Photosynthesis and the Calvin Cycle: A Comprehensive Analysis

This essay provides a comprehensive overview of the Calvin cycle, a vital component of photosynthesis, occurring in the chloroplast stroma of plants. The essay delves into the cycle's three main phases: carbon fixation, reduction, and regeneration, detailing the enzymatic reactions and intermediate compounds involved. It explains how carbon dioxide is incorporated into carbohydrates, specifically glyceraldehyde-3-phosphate (G3P), and the role of key enzymes like RuBisCO. The essay also highlights the significance of the Calvin cycle in producing organic compounds, such as glucose, and its broader impact on the ecosystem, including the storage of energy and regulation of atmospheric carbon dioxide levels. Figures are included to illustrate the process. The essay references several scientific publications to support the information presented.

Running head: ESSAY
The Calvin Cycle
Name of the Student
Name of the University
Author Note

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Introduction
The Calvin cycle refers to the metabolic pathway that helps in incorporation of carbon
dioxide into carbohydrate. In other words, the photosynthetic Calvin cycle can be referred to as
the primary carbon fixation pathway and it occurs in the chloroplast stroma of higher
plants. Stroma predominantly refers to the fluid filled region of chloroplast that is present in the
outer part of the thylakoid membranes. Nobel laureate Melvin Calvin and his colleagues
highlighted the series of reactions that occur in the cycle during the 50s (Sharkey, 2019). This
was followed by identification of the enzymes that catalyze reactions, which occur in the
pathway and their kinetic properties were investigated in vitro. It is also known as light
independent reaction, or dark reaction, or biosynthetic phase, and the chemical reactions help in
conversion of carbon dioxide into glucose. In this assignment, the mechanism of the Calvin cycle
will be discussed in details.
Discussion
Original referred to as the Calvin Benson Bassham (CBB) cycle, this is C3 cycle or
Reductive Pentose Phosphate cycle encompasses a sequence of biochemical redox reactions,
which had been identified with the use of radioactive isotope carbon 14. Photosynthesis
primarily occurs in two different phases in a particular plant cell. During the first phase, light
dependent reactions play an important role in capturing energy obtained from sunlight, and
utilise it for the generation of transport and energy storage molecules namely, NADPH and ATP
(Sharkey, 2019). The Calvin cycle utilizes energy that is generated from carriers, which are
short lived and electronically agitated, in order to transform carbon dioxide, in addition to water
into different organic compounds, which in turn can be utilized by the plants and the animals that

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feed on plants. The sequences of reactions are also referred to as carbon fixation, and the
primary enzyme of the cycle is known as RuBisCO. In the ensuing biochemical reactions,
chemical compounds such as, carboxylic acid and phosphate remain in equilibrium and are
controlled by pH (Nelson, Lehninger & Cox, 2017).
Figure 1- Fixation of CO2
Source- (Nelson, Lehninger & Cox, 2017)
There is a growing body of evidence that highlights the functional equivalence of
enzymes, which catalyze the Calvin cycle to different enzymes that play an important role in
metabolic pathways of the Pentose Phosphate Pathway and gluconeogenesis. However, the
enzymes of Calvin cycle are predominantly found in the stroma of chloroplast, rather than the

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cytosol of the cell, thereby sorting out the reactions. These enzymes generally get activated in the
presence of light and also by products that are formed from light dependent reactions. CO2 enters
the plants through stomata present in the leaf, following which it gets diffused over short
distances through the major intercellular spaces, and eventually reaches mesophyll cells of the
leaves. Upon reaching the mesophyll cells, there occurs diffusion of CO2 into the chloroplast
stroma that is the primary reaction of light independent photosynthetic reactions (Lopez &
Barclay, 2017). Below given is the sum of reactions of the Calvin cycle:
3 CO2 + 6 NADPH + 6 H+ + 9 ATP → glyceraldehyde-3-phosphate (G3P) + 6 NADP+ +
9 ADP + 3 H2O + 8 Pi (Pi = inorganic phosphate)
It must be noted that 6-carbon sugar hexose are generally not produced at the end of
Calvin cycle. The carbohydrates that are produced from the cycle are generally 3-carbon sugar
phosphate, commonly referred to as triose phosphate, such as, glyceraldehyde 3 phosphate
(G3P).
Calvin cycle is categorized into three principal phases that are (i) carbon fixation,
(ii) reduction, and (iii) regeneration. During the first stage of the cycle there occurs incorporation
of a CO2 molecule into one of the two G3P molecules where two ATP molecules and two
NADPH molecules that had been generated during light dependent phase are used up. The
carboxylation of the five-C compound ribulose-1,5-bisphosphate (RuBP) is catalyzed by the
enzyme Rubisco in a two-step reaction (Gontero, Avilan & Lebreton, 2018). Enediol-enzyme
complex is produced during the first step that captures CO2, following which in the second step,
3-keto-2-carboxyarabinitol 1,5-bisphosphate, an unstable six-C compound is generated that
eventually splits to form two 3-phosphoglycerate (3-PGA) molecules.

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Figure 2- Carbon fixation
Source- (Nelson, Lehninger & Cox, 2017)
The reduction phase is marked by phosphorylation of 3-PGA that is catalyzed by
phosphoglycerate kinase, in the presence of ATP, following which ATP and 1, 3-
Bisphosphoglycerate (1,3 BPGA) are generated. This is followed by reduction of 1,3 BPGA in
the presence of glyceraldehydes-3-phosphate dehydrogenase and NADPH to produce G3P.
Following oxidation of NADPH, NADP+ gets formed. The phase obtains its name owing to the

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fact that NADPH donates its electrons or reduces a three-C compound for producing G3P
(Mcfarlane et al., 2018).
Figure 3- Reduction phase
Source- (Nelson, Lehninger & Cox, 2017)
The final step in Calvin cycle involves regeneration of RuBP. Three RuBP molecules are
produced from five G3P molecules, thereby utilizing 3 ATP molecules. Considering the fact that
each molecule of carbon dioxide results in the generation of two molecules of G3P, three
molecules of CO2 eventually produce six molecules G3P, of which five molecules are generally

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utilized for RuBP regeneration and one G3P molecule is left for the three molecules of CO2.
During regeneration phase, all G3P are reversibly converted by triose phosphate isomerase into
the three-C compound, dihydroxyacetone phosphate (DHAP). DHAP and G3P are converted by
Fructose-1,6-bisphosphatase and aldolase into Fructose-6-phosphate.
This is followed by fixation of another molecule of CO2 to additional two G3P molecules.
Two carbon atoms are removed from Fructose-6-phosphate by transketolase, thereby forming
erythrose-4-phosphate (E4P), which together with DHAP get converted by aldolase enzyme to
a seven-C compound, called sedoheptulose-1,7-bisphosphate. Two carbons remain on the
transketolase enzyme and get added to G3P, thus generating xylulose-5-phosphate (Xu5P).
Sedoheptulose-1,7- bisphosphate gets converted to sedoheptulose-7-phosphate (S7P) by
the sedoheptulose-1,7-bisphosphatase, which leads to the release of an inorganic phosphate ion.
Two additional G3P are produced, following the fixation of third CO2 molecule. Transketolase
enzyme removes two carbons from the S7P, thus generating ribose-5-phosphate (R5P), which
soon gets converted into ribulose-5-phosphate (Ru5P), through the action of the enzyme
phosphopentose isomerase (Lopez & Barclay, 2017). The two carbon that remain on the enzyme
transketolase get transferred to a G3P, thus leading to the production of an additional Xu5P,
which converts to Ru5P through the enzyme phosphopentose epimerase. The final step involves
phosphorylation of Ru5P to RuBP by the action of phosphoribulokinase, thereby completing the
cycle. One ATP molecule is required for this step. Therefore, the immediate products that are
obtained after one Calvin cycle are three ADP, two NADP+, and two G3P molecules (Nelson,
Lehninger & Cox, 2017).

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Figure 4- Regeneration phase
Source- (Nelson, Lehninger & Cox, 2017)
Hence, in the most general sense the major function of Calvin cycle is to produce
organic compounds from the products generated after light reaction of photosynthesis that can be
used by plants. The organic compounds typically include glucose, the carbohydrate that is
produced utilizing water, CO2, lipids, and proteins. The carbohydrate produced at the end of
Calvin cycle is generally used for the production of other sugars such as, cellulose and starch
which in turn are made use of by plants in the form of structural materials (Nelson, Lehninger &

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Cox, 2017). In absence of Calvin cycle, the plants will not be able to store energy in any form
that can be digested by the herbivores. Eventually the carnivorous will not gain access to energy
stored in herbivores and the entire ecosystem will get disrupted. The cycle also controls the
amount of CO2, which is a major greenhouse gas, in the Earth's atmosphere.
Figure 5- CO2 assimilation in plants
Source- (Nelson, Lehninger & Cox, 2017)
Conclusion
Thus it can be concluded that the Calvin cycle is the second step in the photosynthetic
mechanism and since it occurs independent of light, it is often referred to as dark reaction. The
reaction is typically slower when compared to light reaction. The cycle results in the synthesis of
sugar from CO2. It involves fixation of energy poor CO2 into carbohydrates that are rich in
energy, with the use of ATP and the NADPH assimilatory power. The three stages of
carboxylation, reduction, and regeneration help in successful completion of the Calvin cycle.

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References
Gontero, B., Avilan, L., & Lebreton, S. (2018). Control of carbon fixation in
chloroplasts. Annual Plant Reviews online, 187-218.
http://priede.bf.lu.lv/grozs/AuguFiziologijas/Augu_audu_kulturas_MAG/literatura/
09_Plaxton_McManusControl%20of%20Primary%20Metabolism.pdf#page=209
Lopez, F. B., & Barclay, G. F. (2017). Plant anatomy and physiology. In Pharmacognosy (pp.
45-60). Academic Press. https://doi.org/10.1016/B978-0-12-802104-0.00004-4
Mcfarlane, C., Shah, N., Kabasakal, B. V., Cotton, C. A., Bubeck, D., & Murray, J. W. (2018).
Structural basis of light-induced redox regulation in the Calvin cycle. bioRxiv, 414334.
https://doi.org/10.1101/414334
Nelson, D. L., Lehninger, A. L., & Cox, M. M. (2017). Lehninger principles of biochemistry.
Macmillan. https://books.google.co.in/books?id=mv5TvgAACAAJ&dq=Nelson,+D.+L.,
+Lehninger,+A.+L.,+%26+Cox,+M.+M.+(2008).
+Lehninger+principles+of+biochemistry.+Macmillan.&hl=en&sa=X&ved=0ahUKEwj_-
7qi35LoAhUJzjgGHSg3CKIQ6AEIMTAB
Sharkey, T. D. (2019). Discovery of the canonical Calvin–Benson cycle. Photosynthesis
research, 140(2), 235-252. https://doi.org/10.1007/s11120-018-0600-2