Question 21

Carrier proteins use shape changes to facilitate passive transport.

Carrier proteins are a specific type of passive transport mechanism that utilize proteins which undergo conformational changes to move target molecules across the membrane.

A) Diffusion

Diffusion refers to the movement of molecules from an area of higher concentration to an area of lower concentration, and it does not involve any proteins or shape changes. It is a passive process but does not match the criteria outlined in the question.

B) Carrier proteins

Carrier proteins are integral membrane proteins that bind to specific molecules and undergo shape changes to transport these molecules across the cell membrane. This process is a form of facilitated diffusion, which directly aligns with the question's description of passive transport involving protein shape changes.

C) Channel proteins

Channel proteins facilitate the passage of ions or small molecules through a pore in the membrane but do not change shape in the process. They allow substances to flow freely based on concentration gradients, which does not meet the requirement of shape alteration specified in the question.

D) None of the above

This option is incorrect because carrier proteins do indeed exist and perform the function described in the question. Thus, stating "none of the above" overlooks the validity of carrier proteins as the correct answer.

Conclusion

The correct answer is carrier proteins, as they specifically utilize shape changes to transport target molecules through the membrane, fulfilling the criteria of the question. Other options, such as diffusion and channel proteins, do not incorporate the necessary mechanism of protein shape alteration, making them incorrect.