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Question: Name two glycerophospholipids and two sphingolipids found in eukaryotic cell membranes, determine the fraction of cholesterol present in most plasma membranes, and describe how the cholesterol content affects the membrane fluidity. Answer: Two examples of glycerophospholipids in eukaryotic cell membranes are phosphatidylethanolamine and phosphatidylcholine. Two examples of sphingolipids in eukaryotic cell membranes are sphingomyelin and a glycosphingolipid like a cerebroside. The fraction of cholesterol in most plasma membranes is about 30-40%. Cholesterol affects membrane fluidity by adjusting its rigidity through inserting itself between the fatty acid chains of lipids. At high temperatures, cholesterol decreases fluidity, and at low temperatures, it prevents tight packing, maintaining or increasing fluidity. Overall, cholesterol maintains membrane fluidity over a range of temperatures.