BMB170a_2011_PS4_key

# BMB170a_2011_PS4_key - BMB 170a 2011 Problem Set 4 Lipids...

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1 BMB 170a 2011 Problem Set 4: Lipids, Membranes, and Membrane Proteins 1. Lipid Molecules and Structures (25pts) a. Below is a representation of the shape of a lipid tail that might be attached to a head group represented as a green sphere. Propose a possible aggregate structure for each that it may self-assemble into. (4 pts) A: reversed micelle, reversed hexagonal, cubic B: lamellar or bilayer C: hexagonal, cylindrical micelles D: micelle b. Describe the three pressure contributions that give rise to the lateral pressure profile of a lipid bilayer. Describe how they add up to give a net zero force. Draw a figure of a bilayer and label the location from where each pressure arises. (5 pts) A repulsive force develops between the positively charged head groups, resulting in a positive pressure on both sides of the bilayer. The hydrophobic tails pack tightly together, giving rise to a positive pressure resulting from entropic repulsion between these flexible chains. To cancel these two pressures out, a negative pressure developsbecause of interfacial tension at eh hydrophilic head/hydrophobic tail interface.

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2 c. Suppose you had a lipid bilayer composed of saturated sphingomyelin and the more unsaturated phosphatidylcholines mixed together, and then you added cholesterol to your membrane. What would happen? What drives your prediction energetically? (5 pts) The addition of cholesterol causes a lateral phase separation into two domains in the membrane: cholesterol associated with the gel-like sphingolipids, and the more fluid glycerophopholipids. This is how “lipid rafts” initially form. This is driven by loss of entropy at would occur from the difficulty of forcing the unsaturated lipid into a highly ordered phase of sphingolipids and cholesterol. d. What are the two main advantages of using triacylglycerols as stored fuels in an organism rather than polysaccharides like starch? (5 pts) 1) Fatty acids’ carbons are more reduced than those of sugars, and oxidation of triacylglycerols yields more than twice as much energy, gram for gram, than does the oxidation of sugars. 2) Triacylglycerols are hydrophobic and unhydrated, whereas polysaccharides need to be hydrated when they are stored in cells. Using triacylglcerols to store energy means that the organism does not need to carry around as much water weight. e. Inside the head of sperm whales is a large organ known as the spermaceti organ, which contains as much as 3,600 kg of spermaceti oil. This oil is made up of triacylglycerols and waxes that are mainly unsaturated, making it a liquid at 37°C that begins to crystallize at about 31°C. These whales dive from 1,000 m to 3,000 m and can float at their chosen depth and wait for squid to swim but that they can eat. Why is their ability to float at any given depth unusual and what makes it possible? (6 pts) In order for a marine mammal to stay at a given depth without swimming effort, it must be able to change its density to that of the water at any given temperature or
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BMB170a_2011_PS4_key - BMB 170a 2011 Problem Set 4 Lipids...

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