The second part or phase-II of photosynthesis is known as the dark reactions. This is because, the reactions in this phase do not require light. These reactions are thermo-chemical and depend on temperature.

The dark reaction involves thermo-chemical reduction of carbon dioxide to form carbohydrates. This was first established by Blackmann (1905), hence it is also called Blackmann reaction. Presence of light is not necessary for the reduction of carbon dioxide. However, the dark reaction utilizes the products of light reaction (i.e., the assimilatory power (ATP and NADPH₂) to reduce carbon dioxide into carbohydrates.)

All the reactions in phase-II are completed in the stroma of the chloroplast. These are biochemical reactions in which every step is controlled by a specific enzyme. The reactions begin with the fixation of carbon dioxide, involve the utilization of the assimilatory power for the reduction of carbon dioxide and end with the formation of carbohydrate, which is the end product of photosynthesis.

The details of the steps involved in the dark reaction (i.e., the complete path of carbon) were worked out by Professor M. Calvin (who received the 1961 Nobel Prize for the same) and hence, the dark reaction came to be called as Calvin cycle.

(1) This is the major pathway for the fixation of carbon dioxide in green plants.

(2) It represents phase-II, i.e., the dark reaction of photosynthesis.

(3) It takes place in the stroma of the chloroplasts.

(4) The reactions are enzyme-controlled and temperature dependent.

(5) It was discovered by Professor M. Calvin and therefore called the Calvin cycle.

(6) After the fixation of carbon dioxide, the first stable compound formed is 3-carbon phosphoglyceric acid (PGA). Hence it is also called the C₃ pathway.

The important events of the Calvin cycle can be studied under the following heads.

(a) The first CO₂ acceptor and fixation of CO₂ : In the Calvin cycle, a 5-C pentose sugar, ribulose diphosphate (RUDP) acts as the first acceptor of CO₂ .

On entering the reactions of the Calvin cycle, CO₂ first combines with 5-C RUDP (fixation of CO₂) to form an unstable and unknown 6-C disphosphate compound. This compound immediately breaks into 2 molecules of 3-C phosphoglyceric acid (PGA). The enzyme carboxy dismutase catalyzes the reaction.

Thus, 3-C PGA is the first stable compound formed in Calvin cycle. Hence, it is also called C-3 pathway.

(b) Utilization of assimilatory power (or reduction of PGA). The 3-C PGA then undergoes reduction with the help of the assimilatory power to form 3-C phosphoglyceraldehyde (PGAL). NADPH₂ provides the hydrogen and ATP supplies energy for the reduction. Enzyme triosephosphate dehydrogenase catalyzes the reaction. Some molecules of PGAL are converted into another triose phosphate called dihydroxyacetone phosphate (DHAP) in presence of enzyme triose phosphate isomerase.

(c) Formation of sugars (end products of photosynthesis) : The 3-C triose phosphates( i.e., PGAL (3-C) and DHAP (3-C)) form 6-C hexose sugar fructose.1, 6 diphosphate in the presence of enzyme aldolase. Fructose diphosphate is then dephosphorylated first to fructose mono-phosphate and then to fructose (6-C) in the presence of enzyme phosphatase. Some fructose monophosphate molecules may be isomerized into glucose monophosphate by enzyme isomerase and then into glucose (6-C).

The hexose sugars may be further converted to sucrose (C₁₂H₂₂O₁₁) or to starch (C₆H₁₀O₅)n and stored in storage cells.

(d) Regeneration of the CO2 acceptor, RUDP : The 5-C RUDP is constantly required for the fixation of CO₂ in the Calvin cycle. It is regenerated through another chain of reactions. Some molecules of triose phosphates and fructose mono phosphates are used from the Calvin cycle for the formation of RUDP.

Chapter 5