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Living organisms need water, an energy source, a carbon source and an electron source in order to survive and reproduce. Living organisms may be classified according to their needs. Metabolism is about how living organisms manage material and energy resources. More specifically, metabolism is the sum of the physical and chemical changes in living organisms. These changes include anabolic and catabolic processes to maintain chemical disequilibrium.


Energy maintains disequilibrium

Living cells are ordered systems with cell membranes that regulate which compounds go in and which compounds go out. In order to stay alive, free energy is used to maintain organization and counteract spontaneous drift towards chemical equilibrium where free energy is zero and no work (hence no metabolism) can be performed (=dead cell). [[Energy transformations increase disorder|Free energy comes from the environment and must be of high enough quality (order) to permit the cell to work. For example, light and chemical energy are more ordered than heat. Few organisms can use heat to do any work but many can use light and all are able to use chemical energy (but not necessarily as metabolic fuel).

The metabolic fuel used by cells is either light energy or chemical energy released from exergonic reactions, usually (always?) redox reactions. The input energy may be transferred and transformed to drive endergonic reactions (that is do endergonic work) in the cell. Exergonic and endergonic reactions are linked by a shared energized intermediate, almost always ATP. Several distinct but linked metabolic pathways are used by cells to transfer the energy released by breakdown of fuel molecules to ATP.

Enzymes catalyze non-spontaneous reactions

In order for a cell to make use of the released energy from an exergonic reaction it must not happen spontaneously. In a spontaneous redox reaction nothing stops the electrons from flowing directly from electron donor to electron acceptor without involvement of the cell that hence is not able to make any use of the flowing electrons. The internal milieu of cells is also not stimulating spontaneous reactions as the temperatures and pressures are moderate, the concentrations of reactant are low and pH is near-neutral. Citation needed

A non-spontaneous reaction, on the other hand, may be catalyzed by an enzyme. Then the cell can make use of the donor electron flow by diverting it to energy harvesting cell systems before it ends up in the electron acceptor.

Prokaryotic versus eukaryotic metabolism

The evolution of metabolism is both cause and effect of changing environments on Earth. Relative to eukaryotes, prokaryotes have evolved over a much longer time and have, as a group, become more metabolically flexible and able to use diverse energy, carbon and electron sources. Collectively, prokaryotes have evolved a huge number of different enzymes that can catalyze redox reactions with a variety of electron donors and acceptors. The enzymes make it possible for prokaryotes to use alternative metabolic pathways so that additional metabolic fuels, carbon and nitrogen sources from the environment can be used. This metabolic flexibility makes it possible for prokaryotes to colonize environments that eukaryotes can’t survive in.

All living organisms use some common metabolic pathways: Glycolysis, Anaerobic respiration, the Krebs cycle and oxidative phosphorylation. Many other metabolic pathways exist though they are not common to all organisms.

Metabolic flexibility among bioleaching microbes

Some bioleaching microbes are extremely restricted to certain substrates (L. ferrooxidans, L. ferriphilum, others?). The growth of such organisms is probably very sensitive to environmental changes affecting these substrates. Other bioleaching microorganisms are metabolically very versatile (At. ferrooxidans). If the environment changes so the original metabolism is no longer possible the microbe might be able to utilize other energy or nutrient sources.

See also

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