Sunday, October 18, 2009
active transport-celltransport-cellbiology-btechbiotechnology-4th unit-jntu syllabus
Active transport is the mediated process of moving particles across a biological membrane against a concentration gradient. If the process uses chemical energy, such as from adenosine triphosphate (ATP), it is termed primary active transport. Secondary active transport involves the use of an electrochemical gradient. Active transport uses energy, unlike passive transport, which does not use any energy.
There are two main types, primary and secondary. In primary transport energy is directly coupled to movement of desired substance across a membrane, independent of any other species. Secondary transport concerns the diffusion of one species across a membrane to drive the transport of another.
Primary active transport directly uses energy to transport molecules across a membrane. Most of the enzymes that perform this type of transport are transmembrane ATPases. A primary ATPase universal to all cellular life is the sodium-potassium pump, which helps maintain the cell potential.
In secondary active transport, there is no direct coupling of ATP; instead, the electrochemical potential difference created by pumping ions out of cells is used. The two main forms of this are counter-transport (antiport) and co-transport (symport).
Specialized trans-membrane proteins recognize the substance and allows it access (or, in the case of secondary transport, expend energy on forcing it) to cross the membrane when it otherwise would not, either because it is one to which the lipid bilayer of the membrane is impermeable or because it is moved against the concentration gradient. The last case, known as primary active transport, and the proteins involved in it as pumps, uses the chemical energy of, usually, ATP. The other cases, which usually derive their energy through exploitation of an electrochemical gradient, are known as secondary active transport and involve pore-forming proteins that form channels through the cell membrane.
When particles are being moved from areas of low concentration to areas of high concentration (i.e., against the concentration gradient) then specific carrier proteins in the membrane are required to move these particles. The carrier proteins bind to specific molecules (e.g., glucose) and transport them into the cell where they are released. Because energy is required for this process, it is known as active transport. Examples of active transport include when sodium is transported out of the cell and potassium into the cell by the sodium-potassium pump. Active transport often takes place in the internal lining of the small intestine.
Plants need to absorb mineral salts from the soil, but these salts are in very dilute solution. Active transport enables these cells to take up salts from this dilute solution against the concentration gradient.
 ABC pumps
ABC class pumps transport small molecules across membranes. They are also called the ABC superfamily and are an example of primary active transporters. 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.