Notes on the Path of Carbon in Photosynthesis



The metabolic process by which conversion of Co2 into carbohydrate takes place with utilization of assimilatory power produced during light reaction through production of a series of intermediate carbon compounds in photosynthesis is called path of carbon or C3 path ways.

This pathway is purely enzymatic reactions which is independent of light hence called as dark-reaction i.e. reaction can take place in absence of light. The whole process takes place inside the stoma of chloroplast and much slower than the primary photochemical or light reaction.

Path of carbon i.e. the intermediate carbon compounds formed during synthesis of carbohydrate in photosynthesis was traced out by using c24 radio isotope of carbon by Melvin Calvin (1961). Calvin and his co-workers were able to formulate the complete metabolic pathway of carbon assimilation in the form of a cycle metabolic pathway of carbon assimilation in the form of a cycle which called as Calvin cycle.

Various sequences of path of carbon assimilation are described under following phases":

i. Carboxylation phase.

ii. Hexose formation phase.

iii. Regeneration phase


a. Starting point for Co2 fixation is the formation of 5 carbon compound called ribulose 1, 2 disphoshate from ribulose monophospate in presence of phophoribulo kinase.

b. Ribulose, 1, 2 di-phospate is the initial compound which is the Co2 acceptor in the chloroplast. It accepts one molecule of Co2 and combines to produce a 6 vsrbon addition compound which is highly constable.

c. 6 - c addition compound react with water to produce stable two molecules of 3-C compounds named 3 - phospglyceeric acid.


a. 3 - Phospolyceric acid reduced to 3 - phosphoglyceral dehyde by the assimilatory power, generated during light reaction. This reduction taken place in presenc3e of enzyme triose phosphate dehydrogenase.

b. 3 phoshoglycerial dehyde isomerise to form dihydroxy acetone phoshate. One of the molecules of 3 phosphogyceral dehyde re unit with the dihydroxy acetone, phosphate forms 6 carbon compound named fructose 1, 6 - diphospate.


a. Fructopse 1, 6 diphoshate is phosprylated to form fructose 6 phospate in presence of phosphate enzyme.

b. Fructosee 6 phospate converted in fructose 1 phosphate which further converted into glucose -1. Phosphate and later converted into glucose as end product of photo synthesis.


Some molecules of 3 phospholyceral dehyde produced during early stages are diverted to regenerate ribulose 1.5 diphospate in following manner.

i. Two molecules of 3 phospolyceral dehyde react with molecules of fructose 6 phosphate of transketolate to form 2 molecules of erythose 4 phoshate and a molecule of xylusose 5 phosphates.

ii. 2 molecules of erythose 4 phosphate react with dihydroxy acetone phosphate in presence of enyme aldolase to produce 2 molecules of sedoheptulose 1, 7 diphospate.

iii. Sedoheptulose diphospate converted into monophosphate in presence of enyme phospatase.

iv. 2 molecules of sedoheptulose 7 phosphophate and 3 phoshoglyceraldehyde react in presence of transketoase enyme produce xylulose 5 phosphates and rinose 5 phosphates.

v. Isomweiation of xylulose 5 phospate takes place by epimerase ezyme into ribulose 5 phosphate.

vi. Riulose 5 phosphates react with ATP regenerate ribulose 1, 5 diphosphate which is the principal intermediate compound of Co2 fixation during photosynthesis.

Continuation of photosynthesis is maintained by the regeneration of initial Co2 accepter, Ribulose 1, 5 diphosphate.

Path of carbon in C4 plants in separate process as proposed by Hatch -Slack (1961). This pathway is an alternative way of Co2 fixation which only found in tropical grasses like sugarcane, maize, and sorghum. A triplex, althea rosea etc. Here phosphoenol pyruvate is the Co2 acceptor and path of carbon is intermediated by dicarboxylic acids.