Which of the following choices correctly describes the composition of the cell membrane?

Analyze the question: You are asked what the cell membrane (plasma membrane) is made of.

Explain the concept: The cell membrane is described by the fluid mosaic model (Singer and Nicolson, 1972). According to this model, the membrane is made up of:

• Phospholipids arranged in a double layer (bilayer). Each has a hydrophilic (water-loving) head and hydrophobic (water-hating) tails.
• Proteins that are embedded in the bilayer (integral proteins) or attached to its surface (peripheral proteins). They act as channels, carriers, and receptors.
• Smaller amounts of cholesterol (in animal cells) and carbohydrates (attached to lipids or proteins as glycolipids and glycoproteins).

Solution (checking each option):

• A. Only lipids: Wrong. Proteins are also essential components, since they carry out transport and signalling.
• B. Lipids and protein: Correct. These are the two main components of the membrane.
• C. Lipids and cytosol: Wrong. Cytosol is the fluid inside the cell, not a structural part of the membrane.
• D. Only proteins: Wrong. The lipid bilayer is the basic framework of the membrane.

Answer: B

Exam tip: Remember "Lipids make the frame, proteins do the work." The membrane is a mosaic because the proteins are scattered through the lipid layer like tiles in a pattern. When you see "only" in an option about biological structures, be suspicious. It is usually wrong.