In response to seasonal changes in temperature, many organisms must alter the composition of their plasma membranes to maintain the proper degree of fluidity. As summer turns to fall and eventually into winter, which of the following would you predict you would observe in the plasma membranes of organisms that are unable to regulate their body temperature? a) an increase in phospholipid fatty acid side chain length and an increase in side chain saturation b) an increase in phospholipid fatty acid side chain length and a decrease in side chain saturation c) a decrease in phospholipid fatty acid side chain length and an increase in side chain saturation d) a decrease in phospholipid fatty acid side chain length and a decrease in side chain saturation

d) a decrease in phospholipid fatty acid side chain length and a decrease in side chain saturation

Generally in   cell membrane Unsaturated fatty acid chains are bent, Kink. This characteristics prevents close packing of the phospholipid  chains which  promotes fluidity of the cell membrane.Thus the more  the  saturated chains in the cell membranes, the less the fluidity, and the more unsaturated the more fluidity.

Chain length is another factor for cell membrane fluidity. The longer the chains length the less  the fluidity  of the cell membrane since the longer chains are well packed together and the shorter the length of the fatty acid chains the  more the fluidity.

Consequently in this  present scenario; as summer turns to fall to winter, organisms that lack ability for insulation  will  have modification of their cell membrane with    decrease in fatty acid side chain length causing  fluidity. And decrease in side chain saturation(  increasing unsaturation  of fatty acid chains)  and thus  fluidity.