ORGANIC MAin^ER AND LIFE ENERGY O 



in thousands of ways. Some of these transformations lead to compounds 

 whose energy is higher than that of the carbohydrates supphed by the 

 plants — for example, when our l)odies convert sugars into fats, unfortu- 

 nately for some of us. However, this accumulation of chemical energy 

 can be achieved only at the cost of degradation of another quantity of a 

 vegetable substrate. For example, in the alcoholic fermentation of starch, 

 one i)art of this carbohydrate is "promoted" to energy-rich alcohol, while 

 another part is "degraded" to carbon dioxide. 



Differences in energy content of organic compounds are small com- 

 pared with the gap which separates the organic world from the stable 

 inorganic compounds. In the reduction of carbon dioxide to carbo- 

 hydrates, the energy content increases by about 112 kcal per gram atom 

 of carbon. In the transformation of carbohydrates into fats, which are 

 richer in energy than all other common constituents of animal bodies, the 

 additional gain in chemical energy is only 30 kcal per gram atom of carbon. 

 To comply with the precepts of thermodynamics, we should have 

 spoken above, not of the total energy (or "heat content") of organic 

 matter, but of its free energy, because free energy (or "working capacity ") 

 is what organisms need to give them a lease of life. Nature favors 

 disorder — it means greater variety, and therefore greater probability, 

 that is, higher entropy. Photosynthesis not only converts a state of 

 lower energy into a state of higher energy; it also converts a more dis- 

 orderly and therefore more probable state, in which the small molecules 

 of carbon dioxide and water are allowed to tumble freely in the rarified 

 gas or liquid, into a denser, more orderly, and therefore less probable state 

 of large organic molecules. In other words, it leads to a decrease in 

 entropy; and since the change in free energy (AF), is equal to the change 

 in total energy (AH) less a term proportional to the increase in entropy, 

 (TAS), the free energy of photosynthesis is even larger than its total 

 energy. (For quantitative data, see Table 3.V, page 49.) 



Chemical reactions which are associated with an increase in total energy 

 (AH > 0) are called endothermal (because they consume heat, if carried 

 out at a constant temperature). For reactions wliich occur with an 

 increase m free energy {AF > 0), the term endergonic has been suggested. 

 The total energy (or "heat effect") is a characteristic constant of a 

 chemical reaction (at a given temperature) ; while the free energy depends 

 on the concentrations of the reaction components and reaction products. 

 Photosynthesis is a strongly endothermal, and (with the usual concen- 

 trations of carl)on dioxide and oxygen), an even more strongly endergonic 

 process. 



Certain bacteria can live autotrophicully, without carrying out true 

 photosynthesis. Some of them sjmthesize organic matter, in the dark, 

 with the help of the free energy of unstable organic or inorganic chemical 



