Sunlight is the ultimate source of energy. Plants use this light energy to prepare chemical energy (food) during the process of photosynthesis. The whole process of photosynthesis takes place in two phases- photochemical phase and biosynthetic phase. Photochemical phase is the initial stage where ATP and NADPH for the biosynthetic phase are prepared. In biosynthetic phase, the end product glucose is produced. Let us focus more on pathways in biosynthetic phase.
During the biosynthetic phase, carbon dioxide and water combine to give carbohydrates i.e. sugar molecules. This reaction of carbon dioxide is termed as carbon fixation. Different plants follow different pathways for carbon fixation. Based on the first product formed during carbon fixation there are two pathways: C3 pathway and C4 pathway.
C3 pathway / Calvin Cycle
The majority of plants produce a 3-carbon acid called 3-phosphoglycericacid (PGA) as a first product during CO2 fixation. Such pathway is known as C3 pathway which is also called Calvin cycle. This cycle occurs in three steps: carboxylation, reduction, and regeneration.
In the first step, the two molecules of 3-phosphoglycericacid (PGA) are produced with the help of the enzyme called RuBP carboxylase. In the following steps, the ATP and NADPH phosphorylate the 3-PGA and ultimately produces glucose. Then the cycle restarts again by regeneration of RuBP.
C-4 Pathway/ Hatch and Slack Pathway
Every photosynthetic plant follows Calvin cycle but in some plants, there is a primary stage to Calvin cycle known as C4 pathway. Plants in tropical desert regions commonly follow this pathway. Here a 4-carbon compound called oxaloacetic acid (OAA) is the first product of carbon fixation. Such plants are special and have certain adaptations as well.
The C4 pathway initiates with a molecule called phosphoenol pyruvate (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 (PEP case). 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 Calvin cycle whereas 3-C molecule is transferred back to mesophyll cells for regeneration of PEP.
Calvin pathway is a common pathway in both C3 plants and C4 plants but it takes place only in the mesophyll cells of the C3plants but not in the C4plants. To know more about C
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