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Membrane transport protein

A membrane transport protein (or simply transporter) is a membrane protein involved in the movement of ions, small molecules, or macromolecules, such as another protein, across a biological membrane. Transport proteins are integral transmembrane proteins; that is they exist permanently within and span the membrane across which they transport substances. The proteins may assist in the movement of substances by facilitated diffusion or active transport. The two main types of proteins involved in such transport are broadly categorized as either channels or carriers. The solute carriers and atypical SLCs are secondary active or facilitative transporters in humans .

Difference between channels and carriers

A carrier is not open simultaneously to both the extracellular and intracellular environments. Either its inner gate is open, or outer gate is open. In contrast, a channel can be open to both environments at the same time, allowing the solutes it transports to diffuse without interruption. Carriers have binding sites, but pores and channels do not.Sadava, David, et al. Life, the Science of Biology, 9th Edition. Macmillan Publishers, 2009. . p. 119.Thompson, Liz A. Passing the North Carolina End of Course Test for Biology. American Book Company, Inc. 2007. . p. 97. When a channel is opened, millions of ions can pass through the membrane per second, but only 100 to 1000 molecules typically pass through a carrier molecule in the same time. Each carrier protein is designed to recognize only one substance or one group of very similar substances. Research has correlated defects in specific carrier proteins with specific diseases.Sadava, David, Et al. Life, the Science of Biology, 9th Edition. Macmillan Publishers, 2009. . p. 119.

Active transport

Facilitated diffusion

s (left) and carrier proteins (three on the right).]] Facilitated diffusion is the passage of molecules or ions across a biological membrane through specific transport proteins and requires no energy input. Facilitated diffusion is used especially in the case of large polar molecules and charged ions; once such ions are dissolved in water they cannot diffuse freely across cell membranes due to the hydrophobic nature of the fatty acid tails of the phospholipids that make up the bilayers. The type of carrier proteins used in facilitated diffusion is slightly different from those used in active transport. They are still transmembrane carrier proteins, but these are gated transmembrane channels, meaning they do not internally translocate, nor require ATP to function. The substrate is taken in one side of the gated carrier, and without using ATP the substrate is released into the cell. They may be used as potential biomarkers


(Grouped by Transporter Classification database categories)

1: Channels/pores

Facilitated diffusion occurs in and out of the cell membrane via channels/pores and carriers/porters. Note:
  • Channels:
Channels are either in open state or closed state. When a channel is opened with a slight conformational switch, it is open to both environment simultaneously (extracellular and intracellular)
  • than what is required to move the green circles so the movement is coupled and some energy is cancelled out. One example is the lactose permease which allows protons to go down its concentration gradient into the cell while also pumping lactose into the cell.]]Pores:
Pores are continuously open to these both environment, because they do not undergo conformational changes. They are always open and active.

2: Electrochemical potential-driven transporters

3: Primary active transporters

  • 3.A: P-P-bond-hydrolysis-driven transporters :
  • * ATP-binding cassette transporter (ABC transporter), such as MDR, CFTR
  • * V-type ATPase ; ( "V" related to vacuolar ).
  • * P-type ATPase ; ( "P" related to phosphorylation), such as :
  • ** Na+/K+-ATPase
  • ** Plasma membrane Ca2+ ATPase
  • ** Proton pump
  • * than what is required to move the blue circles so the movement is coupled and some energy is cancelled out. One example is the sodium-proton exchanger which allows protons to go down their concentration gradient into the cell while pumping sodium out of the cell.]] F-type ATPase; ("F" related to factor), including: mitochondrial ATP synthase, chloroplast ATP synthase1
  • 3.B: Decarboxylation-driven transporters
  • 3.C: Methyltransfer-driven transporters
  • 3.D: Oxidoreduction-driven transporters
  • 3.E: Light absorption-driven transporters, such as rhodopsin

4: Group translocators

The group translocators provide a special mechanism for the phosphorylation of sugars as they are transported into bacteria (PEP group translocation)

5: Electron carriers

The transmembrane electron transfer carriers in the membrane include two-electron carriers, such as the disulfide bond oxidoreductases (DsbB and DsbD in E. coli) as well as one-electron carriers such as NADPH oxidase. Often these redox proteins are not considered transport proteins.


Each carrier protein, even within the same cell membrane, is specific to one type or family of molecules. For example, GLUT1 is a named carrier protein found in almost all animal cell membranes that transports glucose across the bilayer. Other specific carrier proteins also help the body function in important ways. Cytochromes operate in the electron transport chain as carrier proteins for electrons.


A number of inherited diseases involve defects in carrier proteins in a particular substance or group of cells. Cysteinuria (cysteine in the urine and the bladder) is such a disease involving defective cysteine carrier proteins in the kidney cell membranes. This transport system normally removes cysteine from the fluid destined to become urine and returns this essential amino acid to the blood. When this carrier malfunctions, large quantities of cysteine remain in the urine, where it is relatively insoluble and tends to precipitate. This is one cause of urinary stones.Sherwood, Lauralee. 7th Edition. Human Physiology. From Cells to Systems. Cengage Learning, 2008. p. 67 Some vitamin carrier proteins have been shown to be overexpressed in patients with malignant disease. For example, levels of riboflavin carrier protein (RCP) have been shown to be significantly elevated in people with breast cancer.Rao, PN, Levine, E et al. Elevation of Serum Riboflavin Carrier Protein in Breast Cancer. Cancer Epidemiol Biomarkers Prev. Volume 8 No 11. pp. 985–990

See also


External links

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This article based upon the http://en.wikipedia.org/wiki/Membrane_transport_protein, the free encyclopaedia Wikipedia and is licensed under the GNU Free Documentation License.
Further informations available on the list of authors and history: http://en.wikipedia.org/w/index.php?title=Membrane_transport_protein&action=history
presented by: Ingo Malchow, Mirower Bogen 22, 17235 Neustrelitz, Germany