46. Using the following diagram as necessary, explain the effect of cholesterol

Question

46. Using the following diagram as necessary, explain the effect of cholesterol on membrane fluidity.

A. How does cholesterol in the membrane decrease fluidity at high

temperatures?

B. Would you predict that cholesterol alone could form a lipid bilayer?

Why or why not?

46. Using the following diagram as necessary, explain the effect of cholesterol on membrane fluidity. A. How does cholesterol in the membrane decrease fluidity at high temperatures? B. Would you predict that cholesterol alone could form a lipid bilayer? Why or why not?

Explanation / Answer

a) Cholesterol Helps Maintain the Fluidity of Cell Membranes!!! While cholesterol adds firmness and integrity to the plasma membrane and prevents it from becoming overly fluid, it also helps maintain its fluidity. At the high concentrations it is found in our cell's plasma membranes (close to 50 percent, molecule for molecule) cholesterol helps separate the phospholipids so that the fatty acid chains can't come together and cyrstallize. Therefore, cholesterol helps prevent extremes-- whether too fluid, or too firm-- in the consistency of the cell membrane. b) Without cholesterol, cell membranes would be too fluid, not firm enough, and too permeable to some molecules. In other words, it keeps the membrane from turning to mush. Cholesterol, an important constituent of cell membranes, has a rigid ring system and a short branched hydrocarbon tail. Cholesterol is largely hydrophobic. But it has one polar group, a hydroxyl, making it amphipathic. So - bottom line about temperature? Cholesterol is abundant in membranes, such as plasma membranes, that include many lipids with long-chain saturated fatty acids. In the absence of cholesterol, such membranes would crystallize at physiological temperatures. The inner mitochondrial membrane lacks cholesterol, but includes many phospholipids whose fatty acids have one or more double bonds, which lower the melting point to below physiological temperature. C) Cholesterol can't form the entire bi-layer because cholesterol is a steroid, and a PHOSPHOLIPID bilayer requires - yep - phospholipids. Not just lipids like steroids (lipids --> steroids --> cholesterol) but also a phosphorus head, which cholesterol doesn't have. Remember - cholesterol just has a hydrocarbon tail.

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