C4 Pathway

A carbon fixation route that is most typically seen in low-latitude plants, such as grasses, that have evolved to high temperatures and high light intensity. The four-carbon chemical oxalo-acetate (OAA) is the first product generated as a consequence of the carboxylation of an acceptor molecule, phosphoenolpyruvate (PEP), by CO2 . OAA is transformed to a four-carbon molecule that exits the mesophyll cells right below the chlorophyll-containing leaf cells, travels through plasmodesmata (spaces between cells), and enters bundle-sheath cells densely packed around the leaf veins. There, the complex releases its CO2 and enters photosynthesis’ light-independent stage.

A Study on c4 pathway

Now we will learn more about C4 pathway, the first product of C4 pathway, C3 and C4 plants and finally will read the difference between C3 and C4 pathway.

C4 pathway

A metabolic route in which CO2 is first added to phosphoenolpyruvate by the enzyme PEP carboxylase, resulting in a four-carbon molecule that is then carried to bundle sheath cells, where the CO2 is liberated for utilisation in the Calvin cycle.

The C4 pathway comprises processes that first convert pyruvate to phosphoenolpyruvate (PEP), which then binds with CO2 to generate a four-carbon molecule, rather than direct carbon fixation as in the Calvin cycle (hence the name C4). As a result, the inefficient CO2 loss associated with the C3 carbon fixation pathway is decreased, and photorespiration is avoided.

Plants that initially go through the C4 route are more suited to drought, high temperatures, and low nitrogen or CO2 concentrations than plants that only go through the C3 pathway.

Bundle sheath cells and mesophyll cells work together to form the C4 pathway. –

1. The phosphoenolpyruvate carboxylase enzyme catalyses the production of oxaloacetate from phosphoenolpyruvate and carbon dioxide in the mesophyll cell.

2. In the presence of the malate dehydrogenase enzyme, oxaloacetate is converted to malic acid (a C4 molecule).

3. Through plasmodesmata, the malic acid is subsequently transported to the bundle sheath cell.

4. Malic acid is decarboxylated in the presence of the malic enzyme in the bundle sheath cell, releasing carbon dioxide.

5. Pyruvate is generated by the decarboxylation of malic acid, a three-carbon molecule. The Calvin cycle takes in carbon dioxide, and pyruvate is returned to the mesophyll cells.

6. Pyruvate is converted to phosphoenolpyruvate in the presence of pyruvate phosphate dikinase enzyme in the mesophyll cell, and the cycle continues.

The first product of C4 pathway

C4 plants are so named because the initial result of CO2 fixation is oxaloacetate, a C4 organic acid produced by PEP carboxylase carboxylation of phosphoenolpyruvate (PEP). The C4 acid cycle, which supports the CO2 concentrating mechanism of C4 photosynthesis, has three substrates: oxaloacetate (OAA), malate, and aspartate (Asp). 

OAA is the immediate, short-lived result of the first CO2 fixation step in C4 leaf mesophyll cells in this cycle. The organic acids transferred to the sites of carbon reduction in the bundle-sheath cells of the leaf, where they are decarboxylated, with the liberated CO2 required to build carbohydrates, are malate and asp, which arise from the fast conversion of OAA.

C3 and C4 plants

C3

The C3 pathway takes its name from the first molecule generated in the cycle, 3-phosphoglyceric acid (a 3-carbon molecule). The Calvin Cycle is used by around 85 percent of all plants on Earth to fix carbon. During the one-step process, the enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase) begins an oxidation reaction, resulting in part of the energy utilised in photosynthesis to be lost via a process known as photorespiration. Growth occurs when the soil temperature is between 4 and 7 degrees. As a consequence, the quantity of carbon fixed by the plant and released back into the environment as carbon dioxide is reduced by around 25%. Wheat, oats, rice, sunflower, and cotton are among the most often planted crops.

C4

For the dark response of photosynthesis, C4 plants employ the C4 pathway, also known as the Hatch-Slack Pathway. These are arid-loving plants that grow in warm climates. The result of the C4 cycle is Oxaloacetic acid, a four-carbon molecule. C4 plants make up around 5% of all plants on the planet. They grow in tropical climates, and these plants are plentiful. Kranz anatomy may be found in leaves of these plants, also even when the stomata are close together, it accomplishes photosynthesis. Amaranthus, maize, and sugarcane are some examples of these.

Difference between C3 and C4 pathway

The Calvin cycle creates a three-carbon molecule from C3 photosynthesis, while the Calvin cycle produces an intermediate four-carbon complex from C4 photosynthesis, which splits into a three-carbon compound for the Calvin cycle. C3 photosynthesis occurs in the majority of plants, but C4 photosynthesis occurs in just around 3% of vascular plants, such as crabgrass, sugarcane, and maize. C3 plants have no specific traits to resist photorespiration, but C4 plants do so by executing carbon dioxide fixation and the Calvin cycle in distinct cells.

Conclusion

In this article we read about the C4 pathway, the first product of the C4 pathway, C3 and C4 plants and finally learned about the difference between C3 and C4 pathway. In hot conditions, C4 plants have a growth advantage over C4 plants due to lower photorespiration rates. Sugarcane plants and crabgrass thrive in a hot, humid tropical environment and employ the C4 pathway to fix CO2 . This system also serves as a method for accumulating significant levels of carbon dioxide in the bundle sheath cells’ chloroplasts. As a consequence of the greater quantity of internal carbon dioxide in these chloroplasts, the ratio of carboxylation to oxygenation increases, reducing photorespiration.