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Biochemical Pathways Used for Obtaining Energy

Oxidation
Oxygen is taken up by an organism and used to oxidize various organic molecules to release energy. Aerobic Respiration takes place in many living things. It is easy to get rid of the CO₂ and H₂O, and maximum energy is released from organic substances such as glucose. A general example for aerobic respiration would be:

C₆H₁₂O₆ (Glucose) + 6O₂ -->6H₂O + 6CO₂ + energy!

Fermentation
If there is no oxygen, then an organism must find a new electron acceptor. This can occur through Fermentation. A common form of fermentation is converting sugar to ethanol or acid. This reaction, however, is not as efficient as oxidation. Only a fraction of the organic carbon is oxidized to CO₂ (the rest remains in the alcohol/acid by-product):

C₆H₁₂O₆ ---> 2C₂H₆O (Ethanol) + 2CO₂ + energy

What are some examples of fermentation that result in alcohol (ethanol) or acid (acetic or lactic acid)?

Heterotrophs (different-feeding) are organisms that obtain their carbon from previously produced organic molecules. All animals, fungi, and many bacteria are heterotrophs.

Where does this organic carbon ultimately come from?

Autotrophs (self-feeding) obtain their carbon from inorganic CO₂.

Photosynthesis
The process by which plants and some bacteria (remember cyanobacteria and stromatolites?) use the energy from sunlight to produce organic molecules. Sunlight energy absorbed by the chlorophyll splits H₂O, with the H⁺ reducing the CO₂ to sugar, and the O₂ given off as a gas. The shorthand equation is:

6H₂O + 6CO₂ + Light ---> C₆H₁₂O₆ + 6O₂

Note, this is the REVERSE of oxidation, shown above!

Chemosynthesis

Some autotrophic microorganisms can live in the dark and obtain energy by oxidizing inorganic chemicals. The shorthand equation is:

6CO₂ + 6H₂0 + Reduced chemical (e.g. H₂S, Fe²⁺) ---> C₆H₁₂O₆ + Oxidized chemical (e.g. S, Fe³⁺)

Examples of this include many of the microbes living at the deep sea vents