Dark reaction – Photosynthesis


Dark reaction – Photosynthesis

Calvin-Benson cycle:

  • 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).
  1. Carboxylation occurs with the help of Rubisco enzyme.

6RuBP + 6CO2 →12 3-PGA.

  1. 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+.

  1. Formation of glucose: by using 2 molecules of G3P.
  2. 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
  • 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).
  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

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)


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