can you give me a short note on calvin cycle and the C4 pathway in plants...?
The path of carbon in the dark reaction was traced by Melvin Calvin using radioactive carbon (14C). The Calvin pathway occurs in all photosynthetic plants, irrespective of whether they have C3 or C4 pathway.
The primary acceptor of CO2 was found to be a 5-carbon ketose sugar called Ribulose bisphosphate (RuBP). This was surprising as it was believed that for the formation of a 3-carbon product, CO2 would have to be accepted by a 2-carbon compound.
RuBP is used in a cyclic manner (regenerated) and a sugar is synthesised.
3 stages of Calvin cycle: Carboxylation, Reduction and Regeneration
- Ribulose 1, 5-bisphosphate combines with CO2, and fixes it to a stable organic intermediate (3-phosphoglycerate – 2 molecules).
- Reaction catalysed by the enzyme RuBisCO (RuBP Carboxylase-Oxygenase)
- 3 PGA is the first stable product of this cycle.
- Here, 2 molecules each of ATP and NADPH are required for fixing 1 molecule of CO2.
- This stage contains a series of reactions.
- Glucose is formed as a result of this series of reactions.
- RuBP regenerates to enable the cycle to continue uninterrupted.
- 1 ATP molecule is required.
C4 Pathway (Hatch and Slack Pathway)
- Occurs in plants like maize, sugarcane – plants adapted to dry tropical regions
- The leaves of C4 plants have Kranz anatomy. They have special large cells around their vascular bundles called bundle sheath cells. These cells form several layers around the vascular bundles and have large number of chloroplasts, thick walls impermeable to gaseous exchange, and lack intercellular spaces.
- First CO2 fixation product is a 4-carbon compound called oxaloacetic acid, but C3 cycle is used as the main biosynthetic phase.
- Mesophyll cells: Occur in the C4 cycle
- Bundle Sheath Cells: Occur in the C3 cycle
- C4 plants can tolerate high temperature and high light intensity, show greater productivity of biomass, and lack photorespiration.
- Primary CO2 acceptor: Phosphoenol pyruvate (PEP) – a 3-carbon molecule
- PEPcase (PEP Carboxylase) fixes CO2 in the mesophyll cells. It forms the 4-carbon compound oxaloacetic acid (OAA), and then other 4-carbon compounds like malic acid and aspartic acid.
- These compounds are transported to the bundle sheath cells. There, C4 acid breaks down to form C3 acid and CO2 (released only in the bundle sheath cells, and enters the C3 cycle).
- C3 acid, so formed, is again transported to the mesophyll cells and regenerated back into PEP.
- C3 cycle cannot directly occur in the mesophyll cells of C4 plants because of the lack of the enzyme RuBisCO in these cells
- RuBisCO is found in abundance in the bundle sheath cells of C4 plants. They lack PEPcase.