‘From dust you came, to dust you shall return,’ is one sober, biblical reminder that complex organisms are built from simple chemical elements to which they will revert. From the beginning to the end of their lives, living things wage a battle against natural forces which break down their highly organized structure:

• At the cell level, complex molecules such as proteins and nucleic acids, to name but two of many hundreds, are continually destroyed by hydrolysis

• Valuable molecules are lost to the environment and have to be replaced because cell membranes are leaky

• Our atmosphere is dominated by the highly reactive molecule, oxygen, as a result of which everything on Earth, organic and inorganic, is subject to corrosive oxidation.

Yet, on all sides, we observe organisms using simple materials from their surroundings to maintain, renew, and build complex structures, to achieve which they need a constant supply of energy.

Organisms have evolved two ways of satisfying their absolute need for energy. The most crucial, photosynthesis, traps light energy from an outside source, the Sun, to fuel the building of complex organic structures from simple inorganic materials. The other, respiration, requires that there be a constant source from which chemical energy can be extracted and used for maintenance and construction.

Photosynthesis and respiration together comprise bioenergetics, how living organisms gain the supply of energy they need, which is the subject of Part I.