Which small molecule decreases the fluidity of the plasma membrane by interacting with $\mathrm{phospholipids}$?

• A. $\mathrm{Glucose}$
• B. $\mathrm{Water}$
• C. $\mathrm{Cholesterol}$
• D. $\mathrm{Ethanol}$

Answer: (C)

Verified step by step guidance

1. Understand the structure of the plasma membrane: The plasma membrane is primarily composed of a phospholipid bilayer, which provides fluidity and flexibility to the membrane.
2. Identify the role of cholesterol in the membrane: Cholesterol is a lipid molecule that is interspersed within the phospholipid bilayer of the plasma membrane.
3. Analyze how cholesterol affects membrane fluidity: Cholesterol interacts with the fatty acid tails of phospholipids, which can either increase or decrease membrane fluidity depending on the temperature.
4. Consider the effect of cholesterol at physiological temperatures: At body temperature, cholesterol decreases membrane fluidity by restricting the movement of phospholipid fatty acid chains, thus stabilizing the membrane structure.
5. Conclude why cholesterol is the correct answer: Among the given options, cholesterol is the molecule that decreases the fluidity of the plasma membrane by interacting with phospholipids, unlike glucose, water, or ethanol.