Secondary active transport
Encyclopedia
In secondary active transport or co-transport
, uses energy to transport molecules across a membrane; however, in contrast to primary active transport
, there is no direct coupling of ATP
; instead, the electrochemical potential difference created by pumping ions out of the cell is used.
The two main forms of this are antiport and symport.
energy to drive the transport of the other solute from a low concentration region to a high one. An example is the sodium-calcium exchanger
or antiporter, which allows three sodium ions into the cell to transport one calcium out.
Many cells also possess a calcium ATPase
, which can operate at lower intracellular concentrations of calcium and sets the normal or resting concentration of this important second messenger. But the ATPase exports calcium ions more slowly: only 30 per second versus 2000 per second by the exchanger. The exchanger comes into service when the calcium concentration rises steeply or "spikes" and enables rapid recovery. This shows that a single type of ion can be transported by several enzymes, which need not be active all the time (constitutively), but may exist to meet specific, intermittent needs.
from high to low concentration to move another molecule uphill from low concentration to high concentration (against its electrochemical gradient
).
An example is the glucose symporter SGLT1
, which co-transport
s one glucose
(or galactose
) molecule into the cell for every two sodium ions it imports into the cell. This symporter
is located in the small intestines, trachea, heart, brain, testis, and prostate. It is also located in the S3 segment of the proximal tubule
in each nephron
in the kidney
s . Its mechanism is exploited in glucose rehydration therapy
and defects in SGLT1 prevent effective reabsorption of glucose, causing familial renal glucosuria.
Co-transport
Co-transport, also known as coupled transport or secondary active transport, refers to the simultaneous or sequential passive transfer of molecules or ions across biological membranes in a fixed ratio...
, uses energy to transport molecules across a membrane; however, in contrast to primary active transport
Primary active transport
Primary active transport, also called direct active transport, directly uses energy to transport molecules across a membrane.Most of the enzymes that perform this type of active transport are transmembrane ATPases. A primary ATPase universal to all cellular life is the sodium-potassium pump, which...
, there is no direct coupling of ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
; instead, the electrochemical potential difference created by pumping ions out of the cell is used.
The two main forms of this are antiport and symport.
Antiport
In antiport two species of ion or other solutes are pumped in opposite directions across a membrane. One of these species is allowed to flow from high to low concentration which yields the entropicEntropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
energy to drive the transport of the other solute from a low concentration region to a high one. An example is the sodium-calcium exchanger
Sodium-calcium exchanger
The sodium-calcium exchanger is an antiporter membrane protein that removes calcium from cells. It uses the energy that is stored in the electrochemical gradient of sodium by allowing Na+ to flow down its gradient across the plasma membrane in exchange for the countertransport of calcium ions...
or antiporter, which allows three sodium ions into the cell to transport one calcium out.
Many cells also possess a calcium ATPase
Calcium ATPase
Calcium ATPase is a form of P-ATPase that transfers calcium after a muscle has contracted. The calcium ATPase are:*Plasma membrane Ca2+ ATPase *Sarcoplasmic reticulum Ca2+ ATPase - Plasma membrane Ca2+ ATPase :...
, which can operate at lower intracellular concentrations of calcium and sets the normal or resting concentration of this important second messenger. But the ATPase exports calcium ions more slowly: only 30 per second versus 2000 per second by the exchanger. The exchanger comes into service when the calcium concentration rises steeply or "spikes" and enables rapid recovery. This shows that a single type of ion can be transported by several enzymes, which need not be active all the time (constitutively), but may exist to meet specific, intermittent needs.
Symport
Symport uses the downhill movement of one solute speciesSolution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
from high to low concentration to move another molecule uphill from low concentration to high concentration (against its electrochemical gradient
Electrochemical gradient
An electrochemical gradient is a spatial variation of both electrical potential and chemical concentration across a membrane; that is, a combination of the membrane potential and the pH gradient...
).
An example is the glucose symporter SGLT1
Sodium-glucose transport proteins
Sodium-dependent glucose cotransporters are a family of glucose transporter found in the intestinal mucosa of the small intestine and the proximal tubule of the nephron . They contribute to renal glucose reabsorption...
, which co-transport
Co-transport
Co-transport, also known as coupled transport or secondary active transport, refers to the simultaneous or sequential passive transfer of molecules or ions across biological membranes in a fixed ratio...
s one glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
(or galactose
Galactose
Galactose , sometimes abbreviated Gal, is a type of sugar that is less sweet than glucose. It is a C-4 epimer of glucose....
) molecule into the cell for every two sodium ions it imports into the cell. This symporter
Symporter
A cotransporter is an integral membrane protein that is involved in secondary active transport. It works by binding to two molecules or ions at a time and using the gradient of one solute's concentration to force the other molecule or ion against its gradient....
is located in the small intestines, trachea, heart, brain, testis, and prostate. It is also located in the S3 segment of the proximal tubule
Proximal tubule
The proximal tubule is the portion of the duct system of the nephron of the kidney which leads from Bowman's capsule to the loop of Henle.-Structure and appearance:...
in each nephron
Nephron
The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system, which is not part of the nephron....
in the kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
s . Its mechanism is exploited in glucose rehydration therapy
Oral rehydration therapy
Oral rehydration therapy is a simple treatment for dehydration associated with diarrhoea, particularly gastroenteritis or gastroenteropathy, such as that caused by cholera or rotavirus. ORT consists of a solution of salts and sugars which is taken by mouth...
and defects in SGLT1 prevent effective reabsorption of glucose, causing familial renal glucosuria.