Membrane proteins enable specific transport mechanisms

Steps:

• Membranes with few proteins lack carriers and channels needed for protein-dependent transport.
• Facilitated diffusion (1) uses protein carriers or channels for polar molecules, so it is reduced.
• Active transport (2) requires protein pumps to move substances against gradients using energy, so it is reduced.
• Simple diffusion (3) occurs directly through the lipid bilayer without proteins, so it is unaffected.

Why B is correct:

• Facilitated diffusion and active transport both depend on integral membrane proteins for carrier and pump functions, as defined in cell membrane transport models.

Why the others are wrong:

• A: Includes 3, but simple diffusion does not require proteins and proceeds normally through phospholipids and cholesterol.
• C: Includes 3 but excludes 2; active transport relies on protein pumps and would be reduced.
• D: Includes 3 but excludes 1; facilitated diffusion needs protein channels and would be reduced.

Final answer: B