The Flow of Electrons Provides Energy for Organisms

  Nearly all living organisms derive their energy, directly or indirectly, from the radiant energy of sunlight, which arises from thermonuclear fusion reactions carried out in the sun. Photosynthetic cells absorb light energy and use it to drive electrons from water to carbon dioxide, forming energy-rich products such as glucose (C₆H₁₂O₆), starch, and sucrose and releasing O₂ into the atmosphere:

  Nonphotosynthetic cells and organisms obtain the energy they need by oxidizing the energy-rich products of photosynthesis and then passing electrons to atmospheric O₂ to form water, carbon dioxide, and other end products, which are recycled in the environment:

  Virtually all energy transductions in cells can be traced to this flow of electrons from one molecule to another, in a “downhill” flow from higher to lower electrochemical potential; as such, this is formally analogous to the flow of electrons in a battery-driven electric circuit. All these reactions involving electron flow are oxidationreduction reactions: one reactant is oxidized (loses electrons) as another is reduced (gains electrons).