4.2.1  Compartments, channels and transporters

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Transport phenomena at the membrane level give an insight into how cells acquire and compartmentalise resources. Elec-trical and chemical gradients across membranes, transport of solutes through membrane proteins and factors controlling these proteins all contribute to precise control of individual solute levels within cells. Little in water and solute transport is left to chance. Even passive diffusion of molecules across mem-branes is influenced by channels whose specificity, frequency and control responses are all encoded genetically. Tight regulation of transport at the cell and organelle level are often dependent on metabolic conversions to optimise resource use in whole organisms. For example, malate delivered to the symbiosomes of N2-fixing nodules is metabolised to provide carbon skeletons and energy while ammonium (NH4+) leaves the symbiosomes through an NH4+ channel, providing a nitrogen source for the plant (Whitehead et al. 1995).

Membranes also participate in control by providing the boundaries of compartments from which solutes can be sequestered or withdrawn — vacuoles are a common example of this kind of compartment. For instance, vacuoles act both as repositories for toxic ions and internal stores of calcium (Ca2+) for secondary messenger signalling (Section 4.1.3(a)).