The Calvin cycle (C3-cycle) or PCR-cycle can be divided into three stages:
(a) Car-boxylation, during which atmospheric CO2 combines with 5-C acceptor
molecule ribulose 1, 5-bisphosphate (RuBP) and converts it into 3-phosphoglyceric
acid (3-PGA);
The CO2 is accepted by ribulose 1, 5-bisphosphate (RuBP) already present in the cells
and a 6-carbon addition compound is formed which is unstable. It soon gets hydrolysed
into 2 molecules of 3-phosphoglyceric acid (3PGA). Both these reactions take place in
the presence of ribulose bisphosphate carboxylase (Rubisco). 3-Phosphoglyceric acid is
the first stable product of dark reaction of photosynthesis.
(b) Reduction, which consumes ATP + NADPH (produced during primary
photochemical reaction) and converts 3-PGA into 3-phosphoglyceraldehyde
(3PGAld) or triose phosphate (TRI- OSE-P); and
3-Phosphoglyceric acid is reduced to 3-phosphoglyceraldehyde by the assimilatory
power (generated in light reaction) in the presence of triose phosphate dehydrogenase.
This reaction takes place in two steps:
(c) Formation of hexose sugar and regeneration of RuBP which consumes additional
ATP, so that the cycle continues (Fig. 11.18).
i)
Some of the molecules of 3-phosphoglyceraldehyde isomerise into
dihydroxyaeetone phosphate, both of which then unite in the presence of the
enzyme aldolase to form fructose 1, 6-bisphophate.
ii) Fructose 1, 6-bisphosphate is converted into fructose 6-phosphate in the presence of
phosphatase.
iii) Some of the fructose-6-phosphate (hexose sugar) is tapped off from the Calvin cycle
and is converted into glucose, sucrose, and starch. Sucrose is synthesized in cytosol
while starch is synthesized in chloroplast.
iv) Some of the molecules of 3-phosphoglyceraldehyde produced in step (ii) instead of
forming hexose sugars, are diverted to regenerate ribulose 1, 5-bisphosphate in the
system as follows:
v) 3-Phosphoglyceraldehyde reacts with fructose-6-phosphate in the presence of enzyme transketolase to form erythrose-4-phosphate (4-C atoms sugar) and xylulose 5phosphate (5-C atoms sugar).
vi) Erythrose-4-phosphate combines with dihydroxyaceotone phosphate in the
presence of the enzyme aldolase to form sedoheptulose 1, 7-bisphosphate (7-C atoms
sugar).
viii) Sedoheptulose 1, 7-bisphosphate loses one phosphate group in the presence of
phosphatase to form sedoheptulose-7-phosphate.
(ix) Sedoheptulose-7 phosphate reacts with 3-phosphoglyceraldehyde in the presence
of transketolase to form xylulose-5-phosphate and ribose-5-phosphate (both 5-carbon
atoms sugars).
(x) Xylulose-5-phosphate is converted into another 5-C atoms sugar ribulose-5phosphate in the presence of the enzyme phosphoketopentose epimerase.
(xi) Ribose-5-phosphate is also converted into ribulose-5-phosphate. The reaction is
catalysed by phosphopentose isomerase.
(xii) Ribulose-5-phosphate is finally converted into ribulose 1, 5-bisphosphate in the
presence of phosphopentose kinase and ATP, thus completing the Calvin cycle.
Structural formulae of various 4, 5 and 7-C atoms sugars involved in the Calvin cycle are
given Fig. 11.19.
Because first visible product of this cycle is 3-phosphoglyceric acid which is a 3-C
compound, Calvin cycle is also known as C3-pathway.
C4 pathway
(i) The first step involves the carboxylation of phosphoenol pyruvic acid in chloroplasts of
mesophyll cells to form C-4 dicarboxylic acid, oxaloacetic acid. This reaction is catalysed by
phosphoenol pyruvate carboxylase.
(The CO2 is first dissolved in water in cytoplasm and ionised into HCO3– probably under the
influence of carbonic anhydrase. The resulting HCO–3 (bicarbonate ion) is then used in
carboxylating phosphoenol pyruvic acid).
(ii) Oxaloacetic acid readily equilibrates with other C4-dicarboxylic acids, aspartic acid and
malic acid in the presence of enzymes transaminase and NADP+ specific malate
dehydrogenase respectively.
(iii) From chloroplasts of mesophyll cells the malic acid is transferred to the chloroplasts of
bundle sheath cells where it is decarboxylated to form CO2 and pyruvic acid in the presence
of a NADP+ specific malic enzyme.
(iv) Now, second carboxylation occurs in chloroplast of bundle sheath cells. Ribulosebisphosphate accepts CO2 produced in step (iii) in the presence of Rubisco (i.e., RuBP-carboxylase) and ultimately yields 3-phosphoglyceric acid as in case of Calvin cycle. Some of
the 3-phosphoglyceric acid is utilised in the formation of hexose monophosphates, sucrose
and starch, while rest regenerates ribulose-bisphosphate in the system (see Calvin cycle).
(v) The pyruvic acid produced in step (iii) is transferred to chloroplasts of mesophyll cells
where it is phosphorylated to regenerate phosphoenol pyruvic acid. This reaction is catalysed
by pyruvate Pi kinase.
The AMP is phosphorylated by ATP under the catalytic influence of the enzyme adenylate
kinase to form 2 ADP molecules.
(These ADPs may be converted back into ATPs in the light reaction of photosynthesis).
i. The enzymes catalysing reactions in chloroplasts of mesophyll cells are not found in
chloroplasts of bundle sheath cells and vice versa.
ii. Hatch-Slack pathway begins with the carboxylation of phosphoenol pyruvate and not of
ribulose-bisphosphate. It is because the former has great affinities with CO2 than the latter.
iii. In contrast to the C4 plants, the other higher plants lack Hatch-Slack pathway and have
only Calvin cycle (C3-pathway) for the fixation of CO2 in photosynthesis. These are called as
C3 plants and have only one type of chloroplasts.
The C4 mode of photosynthesis (Hatch-Slack pathway) is less efficient in itself in comparison
to the C3 mode (Calvin cycle in C3 plants). It is because the fixation of 1 CO2 mol. in C3
mode of photosynthesis requires 2NADPH + 3ATP molecules while in C4 mode of
photosynthesis 2NADPH + 5ATP molecules are required (additional 2 ATPs are needed in
reaction no. v) for the fixation of 1CO2 mol.
However, C4 plants are more efficient photosynthetically than C3 plants because of the
absence (or negligible presence) of photorespiration in C4 plants. Thus, net requirement of
ATP + NADPH per CO2 mol. fixed (i.e. resultant of photosynthesis minus photorespiration)
is considerably lower in C4 plants than in C3 plants.