Which statement best describes the basic structure of the cell membrane and its contribution to selective permeability?

• A. The cell membrane is a rigid protein sheet with no lipid components.
• B. The lipid bilayer is impermeable to all substances, with no diffusion across.
• C. The cell membrane is a phospholipid bilayer but lacks cholesterol or proteins, making it highly permeable to ions.
• D. The cell membrane is a phospholipid bilayer with hydrophobic tails inside and hydrophilic heads outside, containing proteins, cholesterol, and carbohydrates; the hydrophobic core restricts polar molecules; small nonpolar molecules diffuse freely, while ions and larger polar molecules require channels or transporters to cross.

Answer: (D)

The idea being tested is that a cell membrane is a phospholipid bilayer with a hydrophobic interior that creates selective permeability. The phospholipid arrangement places hydrophobic tails inward and hydrophilic heads outward, forming a barrier that restricts polar and charged substances. Proteins embedded in or associated with the membrane provide channels and transporters that let specific substances cross, while cholesterol modulates membrane fluidity and stability, and carbohydrates on the outer surface aid in cell recognition. Because of the hydrophobic core, polar molecules and ions don’t diffuse freely, whereas small nonpolar molecules can pass through the bilayer more easily. For ions and larger polar molecules, crossing the membrane typically requires channels or transporters. This combination of structure and embedded molecules explains how the membrane creates selective permeability, allowing some substances to diffuse and requiring others to use specific gateways. The other descriptions misstate the membrane’s composition or its permeability, such as claiming there are no lipids, or that the bilayer is impermeable to all substances, or that it lacks cholesterol and proteins, which would not reflect its true selective nature.