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.