Chapter_11_Solutions - Chapter 11 Membrane Transport of...

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DEFINITIONS 11–1 Channel 11–2 Passive transport (facilitated diffusion) 11–3 Membrane transport protein 11–4 Active transport 11–5 Electrochemical gradient TRUE/FALSE 11–6 False. Lipid bilayers are impermeable to ions, but the plasma membrane contains specific ion channels and transporters that make it very permeable to particular ions and charged solutes under certain circumstances. 11–7 True. Transporters bind specific molecules and undergo a series of confor- mational changes to move the bound molecule across a membrane. They can transport passively down the electrochemical gradient, or the trans- porters can link the conformational changes to a source of metabolic energy such as ATP hydrolysis to drive active transport. By contrast, channels form aqueous pores that can be open or shut, but always transport downhill; that is, passively. Channels interact much more weakly with the solute to be transported, and they do not undergo conformational changes to accom- plish transport. As a consequence, transport through channels cannot be linked to an energy source and is always passive. THOUGHT PROBLEMS 11–8 The order is CO 2 (small and nonpolar) > ethanol (small and slightly polar) > H 2 O (small and polar) > glucose (large and polar) > Ca 2+ (small and charged) > RNA (very large and highly charged). This list nicely illustrates the two basic properties that govern the capacity of molecules to diffuse through a lipid bilayer: size (small > large) and polarity (nonpolar > polar > charged). 11–9 Specific solutes move through the membrane much more slowly via trans- porters than by channels because transporters must bind the solute and undergo a series of conformational changes to transfer the solute across the membrane. Transport through channels is much faster because they are ion- specific pores that neither bind the ion nor undergo any conformational changes in order to move it across the membrane. 11–10 To behave like a revolving door, the transporter would have to rotate from one side of the bilayer to the other. It is now known that membrane proteins PRINCIPLES OF MEMBRANE TRANSPORT In This Chapter PRINCIPLES OF A233 MEMBRANE TRANSPORT TRANSPORTERS AND A237 ACTIVE MEMBRANE TRANSPORT ION CHANNELS AND A243 THE ELECTRICAL PROPERTIES OF MEMBRANES A233 Chapter 11 11 Membrane Transport of Small Molecules and the Electrical Properties of Membranes
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A234 Chapter 11: Membrane Transport of Small Molecules and the Electrical Properties of Membranes rarely, if ever, flip across the membrane. Transporters appear to be no dif- ferent; none of the characterized transporters alter their orientation relative to the lipid bilayer. 11–11 A. Transport by a transporter can be described by an equation that is strictly analogous to the one for a simple enzyme reaction: T + S high TS Æ T + S low Where S high is the solute on the high side of the concentration gradient and S low is the solute on the low side. For a transport process the solute remains unchanged—unlike the substrate in an enzyme reaction—but it is moved to the other side of the membrane.
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Chapter_11_Solutions - Chapter 11 Membrane Transport of...

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