Secondary active transport is the transport of a substance with sodium ion, by means of a common carrier protein. When sodium is transported by a carrier protein, another substance is also transported by the same protein simultaneously, either in the same direction (of sodium movement) or in the opposite direction. Thus, the transport of sodium is coupled with transport of another substance. Secondary active transport is of two types:

1. Cotransport

2. Counter transport.

Sodium Cotransport

Sodium cotransport is the process in which, along with sodium, another substance is transported by a carrier protein called symport. Energy for movement of sodium is obtained by breakdown of ATP. And the energy released by the movement of sodium is utilized for movement of another substance. Substances carried by sodium cotransport are glucose, amino acids, chloride, iodine, iron and urate.

Carrier protein for sodium cotransport

Carrier protein for the sodium cotransport has two receptor sites on the outer surface.

Among the two sites, one is for binding of sodium and another site is for binding of other substance.

Sodium cotransport of glucose

One sodium ion and one glucose molecule from the ECF bind with the respective receptor sites of carrier

protein of the cell membrane. Now, the carrier protein is activated. It causes conformational changes in the carrier protein, so that sodium and glucose are released into the cell . Sodium cotransport of glucose occurs during absorption of glucose from the intestine and reabsorption of glucose from the renal tubule.

Sodium cotransport of amino acids

Carrier proteins for the transport of amino acids are different from the carrier proteins for the transport of

glucose. For the transport of amino acids, there are five sets of carrier proteins in the cell membrane. Each one carries different amino acids depending upon the molecular weight of the amino acids.

Sodium cotransport of amino acids also occurs during the absorption of amino acids from the intestine

and reabsorption from renal tubule.

Sodium Counter Transport

Sodium counter transport is the process by which the substances are transported across the cell membrane in exchange for sodium ions by carrier protein called antiport.

Various counter transport systems are:

i. Sodium-calcium counter transport: In this, sodium and calcium ions move in opposite

directions with the help of a carrier protein. This type of transport of sodium and calcium ions is

present in all the cells

ii. Sodium-hydrogen counter transport: In this system, the hydrogen ions are exchanged for

sodium ions and this occurs in the renal tubular cells. The sodium ions move from tubular lumen

into the tubular cells and the hydrogen ions move from tubular cell into the lumen.

iii. Other counter transport systems: Other counter transport systems are sodium-magnesium

counter transport, sodium-potassium counter transport, calcium-magnesium counter transport,

calcium-potassium counter transport, chloridebicarbonate counter transport and chloridesulfate

counter transport.

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