While most texts have a section on membrane transport, they tend to focus on ion transport and virtually ignore the significance of metabolite transport, especially for the intracellular organelles. Cells can't exist in total isolation, so provision is made for transport of specific metabolites. For example almost all organisms have sugar and amino transporters in the cell membrane. Transporters are transmembrane proteins forming a hydrophilic pore through the phospholipid bilayer. This gated pore mechanism limits transport only to the intended substrates.
Many transporters elaborate the basic gated pore mechanism by transporting two substrates.
Cotransport processes occur in strict stoichiometry so the number of molecules transported is matched for each substrate. Coupling energy to transport The transport process can be made to favour a specific direction of transport by coupling to an energy source. The energy change of the coupled process determines the equilibrium constant. Active transport processes, especially ion pumps may couple transport directly to ATP hydrolysis or redox reactions. Similarly, redox reactions couple to proton pumping in oxidative phosphorylation.