320 PHOTOSYNTHESIS 



The chemical equation representing the process of photosynthesis may be 

 written as follows : 



6 CO2 + 6 H2O + 673 kg.-cal. -> CeHisOe + 6 O2 



of radiant energy 



As shown in this equation six molecules each of water and carbon dioxide 

 combine in the production of each molecule of hexose sugar, six moiCCules 

 of oxygen being released as a by-product. For each mol of hexose synthesized 

 radiant energy equivalent to 673 kg.-cal.^ is converted into the chemical 

 energy of sugar molecules. 



It is inconceivable that such a complex chemical process as photosynthesis 

 could take place in one simple step as this equation would seem to indicate. 

 Possible intermediate steps in the process will be considered later. This equa- 

 tion is to be regarded as only a convenient summary statement in the short- 

 hand of chemical symbols. 



Leaf Anatomy in Relation to Photosynthesis. — In the vascular plants 

 photosynthesis occurs chiefly in the leaves, which in the majority of species 

 are thin, expanded organs possessing a large external surface in proportion 

 to their volume. This type of structure permits the display of a large number 

 of chloroplast-containing cells to light in proportion to the volume of the leaf. 

 The labyrinth of intercellular air passages in the interior of the leaf is so 

 extensive that practically every green cell is in contact with the internal 

 atmosphere of the leaf. As a result of this loose cellular structure the internal 

 leaf surface (surface of the leaf cells in contact with the intercellular spaces) 

 is much greater than the surface of the epidermal cells exposed to the outside 

 atmosphere. In a lilac leaf, for example, the internal surface is about thirteen 

 times as great as its external surface (Turrell, 1936). Most of the carbon 

 dioxide absorbed by the mesophyll cells diffuses into the cells from the inter- 

 cellular spaces rather than directly from the outside atmosphere. The presence 

 of intercellular spaces in a leaf therefore provides a much more extensive 

 carbon dioxide absorbing surface than if this gas were absorbed directly 

 through the external leaf surfaces. Since the walls of all of the cells within 

 a leaf are normally more or less saturated with water the vapor pressure 

 of the internal air spaces is usually higher than that of the outside atmosphere. 

 This makes it possible for the leaf cells to absorb atmospheric carbon dioxide 

 without being exposed to the usually relatively dry external atmosphere. 

 Whenever the stomates are open the internal atmosphere of the intercellular 



^ This is the quantity of energy required if glucose is the only sugar synthe- 

 sized. As the later discussion shows, it is not known with certainty what the 

 first sugar of photosynthesis is. Approximately the same quantity of energy is 

 required, however, for the synthesis of any of the hexoses. 



