Introductory Crop Physiology (1+1)

C4 cycle or Hatch and Slack pathway

C4 Cycle

C₄ Cycle or Hatch slack Pathway
It is the alternate pathway of C₃ cycle to fix CO₂. In this cycle, the first formed stable compound is a 4 carbon compound viz., oxaloacetic acid. Hence it is called C₄ cycle. The path way is also called as Hatch and Slack as they worked out the pathway in 1966 and it is also called as C₄ dicarboxylic acid pathway. This pathway is commonly seen in many grasses, sugar cane, maize, sorghum and amaranthus.
The C₄ plants show a different type of leaf anatomy. The chloroplasts are dimorphic in nature. In the leaves of these plants, the vascular bundles are surrounded by bundle sheath of larger parenchymatous cells. These bundle sheath cells have chloroplasts. These chloroplasts of bundle sheath are larger, lack grana and contain starch grains. The chloroplasts in mesophyll cells are smaller and always contain grana. This peculiar anatomy of leaves of C₄ plants is called Kranz anatomy. The bundle sheath cells are bigger and look like a ring or wreath. Kranz in German means wreath and hence it is called Kranz anatomy. The C₄ cycle involves two carboxylation reactions, one taking place in chloroplasts of mesophyll cells and another in chloroplasts of bundle sheath cells.
There are four steps in Hatch and Slack cycle:

1. Carboxylation
2. Breakdown
3. Splitting
4. Phosphorylation



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The C4 pathway is known to operate in two types of cells:

1. Chloroplasts of mesophyll cells.
2. Chloroplasts of bundle sheath cells.


1. Carboxylation
It takes place in the chloroplasts of mesophyll cells. Phosphoenolpyruvate, a 3 carbon compound picks up CO₂ and changes into 4 carbon oxaloacetate in the presence of water. This reaction is catalysed by the enzyme, phosphoenol pyruvate carboxylase.



2. Breakdown
Oxaloacetate breaks down readily into 4 carbon malate and aspartate in the presence of the enzyme, transaminase and malate dehydrogenase.

These compounds diffuse from the mesophyll cells into sheath cells.



3. Splitting
In the sheath cells, malate and aspartate split enzymatically to yield free CO2 and 3 carbon pyruvate. The CO₂ is used in Calvin’s cycle in the sheath cell.

The second carboxylation occurs in the chloroplast of bundle sheath cells. The CO₂ is accepted by 5 carbon compound ribulose diphosphate in the presence of the enzyme, carboxydimutase and ultimately yields 3 phosphoglyceric acid. Some of the 3 phosphoglyceric acid is utilised in the formation of sugars and the rest regenerate ribulose diphosphate.

4. Phosphorylation
The pyruvate molecule is transferred to chloroplasts of mesophyll cells where, it is phosphorylated to regenerate phosphoenol pyruvate in the presence of ATP. This reaction is catalysed by pyruvate phosphokinase and the phophoenol pyruvate is regenerated.



In Hatch and Slack pathway, the C₃ and C₄ cycles of carboxylation are linked and this is due to the Kranz anatomy of the leaves. The C₄ plants are more efficient in photosynthesis than the C₃ plants. The enzyme, phosphoenol pyruvate carboxylase of the C₄ cycle is found to have more affinity for CO₂ than the ribulose diphosphate carboxylase of the C₃ cycle in fixing the molecular CO₂ in organic compound during carboxylation.

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