A scientist is studying the passive movement of a molecule into a cell. He measures the rate at which this chemical moves into the cell, as a function of its external concentration. The chemical's rate of movement is directly proportional to the extracellular concentration but eventually reaches a transport maximum. What type of transport seems to be used by this molecule

The correct answer is - facilitated diffusion.

Explanation:

First let's understand about FACILTATED DIFFUSION.

Normally diffusion occurs from higher concentration to lower concentration area, that means, along the concentration gradient.

In the process of FACILTATED DIFFUSION, for a substance to get diffued, it should occur with help of a protein in the membrane. Of course this process occurs with out consuming energy in the form of ATP therefore its a PASSIVE process.

Now lets understand the term TRANSPORT MAXIMUM, here lies the answer of above question.

Transport maximum is a condition when there is no more movement of chemicals/ any particles via mebrane because of SATURATION of available proteins required to transport chemicals across the membrane. As there are no more proteins available to transport chemicals across membrane, diffusion wont takes place anymore untull those proteins are regenerated.

Therefore as mentioned in question that CHEMICAL-A reached transport maximum. This says that it is transported by a process called facilitated diffusion.

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