The products of light reaction are ATP, NADPH and O2. The oxygen diffuses out of the chloroplast. ATP and NADPH are used to drive the processes which lead to the synthesis of food. These processes depend on the products of light reaction; apart from being dependent on CO2 and H2O.
The Calvin cycle can be described under three stages: carboxylation, reduction and regeneration.
Carboxylation: Fixation of CO2 into a stable organic intermediate is called carboxylation. In this step, carbon dioxide is utilised for the carboxylation of RuBP. The enzyme RuBP carboxylase catalyses this reaction. The reaction results in the formation of two molecules of 3-PGA. This enzyme is also called RuBP carboxylase-oxygenase or RuBisCO; because it also has an oxygenation activity.
Reduction: This step involves utilization of 2 molecules of ATP for phosphorylation and two of NADPH for reduction of each CO2 molecule fixed. For the removal of one molecule of glucose from the pathway, fixation of six molecules of CO2 and 6 turns of the cycle are required.
Regeneration: This step involves regeneration of CO2 acceptor molecule RuBP. This is necessary for the cycle to continue without interruption. This step requires one ATP for phosphorylation to form RuBP.
Thus, for each CO2 molecule entering the Calvin Cycle, 3 molecules of ATP and 2 of NADPH are required. Cyclic phosphorylation probably takes place to meet this difference in number of ATP and NADPH used in the dark reaction.
Plants which are adapted to dry tropical regions use the C4 pathway. While 3-PGA is the first carbon fixation product in C3 plants, it is oxaloacetic acid (4 carbon atoms) which is the first carbon fixation product in C4 plants. However, the main biosynthetic pathway remains the Calvin cycle; as in C3 plants.
Kranz Anatomy: In C4 plants, large cells are found around the vascular bundles. These are called sheath cells. Leaves have special anatomy; called Kranz anatomy. The bundle sheath cells may form several layers around the vascular bundles. The cells are characterized by large number of chloroplasts, thick walls (impervious to gaseous exchange) and no intercellular spaces.
This pathway is known as Hatch and Slack Pathway. It happens in following steps.
Phoshpenol pyruvate (PEP) which is a 3-carbon molecule is the primary CO2 acceptor. It is present in the mesophyll cells. The enzyme PEP carboxylase or PEPcase is responsible for this fixation. RuBisCO enzyme is absent in mesophyll cells. Oxaloacetic acid (OAA) is formed in the mesophyll cells.
Then other 4-carbon compounds; like malic acid or aspartic acid are formed in the mesophyll cells. These are then transported to the bundle sheath cells. These C4 acids are broken down in the bundle sheath cells; to release CO2 and a 3-carbon molecule.
The 3-carbon molecule is transported back to the mesophyll. In the mesophyll, it is converted into PEP again. Thus, the cycle is completed.
The CO2 released in the bundle sheath cells enters the Calvin cycle. The bundle sheath cells are rich in RuBisCO but lack PEPcase.
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