Final answer:
Cholesterol insertion into a crystalline gel membrane decreases fluidity by filling in gaps between phospholipids, making the membrane less fluid and helping to maintain optimal fluidity at various temperatures.
Step-by-step explanation:
Cholesterol insertion into a crystalline gel membrane will B) Decrease fluidity of the membrane. Cholesterol molecules interspersed between phospholipids tend to fill the space between fatty acids in the hydrophobic interior of the membrane. This reduces lateral mobility thus making the membrane less fluid at room temperature. Cholesterol functions as a buffer for the membrane, preventing high temperatures from increasing the fluidity too much and also preventing low temperatures from reducing it excessively. Therefore, cholesterol helps in maintaining the membrane's fluidity within an optimal range.
At higher temperatures, whereas membranes might normally become more fluid, the presence of cholesterol helps to maintain fluidity by reducing movement within the membrane. It's important to note that while cholesterol can decrease fluidity at moderate temperatures, it can also prevent the membrane from becoming too rigid at lower temperatures, demonstrating its dual role in maintaining cell membrane integrity.