What is the correct answer to the listed problem and why. Also, why is my answer

Question

What is the correct answer to the listed problem and why. Also, why is my answer (marked with a red dash through it) wrong.

9/The figure to the right shows data regarding the movement of molecule X from outside of the cell to the inide. The data allows which of the following nclusions? A. molecule X is moving by simple diffusion B. movement of molecule X requires the expenditure of ATP C. movement of molecule X requires a protein carrier or transporter D. none of the above is consistent with the data E. all of the above are consistent with the data Amount of X added to extracellular environment

Explanation / Answer

Answer is C. Movement of molecule X requires a protein carrier or transporter.

The rate of transport increases at. first, but as the concentration of the molecule X rises, the rate reaches a plateu. This is due to the fact that the number of transporters is fixed. So, it can carry X up to a certain extent, after which there is no binding sites available to bind with X and carry it. This type of transport is called facilitated diffusion. This doesn't require energy, but follows a saturation kinetics.

If the molecule had moved by simple diffusion, then the rate of transport would have increased aswe add more and more number of X molecules to the medium. It doesn't follow saturation kinetics. So the option A is wrong.

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