SYNTHESIS OF CARBOHYDKATE 17 



(e) ^ ~^ - = ? = assimilation coefficient. 



^^^ {d - c) O2 



(Cf. respiratory quotient, see Chap. III.). 



They find that under all conditions of light the assimilation 

 coefficient is 1. That is, for every volume of CO2 assimilated, there 

 is evolved a volume of O2. In short, the chloroplast acts as a 

 machine for converting CO.2 into C and O2 which is evolved. 

 The carbon then unites with water to form some simple sugar: 

 — which one is not known. The final product is starch. The 

 process may be represented by the equation : 



xR^O + crCOg + light energy = C^Hg^O^ + xO^. 



It has been proved that the first step is the formation of 

 formaldehyde, i.e. a; = 1. 



(i) HgO + CO2 + 112,090 gram cals (?) := O2 + CHgO 



(formaldehyde). 



Formaldehyde is injurious to plant tissues, and it is rapidly 

 transformed into other products. 



If a^ = 2 then the formaldehyde would condense to form 

 glycollic aldehyde — a diose. 



(ii) CH2O + CH2O + energy — C2H4O2. 



Similarly, glucose could be formed. 



(iii) C2H4O2 + C2H4O2 + C2H4O2 + energy ^ Q^YL^f^^. 

 Two molecules of glucose combine to form maltose. 



(iv) C^HiaOg + CgHiaOe + 3.300 gram-calories 



;= t!l2H220ll + HoO. 



(v) The gums and dextrins are composed of condensed molecules 

 of maltose, e.g. 



C12H20O11 + C12H22O11 + H2O + energy z:; C24H4e023, 



and so on. 



(3) Carbon-dioxide and water have small energy contents (2-1 

 and 6-5 gram calories per gram respectively, and these amounts are 

 not available for food), while starch has a value of 4-1 large 

 Calorics per gram. It is evident, therefore, that in some way the 

 plant has converted a certain amount of kinetic energy into poten- 

 tial energy. Now, as the formula for starch is imcertain, let us 

 consider the amount of energy required to form glucose from COo 

 and HoO. Carbon dioxide and water are fullv oxidised. Theo- 

 reticallv, they may be considered as undergoing a process of 

 reduction before combining to form the aldehyde, but as the energy 

 evolved during reduction would be balanced by the energy absorbed 



