Transport Systems in Mycoplasmas
Transport processes are classified on the basis of kinetics and energetics (Figure 1). Using D and L isomers of glucose as examples, simple, Fickian diffusion kinetics are characteristic of unmediated transport which does not discriminate between the D and L isomers. Unmediated transport represents transport through the lipid bilayer. Discrimination between D and L isomers and saturation kinetics are characteristic of mediated transport. Mediated transport is carried out by intrinsic membrane proteins and involves interaction between the solute and the proteins. The classification of mediated transport processes is based on biochemical and energetic criteria. In group translocation, the transported solute is derivatized by a membrane enzyme; in the group translocation of D-glucose, the sugar is transported and phosphorylated in a coordinated process. In carrier transport, the free solute, not a derivative, is transported. Carrier transport is further classified on the basis of energetic criteria. If the free sugar is transported energetically uphill (i.e., against its concentration gradient), it is classified as active transport; if the sugar is transported energetically downhill, it is classified as facilitated diffusion.
KeywordsCholesterol Permeability Arsenate Polypeptide Pyruvate
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