Short notes on C3 path way of Co2 fixation

The essentials of the Calvin cycle are as follows. The cycle should be self exclusive in that it should remain unaltered, but provide for the continuous entry of 6 molecules of CO₂, formation of one molecule of glucose and regeneration of six molecules of Ribulose 1.5 diphosphate (RUDP), the initial acceptor molecule. The important events of CO₂ cycle can be detailed as under:

a) Fixation of CO₂,

b) Reduction of PGA,

c) Formation of sugars, and

d) Regneration of RUDP.

Fixation of CO₂:

CO₂ is first accepted by molecules of a pentose (5C) sugar Ribulose 1 -5 diphosphate and an unstable 6 carbon compound is formed. This compound (tentatively called carboxy dimsutase system) immediately breaks up into 2 molecules of a triose (3C) called phosphoglyceric acid (PGA). The reaction is catalyzed by the enzyme carboxy dismutase or Rudp car­boxylase (Rubisco). For every molecule of CO₂, 2 mols of PGA are formed; hence 12 PGA moles for 6 CO₂ mols.

Reduction of PGA:

The molecules of PGA are reduced to phosphoglyceraldehyde (PGAI) by NADPH + H. The reaction requires energy and hence ATP participates. The enzyme triose phosphate dehydrogenase catalyses this reaction.

1.3 diphosphoglyceric acid + NADPH + H -> 3 PGAI + NADP

Since 6 molecules of CO₂ are taken into the system (by 4 mols of Rudp) jj PGAL mols are fomred after reduction of the 12 molecules of PGA. Onl two are utilized for the formation of sugars, while the rest are channeled back for the regeneration of Rudp molecules. Several intermediate compounds are formed in the process.

Regeneration of Rudp:

In the above explanation of formation of sugars, it has been pointed out that Rudp accepts a molecule of CO₂ to ultimately form a molecule of sugar. But the overall formula for photosynthesis accounts for the entry of 6 molecules of C0₂, hence there must be 6 molecules of Rudp to accept the CO₂ molecules and for every turn of the cycle, this should result in the formation of 6 molecules of glucose. But the output of the cycle for every 6 CO₂ molecules is only one molecule of glucose. Then what happens to the remaining 6 molecules of hexose (5 moles of 6C).

Obviously they undergo transformation to regenerate 6 molecules of Rudp. This is neces­sary because, the cycle should be self-sufficient and there must always be 6 molecules of Rudp ready to accept 6 molecules of CO₂. If all the six mol­ecules of Rudp combine with 6 molecules of CO₂ and form 6 molecules of glucose, the cycle will cease to operate; for the next turn of the cycle there
will be no Rudp to accept CO, molecules. Thus for the entry of every 6 Co molecules, the output is one mole of hexose and the rest go for the regeneration of Rudp.