Final answer:
Phospholipids in biological membranes commonly have one saturated and one unsaturated fatty acid; the unsaturated fatty acids contribute to membrane fluidity, which is crucial especially in cold environments.
Step-by-step explanation:
Phospholipids in biological membranes typically contain one saturated and one unsaturated fatty acid. The saturated fatty acids have tails that are straight because they contain no double bonds between carbon atoms. In contrast, unsaturated fatty acids include one or more double bonds, introducing bends or "kinks" in the tails.
These kinks prevent the phospholipid molecules from packing tightly together, which contributes to the fluidity of the membrane. This characteristic is particularly important in colder environments where many organisms, like fish, adapt by changing the proportion of unsaturated fatty acids in their membranes, thus maintaining membrane fluidity and function.