secondary active transport

In secondary active transport, 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. ^[1] example of Primary active transport Primary active transport directly uses energy to transport molecules across a membrane. ... Adenosine 5-triphosphate (ATP) is a multifunctional nucleotide that is most important as a molecular currency of intracellular energy transfer. ... Electrochemical potential is a thermodynamic measure that reflects energy from entropy and electrostatics and is typically invoked in molecular processes that involve diffusion. ...

The three main forms of this are uniport, counter-transport (antiport) and co-transport (symport). An antiporter is an integral membrane protein that is involved in facilitated diffusion. ... A symporter, also known as a coporter, is an integral membrane protein that is involved in facilitated diffusion. ...

Counter-transport

In counter-transport 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 entropic 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. Ice melting - classic example of entropy increasing[1] described in 1862 by Rudolf Clausius as an increase in the disgregation of the molecules of the body of ice. ... The sodium-calcium exchanger (often denoted Na+/Ca2+ exchanger or exchange protein) is an antiporter ion pump membrane protein which removes calcium from cells. ...

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. Calcium ATPase is a form of ATPase which transfers calcium after a muscle has contracted. ... In biology, second messengers are low-weight diffusible molecules that are used in signal transduction to relay signals within a cell. ...

Co-transport

Co-transport also uses the flow of one solute species from high to low concentration to move another molecule against its preferred direction of flow. An example is the glucose symporter, which co-transports two sodium ions for every molecule of glucose it imports into the cell. It contains water and also has no ATP Impact of a drop of water. ...

References

1. ^ Physiology at MCG 7/7ch05/7ch05p12