The C3 cycle occurs in algae, bryophyta, pteridophyta and gymnosperms. The C3 cycle occurs in most dicots and some monocots. C3 cycle occurs in plants like rice, wheat, barley/sorghum, mango, yam, jackfruit, litchi, banana, jute, algae, bryophyta, pteridophyta, gymnosperms etc.

The acronym for ribulose bisphosphate carboxylase or oxygenase enzyme is rubisco enzyme. Rubisco enzyme is the most important enzyme in the world. These enzymes create chemical bonds between the natural world and living organisms. Hence it is called a bridge or bonding enzyme. 50% or more of the total protein in leaves is rubisco enzyme. The amount of rubisco enzyme in the world is about 40 million tons. With the help of this enzyme, 100 billion tons of CO2 are converted into carbohydrates every year on earth.

1. Carboxylation
(i) Ribulose 1, 5 bisphosphate reacts with atmospheric CO2 to produce 6-carbon (C6) keto acids under the action of rubisco enzyme. Keto acid is rapidly broken down to 3-phosphoglyceric acid. (RuBisCO is the most important enzyme in the world)
2. Phosphorylation
(ii) 3-phosphoglyceric acid is converted to 1,3-bisphosphoglyceric acid with the help of the enzyme phosphoglycerokinase. At this time, ATP takes part in the reaction and produces ADP.
3. Reduction
(iii) Phosphoglyceraldehyde dehydrogenase enzyme produces 3-phosphoglyceraldehyde and dihydroxyacetone phosphate from 1,3 bisphosphoglyceric acid. At this time, NADPH+H+ takes part in the reaction to form NADP.
4. Carbohydrate production
(iv) Fructose 1, 6 bisphosphate is produced from 3-phosphoglyceraldehyde by the action of aldolase enzyme.
(v) Fructose 1, 6 bisphosphate is converted to fructose 6-phosphate in the presence of phosphofructokinase enzyme. During this reaction, ADP takes part and produces ATP.
(vi) Glucose 6-phosphate is produced from fructose 6-phosphate under the action of phosphoglucoisomerase enzyme.
(vii) Glucose is produced from glucose 6-phosphate under the action of hexokinase enzyme. At this time, ADP takes part in the reaction to form ATP.
5. Regeneration of RuBP
(viii) 3-phosphoglyceraldehyde reacts with fructose 1,6 bisphosphate to produce ribulose 5 phosphate.
(ix) Ribulose 1, 5-bisphosphate is produced from ribulose 5-phosphate. Ribulose 1, 5 bisphosphate re-enters the Calvin-Basham cycle and keeps the cycle going.

1. Carboxylation
(i) Ribulose 1, 5 bisphosphate reacts with atmospheric CO2 to produce 6-carbon (C6) keto acids under the action of rubisco enzyme. Keto acid is rapidly broken down to 3-phosphoglyceric acid. (RuBisCO is the most important enzyme in the world)
2. Phosphorylation
(ii) 3-phosphoglyceric acid is converted to 1,3-bisphosphoglyceric acid with the help of the enzyme phosphoglycerokinase. At this time, ATP takes part in the reaction and produces ADP.
3. Reduction
(iii) Phosphoglyceraldehyde dehydrogenase enzyme produces 3-phosphoglyceraldehyde and dihydroxyacetone phosphate from 1,3 bisphosphoglyceric acid. At this time, NADPH+H+ takes part in the reaction to form NADP.
4. Carbohydrate production
(iv) Fructose 1, 6 bisphosphate is produced from 3-phosphoglyceraldehyde by the action of aldolase enzyme.
(v) Fructose 1, 6 bisphosphate is converted to fructose 6-phosphate in the presence of phosphofructokinase enzyme. During this reaction, ADP takes part and produces ATP.
(vi) Glucose 6-phosphate is produced from fructose 6-phosphate under the action of phosphoglucoisomerase enzyme.
(vii) Glucose is produced from glucose 6-phosphate under the action of hexokinase enzyme. At this time, ADP takes part in the reaction to form ATP.
5. Regeneration of RuBP
(viii) 3-phosphoglyceraldehyde reacts with fructose 1,6 bisphosphate to produce ribulose 5 phosphate.
(ix) Ribulose 1, 5-bisphosphate is produced from ribulose 5-phosphate. Ribulose 1, 5 bisphosphate re-enters the Calvin-Basham cycle and keeps the cycle going.

1. ATP: ATP produced by cyclic and acyclic photophosphorylation provides the energy required for the Calvin cycle.
2. NADPH+H+ : The acyclic photophosphorylation process produces NADPH+H+ which acts as a catalyst in the cycle.
3. CO2 : Atmospheric CO2 plays a special role in the production of sugars in this cycle.
4. RuBP: RuBP combines with CO2 to initiate the Calvin cycle.

1. The CO2 consumer in the Calvin cycle is 1,5-ribulose bisphosphate.
2. The first stable compound produced in this cycle is 3-phosphoglyceric acid with three carbons.
3. This cycle occurs at higher concentrations of CO2 (50-150 ppm).
4. The cycle occurs at low temperatures (10-250C).
5. The cycle occurs only in mesophyll chloroplasts. ie occurs in the stroma of chloroplasts.
6. This happens in low light.
7. It mainly occurs in cold climate plants.
8. The process occurs in C3 and C4 plants.
9. The CO2 fixing enzyme in this cycle is Rubisco.
10. Photorespiration occurs in plants during this cycle.
11. The rate of photosynthesis of plants in this cycle is low.

Professor M. Calvin of the University of California in the United States, along with his two colleagues Bessham and Benson Benson, described the process of carbon assimilation in the light neutral stage of photosynthesis, called the Calvin-Besham cycle. The first stable compound produced in this cycle is the three-carbon 3-phosphoglyceric acid and is called the C3 cycle. They discovered this cycle in Chlorella and Scenedesmas algae in 1947 using radioactive carbon (14C) (Tracer method). They won the Nobel Prize in Medicine and Physiology in 1961 for their discovery.

The stage of photosynthesis that does not require light is called the dark or light neutral stage. At this stage CO2 is oxidized to produce sugar, so it is called carbon oxidation reaction or carbon oxidation pathway or carbohydrate making process or carbon oxidation stage. In this process, ATP and NADPH+H+ are called energy compounds for carbon assimilation. Modern scientists have discovered three pathways of carbon assimilation in the photoneutral phase using methods such as radioactive isotopes, autoradiography, paper chromatography etc. Calvin-Basham or C3 cycle, Hatch-Slack or C4 cycle and CAM cycle.

In the process of photophosphorylation, the high energy electrons (e-) released from chlorophyll molecules are carried through different carriers, lose energy, become dull and are transferred to another chlorophyll molecule without returning to that chlorophyll molecule is called acyclic photophosphorylation. In 1964 scientists Hill and Bendall explained the reactions of this process by Z diagram. Hence it is called Z scheme reaction. The process of acyclic photophosphorylation is-
(i) Both photosystem-I and photosystem-II also participate in the acyclic photophosphorylation process. Photosystem-II absorbs light energy from the sun. This light energy is transferred to the reaction center P-680.
(ii) Two high energy electrons (2e-) are emitted from P-680. Two electron carriers are accepted by plastoquinin (PQ).
(iii) Two electrons from plastoquinin come to cytochrome-f (Cyt. f). At this time the two electrons lose some energy. Under the influence of this energy, ATP is produced by combining ADP and Pi.
(iv) Electrons then pass from two cytochrome-f to photosystem-I through plastocyanin. Because previously two electrons have been released from photosystem-I. As a result there is a shortage of electrons. In this way photosystem-I regains its lost electrons.
(v) Photosystem-I absorbs light energy from the sun. This light energy is transferred to the reaction center P-700.
(vi) Two high energy electrons (2e-) are emitted from P-700. Electrons are accepted by the two carriers ferridoxine (Pd). Electrons from ferridoxine enter two NADP-reductases.
(vii) During this period photolysis of H2O molecules takes place in the presence of light to produce O2, 2H+ and 2e-. O2 escapes to the atmosphere. 2e- enters photosystem-II. In this way photosystem-II regains its lost electrons. 2H+ combines with NADP-reductase to produce NADPH+H+.
Both cyclic and acyclic processes can occur simultaneously in higher plants. However, in cyanobacteria, algae and green plants cyclic processes occur when the supply of NADP is cut off. When the water supply is stopped, the acyclic process stops and the cyclic process takes place.
[Applying herbicides to the soil stops the photosynthetic electron flow of the weed, so the weed dies]

In the process of photophosphorylation, high energy electrons (e-) released from chlorophyll molecules are carried through various carriers, lose energy and return to the same chlorophyll molecule, it is called cyclic photophosphorylation. Cyclic photophosphorylation process is as follows.
(i) Only photosystem-I participates in the cyclic photophosphorylation process. Photosystem-II absorbs light energy from the sun. This light energy is transferred to the reaction center P-700.
(ii) Two high energy electrons (2e-) are emitted from P-700. Electrons are accepted by the two carriers ferridoxine (Pd).
(iii) Two electrons from ferridoxine come to cytochrome b6 (Cyt. b6). Electrons from cytochrome-b6 pass two very quickly to plastoquinin (PQ).
(iv) Then two electrons come from plastoquinin to cytochrome-f (Cyt. f). At this time the two electrons lose some energy. Under the influence of this energy, ATP is produced by combining ADP and Pi.
(v) Finally electrons pass from cytochrome-f to photosystem-I via plastocyanin. Thus photosystem-I regains its lost electrons.