Dark reaction – Photosynthesis

Calvin-Benson cycle:

• Also, known as the C₃ cycle as the 1^st 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).

1. Carboxylation occurs with the help of Rubisco enzyme.

6RuBP + 6CO₂ →12 3-PGA.

2. 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⁺.

3. Formation of glucose: by using 2 molecules of G3P.
4. 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 – 25^o 
• CO₂ compensation point is 30 -70 ppm.

Hatch & Slack pathway:

• Also, known as C₄ cycle as the 1^st 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).

1. Carboxylation occurs in mesophyll cells with the help of PEP carboxylase enzyme to form OAA.
2. Malic acid is formed from OAA & is transferred to the bundle sheath cells through plasmodesmata.
3. Malic acid undergoes oxidative decarboxylation to form Pyruvic acid.
4. The CO₂ released from Malic acid enters the Calvin cycle to form sugar. Rubisco occurs in bundle sheath cells.
5. 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 – 45^o 

CAM pathway:

• 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.

It undergoes oxidative decarboxylation. CO₂ released enters Calvin cycle to form sugar.

Also, read Photophosphorylation (Light reaction)