Final answer:
The molecules that pass through specific carrier proteins are determined by their unique fit to the binding sites on the carrier protein, involving conformational changes in the protein structure. Option B) the unique fit to binding sites on the channel or carrier protein is the correct answer.
Step-by-step explanation:
The molecules that move through specific carrier proteins are determined by the unique fit between the molecule and binding sites on the carrier protein. When a carrier protein binds a solute, such as glucose or amino acids, it undergoes an allosteric or conformational change in shape.
This change allows the bound molecule to be carried across the cell membrane. Once on the other side, the molecule is released because it no longer has a high affinity for the carrier protein, and the protein can return to its original shape, ready to transport another molecule.
The specificity of carrier proteins to their ligands ensures that only certain molecules can be transported via this mechanism. Therefore, the correct option for the question is B) the unique fit to binding sites on the channel or carrier protein.