins: Transmembrane Proteins That Allow Rapid Passage of Various Solutes Active Transport Active Transport: Protein-Mediated Movement Up the Gradient The Coupling of Active Transport to an Energy Source May Be Direct or Indirect Direct Active Transport Depends on Four Types of Transport ATPases P-type ATPases V-type ATPases F-type ATPases ABC-type ATPases Indirect Active Transport Is Driven by Ion Gradients Examples For Active Transport Direct Active Transport: The Na/K Pump Maintains Electrochemical Ion Gradients Indirect Active Transport: Sodium Symport Drives the Uptake of Glucose The Bacteriorhodopsin Proton Pump Uses Light Energy to Transport Protons Summary Cells and Transport Processes ■ The selective transport of molecules and ions across membrane barriers ensures that necessary substances are moved into and out of cells and cell compartments at the appropriate time and at useful rates. ■ Nonpolar molecules and small, polar molecules cross the membrane by simple diffusion. Transport of all other solutes, including ions and many molecules of biological relevance, is mediated by specific transport proteins that provide passage through an otherwise impermeable membrane. ■ Each such transport protein has at least one, and frequently several, hydrophobic membrane-spanning sequences that embed the protein within the membrane and often act as the channel itself. Typically, a separate regulatory domain controls channel opening and closing. Summary Simple Diffusion: Unassisted Movement Down the Gradient ■ Simple diffusion through biological membranes is limited to small or nonpolar molecules such as , , and lipids. Water molecules, although polar, are small enough to diffuse across membranes in a manner that is not entirely understood. ■ Membranes are permeable to lipids, which can pass through the nonpolar interior of the lipid bilayer. Membrane permeability of most compounds is directly proportional to their partition coefficient—their relative solubility in oil versus water. ■ The direction of diffusion of a solute across a membrane is determined by its concentration gradient and always moves toward equilibrium. The solute will diffuse down the gradient from a region of high concentration to a region of low concentration Facilitated Diffusion: Protein-Mediated Movement Down the Gradient Transport can either be downhill or uphill in relation to an uncharged solute’s concentration gradient. For an ion, we must consider its electrochemical potential—the combined effect of the ion’s concentration gradient and the charge gradient across the membrane. ■ Downhill transport of large, polar molecules and ions, called facilitated diffusion, must be mediated by carrier proteins and channel proteins because these molecules and ions cannot diffuse through the membrane directly. Facilitated Transport Carrier proteins function by alternating between two conformational states. Examples include the glucose transporter and the anion exchange protein found in the plasma membrane of the erythrocyte. ■ Transport of a single kind of molecule or ion is called uniport. The coupled transport of two or more molecules or ions at a time may involve movement of both solutes in the same direction (symport) or in opposite directions (antiport). ■ Channel proteins facilitate diffusion by forming transmembrane channels lined with hydrophilic amino acids. Three important categories of channel proteins are ion channels (used mainly for transport of , , , , , and ), porins (for various high-molecular-weight solutes), and aquaporins (for water). Active Transport: Protein-Mediated Movement Up the Gradient Active transport—the uphill transport of large, polar molecules and ions—requires a protein transporter and an input of energy. It may be powered by ATP hydrolysis, the electrochemical potential of an ion gradient, or light energy. Active transport powered by ATP hydrolysis utilizes four major classes of transport proteins: P-type, V-type, F-type, and ABC-type ATPases. One widely encountered example is the ATP-powered pump (a P-type ATPase), which maintains electrochemical potentials for sodium and potassium ions across the plasma membrane of animal cells. Active Transport Active transport driven by an electrochemical potential usually depends on a gradient of either sodium ions (animal cells) or protons (plant, fungal, and many bacterial cells). For example, the inward transport of nutrients across the plasma membrane is often driven by the symport of sodium ions that were pumped outward by the / pump. As they flow back into the cell, they drive inward transport of sugars, amino acids, and other organic molecules. ■ In Halobacterium, active transport is powered by light energy. As photons of light are absorbed by bacteriorhodopsin, protons are pumped across the cell membrane and out of the cell. As the protons flow back into the cell, ATP is synthesized.