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Active transport requires membrane protein transporters that use energy to transport molecules against their concentration gradients. At low concentrations of substrate outside the cell, many transporters are available, and the rate increases as the substrate concentration outside the cell increases. At higher concentrations of substrate, the transporters become more fully occupied, and the rate of import plateaus until all the transporters are transporting molecules as fast as they can. At that point, the only way to increase the rate of transport is to increase the number of transporters. So the kinetics will look like facilitated diffusion, but with the availability of energy the transporters work against the concentration gradient. An example is phosphate transport, where transporters can import submicromolar levels of phosphate even when the cytoplasmic phosphate concentration is much higher.

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