In response to seasonal changes in temperature, many organisms must alter the composition of their plasma membranes to maintain the proper degree of fluidity. Which change in the fatty acids of phospholipids would be most effective in maintaining membrane fluidity in a colder environment?

A. an increase in phospholipid fatty acid side chain length and an increase in side chain saturation
B. a decrease in phospholipid fatty acid side chain length and an increase in side chain saturation
C. a decrease in phospholipid fatty acid side chain length and a decrease in side chain saturation
D. an increase in phospholipid fatty acid side chain length and a decrease in side chain saturation

The plasma membrane is made of lipid bilayer and proteins. Carbohydrates are present in association with lipids and proteins. Cholestrol is also present in the membrane.

The composition of the bilayer changes in the organism living in cold climate. The the phospholipid fatty acid chain increases that provides the warm to the organism. Saturation level decreases and unsaturated carbon increases to maintain the membrane fluidity.

Thus, the correct answer is option (D).