D. Molecules called pumps function to control active transport.
Explanation:
Active transport is one of the two types of transport (the other being passive transport). It is the type of transport in which substances move against their concentration gradient i.e. from a lower concentration to a higher concentration. Hence, due to this, energy input in form of ATP is required by the cell.
However, the process of active transport makes use of certain molecules called PUMPS to control it. The pumps moves ions/molecules against their concentration gradient. An example is the sodium-pottasium pump which moves sodium ions from a region of low conc. to a region of high conc.
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