Class 11 Biology Photosynthesis In Higher Plants The C4 pathway

The C4 pathway

  • Though these plants have the C4 oxaloacetic acid as the first CO2 fixation product, plants use the C3 pathway or the Calvin cycle as the main biosynthetic pathway.
  • C4 plants are special because
  • they have a special type of leaf anatomy,
  • they tolerate higher temperatures,
  • they show a response to highlight intensities,
  • they lack a process called photorespiration and have greater productivity of biomass.
  • The particularly large cells around the vascular bundles of the C4 pathway plants are called bundle sheath cells, and the leaves which have such anatomy are said to have ‘Kranz’ anatomy.
  • Kranz’ means ‘wreath’ and is a reflection of the arrangement of cells.
  • The bundle sheath cells may form several layers around the vascular bundles.
  • Bundle sheath cells are characterised by having a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces.
  • The presence of the bundle sheath would help to identify the C4
  • The pathway discovered by Hatch and Slack is called Hatch and Slack Pathway, which is again a cyclic process.
  • The primary CO2 acceptor is a 3-carbon molecule phosphoenol pyruvate (PEP) and is present in the mesophyll cells and the enzyme responsible for this fixation is PEP carboxylase or PEPcase.
  • The mesophyll cells lack RuBisCO enzyme and the C4 acid OAA is formed in the mesophyll cells.
  • It then forms other 4-carbon compounds in the mesophyll cells itself, which are transported to the bundle sheath cells.
  • In the bundle sheath cells, these C4 acids are broken down to release CO2 and a 3-carbon molecule.
  • The 3-carbon molecule is transported back to the mesophyll where it is converted to PEP again, thus, completing the cycle.
  • The CO2 released in the bundle sheath cells enters the C3 or the Calvin pathway.
  • The bundle sheath cells are rich in an enzyme ribulose bisphosphate carboxylase-oxygenase (RuBisCO), but lack PEPcase.
  • The Calvin pathway is common to the C3 and C4
  • In the C4 plants, it does not take place in the mesophyll cells but does so only in the bundle sheath cells.

 

Class_11_Biology_Photosynthesis_in_Plants_Hatch_Slack_Pathway

Fig. Hatch-Slack pathway

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