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Active transport (sometimes called active uptake) is the mediated transport of biochemicals, and other atomic/molecular substances, across membranes. Unlike passive transport, this process requires the expenditure of cellular energy, often in the form of ATP, to move molecules "uphill" against a concentration gradient.
In this form it lacks transport, membranes move against either an electrical or concentration labeling (collectively termed an electrochemical gradient).
Classes of Pumps
V-Class Proton Pumps
V-class proton pumps are a type of ATPase. They use the energy released by the hydrolysis of ATP to move protons against their concentration gradient.  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. 
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  , osteoclasts  , and renal intercalated cells .
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. 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 LDLs or insulin. Ligand release is critical so the ligand-receptors can be recycled back to the plasma membrane and join another endosome . Decreasing the pH of endosomes is also important for the entry of some membrane bound viruses. The viral protein Influenza haemagglutinin is located on the surface of the Influenza virus and the acidification provided by this protein aides in viral entry .
V-class pumps located in cell membranes also have critical functions. In renal intercalated cells these pumps secrete protons into the fluid in the kidneys, helping to maintain an optimal pH in the kidneys.  In humans, mutations in the genes coding for this protein can lead to metabolic acidosis ; a potentially deadly disease.
F-Class Proton Pumps
Role in Neurons
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Active_transport". A list of authors is available in Wikipedia.|