The initial product of carbon dioxide fixation in some of the tropical and subtropical plants is malic acid or malate. Malate is 4-C compound. Therefore these plants are called C-4 plants. This cycle is also called Hactch —Slack cycle. C.1 cycle takes place in bundle sheath cells. These are the sheath of cells around the vascular bundles of the leaf. These cells contain a large number of chloroplasts. These chloroplasts contain enzyme PEP carboxylase. Rubisco is absent in them.


Following reactions take place during C4 cycle:



  1.  The CO, reacts with phosphoenol pyruvic acid (PEP) to form oxaloacetic acid. This reaction takes place in the presence of the enzyme phosphoenol pyruvate carboxylase (PEP carboxylase).
  1. The NADPH, reduce oxaloacetic acid into malate (malic acid). This reaction is catalyzed by malate dehydrogenase.
  2. The malate is transported to the bundle-sheath cells. It releases CO, to from pyruvate (pyruvic acid). The CO, released is used in the Calvin cycle
  3. Pyruvic acid reacts with ATP. It regenerates phosphoenol pyruvate (PEP). Thus it completes the cycle. Not included in syllabus)

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