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A facilitated diffusion protein speeds the movement of a chemical through a membrane in the absence of energy input; therefore, the transported chemical can only move down a concentration gradient. This can be accomplished by the formation of a high-specificity pore or channel that spans the membrane.
Transport proteins are also used in active transport, which by definition requires an energy input.
Chemiosmotic transport utilizes electrochemical gradients to drive transport. As the creation and maintenance of chemiosmotic gradients require energy input from the cell, this is a form of active transport. Prokaryotes typically use hydrogen ions as force to drive chemiosmotic transport, while eukaryotes typically use sodium ions. A symporter/coporter transports a chemical in the same direction as the electrochemical gradient, while an antiporter moves the target chemical in a direction opposite to the gradient.
Binding dependent active transport also moves the targeted chemical against a concentration gradient, but uses stored chemical energy, typically in the form of ATP, to power the transport. Generally speaking, a binding dependent transport system consists of a membrane spanning component with a high degree of specifity. The membrane spanning component changes configuration with the aid of chemical energy input (often through the use of an associated ATPase protein), thus translocating the chemical from one side of the membrane to the other.
By some definitions, proteins that catalyze the ligation of phosphate or coenzyme groups to a catabolized chemical can be considered active transport proteins in that they drive the uptake of a chemical by maintaining a steep functional concentration gradient. This pheonomenon is termed group translocation in the case of sugar phosphorylation and vectoral acylation or vectoral esterification in the case of fatty acid coenzyme A ligation.
Examples[edit | edit source]
- ATP-binding cassette transporter genes
- Glutamate transporter
- Vesicular monoamine transporter
- Glutamate transporter
- Proton pump
- Hydrogen potassium ATPase
- Norepinephrine transporter
- Sodium-chloride symporter
- Glucose transporter
- Adaptor protein
See also[edit | edit source]
- Carrier protein
- Ion channel
- Ion pump (biology)
- TC number (classification)
- Wikipedia:MeSH D12.776#MeSH D12.776.157.530 --- membrane transport proteins
- Wikipedia:MeSH D12.776#MeSH D12.776.543.990 --- vesicular transport proteins
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