Photosynthesis is the biological process by which all green plants, photosynthetic bacteria and other autotrophs convert light energy into chemical energy. In this process, glucose is synthesised from carbon dioxide and water in the presence of sunlight. Furthermore, oxygen gas is released out into the atmosphere as the byproduct of photosynthesis.
The balanced chemical equation for the photosynthesis process is as follows:
6CO2 + 6H2O —> C6H12O6 + 6O2
Sunlight is the ultimate source of energy. Plants use this light energy to prepare chemical energy during the process of photosynthesis.
C4 Pathway (Hatch and Slack Pathway)
Every photosynthetic plant follows Calvin cycle but in some plants, there is a primary stage to the Calvin Cycle known as C4 pathway. Plants in tropical desert regions commonly follow the C4 pathway. Here, a 4-carbon compound called oxaloacetic acid (OAA) is the first product by carbon fixation. Such plants are special and have certain adaptations as well.
The C4 pathway initiates with a molecule called phosphoenolpyruvate (PEP) which is a 3-carbon molecule. This is the primary CO2 acceptor and the carboxylation takes place with the help of an enzyme called PEP carboxylase. They yield a 4-C molecule called oxaloacetic acid (OAA).
Eventually, it is converted into another 4-carbon compound known as malic acid. Later, they are transferred from mesophyll cells to bundle sheath cells. Here, OAA is broken down to yield carbon dioxide and a 3-C molecule.
The CO2 thus formed is utilized in the Calvin cycle whereas 3-C molecule is transferred back to mesophyll cells for regeneration of PEP.
Corn, sugarcane and some shrubs are examples of plants that follow the C4 pathway. Calvin pathway is a common pathway in both C3 plants and C4 plants but it takes place only in the mesophyll cells of the C3 Plants but not in the C4 Plants.