SECONDARY ACTIVE TRANSPORT
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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