3.3 Active transport and role of electrochemical gradients
Solute transport coupled to energy transduction is divided into primary and secondary transport according to the energy source. Primary transport systems are driven by energy-generating metabolism. Primary transport includes proton export driven by electron transport in respiration and photosynthesis, by ATP hydrolysis, and by light in halophilic archaea. Also included in primary transport are chloride ion import in halophilic archaea driven by light , sodium ion export coupled to decaboxylationreactions, proton export coupled to fumarate reduction and fermentation product excretion, and import of sugars through group translocation. These ion transport mechanisms are energy-conserving processes, except for sugar transport by group translocation, and will be discussed in the appropriate sections. This section is devoted to energy-dependent transport of materials needed for growth and survival and includes the secondary transport and group translocation of sugars.
Energy needed for secondary transport is supplied as an electrochemical gradient(a proton motive or sodium motive force) or from high energy phosphate bonds, as the results of primary transport (the proton(acidic internal pH) gradient and membrane potential) are established. Since sodium ions are exported due to their coupling with some energy-yielding reactions such as decarboxylation and anaerobic respiration, a sodium gradient across the prokaryotic membrane is also established. Proton and sodium gradients are collectively termed electrochemical gradient, and they are used as energy for many secondary transport processes. All te carrier proteins studied have 12 helices spanning the membrane, some of which function as binding sites for solutes and others for proteins(or sodium ions).
The proton motive force consists of a proton gradient (pH) and membrane potential =. Depending on the nature of the solute, transport requires energy in the form of either สามเหลียมpH or สามเหลี่ยม, or both. According to the carrier proteins involved, electrochemical gradient-dependent solute transport can be classified as symport, antiport and uniport mechanisms. A solute crosses the membrane in the same direction with protons( or sodium ions) in the symport mechanism but in the opposite direction in an antiport system. Uniporters transport ions along the electrochemical gradient without involving protons or sodium ions. Uniporters consume only the สามเหลี่ยม part of the proton motive (or sodium motive) force. This is called electrogenic transport. When a monovalent anion is symported (cotransported) with a proton, the สามเหลี่ยม pH is reduced without any change in the สามเหลี่ยม. This is called electroneutral transport. When an uncharged solute is symported with protons, the สามเหลี่ยม pH as well as the สามเหลี่ยม supply the energy needed for the accumulation of the solute. Since สามเหลี่ยม is reduced this becomes an electrogenic transport system.