Solute Transport across Membranes:- Passive Transport Is Facilitated by Membrane Proteins

When two aqueous compartments containing unequal concentrations of a soluble compound or ion are separated by a permeable divider (membrane), the solute moves by simple diffusionfrom the region of higher concentration, through the membrane, to the region of lower concentration, until the two compartments have equal solute concentrations (Fig. 11–27a). When ions of opposite charge are separated by a permeable mem brane, there is a transmembrane electrical gradient, a membrane potential, V_m (expressed in volts or milli volts). This membrane potential produces a force opposing ion movements that increase V_m and driving ion movements that reduce V_m (Fig. 11–27b). Thus, the di rection in which a charged solute tends to move spontaneously across a membrane depends on both the chemical gradient (the difference in solute concentration) and the electrical gradient (Vm) across the mem brane. Together, these two factors are referred to as the electrochemical gradient or electrochemical potential. This behavior of solutes is in accord with the second law of thermodynamics: molecules tend to spontaneously assume the distribution of greatest random ness and lowest energy.

FIGURE 11–27 Movement of solutes across a permeable membrane. (a) Net movement of electrically neutral solutes is toward the side of lower solute concentration until equilibrium is achieved. The solute concentrations on the left and right sides of the membrane are designated C1 and C2. The rate of transmembrane movement (indicated by the large arrows) is proportional to the concentration gradient, C1/C2. (b) Net movement of electrically charged solutes is dictated by a com bination of the electrical potential (Vm) and the chemical concentration difference across the membrane; net ion movement continues until this electrochemical potential reaches zero.

To pass through a lipid bilayer, a polar or charged solute must first give up its interactions with the water molecules in its hydration shell, then diffuse about 3 nm (30 Å) through a solvent (lipid) in which it is poorly soluble (Fig. 11–28). The energy used to strip away the hydration shell and to move the polar compound from water into and through lipid is regained as the com pound leaves the membrane on the other side and is re hydrated. However, the intermediate stage of trans membrane passage is a high-energy state comparable to the transition state in an enzyme-catalyzed chemical re action. In both cases, an activation barrier must be overcome to reach the intermediate stage (Fig. 11–28; com pare with Fig. 6–3). The energy of activation (ΔG‡) for translocation of a polar solute across the bilayer is so large that pure lipid bilayers are virtually impermeable to polar and charged species over periods of time relevant to cell growth and division. Membrane proteins lower the activation energy for transport of polar compounds and ions by providing an alternative path through the bilayer for specific solutes. Proteins that bring about this facilitated diffusion, or passive transport, are not enzymes in the usual sense; their “substrates” are moved from one compartment to another, but are not chemically altered. Membrane proteins that speed the movement of a solute across a mem brane by facilitating diffusion are called transporters or permeases.

Like enzymes, transporters bind their substrates with stereochemical specificity through multiple weak, noncovalent interactions. The negative free-energy change associated with these weak interactions, Δ_Gbinding, counterbalances the positive free-energy change that accompanies loss of the water of hydration from the substrate, ΔG_dehydration, thereby lowering ΔG‡ for transmembrane passage (Fig. 11–28). Transporters span the lipid bilayer several times, forming a trans membrane channel lined with hydrophilic amino acid side chains. The channel provides an alternative path for a specific substrate to move across the lipid bilayer without its having to dissolve in the bilayer, further lowering ΔG‡ for transmembrane diffusion. The result is an increase of several orders of magnitude in the rate of transmembrane passage of the substrate.

FIGURE 11–28 Energy changes accompanying passage of a hydrophilic solute through the lipid bilayer of a biological membrane. (a) In simple diffusion, removal of the hydration shell is highly endergonic, and the energy of activation (ΔG‡) for diffusion through the bilayer is very high. (b) A transporter protein reduces the ΔG‡ for transmem brane diffusion of the solute. It does this by forming noncovalent in teractions with the dehydrated solute to replace the hydrogen bond ing with water and by providing a hydrophilic transmembrane passageway.

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