90 BIOLOGICAL CHEMISTRY 



In studying this process we can begin with comparatively 

 simple observations. A leaf taken from a plant, which has 

 been kept in the dark, after decolorisation and on treatment 

 with iodine shows very little or no starch. After Exposure 

 to light most leaves show an accumulation of starch. Those 

 leaves that do not form starch show the presence of other 

 carbohydrates, mainly ^-glucose. This illustrates the necessity 

 of light for the energy accumulation, and it can be shown that 

 only the green parts of plants accomplish this synthesis in a 

 measurable degree. 



If, however, the leaf be exposed to light in an atmosphere 

 free from carbon dioxide no formation of carbohydrate will 

 occur. If the leaf be kept in a closed space containing a 

 known amount of carbon dioxide the amount of carbon dioxide 

 in the space decreases and the amount of oxygen increases. 



The reverse process occurs when the leaf is kept in the 

 dark : the carbohydrate is used up, oxygen disappears and 

 carbon dioxide accumulates. The latter is the process of 

 respiration, and it will be studied in a later chapter. The 

 processes of photosynthesis and respiration may not proceed 

 by the same intermediate steps and they cannot be said to 

 be strictly reversible, but the energy exchanges are reversible. 



by synthesis 



6CO 2 + 6H 2 O -f 738,000 calories* ^1 C 6 H 12 O 6 +6O 2 . 



by respiration - 



This reaction shows an accumulation of energy equivalent 

 to 4100 calories for every gram of carbohydrate formed, 

 and a similar amount can be liberated by the decomposition 

 of the carbohydrate. As already mentioned this energy comes 

 from the sunlight. 



In studying this process we can gain some insight into the 

 mechanism by finding out what conditions affect the change, 

 and to what degree they affect it. By altering one factor at 

 a time and by keeping the others constant the influence of 

 the various factors can be determined. We shall consider 

 the following factors : (i) Concentration of substrate ; 

 (2) Intensity of illumination and quality of the light ; (3) 

 temperature, f 



* A calorie (c) is the amount of heat required to raise i g. of water 

 from 15 C.-i6 C. A large calorie (C) = 1000 c. 

 C 6 H i26 = 180 g. glucose .'. for i g. glucose the energy value is 



73 00 = 4100 calories or 4-1 C. 

 180 



JF. F. Blackman and G. L. C. Matthaei, Proc. Roy. Soc., 1905, B 

 ?6 p. 402. 



