Final answer:
Unsaturated fatty acids maintain fluidity in cell membranes due to the kinks in their carbon chains, which prevent tight packing and solid aggregates, especially at lower temperatures.
Step-by-step explanation:
Unsaturated fatty acids reduce membrane rigidity because they do not form solid aggregates. Unlike saturated fatty acids with their straight tails, unsaturated fatty acids contain double bonds that introduce kinks or bends into the fatty acid tails. These kinks prevent the fatty acids from packing tightly together, which means that when temperatures decrease, unsaturated fatty acids elbow adjacent phospholipid molecules away, thus maintaining fluidity in the membrane.
Higher temperatures also increase membrane fluidity, as they cause the phospholipid and protein components of the membrane to move more freely. In addition, the presence of cholesterol can fill gaps in the hydrophobic interior of the membrane, which leads to reduced fluidity by hindering the lateral mobility of phospholipid and protein components. Nonetheless, membranes with high concentrations of unsaturated fatty acids remain more fluid, which is crucial in cold environments where increased fluidity reduces the susceptibility to damage.