C4 Pathway: Steps, Differences and FAQ

What Is C4 Pathway?

The C4 pathway is one of the metabolic procedures in photosynthesis that allows high efficiency in photosynthesis in certain plants, mainly of tropical origin and those growing in hot, dry climates. First explained by M. D. Hatch and C. R. Slack in 1966, this process allows plants to fix carbon dioxide better than the plants having the C3 pathway in certain environmental conditions. This paper provides information about the mechanisms, steps and the importance of C4 pathway

Hatch And Slack Pathway

The C4 pathway starts by mainly fixing carbon dioxide with phosphoenolpyruvate in the mesophyll cells. This step is catalyzed by the PEP carboxylase. The first reaction produces a four-carbon compound that is oxaloacetic acid. It is then diffused into the bundle sheath cells for further fixation in the Calvin cycle

C4 Cycle

In the C4 cycle, the first stable product that is formed is the oxaloacetic acid. The wide distribution of the C4 pathway can be noted in some families of plants including chenopodiaceae, Gramineae and Cyperaceae. All these belong to both dicots and monocots.

Hatch And Slack Pathway In C4 Plants

C4 is the alternate route to the C3 cycle in carbon fixation. The process gets its name "C4 cycle" from the first compound formed, which is a four-carbon molecule: oxaloacetic acid. This pathway dominates most grasses, including maize and sugarcane; the characteristic anatomy of the leaves in this pathway is called Kranz anatomy.

Anatomy Of Kranz

In leaves of C4 plants, each vascular bundle or rib is enclosed with a bundle sheath composed of larger parenchymatous cells. The bundle sheath cells have larger chloroplasts, lacking intergranal lamellae, and starch grains, while the mesophyll cells have small chloroplasts with grana. The anatomy is thus especially suited to increase carbon fixation efficiency due to the arrangement, so it is called Kranz anatomy from the German for "wreath".

Steps Of The C4 Cycle

The C4 cycle takes four steps as follows:

1. Carboxylation

Carbon dioxide is captured in the chloroplasts of mesophyll cells by the three-carbon compound, phosphoenolpyruvate, producing oxaloacetic acid. This is made possible through the action of an enzyme called PEP carboxylase.

2. Breakdown

The newly formed oxaloacetic acid is reduced by two enzymes, namely transaminase and malate dehydrogenase, to form malate and aspartate, respectively. These C4 products diffuse out from the mesophyll cells into the bundle sheath cells.

3. Cleavage

This will be further cleaved by the bundle sheath cell enzymes to release free carbon dioxide and three-carbon pyruvate. The released CO₂ here will diffuse in the Calvin cycle, get combined with RuBP and get reduced into the formation of 3-PGA.

4. Phosphorylation

The formed pyruvate molecules diffuse back into the mesophyll cells and in the presence of ATP get phosphorylated to re-generate phosphoenolpyruvate, which is catalyzed by the enzyme pyruvate phosphokinase.

Importance Of The C4 Pathway

The C4 pathway thus confers an increased efficiency in carbon fixation—especially when photorespiration is high. From the energy point of view, it yields high growth rates for C4 plants in certain biomes as a consequence of reduced losses due to RuBisCO's oxygenase reaction. There are broad implications for agriculture also, with scientists field-testing whether key commercial crops can be genetically altered to produce C4 plants.

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