Final answer:
Unsaturated fatty acids increase membrane fluidity, while saturated fatty acids decrease it. In cold temperatures, oleic acid is increased to maintain fluidity, whereas in warm temperatures, palmitic acid may be more prevalent.
Step-by-step explanation:
The type of fatty acids that increase membrane fluidity are unsaturated fatty acids, due to the presence of double bonds or "kinks" in their hydrocarbon tails, which prevent them from packing tightly together. Conversely, saturated fatty acids with their straight tails decrease fluidity as they pack together more densely. In response to cold temperatures, organisms increase the proportion of unsaturated fatty acids in their membranes to maintain fluidity. Specifically, they increase the levels of fatty acids like oleic acid (a monounsaturated fatty acid), which helps prevent membranes from becoming too rigid and susceptible to rupturing. In warm temperatures, the proportion of saturated fatty acids such as palmitic acid may increase to maintain proper membrane structure and function. Therefore, the correct answer to which fatty acids increase and decrease membrane fluidity, and which increase in cold versus warm temperatures, is: b) Unsaturated; Saturated; Oleic acid; Palmitic acid.