Questions: How would the membrane lipid composition of fish found in very cold waters compare with that of a warm-blood mammal like a polar bear? (Select all that apply.) A fish living in cold waters would have more unsaturated fatty acids in their cell membranes because those fatty acids pack together less tightly and remain fluid at cooler temperatures. A warm-blooded animal like a polar bear would have more cholesterol, since this would make cell membranes more fluid at higher temperatures and therefore helping them to stay intact.

The answer is the first one (A): A fish living in cold waters would have more unsaturated fatty acids in their cell membranes because those fatty acids pack together less tightly and remain fluid at cooler temperatures.

Explanation for each option:

A. Correct. Fish living in cold waters need to maintain membrane fluidity despite the low temperatures. Unsaturated fatty acids have one or more double bonds, which introduce kinks in the fatty acid chains, preventing them from packing tightly. This helps maintain fluidity in cold environments.

B. Incorrect. Unsaturated fatty acids do not pack together more tightly; they pack less tightly due to the kinks caused by double bonds, which is why they help maintain fluidity at lower temperatures.

C. Incorrect. Saturated fatty acids pack together more tightly and are more likely to be solid at lower temperatures, which would not be beneficial for fish in cold waters that need fluid membranes.

D. Incorrect. Cholesterol does not make cell membranes more solid at higher temperatures; it actually helps maintain membrane fluidity by preventing fatty acid chains from packing too closely in warm conditions.

E. Correct. Cholesterol helps maintain membrane fluidity by preventing the fatty acid chains from packing too closely together, which is important for warm-blooded animals like polar bears to keep their membranes intact at higher body temperatures.

F. Incorrect. While unsaturated fatty acids do increase fluidity, warm-blooded animals like polar bears rely more on cholesterol to regulate membrane fluidity at higher temperatures rather than increasing unsaturated fatty acids.

In summary, fish in cold waters have more unsaturated fatty acids to maintain membrane fluidity, while warm-blooded animals like polar bears use cholesterol to regulate membrane fluidity at higher temperatures.