The transport of substances across the membrane from a region of high concentration to low concentration against the concentration gradient is called Active transport.
The active transport of substance across cell membranes can be explained by using the example of Na+-K+ -ATPase. It is an ion pump in which the Na+ ions are released outside the cell membrane and K+ ions are imported inside the cell. The energy required to carry out this process is obtained from ATP.
This ions exchanges across the cell membrane takes place as follows:
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The Na+ ions present in the cytoplasm attaches itself to the proteins of the membrane. This stimulates phosphorylation by ATP molecules.
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The phosphorylation of ATP causes conformational changes in the protein structure
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These conformational changes release the Na+ ions to the outside and the extracellular K+ ions bind to the protein
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Binding of K+ ions causes release of phosphate group
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Due to loss of phosphate, the protein returns to its original conformation (shape)
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This causes the K+ ions to get released inside the cell and the binding sites for the attachment of Na+ ions are active once again and the cycle repeats once again.
This is the active transport of substances across cell membranes.
