10 I The Process of Evolution 



conditions (temperature, pressure) in a fashion distinct from non- 

 living reactions; and demonstrate a degree of conservation of energy 

 not often equaled in inorganic reactions. Such reactions in living 

 systems are referred to as biological oxidations. The energy needed 

 by the heterotrophic organism is almost universally mediated by a 

 single type of organophosphate bond, that in the energy-rich com- 

 pound called adenosine triphosphate (ATP). When ATP releases 

 energy in a biological reaction, it releases one phosphate and be- 

 comes adenosine diphosphate (ADP). The latter still includes one 

 energy-rich phosphate bond and is particularly susceptible to being 

 rephosphorylated into ATP. But the rephosphorylation (called 

 oxidative phosphorylation) is mediated by the system of enzymes 

 referred to above. These enzymes in their turn depend upon energy 

 from the cleavage of ATP bonds, but the important point is that, for 

 each ATP bond that releases its energy for these enzymes to function, 

 more than one ATP bond is formed. The extra energy is derived 

 from the energy stored in the glucose (or other) molecule upon 

 which the enzymes are acting directly. By this interlocked series of 

 reactions, chemical energy supplied to the organism as molecules 

 of carbohydrate, lipid, or protein (which the organism cannot use, 

 as such) is transformed into ATP-bond energy that the organism 

 can use. 



In nonliving systems, energy transfer by molecular degradation 

 yields smaller molecules plus much heat. In living systems, the 

 products are high-energy organophosphate bonds, smaller mole- 

 cules, and surprisingly little heat. In fact, one of the salient features 

 of the living energetic machinery is the closeness of the coupling 

 between energy-yielding and energy-storing reactions and the result- 

 ant conservation of energy. Of course the ultimate source of energy 

 in existing organisms (except chemosynthetic bacteria) is that of the 

 sun trapped by photosynthetic organisms and eventually stored in 

 phosphate bonds by a related process involving photophosphoryla- 

 tion in the photosynthetic organism or by synthesis into carbo- 

 hydrate with phosphorylation. Many important enzymes or catalysts 

 in both photosynthesis and biological oxidations are colored com- 

 pounds involving metal ions (Fe, Mg) and the organic substances 

 known as porphyrins. Calvin has diagrammed (Fig. 1.2) how such 

 important biological materials might have arisen in the course of 

 chemical selection involving autocatalysis. In the sequence from 

 simpler to more complex molecules, later stages are catalysts for 

 succeeding stages. Since the use of porphyrins by nonphotosynthetic 

 organisms is widespread, Calvin feels that the presumably random 



