Living organisms, even the most simple, are extremely improbable structures. Their highly complex forms are not at all likely to appear by the random associations of small molecules (spontaneous generation); and the elaborate organic compounds that make up an organism are thermodynamically unstable – they tend to break down with a release of energy and an increase of disorder (entropy). Energy must be available to drive forward the reactions of biosynthesis, that is to move from disorder to order, and to maintain the state of order against a hostile Nature. Energy may be needed for other purposes too, such as movement, uptake of nutrients, or maintenance of body temperature by some animals.

Sources of energy

Energy is gained by living organisms from two main sources:

1. (1) Light (i.e. by photosynthesis). Photosynthesis may be considered as the fundamental process by which most living organisms gain energy. However, it is easier to approach the complicated ideas within the subject of energy metabolism by first considering oxidation reactions.

2. (2) Oxidation reactions (usually of an organic molecule, and usually with oxygen as the final electron acceptor).

Many organic molecules can be broken down by microbes (and other living organisms) in the presence of oxygen, to yield CO2 and water and to release energy.