What best describes the role of membrane transport proteins?

• A. Form permanent pores for hydrophobic molecules
• B. Facilitate passage of certain molecules across membranes that cannot easily cross lipid bilayer
• C. Only present in plant cells
• D. Convert charged molecules into neutral to diffuse

Answer: (B)

Membrane transport proteins provide selective pathways that allow specific substances to cross the cell membrane when the lipid bilayer by itself is not permeable to them. They come in two main forms: channels that create gated pores and carriers that bind substrates and undergo conformational changes to shuttle them across. This enables facilitated diffusion or active transport, moving polar, charged, or larger molecules that cannot easily cross the lipid core on their own.

That is why the best description is that they facilitate passage of certain molecules across membranes that cannot easily cross the lipid bilayer. They are not simply permanent pores for hydrophobic molecules, since hydrophobic substances diffuse through the lipid bilayer without needing a pore; they are not exclusive to plant cells, as these proteins are found in all cell types; and they do not chemically convert charged molecules to neutral to enable diffusion—transport proteins move or couple the transport of substrates without such chemical modification.