What is the role of lipids in membranes and energy storage?

• A. Phospholipids form membranes; cholesterol modulates fluidity; triglycerides store energy.
• B. Triglycerides form membranes; phospholipids store energy; cholesterol stores energy.
• C. Cholesterol modulates fluidity; phospholipids form membranes; triglycerides store energy.
• D. Phospholipids form membranes; cholesterol modulates fluidity; triglycerides store energy.

Answer: (D)

Lipids play two main roles here: building membranes and storing energy. Phospholipids are amphipathic, with hydrophilic heads and hydrophobic tails, so they naturally form a bilayer that acts as a selective barrier in cell membranes. Cholesterol sits among the phospholipids and modulates membrane fluidity and stability, helping the membrane stay flexible at low temperatures and not become too fluid at high temperatures. Triglycerides store energy; they are composed of glycerol and three fatty acids and are packed in adipose tissue to be mobilized for fuel when needed. They do not form membranes, which is why triglycerides aren’t part of the membrane structure.