Final answer:
Cholesterol decreases the fluidity of a liquid crystal membrane by filling in gaps between phospholipids and reducing the mobility of membrane components.
Step-by-step explanation:
The insertion of cholesterol into a liquid crystal membrane results in a decrease in fluidity. Cholesterol molecules fit in between the fatty acid tails in the hydrophobic interior of the membrane, effectively filling in gaps and reducing the lateral mobility of phospholipid and protein components. This action stiffens the membrane, making it more rigid. While cholesterol does prevent low temperatures from excessively decreasing fluidity, it also prevents high temperatures from making the membrane too fluid, thus acting as a buffer. However, the presence of cholesterol generally makes the membrane less permeable to certain ions.