170 THE INDIVIDUAL ORGANISM 



has been made in its study in recent years, especially since radioactive 

 tracer isotopes of carbon, hydrogen, and oxygen have become available 

 as tools of investigation. 1 



It is now known that chlorophyll is not a single substance but a group 

 of related substances, just as are many of the vitamins and hormones. 

 At least 10 different chlorophylls have been differentiated. Two of these, 

 chlorophyll a and chlorophyll b (C55H 72 05N 4 Mg and CssHToOe^Mg 

 respectively) are present in all the higher plants and the green algae. 

 Furthermore, it has been found that chlorophyll is not solely responsible 

 for photosynthesis but is a part of a system involving at least two addi- 

 tional catalysts; that the reaction occurs in steps, one requiring light and 

 the other(s) not; and that 12 molecules of water instead of 6 take part 

 in the reaction 



6C0 2 + 12H 2 + Energy -> C 6 H 12 6 + 6H 2 + 60 2 



Water does not combine directly with carbon dioxide to form sugar but is 

 decomposed to liberate the free oxygen of the right-hand side of the 

 equation and to furnish hydrogen that reduces the C0 2 to sugar and 

 water. Beyond this point we need not go, but enough has been said to 

 indicate the nature of the problems encountered when we try to discover 

 how photosynthesis works. 



Any cell containing chlorophyll is capable of manufacturing glucose; 

 but the greater part of the work of photosynthesis is carried on by the 

 parenchyma of the leaves. A leaf is an organ especially adapted for 

 efficient utilization of the energy of light in the synthesis of glucose. 

 The required water is brought from the roots through the xylem con- 

 duits. The carbon dioxide is almost wholly taken from the air through 

 the stomata, though small amounts may be derived from the soil, and a 

 part is released within the plant by oxidation of previously elaborated 

 food. As the carbon dioxide is used up in the cells, that in the inter- 

 cellular spaces diffuses into the cells, and additional quantities diffuse 

 through the stomata into the air spaces. 



The absorption area of the plant is so great that, although air contains 

 only about 3 parts in 10,000 of carbon dioxide, relatively great quantities 

 of this gas can be obtained from the atmosphere. Thus a moderately 

 large oak tree may weigh, when dry, as much as 14,000 pounds. About 

 4 per cent of this dry weight consists of nitrogen and other materials 

 derived from the soil, and about 60 per cent is hydrogen and oxygen. 

 The remaining 36 per cent, or about 5,200 pounds, consists of carbon, 

 originally taken in as carbon dioxide. This amount does not represent 



1 Results of experiments with radioactive isotopes, published in 1949, indicate that 

 in certain simple plants the first free carbohydrate to appear following photosynthesis 

 is sucrose, not glucose or fructose. The significance of this observation is still uncertain. 



