C4 Pathways  – Class 11 | Chapter – 13 | Biology Short Notes Series PDF

C4 Pathways: M. D. Hatch and C. R. Slack first outlined this metabolic pathway in depth. In this, the carbon dioxide is added first to the phosphoenolpyruvate by the action of the enzyme PEP carboxylase. Thus, producing the four-carbon compound in the mesophyll cells, which is transported later on to the bundle sheath cells to release the carbon dioxide to be used in the Calvin cycle.

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In 1966, Hatch and Slack discovered the C4 cycle, hence the name. It is also referred to as the ß-carboxylation pathway and co-operative photosynthesis. The 4-carbon oxaloacetic acid is the first stable compound of the Hatch and Slack cycle, hence is called the C4 cycle.

C4 plants are plants possessing a C4 cycle. Such plants are inclusive of dicots and monocots, the C4 cycle is evident in the Chenopodiaceae, Gramineae and Cyperaceae family.

Hatch and Slack C4 Pathways in Plants

To fix carbon dioxide, this pathway is the alternate to the C3 cycle. Here, the first formed stable compound – oxaloacetic acid is a 4 carbon compound, hence the name C4 cycle. This pathway is a common sight in several grasses, maize, sugarcane, amaranthus, sorghum. The C4 plants depict a different kind of leaf anatomy (Kranz anatomy).

The chloroplasts are dimorphic and in the leaves, vascular bundles are wrapped by a bundle sheath of larger parenchymatous cells. Such bundle sheath cells possess chloroplasts, which are larger, containing starch grains, lacking grana while the chloroplasts in the mesophyll cells always possess grana and are smaller. The bundle sheath cells appear as a wreath or a ring while cells are larger. This characteristic leaf anatomy of the C4 plants is referred to as Kranz Anatomy. In German, Kranz corresponds to the wreath, hence the name Kranz Anatomy.

The C4 Cycle depicts two carboxylation reactions occurring in the chloroplasts of the mesophyll cells and others in the chloroplast of the bundle sheath cells. The Hatch and Slack Cycle involves four steps –

  • Carboxylation
  • Breakdown
  • Splitting
  • Phosphorylation

Carboxylation

Occurs in the chloroplasts of the mesophyll cells. A 3-carbon compound, Phosphoenolpyruvate, collects carbon dioxide and in the presence of water, transforms to 4 carbon oxaloacetate. The enzyme phosphoenolpyruvate carboxylase catalyzes the reaction.

 

 

 

Breakdown

Readily, oxaloacetate disintegrates into 4 carbon malate and aspartate. The enzyme involved in the reaction is transaminase and malate dehydrogenase. The compounds formed diffuse into the sheath cells from the mesophyll cells.

 

 

 

Splitting

The malate and aspartate in the sheath cells enzymatically split to produce free carbon dioxide and 3-carbon pyruvate. The carbon dioxide is made use of Calvin’s cycle in the sheath cells. The second carboxylation takes place in the chloroplasts of the bundle sheath cells. The carbon dioxide is accepted by the 5-carbon compound ribulose diphosphate with the activity of the carboxy dismutase enzyme, finally producing 3 phosphoglyceric acid. For the formation of sugars, some of the 3 phosphoglyceric acid is used and the remaining regenerates ribulose diphosphate.

 

 

Phosphorylation

The pyruvate molecules are moved to the chloroplasts of the mesophyll cells wherein, in the presence of ATP, it is phosphorylated for the regeneration of phosphoenolpyruvate. Pyruvate phosphokinase catalyzes the reaction and phosphoenolpyruvate is regenerated.

In this pathway, the C3 and C4 cycles of the carboxylation are associated as a result of the Kranz anatomy of the leaves. Compared to C3 plants, the C4 plants are more efficient in photosynthesis. Phosphoenolpyruvate carboxylase enzyme of the C4 cycle is seen to possess more affinity for carbon dioxide compared to ribulose diphosphate carboxylase of the C3 cycle when it comes to fixing molecular carbon dioxide in the organic


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