Question

The presence of cholesterol in plasma membranes of some animals

• A. enables the membrane to stay fluid more easily when there is drop in temperature
• B. enables the animal to remove hydrogen atoms from saturated phospholipids
• C. enables the animals to add hydrogen to unsaturated phospholipids
• D. makes the membrane more flexible, allowing it to sustain greater pressure from within the cell

Hint:

Think of cholesterol as an "anti-freeze" for biological membranes.
By packing between phospholipid tails, it prevents them from crystallizing into a rigid, non-functional gel state when temperatures drop.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Biological membranes must maintain a specific level of fluidity to function properly.
Membrane fluidity is regulated by temperature, fatty acid composition, and the presence of sterols like cholesterol.

Step 2: Detailed Explanation:
Cholesterol acts as a bidirectional "fluidity buffer" in animal cell membranes:
- At high temperatures: Cholesterol stabilizes the membrane by restricting the lateral movement of phospholipid fatty acid chains, preventing the membrane from becoming too fluid or disintegrating.
- At low temperatures: When the temperature drops, phospholipids naturally tend to pack tightly together and crystallize into a rigid gel phase, which would render the membrane non-functional.
Cholesterol prevents this rigid packing by inserting its bulky hydrophobic steroid ring structure between the fatty acid tails, disrupting their orderly arrangement.
This allows the membrane to remain in a fluid state at lower temperatures.
The other options are incorrect: cholesterol does not participate in chemical hydrogenation or dehydrogenation of fatty acid chains (B and C), nor does it directly increase pressure resistance from within the cell (D).

Step 3: Final Answer:
Thus, the presence of cholesterol prevents the membrane from solidifying at low temperatures, making option (A) the correct choice.