Q15 - Structure and function of the cell membrane

The correct answer is D) Phospholipids and proteins.

According to the fluid mosaic model, the plasma membrane is a phospholipid bilayer with proteins embedded in it or attached to its surface. Smaller amounts of cholesterol and of carbohydrates (bound to lipids and proteins) are also present.

A) Incorrect: Glycogen is a storage polysaccharide found in the cytoplasm of liver and muscle cells, not in the membrane.

B) Incorrect: Fatty acids are parts of phospholipids, and sugars appear only as short chains attached to lipids and proteins. Neither is a main component on its own.

C) Incorrect: RNA is found in the nucleus, cytoplasm and ribosomes. Free amino acids are the building blocks of proteins, not membrane components.

D) Correct: Phospholipids form the bilayer and proteins carry out most membrane functions.

E) Incorrect: Carbohydrates are only a minor component, and DNA is found in the nucleus (and mitochondria), not in the membrane.