The C4 pathway is an alternative photosynthetic pathway that occurs in certain plants as a mechanism to increase carbon dioxide concentration around the enzyme Rubisco, thus enhancing photosynthetic efficiency. Unlike the C3 pathway, which occurs in the majority of plants, the C4 pathway involves additional biochemical steps and anatomical adaptations.

The Occurrence of the C4 Pathway:
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The C4 pathway occurs in plants known as C4 plants, which include various grasses (such as maize, sugarcane, and sorghum) as well as some dicotyledonous plants like Amaranthus and Chenopodium. These plants have evolved the C4 pathway as an adaptation to their environments, typically characterized by high temperatures and low carbon dioxide concentrations.

The Process of the C4 Pathway:
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1. Initial Fixation: In C4 plants, the initial fixation of carbon dioxide occurs in the mesophyll cells, where phosphoenolpyruvate (PEP) carboxylase catalyzes the conversion of carbon dioxide into a four-carbon compound called oxaloacetate (OAA).

2. Formation of Bundle Sheath Cells: Unlike C3 plants, C4 plants possess specialized bundle sheath cells surrounding the vascular tissue. The CO2 released from OAA is then fixed by Rubisco in the bundle sheath cells, leading to the formation of a three-carbon compound called 3-phosphoglycerate (PGA).

3. Regeneration of PEP: The remaining two carbons from OAA are converted back to phosphoenolpyruvate (PEP), which is then transported back to the mesophyll cells for the continuous cycling of carbon dioxide.

Advantages of the C4 Pathway:
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1. Concentration of Carbon Dioxide: The C4 pathway allows C4 plants to concentrate carbon dioxide around Rubisco, reducing the oxygenation reaction and minimizing photorespiration. This enhances their photosynthetic efficiency, especially under high temperatures and low carbon dioxide conditions.

2. Water Efficiency: C4 plants exhibit reduced transpiration rates due to their specific leaf anatomy, which helps in conserving water. This adaptation is particularly advantageous in arid and hot environments.

Conclusion:
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The C4 pathway occurs in certain plants as an alternative photosynthetic pathway. It involves additional biochemical steps and anatomical adaptations, allowing C4 plants to concentrate carbon dioxide and enhance photosynthetic efficiency. However, it is important to note that the occurrence of the C4 pathway is limited to specific plant species, and it does not occur six times like the cycles of the C3 pathway.

Then why do we multiple 5 ATP molecules by 6 in balance sheet of C4 cycle...?