110 TEXTBOOK OF BOTANY 



Chemical reactions that are brought about bv hght are often referred 

 to as photochemical reactions — reactions initiated by the collision of 

 photons (units of radiant energy) with some part of a molecule. There 

 are, of course, several kinds of photochemical reactions; but by tradition 

 in botany the term photosynthesis usually refers to the making of sugar 

 from COi- and Hi>0 in the chlorophyll-containing cells of plants. 



Through the chemical union of 6 molecules of water with 6 molecules 

 of carbon dioxide 1 molecule of sugar is formed, and the excess 6 mole- 

 cules of free oxygen are set free. For every molecule of CO2 used, one 

 molecule of O- is released. From the principle first formulated bv 

 Avogadro — that equal volumes of gases under the same conditions of 

 temperature and pressure contain the same number of molecules — we 

 may infer that the volume of CO- consumed in photosynthesis is equal 

 to the volume of O2 liberated. This inference has been verified bv experi- 

 mentation. The comparative weights of the materials used and formed 

 during photosynthesis may also be indicated: 



264 gm. CO2 -\- 108 gm. H2O > 180 gm. CeHiaOe + 192 gm. O2 



Obviously if one knows the amount of COj used in photosynthesis, he 

 can compute the amount of sugar made and the amount of oxygen set 

 free. Similarly, if he knows the amount of oxygen set free or the amount 

 of sugar made, he can calculate the amount of any one of the other sub- 

 stances involved in the process. How can one calculate the amount of 

 energy that is chemically bound during photosynthesis? 



It should be noted that tlie material and energy transformations that 

 occur in the making of sugar are the con\'erse of those that occur in the 

 burning of sugar. If we regard the burning of sugar as an oxidation 

 process, we may regard photosynthesis as a reduction process. It is the 

 primary energy-storing process of the organic world. 



Oxidation processes are often regarded as exothermic ( heat-releasing ) , 

 and reduction processes as endothermic ( heat-storing ) . These terms are 

 quite appropriate when transformations of heat energy only are involved. 

 They are quite confusing, however, when these processes of living cells 

 result in transformations to other forms of energy, as, for instance, 

 radiant energy to chemically bound energy, chemically bound energy to 



nutrition. In order to avoid confusion of terms we shall refer to the above process as photo- 

 synthesis. The term carbon fixation is of course entirely ruled out, because carbon is 

 already "fixed" in CO„ before the process begins. If we must substitute another term for 

 photosynthesis in botany, it should be "sugar syntliesis." 



