SYNTHESIS OF CARBOHYDRATE 17 



They find that under all conditions of light the assimilation 

 coefficient is 1. From this it may be inferred that C not CO 2 is 

 being built into the leaf. In short, the chloroplast acts as a 

 machine for converting CO 2 into C and O 2 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 : 



*H 2 +#C0 2 +light energy ^C^H^ +zO 2 . 



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

 formaldehyde, i.e. ccl. 



(i) H 2 O +CO 2 +112,090 gram cals (?)^O 2 +CH 2 O 



(formaldehyde). 



Formaldehyde is injurious to plant tissues, and it is rapidly 

 transformed into other products. 



If x =2 then the formaldehyde would condense to form gly collie 

 aldehyde a diose. 



(ii) CH 2 +CH 2 +energy^C 2 H 4 O 2 . 



Similarly, glucose could be formed. 



(iii) C 2 H 4 2 +C 2 H 4 2 +C 2 H 4 2 +energy^C 6 H 12 O 6 . 

 Two molecules of glucose combine to form maltose. 



(iv) C f H 12 O 6 +C 6 H 12 O 6 +3300 gram-calories 



^C 12 H 22 O n +H 2 0. 



(v) The gums or dextrins are composed of condensed mole- 

 cules of maltose, e.g. 



C 12 H 22 O n +C 12 H 22 O n +H 2 +energy^C 24 H 46 23 , 



and so on. 



(3) The energy value of CO 2 is 2-1 cals. per gram, that of H 2 O 

 is 6-5, while starch has a value of 4191 gram cals. It is evident, 

 therefore, that in some way the plant has converted a certain 

 amount of kinetic energy into potential energy. Now, as the 

 formula for starch is uncertain, let us consider the amount of 

 energy required to form glucose from CO 2 and H 2 O. Carbon 

 dioxide and water are fully oxidised. Theoretically, they may 

 be considered as undergoing a process of reduction before com- 

 bining to form the aldehyde, but as the energy evolved during 

 reduction would be balanced by the energy absorbed during 

 formation, we may limit our problem to the total energy change 

 according to the two equations given above, viz. (i) and (iii). 



