Answerkeyquiz_2 - region of the bilayer(4pt If they state...

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Quiz for third discussion 1) While phospholipids can diffuse laterally in a membrane bilayer, they rarely (once a day) “flip” from one layer to the other in most membranes. Explain why. (5 pts) Phosopholipids have a large polar, charged, head group. For these molecules to flip, the head group would have to traverse the hydrophobic interior of the plasma membrane to get to the other side. This causes hydrophilic regions to interact with hydrophobic regions which is not a stable situation (energically unfavorable). (2pts) If there is a good drawing or description of a phospholipid bilayer without an explanation as to why it doesn’t flip. 2) In contrast to phospholipids, cholesterol flips from one layer to another very rapidly (seconds). Explain why. (5pts) Cholesterol’s hydrophilic end (a hydroxyl group) is very small and uncharged. It can therefore pass much more readily through the hydrophobic
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Unformatted text preview: region of the bilayer. (4pt) If they state that cholesterol has a smaller polar, uncharged hydrophilic end (compared to phospholipids) but then don’t directly state that this will enable it to pass through the hydrophobic portion of the bilayer. Instead, they postulate other odd mechanisms, focused on the size or structure of the molecule. (2-3 pts) recognize that cholesterol is amphiphatic but do not relate the small size of the polar group with that of phospholipids (1pt) answer that doesn’t focus on the polar portion of the molecule. No points deducted if they include information about cholesterol filling gaps in the membrane or decreasing the fluidity (permeability) of the plasma membrane. This is technically correct, though irrelevant to the question. The size of the hydrophobic region of cholesterol or phospholipids has nothing to do with the rate of flipping. So it’s not a factor in deciding the grade....
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This note was uploaded on 12/06/2009 for the course BIO 172 taught by Professor Clark during the Spring '08 term at University of Michigan.

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