Active Transport:
* Accumulation of solute against its electro-chemical potential through an energy - linked process is called as 'Active transport.'
* Active transport system differs from FD system, in that they consist of at least two components.
* One of them is solute specific carrier which acts in a catalytic manner and the other is a continuous supply of metabolic energy.
* A carrier coupled to a source of metabolic energy can transport solute against a concentration gradient or electro-chemical gradient.
* Active transport can maintain a concentration of an ion or a molecule several times higher on one side of the membrane than the other.
* Active transport is carried out in at least three different ways in bacteria.
* These include group translocation, membrane bound transport system and binding protein transport systems.
* Group translocation system utilize the chemical energy of the modification reaction.
* membrane bound transport systems utilize the proton- motive force across the membrane .
* Binding protein transport system utilize ATP or a related compound.
Mechanism of Active Transport:
* Active transport and group translocation share with facilitated diffusion, the participation of substrate specific protein.
* They differ from FD, however by their dependence on energy.
* When metabolic energy is available, the substrate can be accumulated inside the cell against the concentration gradient.
* The basic difference between active transport and group translocation is the nature of the product that is released inside the cell.
* In active transport, the molecule released into the cytoplasm is identical with that taken up from the medium.
* In group translocation, the molecule is modified during the transport process, for example by phosphorylation.
* Various models have been proposed for active transport, all of these include specific transport proteins in the membrane.
* these have been given names that indicate their presumed functions (permeases, translocase, translocator proteins & carriers).
* Different transport process are distinguished mainly by the way in which the energy necessary to drive the transport process is made available by proton potential, by ATP or by phosphoenol pyruvate.
* Trasnport of many substances, including inorganic & organic ions, as well as sugars, is driven by the proton potential.
* The bacterial cell maintains a proton potential by constantly pumping out protons & other ions (Na+).
* This is mediated by transport proteins located in the membrane, each of these proteins has a specific functions.
* one protein is known, for e.g. to catalyse the simultaneous transport of a sugar molecule and a proton in the same direction. This is referred as a Symport of two or more substances.
* Other transport protiens catalyse the simultaneous transport of two substances in opposite directions, this is called as Antiport.
* The ions that drive the entry of sugar into the cell are probably always H+ or NA+.
* In prokaryotes the H+ coupled symport system seems to predominate, where as in eukaryotes Na+ coupled symport is the rule.
* Active transport driven by a proton potential is probably the most common mechanism for the active uptake of metabolic substrates.
* In addition to transport system that depends on a proton potential, there are others that depends on ATP.
* In these the periplasmic binding proteins play a part.
* ATP-powered pumps (ATPases) couple the splitting, or hydrolysis, of ATP with the movement of ions across a membrane against a concentration gradient.
* ATP is hydrolyzed directly to ADP and inorganic phosphate, and the energy released is used to move one or more ions across the cell membrane.
* As much as 25% of a cell's ATP reserves may be spent in such ion transport.
* Examples include: The Na+ -K + ATPase pumps Na+ out of the cell while it pumps K+ in.
* The membrane potential is probably generated by ATP- dependent pumping mechanism, such as the sodium- potassium pump, and the Na+ potential then drives the Na+ nutrient symport.
* All the amino acids can be active transported, sodium- driven symport pumps.
