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:
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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