Notes on the C4 Cycle of Dark Reaction of Photosynthesis

Till 1965 it was believed that Calvin cycle was the only path way of CO2 fixation in photosynthesis. In 1957 kortschak and co-workers reported synthesis of a 4-C organic acid as the first stable product of photosynthesis in sugar cane.

In 1967 two Australian scientists of the north plant research centre, Queensland namely M,O. Hathch and C.R. slack thoroughly investigated the complete pathway in these plants where the first stable product of CO2 fixation was a $-C compound. This path way was knows as C4 cycle because the first stable product was a 4-C compound. It is also knows as hatch slack pathway in honour of the two scientists.

The plants which exhibit this cycle are knows as C4 plants. The common example of C4 plants are tropical grasses, sugar cane, maize cynodon etc.

The anatomy of C4 leaves is known as kanz anatomy. In this case the leaves have two types of cells – the mesolphyll cells and the bundle sheath cells. The bundle sheath cells are single leyered and aurround the vescular bundles. They contain few large chloroplasts and lack grna. On the other hand the mesophyll cells contain large number of normal chloroplasts. They lack enzymes of Calvin cycle and donot contain starch.

Mechanism: The steps involved in the C4 path way are as follows:

1) In the mesophyll cells the C4 cycle occurs, the primary acceptor of CO2 is a 3-C compound phosphoenol pyrvic acid. It combines with CO2 in presence of the enzymephosphoenol pyrvate carboxylase to form a 4-c compound oxalo acetic acid. It is the first stable product of c4 pathway.

2) Oxaloacetic acid is then reduced to lalic acid using NANDPH produced during light reaction. The reaction is catalysed by the enzyme malic-dehydrogenase.

3) Sometimes the oxaloacetic acid is converted to saspatic acid by a transmynation reaction. However aspatic acid has no role in the cycle.

4) The malic acid formed in mesoph7ull cell is transported to bundle sheath cells where they are decarboxylated in presence of NANDP sheath cells where they are decarboxylated in presence of specific malic enyme tto produce pyruvic acid.

5) The co2 so liberated by decarboxylation of malic acid is accepted by ribulose 1, 5 disphophate and enters the Calvin cycles.

6) The pyrvic acid formed in the bundle sheath cells is transported back to mesophyll cells where they are phosporylated in presence of ATP produced in light reaction to form phospoenol pyruvic acid in presence of enzyme pyruvate phosphate dikinase.

Thus the phospoenol pyrvic acid is regenerated which can take part again in the cycle.

Significance:

1) The C4 plants can absorb CO2 from a low concentration of CO2.

2) It requires more light light energy for photosynthesis.

3) Photorespiration does not take place.

4) The plants are better adapted to deserts.

Differences Between C3 and C4 Cycle:

C3 CYCLE

1) The CO2 acceptor is ribulose 1, 5 diphosphate (RUDP).

2) The first stable product is phophoglyceric acid (PGA).

3) Enzyme catalysing carboxylation is rubisco or carboxy dismutase.

4) Photo-respiration occurs.

5) Optimum temperature is 10′ 25’C.

C4 CYCLE

1) The CO2 acceptor is phoshenol pyruvic acid (PEPA).

2) The first stable procduct is wxalo acetic acid (OAA).

3) Enzyme catalysing carbxylation is phosphoenol pyruvate carboxylase.

4) Photorepiration does not occur.

5) Optimum temperature is 30′ – 45’C.