Which protein allows a molecule to pass through the plasma membrane without energy input by providing direct passage?

Which protein allows a molecule to pass through the plasma membrane without energy input by providing direct passage?

• A. Enzymatic Proteins
• B. Channel Protein
• C. Carrier Protein
• D. Receptor Protein

Answer: (B)

A channel protein forms a pore in the membrane that lets ions or water pass directly through, down their concentration or electrochemical gradient, without the cell using energy. This direct tunnel means molecules move by diffusion through the protein’s opening rather than by undergoing a shape change of the transporter itself. Channel proteins can be gated, opening or closing in response to voltage, ligands, or mechanical forces, but the energy required for transport isn’t provided by the cell. Carrier proteins, by contrast, do not create a continuous pore. They grab the molecule and undergo conformational changes to shuttle it across the membrane, which is not a direct passage and typically still relies on the gradient rather than ATP. Enzymatic proteins catalyze chemical reactions, not transport. Receptor proteins bind signaling molecules to trigger responses, not to move substances across the membrane.

A channel protein forms a pore in the membrane that lets ions or water pass directly through, down their concentration or electrochemical gradient, without the cell using energy. This direct tunnel means molecules move by diffusion through the protein’s opening rather than by undergoing a shape change of the transporter itself. Channel proteins can be gated, opening or closing in response to voltage, ligands, or mechanical forces, but the energy required for transport isn’t provided by the cell.

Carrier proteins, by contrast, do not create a continuous pore. They grab the molecule and undergo conformational changes to shuttle it across the membrane, which is not a direct passage and typically still relies on the gradient rather than ATP.

Enzymatic proteins catalyze chemical reactions, not transport. Receptor proteins bind signaling molecules to trigger responses, not to move substances across the membrane.