Cotransport

Cotransport

A type of secondary active transport across a biological membrane in which a transport protein couples the movement of an ion (usually Na⁺ or H⁺) down its electrochemical gradient to the movement of another ion or molecule against a concentration or electrochemical gradient. The ion moving down its electrochemical gradient is referred to as the driving ion. The ion/molecule being transported against a chemical or electrochemical gradient is referred to as the driven ion/molecule.

In cotransport, the direction of transport is the same for both the driving ion and driven ion/molecule (into the cell or out of the cell).

An example is the Na⁺/glucose cotransporter (SGLT), which couples the movement of Na⁺ into the cell down its electrochemical gradient to the movement of glucose into the cell against its concentration gradient.

Cotransport is also commonly referred to as symport.

Transport proteins that are involved in this type of transport are referred to as cotransporters or symporters.

Symport

Secondary active transport
Exchange

Lecture notes on Secondary Active Transport

Refers to the ability of the thyroid gland to accumulate iodide (I⁻) against a steep electrochemical gradient. While the iodide concentration in plasma and interstitial fluid is approximately 300 nL, iodide concentration in the cytoplasm of thyroid follicular cells, as well as the lumen of thyroid follicles can be many folds higher. The protein that enables iodide transport into the thyroid gland against an electrochemical gradient is the Na⁺/iodide symporter (NIS), which is located in the basolateral membrane of thyroid follicular cells. Within the lumen of thyroid follicles, iodide is incorporated into the tyrosine residues of thyroglobulin during thyroid hormone biosynthesis, hence, allowing very high iodide concentrations in the colloid.

Secondary active transport is a type of active transport across a biological membrane in which a transport protein couples the movement of an ion (typically Na⁺ or H⁺) down its electrochemical gradient to the movement of another ion or molecule against a concentration or electrochemical gradient. The ion moving down its electrochemical gradient is referred to as the driving ion. The ion/molecule being transported against a chemical or electrochemical gradient is referred to as the driven ion/molecule.

This transport process is referred to as active transport because the driven ion/molecule is transported against a concentration or electrochemical gradient. It is referred to as secondary active transport because no ATP hydrolysis is involved in this process (as opposed to primary active transport). The energy required to drive transport resides in the transmembrane electrochemical gradient of the driving ion.

Secondary active transport is also referred to as ion-coupled transport. Those utilizing Na⁺ as the driving ion are called Na⁺-coupled transporters. Those utilizing H⁺ as the driving ion are called H⁺-coupled transporters.

Two types of secondary active transport exist: cotransport (also known as symport) and exchange (also known as antiport). Na⁺/glucose cotransporter and H⁺/dipeptide cotransporter are examples of cotransporters. Na⁺/Ca²⁺ exchanger and Na⁺/H⁺ exchanger are examples of exchangers.

Cotransport
Symport
Exchange
Antiport

Lecture notes on Secondary Active Transport