[Chap. XIII SYNTHESIS OF SUGAR— PHOTOSYNTHESIS 109 



creased and the amount of carbon dioxide increased, both in hght and 

 in darkness. The increase in oxygen and decrease in carbon dioxide in 

 the surrounding air occurred only when some green tissue of the plant 

 was exposed to light. When animals and non-green plants were used in 

 these experiments, the results were like those obtained for roots and 

 other non-green parts of the plant. How was one to account for this 

 unique effect of the green tissues of plants? 



The phenomenon was not satisfactorily explained until after the 

 middle of the 19th century. So little was known about the substances 

 and processes within plants and about the factors in the environment of 

 plants, that numerous facts had to be discovered bv experimentation 

 before any of the various hypotheses proposed could be eliminated or 

 converted into a scientificallv established conclusion. The facts finallv 

 established mav be represented brieflv bv the following equation:^ 



Light energy + Water + Carbon dioxide > Sugar (bound energy) + Oxygen 



Light energy + 6 H2O + 6 CO2 > CgHizOe + 6 O2 



This equation indicates the more obvious material and energy trans- 

 formations that occur in the making of sugar. It represents the initial 

 substances used and the final products formed, but it does not indicate 

 the intermediate reactions that occur. Free energy of light is transformed 

 to potential energy in the sugar. For every 180 grams of sugar made, 674 

 Calories of light energy are transformed to chemicallv bound energy. 

 One gram of sugar contains about 3.75 Calories of chemically bound 

 energy. 



Photosynthesis. This process of making sugar in the green parts of 

 plants is called photosynthesis (Greek, photos, light; and synthesis, put- 

 ting together), because light is necessary for this building of large mole- 

 cules (CiiHii'Oti) through the chemical union of smaller ones (COj 

 and H2O).- 



^ One reservation to this statement should be made. The equation indicates that a 

 hexose sugar (the molecule being composed of 6 atoms of carbon, 12 atoms of hydrogen, 

 and 6 atoms of oxygen) is formed. This is generally considered to be glucose, though the 

 known facts are not sufficient to exclude fructose entirely, at least in some cases. These 

 two sugars are the only simple sugars found widely distributed in detectable amounts as 

 free sugars in plants. Either of them may be formed from the other one in plants. See 

 the structural formulas in the footnote on p. 104. Some in\ estigators ha\ e suggested sucrose 

 (C12H00O11) as the first sugar made, but the evidence is not con\incing. 



^ The older term, carbon assimilation, is still used by some writers; but, as we shall see 

 later, the term assimilation is also used to designate the conversion of food into the 

 substances of which cells are composed, a process that occurs in all plants and animals, 

 whereas the process we are describing here occurs only in the green tissues of plants. 

 Furthermore, the term carbon assimilation is a heritage of the early misconceptions of plant 



