Active transport
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Active transport (sometimes called active intake because of the absorbing movement of particles) is an energy-requiring process that moves material across a cell membrane and up the concentration gradient. The cell uses active transport in three situations: when a particle is going from low to high concentration, when particles need help entering the membrane because they are selectively impermeable, and when very large particles enter and exit the cell.
Active transport is the mediated process of moving molecules and other substances across membranes. Use this tutorial to learn about active transport, including such key terms as adenosine triphosphate (ATP), carrier mediated transport, and concentration gradients.
When particles are being moved from areas of low concentration to areas of high concentration(ie. against the concentration gradient) then specific carrier proteins in the membrane are required to move these particles. the carrier proteins bind to specific molecules (eg. glucose) and transport them into the cell where they are released. Energy is required for this process so this is known as Active Transport. Examples: sodium is transported out of the cell and potassium into the cell by the sodium/potassium pump, a form of active transport.
Active transport can be:
Primary: Uses the chemical energy from ATP or other sources.
- or
Secondary: Uses the electrochemical gradient to power transport.
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[edit] Classes of primary transporters (pumps)
[edit] ABC pumps
ABC class pumps transport small molecules across membranes. They are also called the ABC superfamily. They consist of two transmembrane domains, and two ATP binding domains. ABC pumps are involved in the transport of small molecules, phospholipids and lipophilic drugs in mammalian cells. In bacteria they transport amino acids, sugars and peptides.[1]
[edit] P-type pumps
P-type pumps use ATP to transport ions against a gradient. They are phosphorylated during transport, which is different from the other classes of active transport pumps.
Some examples of P-class pumps are the sodium-potassium pump, proton pump, calcium transport in muscle cells and the hydrogen-potassium pump in the apical membrane of the stomach.
[edit] V-type pumps
V-class proton pumps are a type of ATPase. They use the energy released by the hydrolysis of ATP to move protons (or in a few cases of bacteria sodium-ions[2]) against their concentration gradient.[3] All proteins that fall into this class have two structural domains. One domain called the V0 domain is made of 5 subunits and is involved in translocation of the protein. The other domain is called the V1 domain which is composed of 8 subunits and is involved in ATP-hydrolysis.[4]
V-class proton pumps are found in a wide variety of organelle membranes. In fungi, yeast and plant cells they are found in Vacuole membranes. In animals they are found in the membranes of lysosomes and endosomes. V-class proton pumps are also found in the plasma membranes of macrophages[5] , osteoclasts[6] , and renal intercalated cells.[7]
The function of this class of pump is strictly to transport protons across the membrane that they are embedded within. Transporting protons across a membrane can decrease the pH on one side of the membrane which can be critical for organelle functioning.[3] This is indeed the case in endosomes. When endosomes bud off from the plasma membrane as they do in receptor-mediated endocytosis V-class pumps increase the acidity within the lumen of the endosome. This increased acidity acts as a signal to the ligand-receptors to release their ligands which can be molecules such as LDL or insulin. Ligand release is critical so the ligand-receptors can be recycled back to the plasma membrane and join another endosome.[8] Decreasing the pH of endosomes is also important for the entry of some membrane bound viruses. The viral protein haemagglutinin is located on the surface of the Influenza virus and the acidification provided by this protein aids in viral entry.[9]
V-class pumps located in cell membranes which also have critical functions to the cell. In renal intercalated cells these pumps secrete protons into the fluid in the kidneys, helping to maintain an optimal pH in the kidneys.[10] In humans, mutations in the genes coding for this protein can lead to metabolic acidosis;[11] a potentially deadly disease.
[edit] F-type proton pumps
Uses H+ gradient in order to produce ATP. Can also work in the reverse direction.
[edit] Role in neurons
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Please help improve this article or section by expanding it. Further information might be found on the talk page or at requests for expansion. (February 2008) |
[edit] See also
[edit] References
- ^ Lodish et al (2008) Molecular Cell Biology, 6, W. H. Freeman
- ^ Murata T, Yamato I, Kakinuma Y, Leslie AG, Walker JE (April 2005). "Structure of the rotor of the V-Type Na+-ATPase from Enterococcus hirae". Science (journal) 308 (5722): 654–9. doi:. PMID 15802565.
- ^ a b Lodish, Harvey; et al. (2003). Molecular Cell Biology, 5, W. H. Freeman. ISBN 0716743663.
- ^ Nishi, T. "The Vacuolar H+-ATPases - Nature's Most Versatile Proton Pumps". Nature Rev. MCB 3: 94-103.
- ^ Brisseau, G.F.; et al. (1996). "IL-1 increases V-ATPase activity in murine peritoneal macrophages". J. Biol. Chem 271: 2005-2001.
- ^ Li, Y.P.; Y. Liang, E. Li, P. Stashenko (1999). "Atp6i-deficient mice exhibit severe osteopetrosis due to loss of osteoclast-mediated extracellular acidification". Nature Genet. 23: 447-451.
- ^ Brown, D.; S. Breton (2000). "V-ATPase dependent lumenal acidification in the kidney collecting duct and the epididymis/vas deferens". J. Exp. Biol 203: 127-145.
- ^ Forjac, M. (1999). "Structure and Properties of the Vacuolar (H+)-ATPases". J. Biol..
- ^ Han, X.; Bushweller, J.H., Cafiso, D.S. & Tamm, L.K. (2001). "Membrane structure and fusion-triggering conformational change of the fusion domain from influenza hemagglutinin". Nature Struct. Biol. 8: 715-720.
- ^ Brown, Dennis\coauthors=Brenton, Sylvie (2000). "H+V-ATPase-Dependent Luminal Acidification in the Kidney Collecting Duct and Epididymis/Vas Deferens: Vesicle Recycling and Transcytotic Pathways". J. Exp. Biol. 203: 137-145.
- ^ Karet, F.E; et al. (1999). "Mutations in ATP6N1B, encoding a new kidney vacuolar proton pump 116-kD subunit, cause recessive distal renal tubular acidosis with preserved hearing". Nature Genet. 21: 84-90.
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