The C4 cycle occurs in monocots and some dicots in summer. The C4 cycle occurs in Gramini and Cyperaceae and in 16 dicotyledonous genera. Currently, the C4 pathway in plants such as sugarcane (Saccharum officinarum), bhutra (Zea mays), mutha grass (Cyperus rotundus), Panicum maximum, Amaranthus cruentus, Chloris gayana, crown of thorns (Euphorbia milii), Triticum etc. can be observed.

1. Reaction of mesophyll cells
(i) Under the action of carboxylase enzyme, phospho-enol pyruvic acid reacts with atmospheric CO2 to produce oxaloacetic acid.
(ii) Oxaloacetic acid is converted to malic acid (aspartic acid) with the help of malic dehydrogenase enzyme. At this time NADPH+H+ takes part in the reaction to produce NADP.
2. Reaction of bundlesheath cells
(iii) Pyruvic acid is produced from malic acid (aspartic acid) under the action of decarboxylase enzyme.
(iv) Phospho-enol pyruvic acid is produced from pyruvic acid under the action of pyruvic acid kinase enzyme. At this time, ATP takes part in the reaction and produces ADP.
Phospho-enol pyruvic acid then re-enters the Hatch-Slack cycle and keeps the cycle going.

1. Reaction of mesophyll cells
(i) Under the action of carboxylase enzyme, phospho-enol pyruvic acid reacts with atmospheric CO2 to produce oxaloacetic acid.
(ii) Oxaloacetic acid is converted to malic acid (aspartic acid) with the help of malic dehydrogenase enzyme. At this time NADPH+H+ takes part in the reaction to produce NADP.
2. Reaction of bundlesheath cells
(iii) Pyruvic acid is produced from malic acid (aspartic acid) under the action of decarboxylase enzyme.
(iv) Phospho-enol pyruvic acid is produced from pyruvic acid under the action of pyruvic acid kinase enzyme. At this time, ATP takes part in the reaction and produces ADP.
Phospho-enol pyruvic acid then re-enters the Hatch-Slack cycle and keeps the cycle going.

(i) CO2 acceptor in C4 cycle is phosphoenol pyruvic acid.
(ii) The first stable compound in the C4 cycle is oxaloacetic acid.
(iii) A cycle occurs at low concentrations of CO2 (0.1-1.0 ppm).
(iv) This cycle occurs at high temperature (32-450C).
(v) The cycle occurs in mesophyll and bundlesheath chloroplasts.
(vi) It occurs in high light.
(vii) It occurs mainly in plants of tropical countries.
(viii) C4 cycle C4 occurs in plants.
(ix) The CO2 fixing enzyme in this cycle is carboxylase.
(x) Plants in cycle A have higher rate of photosynthesis.

The C4 cycle occurs in monocots and some dicots in summer. The C4 cycle occurs in Gramini and Cyperaceae and in 16 dicotyledonous genera. Currently, the C4 pathway in plants such as sugarcane (Saccharum officinarum), bhutra (Zea mays), mutha grass (Cyperus rotundus), Panicum maximum, Amaranthus cruentus, Chloris gayana, crown of thorns (Euphorbia milii), Triticum etc. can be observed.

In 1966 Australian scientists M. D. Hatch and C. R. Slack described the pathway of carbon assimilation in the light neutral phase of photosynthesis called the Hatch-Slack cycle. They proved that the first stable compound produced in photosynthesis was four-carbon oxaloacetic acid (OAA). Hence it is called C4 cycle. This cycle is also known as dicarboxylic cycle. In 1970, it was recognized as Hatch-Slack cycle. In 1965, Hugo Kortschak, Hart and George Burr first observed that oxaloacetic acid was the first stable substance produced during CO2 oxidation in sugarcane leaves. The cycle is also called the HSK pathway because scientists Hatch, Slack and Kortschak discovered the series of reactions in the C4 pathway. Currently, this pathway has been discovered in many plants of 16 genera.

All plants except C4 plants are C3 plants. E.g.- Paddy, Wheat, Barley/Sorghum, Mango, Yam, Jackfruit, Litchi, Banana, Jute, Algae, Bryophyta, Pteridophyta, Gymnosperms etc.

1. C3 plants originate from cold temperate countries.
2. C3 plants are unable to adapt to high temperatures.
3. A temperature of 10-25 degrees Celsius is easy for them to survive.
4. Leaves of C3 plants do not have cranial anatomy.
5. They do not have a layer of mesophyll cells surrounding the leaf bundle sheath.
6. Calvin-Basham cycle occurs in the mesophyll cells of C3 plants.
7. Their CO2 emission rate is low.
8. C3 plants have lower rates of photosynthesis.
9. Their respiration and photorespiration occur more.
10. Their chloroplasts are structurally similar.
11. Their photosynthesis is best when atmospheric CO2 concentration is 50-150 ppm.
12. Stomata of C3 plants are open during the day and closed at night.
13. Most reactions in C3 plants are unidirectional.
14. They tend to waste water more and may not be adapted to dry areas.
15. Ribulose bisphosphate carboxylase enzyme activity is substantial in mesophyll cells of C3 plants.
16. Bundlesheath chloroplasts and mesephyll chloroplasts contain starch granules.
17. Rubisco enzyme activity occurs in both bundlesheath and mesophyll cells.
18. Carbon oxidation by C3 plants is inhibited when the atmosphere contains more than 20% O2.

Plants that produce three-carbon 3-phosphogluceric acid as the first stable compound during carbon assimilation in photosynthesis are called C3 plants.

1. In this cycle RuBP combines with CO2 to produce PGA. PGA is formed from PGAld.
2. Complex sugars are produced from PGAld through a series of reactions.
3. CO2 is absorbed in the Calvin cycle. As a result, the amount of CO2 in the environment remains normal.
4. RuBP is regenerated through the Calvin cycle and maintains a photoneutral state.
5. Various compounds produced in the Calvin cycle participate in plant metabolism.
6. Xylulose produced in the Calvin cycle helps form plant cell walls.
7. Glucose produced in the Calvin cycle forms cellulose and cellulose through polymer formation.