158 TEXTBOOK OF BOTANY 



of intermediate steps in which successive transformations of materials 

 and energy occur. The hberation of heat energy is just a consequence of 

 this series of energy transformations in which some of the intermediate 

 steps are the essential ones. For instance, the electrical potentials char- 

 acteristic of all living active cells are dependent upon continuous 

 oxidation-reduction processes in the cells. The formation of the proto- 

 plasmic fats and proteins, tlie pigments, enzymes, and hundreds of other 

 compounds in cells is likewise dependent upon the energy of respiration. 

 The intermediate steps by which these compounds are formed are much 

 too complicated to be discussed here. 



All the organic compounds of plants in which some of the carbon 

 atoms are either more oxidized or less oxidized than the carbon atoms 

 in the molecule of sugar, have been formed from sugar or from some of 

 its derivatives by oxidation-reduction processes and are, to that extent, 

 products of respiration. With these we may also include all the organic 

 compounds of plants that contain reduced nitrogen or reduced sulfur. 

 All the processes and compounds we recognize in living cells are the 

 results of series of intermediate processes and unstable compounds. Some 

 of these intermediate compounds are highly unstable; and unless they 

 are continuously renewed by oxidation-reduction processes the other 

 processes that we recognize in living cells will cease. The essential 

 features of respiration appear to be in the intermediate transformations 

 of energy and materials by which these unstable compounds are formed. 



Summary. Respiration is an oxidation-reduction process by which the 

 chemically bound energy in food is transformed to other kinds of 

 energy upon which certain processes (chemical transfomiations ) in all 

 living cells are dependent. The best known of these processes are those 

 in which some of the food is converted to the partially reduced and 

 partially oxidized compounds of which the protoplasm, parts of cell 

 walls, pigments, enzymes, and other fundamental cell substances are 

 composed. Some of the energy involved in the formation of these sub- 

 stances remains bound within them as long as they exist, but usually a 

 much larger amount of the energy that is released by respiration eventu- 

 ally escapes from the plant as free heat energy. This free heat energy, 

 together with the escape of carbon dioxide, the formation of water, and 

 the loss in dry weight, should be regarded as an accessory consequence 

 of respiration in plants. The escape of heat energy is the only one of the 

 three that is known to occur in all cases of respiration, and there is no 

 evidence that it is of anv essential value to the plant. The kinds of 



