Energy may be generated by the oxidation of inorganic molecules

In the previous pages, we have seen how electrons derived from a variety of organic sources can be channelled into the glycolytic pathway (or one of its alternatives), and how energy is generated by either oxygen or an organic/inorganic molecule acting as an electron acceptor. Certain bacteria, however, are able to derive their energy from the oxidation of inorganic substrates; these are termed chemolithotrophs (see Chapter 4). Molecules such as NH4+, NO2-, Fe2+ and S0 can be oxidised, with the concomitant generation of ATP.

The AG (change in free energy) for each of these reactions is much smaller than that for aerobic respiration. The value of AG is a measure of how much energy is released by a reaction. Thus bacteria using this form of metabolism need to oxidise a larger amount of their substrate in order to synthesise the same amount of cellular material. In most cases, such bacteria are autotrophs, fixing carbon from carbon dioxide via the Calvin cycle. This is also used by phototrophic organisms, and is described at greater length in the section on photosynthesis below. If organic carbon is available, however, some

The Calvin cycle is a pathway for the fixation of carbon dioxide used by photosynthetic organisms and some chemolithotrophs.

organisms are able to act as heterotrophs, deriving their carbon, but not energy, from such molecules. The overall energy yield from inorganic oxidation is much lower than that from aerobic respiration.

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