What is understood by active transport?

Active transport is used to describe the transport of molecules across the cell membrane, at an expense of energy. The phopholipid bilayer which forms the cell membrane is highly selective and does not allow the entry of a large number of molecules into the cell (from external environment). The transport of molecules across this highly-selective membrane takes by either active transport or passive transport. In active transport, molecules move from an area of low concentration to an area of high concentration. This movement across the concentration gradient cannot occur without energy expenditure. An example is the transport of potassium ion, that has a higher concentration inside the cell and lower concentration outside it. In contrast, sodium ion has to be transported out of cell and the concentration inside the cell is less than that on the outside. In comparison, passive transport takes place from an area of high concentration to an area of low concentration and hence does not require energy expenditure.

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