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BCH2011: Textbook summary - Lecture 29

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LECTURE 29 Simple Diffusion: The solute moves by simple diffusion from the region of higher concentration, through the membrane, to the region of lower concentration, until the two compartments have equal solute concentrations. When ions of opposite charge are separated by a permeable membrane, there is a transmembrane electrical gradient, a membrane potential, Vm. This membrane potential produces a force opposing ion movements that increase Vm and driving ion movements that reduce Vm. Thus, the direction 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 membrane. Together these two factors are referred to as the electrochemical gradient or electrochemical potential. Facilitated Diffusion: Membrane proteins lower the activation energy for transport of polar compounds and ions by providing an alternative path across the membrane 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 membrane by facilitating diffusion are called transporters or permeases. Transporters and Ion Channels are Fundamentally Different: Transporters for molecules and ions bind their substrates with high specificity, catalyse transport at rates well below the limits of free diffusion, and are saturable in the same sense as are enzymes: there is some substrate concentration above which further increases will not produce a greater rate of transport. Channels generally allow transmembrane movement of ions at rates that are orders of magnitude greater than those typical of transporters, approaching the limit of unhindered diffusion. Channels typically show some specificity for an ion, but are not saturable with the ion substrate, in contrast to the saturation kinetics seen with transporters. The direction of ion movement through an ion channel is dictated by the ion’s charge and the electrochemical gradient across the membrane. Among the transporters, some simply facilitate diffusion down a concentration gradient; they are the passive transporters. Active transporters: They can drive substrates across the membrane against a concentration gradient, some using energy provided directly by a chemical reaction (primary active transporters) and some coupling uphill transport of one substrate with downhill transport of another (secondary active transporters). The Glucose Transporters of Erythrocytes Mediates Passive Transport: Energy-yielding metabolism in erythrocytes depends on a constant supply of glucose from the blood plasma, where the glucose concentration is maintained. Glucose enters the erythrocyte by facilitated diffusion via a specific glucose transporter, at a rate about 50,000 times greater than uncatalysed transmembrane diffusion. The glucose transporter of erythrocytes called GLUT1 is a type III integral protein with 12 hydrophobic segments, each of which is believed t
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