You have just discovered an organism that lives in extremely cold environments. Which of the following would you predict to be TRUE about the phospholipids in its membranes, compared to phospholipids in the membranes of organisms that live in warmer environments? a) The membrane phospholipids of cold-adapted organisms will have longer hydrocarbon tails. b) The membrane phospholipids of cold-adapted organisms will have more saturated hydrocarbon tails. c) The membrane phospholipids of cold-adapted organisms will have more unsaturated hydrocarbon tails.

The correct answer is c) The membrane phospholipids of cold-adapted organisms will have more unsaturated hydrocarbon tails.

Explanation:

The lipid bilayer is made up of two phospholipid layers.Each phospholipid is composed of a glycerol moeity which is attached to a phosphate group and two fatty acid (saturated and unsaturated) chains using ester bonds.The phosphate groups are hydrophilic and hence they face outside the lipid bilayer.The fatty acid chains are hydrophobic and hence they face inside the lipid bilayer.This lipid bilayer is fluid in nature where the fatty acid molecules can move laterally with respect to each other. This is essential to promote the diffusion of gases and movement of nutrient and other molecules across the cell.Lower temperature reduces the energy of the system. Therefore, the kinetic energy required the movement of the phosholipid molecules reduces and hence the lipid molecule tend to reduce their lateral movement and become stiff. This can prevent the movement of nutrients and gases across the lipid bilayer, thereby causing death of the cell and hence the organism.However, organisms living in cold environment are prevented from dying. This is because their plasma membrane have a high concentration of unsaturated fatty acids in them. Unsaturated fatty acids are those that have single or multiple double double bonds in them that make their structure "kinked" or bent. As a result of this they occupy the space in the lipid bilayer such that a lot of free space is available for the lateral movement of the molecules.Also they are interspersed with cholesterol molecules that prevent the phospholipid molecules from "sticking" to each other.This helps in maintaining the fluidity of the membrane at low temperature.