Active transport

Active transport (sometimes called active uptake) is the mediated transport of biochemicals, and other atomic/molecular substances, across membranes. Unlike passive transport, this process requires the expenditure of cellular energy to move molecules "uphill" against a gradient.

Process
In this form of transport, molecules move against either an electrical or concentration gradient (collectively termed an electrochemical gradient).
 * The active transport of small molecules or ions across a cell membrane is generally carried out by transport proteins that are found in the membrane.
 * Larger molecules such as starch can also be actively transported across the cell membrane by processes known as endocytosis and exocytosis.
 * '' The process whereby particles are moved through a membrane from a region of low concentration to a region of high concentration is known as active transport.

Types

 * In primary transport, energy from hydrolysis of ATP is directly coupled to the movement of a specific substance across a selectively permeable membrane independent of any other species.
 * In secondary active transport, the required energy is derived from energy stored in the form of concentration differences in a second solute. Typically, the concentration gradient of the second solute was created by primary active transport, and the diffusion of the second solute across the membrane drives secondary active transport of the first solute.

- Active transport carries water, food and other materials in and out of the cell.

Relation to Cellular Energy
Using conservative assumptions, it has been calculated that the amount of energy available to the cell is insufficient to power the estimated degree of active transport, e.g. by a factor of 15-30 times for the sodium pump alone. Following vigorous debate in Science and elsewhere, this discrepancy has yet to be resolved.