Dark reaction – Photosynthesis
- Also, known as the C3 cycle as the 1st stable compound produced is a 3C compound Phosphoglyceric acid (PGA).
- Occurs in the chloroplast of mesophyll cells.
- Initial acceptor of CO2 is a 5C compound Ribulose bisphosphate (RuBP).
- Carboxylation occurs with the help of Rubisco enzyme.
6RuBP + 6CO2 →12 3-PGA.
- Reduction: ATP & NADPH produced during light reaction are used to convert PGA to Glyceraldehyde-3-phosphate (G3P).
12 3-PGA + 12 ATP + 12NADPH → 12G3P + 12ADP + 12iP + 12NADP+.
- Formation of glucose: by using 2 molecules of G3P.
- Regeneration of 6RuBP: by using10 molecules of G3P & 6ATP
- To fix a molecule of CO2, 3ATP & 2NADPH are required.
- Six Calvin cycles i.e., 6 CO2, 18ATP & 12NADPH are required to form a molecule of sugar.
- Overall reaction:
6RuBP + 6CO2 + 18ATP + 12NADPH → C6H12O6 + 6H2O + 18ADP + 18iP + 12NADP+ + 6RuBP.
- Rubisco is inefficient at low CO2
- Hence photorespiration occurs at high temperature & high O2 concentration decreasing the rate of photosynthesis.
- Rubisco is the most abundant protein on earth.
- Optimum temperature is 20 – 25o
- CO2 compensation point is 30 -70 ppm.
Hatch & Slack pathway:
- Also, known as C4 cycle as the 1st stable compound is a 4C compound Oxaloacetic acid (OAA).
- C4 cycle occurs in tropical plants e.g., Sugarcane, Maize, Sorghum
- Leaves of these plants show Kranz anatomy.
- It occurs in the chloroplast of mesophyll cells & bundle sheath cells.
- Initial acceptor of CO2 is a 3C compound Phosphoenolpyruvate (PEP).
- Carboxylation occurs in mesophyll cells with the help of PEP carboxylase enzyme to form OAA.
- Malic acid is formed from OAA & is transferred to the bundle sheath cells through plasmodesmata.
- Malic acid undergoes oxidative decarboxylation to form Pyruvic acid.
- The CO2 released from Malic acid enters the Calvin cycle to form sugar. Rubisco occurs in bundle sheath cells.
- Regeneration of PEP: Pyruvic acid is transferred to mesophyll cells & is used to regenerate back 6PEP, 12ATP are utilized.
- It utilizes 30ATP in total.
- PEP carboxylase has a high affinity for CO2 at low concentration, hence photorespiration does not occur.
- CO2 compensation point is 10 ppm.
- Optimum temperature is 35 – 45o
- Crassulacean acid metabolism occurs in members of family Crassulaceae, these are xerophytic plants. E.g., Opuntia (cactus).
- Stomata in these plants open at night & closes during the day to reduce the rate of transpiration.
- During night: CO2 fixation occurs.
CO2 reacts with PEP to form OAA. OAA is converted in Malic acid & is stored in vacuoles.
- During day: Malic acid is released from vacuoles.
Also, read Photophosphorylation (Light reaction)
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