Active Transport in the Cell

Active transport occurs when a cell moves materials against the flow of a concentration gradient. A cell may do this in order to concentrate certain types of molecules on one or the other side of a cell wall when this cannot be accomplished using simple diffusion. The cell may also expel waste, consume food, or communicate with other cells by the transfer of hormones or prostaglandins.

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Active transport can be accomplished by the cell in several ways. It may use an ion pump, co-transport, or endocytosis.

The Ion Pump or Direct Active Transport

The ion pump works by taking advantage of the ATP-ADP cycle. When a phosphate is split from an ATP molecule it releases energy. This energy is used to propel material through a channel in the cell membrane.

In one important process (The Na+/K+ ATPase), positively charged sodium (Na) ions are transported out of the cell, while positively charged potassium (K) ions are transferred into the cell. This is done to facilitate nerve impulses and muscle contraction. It also keeps cells from absorbing too much water from diffusion and osmosis. The same process is used to inject gastric juices into the digestive tracts (only this time it is hydrogen instead of sodium that is transported out of the cell).1

Co-transport or Indirect Active Transport

In co-transport an ion moving from a higher concentration to a lower concentration takes some other material with it. This happens when glucose is transferred from outside of the cell to inside of the cell. The concentration of sodium is much higher outside the cell than it is inside the cell. This means that the sodium would naturally flow into the cell. As the cell needs glucose, it allows the sodium (which it has pumped out through active transport) to flow back, bringing with it more glucose.

Endocytosis

Though not considered a form of active transport by all biologists, endocytosis is a process of induction into a cell in a manner requiring energy. Endocytosis, is accomplished by some cells (for example amoebas, or phagocytes) by surrounding materials with the cell membrane and then inducting the material into the cell by creating a vacuole around it. This vacuole is then broken down and the substance used by the cell.


  1. Transport Across Cell Membranes

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Peristaltic Motion

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