### Falling through the respiratory chain

This diagram provides an overview of the major redox couples that provide the energy that drives the life process. Most organisms derive their metabolic energy from respiration, a process in which electrons from foodstuffs (nominally glucose, shown at the top left), fall to lower-free energy acceptors on the right.

In eucaryotic organisms this electron sink is dioxygen (you will have to scroll the adjacent pane down to see it.) Aerobic respiration is the most efficient of all because the electron falls so far. (As it does so, part of the energy is captured by a series of intermediates and used for the synthesis of ATP, but that's another story.)

To make the glucose, animals rely on plants, which utilize the energy of sunlight to force electrons from O2 back up to the top left of the diagram. This of course is photosynthesis, which is just respiration driven in reverse. (Have you thanked a green plant today?)

Aerobic respiration is a fairly recent development in the history of life. There still exist a host of primitive organisms (all bacteria) that inhabit anoxic environments and which must employ other electron sinks that reside higher on the scale, and thus yield smaller amounts of energy.

Among the more familiar of these sinks:

- NO3– can produce either N2 or NH3, depending on the number of electrons. Formation of N2 from NO3– is known as denitrification and is the reason that lawns and fields must be re-fertilized every year. Ammonia, NH3 is the more commonly formed product, as anyone who has worked around manure (or a diaper pail) knows.
- If nitrate is lacking, partly oxided carbon (shown as CH3O is usually available and leads to the production of methane, CH4. This forms in large quantities in marshy areas (hence the name "marsh gas" and in the intestinal tracts of animals.
- Farther still up the diagram you will see the sulfate ion. This is present in all soils, and its reduction product H2S produces the rotten-egg odor that is often noticed in heavy muds into which O2 is unable to penetrate.

Not all organisms start with glucose; H2, just below it, can serve as an elecron source and was likely an important one during the earliest stages of life, as were most of the sources below it.

For more on this topic, please see the page Chemistry and energetics of the life process, which is Part 10 of the Survey of Elementry Geobiochemistry.

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