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Concentrations of lipid below the critical micelle concentration do not formmicelles. Lipid solutions whose concentration is greater than the critical micelleconcentration contain micelles in equilibrium with free lipid molecules. The concentration ofthe free lipid is equal to the critical micelle concentration.5. The presence of D-amino acids, the unique linkage of amino acid groups, and the definedamino acid composition all make the peptide portion of bacterial cell walls difficult tohydrolyze by proteases.6. a./P and F; b./P; c./F and A; d./P and F; e./A; f./A; g./F; h./P; i./P; j./A.7. a./4; b./3; c./5; d./1; e./2.8. A symport is a transport system that couples movement of two substances in the samedirection. An antiport couples the movement of two substances in opposite directions. Theycan be used to move substances against a concentration gradient if transport of the coupledsubstance is down a concentration gradient. In this case the energy of the concentrationgradient of the co-transported substance is used to drive uptake.9. Light.10. One of the intermediates in glycolysis, PEP, is used as a phosphoryl group donor tophosphorylate glucose. Glucose phosphorylation is coupled to glucose uptake by cells. 11. Ionophores are substances that can transport ions across a biological membrane. Carrierionophores form a lipid-soluble complex with the ion and the complex diffuses from one sideof a membrane to the other. A channel-forming ionophore is a lipid soluble compound thatcan dissolve in biological membranes and spans the membrane with a channel or porethrough which an ion diffuses.Additional Problems1. The transport properties of two potassium ionophores were being studied in synthetic lipidbilayers with a phase transition temperature of 50°C. Ionophore X transports potassium at arate proportional to temperature from 20°C to 70°C. In contrast, ionophore Y transportspotassium very well above 60°C; however, from 60°C to 40°C the rate of transport falls off138
Chapter 9 . Membranes and Membrane Transportprecipitously to very low values below 40°C. Based on this information, can you suggestmodes of action for these two ionophores?2. For integral membrane proteins, -helices play an important role in anchoring the proteinto the membrane. Explain why a helix is a thermodynamically stable structural element toembed in a membrane.3. How might you expect helical wheel plots of -helical segments from the following proteinsto differ: (a) a typical globular protein, (b) an integral membrane protein with a singletransmembrane segment, and (c) an integral membrane protein with several -helicesforming a channel through which a water-soluble compound is transported?4. Explain how soaps and detergents help to remove water insoluble substances.5. Explain how lipid membrane asymmetry might arise in a natural membrane through actionof a flippase that does not couple lipid movement to another thermodynamically favorableprocess (like ATP hydrolysis).6.