512 



RESPIRATION 



TABLE 48 COMPARISON OF THE TOTAL ENERGY RELEASE OF ONE KILOGRAM OF WHEAT SEED- 

 LINGS AND OF THE ENERGY DIRECTLY MEASURABLE AS HEAT (oATA OF DOYER, I9I5) 



Energy becomes manifest in living cells as well as in inorganic systems 

 as chemical energy, heat energy, radiant energy, surface energy, mechanical 

 energy, potential energy, etc. All of the chemical energy released from mole- 

 cules during respiration represents radiant energy which was previously en- 

 trapped in the process of photosynthesis. Upon release this energy may be 

 transformed into any of the kinds listed above. 



The present state of our knowledge makes it impossible to say precisely 

 in what manner all of the energy released in respiration which does not ap- 

 pear as heat is utilized in plant cells. A supply of respiratory energy is indis- 

 pensable in the maintenance of living cells, and at least some of the ways 

 in which this energy is utilized have been recognized. The synthesis of fats, 

 amino acids, and many of the other metabolic products of plant cells requires 

 the energ}^ of respiration. Other energy-requiring processes occurring in plant 

 cells include migration of chromosomes and translocation of other cell con- 

 stituents during cell division, streaming of the protoplasm, growth of stems 

 in opposition to the pull of gravity, growth of root tips through the soil, 

 maintenance of differences of electrical potential in plant tissues and organs, 

 and the accumulation of ions or molecules by plant cells (Chap. XXIV). 

 Most, if not all of the energy utilized in these processes is made available by 

 the process of respiration. 



However, not all plant processes rely upon the energy derived from respira- 

 tion for their motive power. Transpiration, for example, is essentially a 

 modified evaporation process, and the energy utilized in the vaporization of 

 water mostly comes either directly from the radiant energy of sunlight, or 

 from the heat energy of the surrounding atmosphere. 



In addition to its fundamental importance as an energy-releasing process, 

 respiration apparently plays at least one other important role in plant metabol- 



