Final answer:
Cholesterol is crucial for cell membrane fluidity, rigidity, and permeability, but it does not directly influence membrane thickness. Its main role is to buffer temperature effects on the cell membrane and maintain the structural integrity of the membrane.
Step-by-step explanation:
Cholesterol plays a critical role in maintaining the fluidity and integrity of mammalian cell membranes. It is positioned within the membrane, alongside phospholipids, and functions as a buffer to dampen the effects of temperature changes. Cholesterol makes the membrane less permeable to very small water-soluble molecules that might otherwise freely pass through. Furthermore, cholesterol helps to maintain the right degree of membrane rigidity and contributes to the membrane thickness by interacting with the fatty acid chains of the phospholipids.
The one function cholesterol does not influence is d. membrane thickness. While cholesterol does insert between the phospholipids and can impact how closely they pack together, which in turn could affect membrane thickness indirectly, its primary roles are to modulate membrane fluidity, permeability, and rigidity. Hence, (d) membrane thickness is not directly influenced by cholesterol.