C3 and C4 cycles........
Asked by harinderamit1234
20th September 2018,
9:32 PM
Answered by Expert
Answer:
The C3 cycle or Calvin cycle comprises a series of reactions controlled by enzymes. In the Calvin cycle, the first product of CO2 fixation is a 3C compound called phosphoglyceric acid. It takes 6 turns of the Calvin cycle to generate one molecule of glucose. Each Calvin cycle is completed in three phases—carboxylation, reduction and regeneration
Carboxylation
- During carboxylation, CO2 combines with ribulose 1,5-biphosphate to produce an intermediate compound, which splits in the presence of water to form two molecules of 3-phosphoglyceric
acid (PGA).
Reduction
- During reduction or the glycolytic reversal phase, carbohydrate is formed at the expense of the photochemically made ATP and NADPH.
Regeneration
- During regeneration, the CO2 acceptor ribulose 1,5-biphosphate is formed again so that the cycle is continuous.
- C4 plants carry out double fixation of CO2—initial and final.
- The product of initial fixation in mesophyll cells is transported to bundle sheath cells for final fixation.
Initial Fixation
- Carbon dioxide is first fixed in the mesophyll cells to form a 3C compound—phosphoenol pyruvic acid (PEP), leading to the formation of a 4C compound, oxaloacetic acid (OAA).
- OAA is reduced to malic acid or transaminated to form aspartic acid.
Transport
- Malic acid or aspartic acid is transported to bundle sheath cells through plasmodesmata.
Final Fixation
- It occurs in two steps:
i. Release of CO2
ii. Carbon assimilation
Release of CO2
- Inside the bundle sheath cells, malic acid is decarboxylated and aspartic acid is deaminated through the following reactions:
CO2 Assimilation
- CO2 released in bundle sheath cells is fixed through the Calvin cycle in which RUBP is the secondary or final acceptor of CO2 in C4 plants.
Regeneration of PEP
- Pyruvic acid or alanine formed in the bundle sheath cells return to the mesophyll cells.
- Alanine is deaminated through a transamination reaction to form pyruvic acid.
- Pyruvic acid is converted to phosphoenol pyruvic acid and inorganic phosphate with the help of ATP
Answered by Expert
21st September 2018,
9:01 AM
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