The C3 cycle, also known as the Calvin cycle, occurs in the dark phase of photosynthesis and involves fixing carbon dioxide into organic molecules like glucose. It consists of three main stages: fixation, reduction, and regeneration. During fixation, the enzyme rubisco incorporates CO2 into ribulose bisphosphate, producing two molecules of 3-phosphoglycerate. These are then reduced using ATP and NADPH in the reduction stage. Finally, the cycle is regenerated as the original ribulose bisphosphate is reformed, allowing it to fix another CO2. The C3 cycle is essential for carbon assimilation in photosynthesis and the primary producer of organic compounds and food energy in plants. It occurs in all photosynthetic organisms

1. C3 CYCLE
Presented by : - Salvi Pinky P.
M.sc sem : - 3
Paper : - Botany 503
Department of Life sciences,
H. N. G. U., Patan

2. CONTENTS
 Introduction
 Stages : —
1) Fixation
2) Reduction
3) Regeneration
 Significance of C3 cycle
 Difference between C3 and C4 cycle

3. Introduction
• It is a cyclic reaction occuring in the dark phase of
photosynthesis.
• In this reaction CO2 is converted in to sugar and hence it is a
process of carbon fixation.
• The Calvin cycle was first observed by Melvin Calvin in
chlorella, unicellular green algae.
• Calvin was awarded Nobel prize for this work in 1961.

4. Other names for Calvin Cycle : —
1) Dark reactions
2) C3 Cycle
3) Reductive Pentose phosphate Cycle
4) Calvin Cycle
5) Photosynthetic Carbon Reduction Cycle [ PCR ]

5. • Calvin and other workers are working at the University of
Berkeley, he used a carbon – 14 isotopes to understand the
Photosynthesis process in plants.
• This radioactive isotope helped him determine how the light
independent reaction works in Singal – celled algae.
• Calvin and co - workers found that after three second,
radioactivity appeared in phosphoglyceric acid or PGA.
• Phosphoglyceric acid is, therefore, the first stable product of
Photosynthesis as known as C3 Cycle.

7. Stages of C3 cycle
• These energy carrying molecules are made in the stroma of
the chloroplast where the Calvin takes place.
• The Calvin cycle is not totally independent of light since it
relies on ATP and NADPH, which are products of the light
dependent reactions.
• The light reactions of the Calvin cycle can be organised in to
three basic stages : - 1) Fixation
2) Reduction
3) Regeneration

8. FIXATION
• In the stroma, in addition to CO2, two other components are
present to initiate the light independent reactions : an enzyme
called ribulose Biphosphate carboxylase( RUBISCO) and three
molecules of Ribulose bisphosphate (RUBP).
• RUBP has five atoms of carbon, flanked by two phosphatase.
• For each CO2 molecules that reacts with one RUBP, two
molecules of 3 phosphoglyceric acid ( 3 PGA) from, 3 PGA has
three carbons and one phosphate.

9. • Each turn of the cycle involves only one RUBP and one carbon dioxide and
forms two molecules of 3 - PGA.
• 3 phosphoglyceric acid ( PGA ) is the first stable product of Calvin cycle.
• This prosses is called carbon fixation because CO2 is " fixed" from an
inorganic form into organic molecules.
3 unstable
intermediate 6C + 3 H2O Carbon 3 phoshpo-
Compound dismutase glyceric
acid

10. REDUCTION
• 6 molecules of 3 PGA are phosphorylated by 6 molecules of 1, 3
diphosphoglyceric acid and 6 molecules of ADP.
• Six molecules of 1,3 diphosphoglyceric glyceric acid are
• 6 molecules of 1, 3 diphosphoglyceric acid are reducedhe use of
molecules of NADPH2 to from 6 Molecules of 3 phosphoglyceric.
3 Phospho Kinase 1,3 diphosphate
glyceric acid + ATP glyceric acid + ADP

11. 1,3 diphospho Triose phosphate
+ NADPH2 Dehydrogenase
glyceric acid
3 Phospho
+ NADP + H3 PO4
glyceraldehyde

12. REGENERATION
I. In the regeneration phase, the ribose diphosphate is
regenerated it involves the following steps.
1. Some of the molecules of 3 phosphoglyceraldyde convert
into dihydroxy acetone phosphate ( DHAP ).
3 Phospho Triose phosphate Dihydroxy acetone
isomerase Phosphate
glyceraldehyde ( DHAP )

13. 2. Both 3 PGAL and DHAP then unite in the presence of enzyme,
aldolase to from fructose 1,6 diphosphate.
3. Fructose 1,6 disphosphate is converted into Fructose 6
phosphate in the presence of phosphorylase.
4. Some of the 4 phosphate combines with DHAP in the presence
of the enzyme aldolase to form sedoheptose 1,7 diphosphate.
3 Phospho Aldolase Fructose 1,6
glyceraldehyde + DHAP diphosphate

14. 5. 3PGAL react with Fructose 6 phosphate in the presence of
enzyme transeketolase to from erythrose 4 phosphate and xylulose
5 phosphate.
6. Erythrose 4 phosphate combines with DHAP in the presence of
the enzyme aldolase to form sedoheptose 1,7 diphosphate.
3 Phospho Fructose 6 Trans Erythrose 4
glyceraldehyde + phosphate ketolase phosphate +
Xylulose 5 phosphate

15. 7. Sedoheptulose 1,7 diphosphate loses one phosphate group in
the presence of the enzyme phosphatase to from sedoheptulose 7
phosphate.
8. Sedoheptulose 7 phosphate react with 3 PGAL in the presence
of transeketolase to form xylulose 5 phosphate and ribose 5
phosphate.
Erythrose 4 Aldolase Sedoheptulose 1,7
phosphate + DHAP diphosphate

16. 9. Ribose 5 phosphate is converted into Ribulose 1,5 bisphosphate
in the presence of enzyme phosphopentose kinese and ATP.
Sedoheptulose + 3 Phospho Trans Xylulose
7 phosphate Glyceraldehyde Ketolase 5 phosphate +
Ribose 5 phosphate
Ribose + ATP Phospho Ribulose
5 phosphate pento 1,5 di- + ADP
kinese Phosphate

17. 10. Two Molecules of xylulose 5 phosphate are also converted
into one molecule of Ribulose 5 phosphate.
• Ribulose 5 phosphate is phosphorylated by ATP to form
Ribulose 1,5 bisphosphate and ADP.
2 moles Phosphopento 2 moles
Xylulose + ATP Kinese Ribulose + ADP
5 phosphate 1,5 diphosphate

18. Net reaction : —
3CO2 + 5H2O + 6NADPH + 9ATP
glyceraldehyde - 3 - phosphate + 6NADP+ +
3H+ + 9 ADP + 8Pi

20. SIGNIFICANCE OF C3 CYCLE
• It is the main biochemical pathway during the dark reaction
(phase – II) of Photosynthesis.
• It result in the synthesis of carbohydrates from CO2 [ assimilation
of carbon ].
• It depends on the light reaction ( phase I ) for the supply of the
assimiatory power ( ATP and NADPH2 ) required for carbon
assimilation.

21. • It stores the ATP energy formed during light reaction in the
carbohydrate molecules as the food energy.
• It is the primary source of organic food and food energy for all
the organisms.
• Calvin cycle [ C3 cycle ] reaction occur in all photosynthetic
plants ; i.e. C3, C4 and CAM plants, during the dark phase of
Photosynthesis.

22. DIFFERENT BETWEEN C3 AND
C4 CYCLE

23. REFERENCES
• Plant physiology By : - Taize & Zeiger
• Plant physiology By : - Verma P. S. and P. K. Agarwal
• Principles of biochemistry By : – Albert lehninger
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 https:// eagri.org
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24. Thank you