Regulation of composition and volume of biological compartments

Living organisms consist of a large number of interdependent fluid compartments (see Chapter 2) and these compartments are bounded by membranes which perform at least two major functions. The first of these is to act as a physical barrier so as to impede the free movement of particles between adjacent compartments. These particles consist of intracellular organelles (for example, mitochondria), macromolecules that exist both intra- and extracellularly (for example, proteins) and small polar molecules (such as water) and ions. The second is that membranes, since they are not passive and impermeable, are able to regulate the volume and composition of the intracellular environment. This regulation means that the composition and volume of the intracellular and extracellular compartments are maintained at constant values despite fluctuations in the external environment.

Membranes regulate the composition of compartments by being selectively permeable. They are also able to utilize free energy (that is stored either in ATP high-energy bonds or in concentration gradients) to transport ions and molecules against electrochemical or chemical potential gradients (see Chapter 3 for discussion of these gradients. For a discussion of free energy see Appendix 25).

Coupling mechanisms in membrane transport

This so-called ‘uphill’ transport against gradients is possible because membranes contain specialized molecular complexes which are able to couple chemical reactions (for example, ATP ⇋ ADP + Pi + free energy) to ion fluxes.