Calvin Cycle Dark Reaction

Edited By Irshad Anwar | Updated on Jul 02, 2025 07:30 PM IST

Definition Of Calvin Cycle

The Calvin cycle, otherwise known as the C3 cycle, is a process in biochemistry that takes place in green plants and autotrophs. It produces organic molecules from CO2. These organic molecules are rich in C–H bonds and highly reduced compared to CO2. Photosynthesis is divided into two major steps: the light-dependent reactions that require light and happen in daylight, and the light-independent reactions (also known as the dark reactions or the Calvin Cycle, C3 Cycle), which occur both in the presence and in the absence of light. This paper describes an overview of the Calvin Cycle, which pertains to its definition, stages of the cycle, products, and important points.

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Definition Of Calvin Cycle
Explanation
Diagram
C3 Cycle stages
Products Of The C3 Cycle
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Explanation

Another name for the Calvin Cycle is the light-independent or dark reaction of photosynthesis. Although it occurs regardless of whether light is present or not, this cycle works more actively during a day when there is plenty of supply of NADPH and ATP. While synthesizing organic molecules, plant cells make use of raw materials the light reactions produce. This includes:

Energy:

The endergonic reactions are powered due to energy provided by ATP produced through cyclic and noncyclic photophosphorylation.

Reducing Power:

It is provided in the form of hydrogen and energy-rich electrons from NADPH created during photosystem I. These bind with carbon atoms.

Plants store the absorbed light energy in the form of carbohydrates, mainly starch and sucrose.
Carbon is derived from CO2, while ATP and NADPH, produced during photosynthesis, provide the energy to fix carbon.
The process of converting CO2 to carbohydrates is called the Calvin Cycle, sometimes referred to as the C3 cycle, named after its discoverer Melvin Calvin.
Plants that use this process to fix carbon through the Calvin Cycle are called C3 plants.

Diagram

The diagram illustrates a scheme of the Calvin Cycle, including carbon fixation, reduction, and regeneration. It indeed explains the details of how CO2 is converted into glucose.

C3 Cycle stages

The Calvin Cycle can be categorized into three major steps:

Carbon fixation

The reduction of CO2 is the most crucial step of the Calvin Cycle. The CO2 binds to RuBP in a step called carbon fixation to yield two three-carbon molecules of 3-phosphoglycerate, 3-PGA. The reactant for this is catalyzed by the enzyme ribulose bisphosphate carboxylase/oxygenase or RuBisCO. This is a large enzyme with four subunits and is found in the stroma. RuBisCO is said to be the most abundant protein on Earth and yet it only processes about three molecules of RuBP per second.

Reduction

In the second step of the Calvin Cycle, 3-PGA formed during carbon fixation is reduced to form glyceraldehyde-3-phosphate (G3P), a simple sugar. That step is energized by the ATP and NADPH from the light-dependent reactions. So the general role of the Calvin Cycle is to offer an avenue for the conversion of sunlight energy into long-term energy storage molecules, in this case, sugars. The reason that this step is called reduction is that electrons are donated to the 3-PGA forming G3P.

Regeneration

The third phase of the light-independent reaction is the regeneration of RuBP from G3P. This step is essential so that the cycle can be repeated as RuBP is used up in the first step of carbon fixation. Several molecules of G3P are used to synthesise glucose and some are re-circulated back to regenerate RuBP. This regeneration consumes some ATP also. RuBP is regenerated so that RuBisCo can continue to fix carbon dioxide in the cycle

Products Of The C3 Cycle

At each turn of the Calvin cycle, one molecule of carbon is fixed.
Three cycles produce a net gain of one molecule of glyceraldehyde-3-phosphate G3P.
One glucose molecule is produced from two molecules of G3P.
In the reduction of 3-PGA to G3P and the regeneration of RuBP, 3 ATP are consumed net along with 2 NADPH.
In the formation of one glucose molecule, a total of 18 ATP along with 12 NADPH are consumed.

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Key Points On C3 Cycle

The C3 cycle is also the dark reaction of photosynthesis.
Light-independent reactions indirectly depend on light as essential energy carriers are products of light-dependent reactions.
The first phase initiates the light-independent reactions of the Calvin Cycle and fixes CO2.
In the second stage of the cycle, ATP and NADPH are reduced to ADP and NADP+ by the reduction of 3-PGA to G3P.
The last step is used to regenerate RuBP to repeat the process of carbon dioxide fixation.

Conclusion

The Calvin Cycle is an integral part of photosynthesis in plants, through which carbon dioxide is turned into glucose, and energy is stored within the organism in the form of carbohydrates. The stages and activities of the Calvin Cycle are, thus, a precondition for a full understanding of how plants make their food and, in turn, help the ecosystem to exist.

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Frequently Asked Questions (FAQs)

1. What is the Calvin Cycle?

The Calvin Cycle refers to the process of biochemical reactions in photosynthesis by which carbon dioxide is converted into glucose.

2. What are the major steps of the Calvin Cycle?

The major steps are Carbon Fixation, Reduction, and Regeneration.

3. In which part of the cell does the Calvin Cycle take place?

It takes place in the stroma of chloroplasts of plant cells.

4. What is the importance of the Calvin Cycle?

It is the vital cycle to transfer CO2 into organic forms required for plant growth and the supply of food to heterotrophic organisms.

5. What factors can affect the efficiency of the Calvin Cycle?

Such factors include light intensity, temperature, and carbon dioxide concentration.

6. What is the primary function of RuBisCO in the Calvin cycle?

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is a crucial enzyme in the Calvin cycle. Its primary function is to catalyze the first major step of carbon fixation, where it adds carbon dioxide to a 5-carbon sugar called ribulose bisphosphate. This reaction is the starting point for converting inorganic carbon into organic compounds.

7. What are the three main stages of the Calvin cycle?

The Calvin cycle consists of three main stages: 1) Carbon fixation, where CO2 is added to ribulose bisphosphate; 2) Reduction, where the fixed carbon is reduced using ATP and NADPH from the light reactions; and 3) Regeneration, where the original CO2 acceptor (ribulose bisphosphate) is recreated to continue the cycle.

8. What role does phosphate play in the Calvin cycle?

Phosphate groups play several crucial roles in the Calvin cycle. They're part of important molecules like ATP, which provides energy for the reactions. Phosphate is also involved in the activation and regulation of enzymes, and it's a component of many intermediates in the cycle, such as 3-phosphoglycerate and ribulose bisphosphate.

9. What is the significance of the 3-phosphoglycerate (3-PGA) molecule in the Calvin cycle?

3-phosphoglycerate (3-PGA) is the first stable product formed when RuBisCO fixes CO2 to ribulose bisphosphate. It's a key intermediate in the Calvin cycle, marking the transition from inorganic carbon to organic compounds. The formation of 3-PGA is a critical step in the process of carbon fixation.

10. How does the Calvin cycle differ in C3 and C4 plants?

In C3 plants, the Calvin cycle occurs directly in the mesophyll cells. In C4 plants, there's an additional step: CO2 is first fixed into a 4-carbon compound in the mesophyll cells, then transported to bundle sheath cells where the Calvin cycle takes place. This adaptation helps C4 plants conserve water and operate more efficiently in hot, dry environments.

11. What would happen to a plant if its Calvin cycle was inhibited?

If a plant's Calvin cycle was inhibited, it would be unable to produce glucose and other organic compounds from CO2. This would lead to a severe energy shortage, as the plant couldn't create its own food. Over time, this would result in stunted growth, reduced productivity, and eventually death if the inhibition persisted.

12. What is the connection between the Calvin cycle and cellular respiration?

The Calvin cycle and cellular respiration are interconnected processes. The Calvin cycle produces glucose, which can then be used as a substrate for cellular respiration to generate ATP. Conversely, cellular respiration produces CO2, which can be used in the Calvin cycle. This demonstrates the cyclical nature of carbon in biological systems.

13. What is photorespiration and how does it relate to the Calvin cycle?

Photorespiration is a process where RuBisCO fixes oxygen instead of carbon dioxide, producing a 2-carbon compound that can't be used in the Calvin cycle. This occurs when O2 concentrations are high relative to CO2. Photorespiration reduces the efficiency of the Calvin cycle, as it consumes energy without producing useful products. C4 and CAM plants have evolved mechanisms to minimize photorespiration.

14. How does the Calvin cycle in aquatic plants differ from that in terrestrial plants?

The Calvin cycle operates similarly in aquatic and terrestrial plants, but aquatic plants face unique challenges. They often have to deal with limited CO2 availability in water, which can slow down the cycle. Many aquatic plants have evolved mechanisms to concentrate CO2 around RuBisCO, similar to C4 plants. Some can also use bicarbonate ions as a carbon source, adapting the initial steps of carbon fixation.

15. How does the Calvin cycle demonstrate the integration of multiple metabolic pathways in plants?

The Calvin cycle demonstrates the integration of multiple metabolic pathways by connecting photosynthesis, carbohydrate synthesis, and various other biosynthetic pathways. It uses products from the light reactions (ATP and NADPH), fixes carbon to produce sugars, and its intermediates feed into pathways for amino acid, lipid, and nucleotide synthesis. This integration allows plants to efficiently coordinate their metabolic processes.

16. How does the Calvin cycle demonstrate the principle of carbon reduction in photosynthesis?

The Calvin cycle demonstrates carbon reduction in photosynthesis by using the energy from ATP and the reducing power of NADPH (both produced in the light reactions) to convert inorganic carbon (CO2) into organic compounds. This reduction process decreases the oxidation state of carbon, storing energy in chemical bonds that can later be used by the plant or other organisms.

17. How does the concentration of CO2 in the atmosphere affect the efficiency of the Calvin cycle?

The concentration of CO2 in the atmosphere directly impacts the efficiency of the Calvin cycle. Higher CO2 concentrations generally increase the rate of carbon fixation, as there's more substrate available for RuBisCO. This is why some greenhouse growers artificially increase CO2 levels to boost plant growth. However, extremely high CO2 levels can have complex effects on overall plant physiology.

18. How do plants regulate the Calvin cycle?

Plants regulate the Calvin cycle through various mechanisms. Key enzymes like RuBisCO are activated by light and deactivated in darkness. The availability of ATP and NADPH from the light reactions also regulates the cycle. Additionally, feedback inhibition occurs when there's an excess of end products, and environmental factors like temperature and CO2 concentration affect the cycle's rate.

19. How does the Calvin cycle demonstrate the concept of energy investment and return in biochemical pathways?

The Calvin cycle clearly shows the concept of energy investment and return. It initially requires an input of energy in the form of ATP and NADPH (produced by the light reactions). This energy is invested in fixing carbon and reducing it to form glucose. The glucose produced represents an energy return, as it can be used by the plant for various metabolic processes or stored for future use.

20. What would happen to the Calvin cycle if there was a mutation in the gene coding for RuBisCO?

A mutation in the gene coding for RuBisCO could have severe consequences for the Calvin cycle. Depending on the nature of the mutation, it could reduce the enzyme's efficiency, alter its substrate specificity, or even render it non-functional. This could significantly impair or completely halt carbon fixation, severely affecting the plant's ability to produce its own food and survive.

21. What is the Calvin cycle and why is it called the "dark reaction"?

The Calvin cycle is the second stage of photosynthesis where carbon dioxide is converted into glucose. It's called the "dark reaction" because it doesn't directly require light, unlike the light-dependent reactions. However, this name can be misleading as the Calvin cycle still occurs during daylight and depends on products from the light reactions.

22. Why is the Calvin cycle also referred to as carbon fixation?

The Calvin cycle is called carbon fixation because it's the process where inorganic carbon (in the form of carbon dioxide) is "fixed" or incorporated into organic compounds. This conversion of atmospheric CO2 into sugar molecules is a fundamental step in the global carbon cycle and the basis of most food chains.

23. How does the Calvin cycle relate to the concept of autotrophy?

The Calvin cycle is fundamental to autotrophy, the ability of organisms to produce their own food from inorganic substances. By fixing atmospheric CO2 into organic compounds, the Calvin cycle enables plants (and other autotrophs) to synthesize their own nutrients, forming the base of most food chains and enabling life as we know it on Earth.

24. How does the Calvin cycle demonstrate the concept of a cyclic pathway in biochemistry?

The Calvin cycle is an excellent example of a cyclic pathway in biochemistry. It starts with a molecule (ribulose bisphosphate), goes through a series of reactions that produce the desired product (glucose), and then regenerates the starting molecule. This cyclic nature allows for continuous carbon fixation as long as the necessary inputs (CO2, ATP, and NADPH) are available.

25. How does temperature affect the efficiency of the Calvin cycle?

Temperature significantly impacts the Calvin cycle's efficiency. As an enzymatic process, it operates optimally within a specific temperature range. Too high temperatures can denature the enzymes involved, particularly RuBisCO, while too low temperatures slow down the reaction rates. This is why plants in different climates have adapted various strategies to maintain efficient photosynthesis.

26. How many carbon dioxide molecules are needed to produce one glucose molecule in the Calvin cycle?

The Calvin cycle requires six carbon dioxide molecules to produce one glucose molecule. This process involves multiple turns of the cycle, as each turn fixes one carbon dioxide molecule. The six-carbon glucose is the end product after these multiple cycles.

27. How does the Calvin cycle relate to the light-dependent reactions of photosynthesis?

The Calvin cycle is closely linked to the light-dependent reactions. It uses ATP and NADPH produced during the light reactions to power the conversion of carbon dioxide into glucose. This interconnection demonstrates how the two stages of photosynthesis work together to produce energy for plants.

28. How does the Calvin cycle demonstrate the principle of energy transfer in biological systems?

The Calvin cycle exemplifies energy transfer in biological systems by using the chemical energy stored in ATP and NADPH (produced during the light reactions) to power the synthesis of glucose from CO2. This process shows how energy can be converted from one form to another in living organisms.

29. What would happen to the Calvin cycle if there was a shortage of ATP or NADPH?

If there was a shortage of ATP or NADPH, the Calvin cycle would slow down or stop. These molecules provide the energy and reducing power necessary for carbon fixation and sugar synthesis. Without sufficient ATP and NADPH, plants couldn't efficiently convert CO2 into glucose, impacting their growth and survival.

30. How does the Calvin cycle contribute to the global carbon cycle?

The Calvin cycle plays a crucial role in the global carbon cycle by removing CO2 from the atmosphere and converting it into organic compounds. This process, known as carbon sequestration, helps regulate atmospheric CO2 levels and provides the basis for most food chains on Earth.

31. What is the role of the enzyme phosphoribulokinase in the Calvin cycle?

Phosphoribulokinase is a key enzyme in the regeneration phase of the Calvin cycle. It catalyzes the phosphorylation of ribulose 5-phosphate to produce ribulose 1,5-bisphosphate, the CO2 acceptor molecule. This step is crucial for the cycle to continue, allowing for the fixation of more carbon dioxide.

32. What is the role of the enzyme transketolase in the Calvin cycle?

Transketolase is an important enzyme in the regeneration phase of the Calvin cycle. It catalyzes the transfer of two-carbon units between sugar phosphates, helping to rebuild the five-carbon ribulose phosphate from three-carbon compounds. This step is crucial for maintaining the cycle and ensuring a continuous supply of the CO2 acceptor molecule.

33. What is the significance of the enzyme phosphoglycerate kinase in the Calvin cycle?

Phosphoglycerate kinase is a crucial enzyme in the reduction phase of the Calvin cycle. It catalyzes the transfer of a phosphate group from ATP to 3-phosphoglycerate, forming 1,3-bisphosphoglycerate. This step is important because it prepares the molecule for subsequent reduction, ultimately leading to the formation of glucose.

34. How does the Calvin cycle contribute to the production of other organic compounds besides glucose?

While glucose is often considered the primary product of the Calvin cycle, its intermediates can be used to synthesize a wide range of organic compounds. For example, 3-phosphoglycerate can be used to produce amino acids, and other cycle intermediates contribute to the synthesis of lipids and nucleotides. This versatility allows plants to create all the organic molecules they need for growth and function.

35. What is the role of fructose-1,6-bisphosphatase in the Calvin cycle?

Fructose-1,6-bisphosphatase is an important regulatory enzyme in the Calvin cycle. It catalyzes the conversion of fructose-1,6-bisphosphate to fructose-6-phosphate by removing a phosphate group. This step is crucial in the regeneration phase of the cycle and is also a key regulatory point, as the enzyme is inhibited by high levels of fructose-2,6-bisphosphate, allowing the plant to control the rate of sugar synthesis.

36. What is the importance of the enzyme glyceraldehyde 3-phosphate dehydrogenase in the Calvin cycle?

Glyceraldehyde 3-phosphate dehydrogenase is a key enzyme in the reduction phase of the Calvin cycle. It catalyzes the reduction of 1,3-bisphosphoglycerate to glyceraldehyde 3-phosphate, using NADPH as the reducing agent. This step is crucial as it's where the actual reduction of the fixed carbon occurs, converting it into a form that can be used to make glucose and other organic compounds.

37. What is the role of sedoheptulose-1,7-bisphosphatase in the Calvin cycle?

Sedoheptulose-1,7-bisphosphatase is an enzyme involved in the regeneration phase of the Calvin cycle. It catalyzes the dephosphorylation of sedoheptulose-1,7-bisphosphate to sedoheptulose-7-phosphate. This reaction is important for regenerating the five-carbon sugars needed to continue the cycle. Like fructose-1,6-bisphosphatase, it's also a regulatory enzyme in the cycle.

38. How does the Calvin cycle in cyanobacteria compare to that in plants?

The Calvin cycle in cyanobacteria is fundamentally similar to that in plants, as both use it for carbon fixation. However, cyanobacteria lack chloroplasts, so the cycle occurs in the cytoplasm or in specialized structures called carboxysomes. Carboxysomes concentrate CO2 around RuBisCO, increasing the efficiency of carbon fixation. Some cyanobacteria also have more efficient forms of RuBisCO, allowing them to fix carbon more effectively in aquatic environments.

39. What is the significance of the enzyme phosphoribulose kinase in the Calvin cycle?

Phosphoribulose kinase, also known as phosphoribulokinase, is a critical enzyme in the regeneration phase of the Calvin cycle. It catalyzes the phosphorylation of ribulose 5-phosphate to produce ribulose 1,5-bisphosphate, the CO2 acceptor molecule. This step is essential for the cycle to continue, as it regenerates the substrate for RuBisCO, allowing for ongoing carbon fixation.

40. How does the Calvin cycle demonstrate the principle of feedback inhibition?

The Calvin cycle demonstrates feedback inhibition through the regulation of key enzymes. For example, when there's an excess of the cycle's end products (like glucose), they can inhibit enzymes earlier in the pathway. Specifically, fructose-1,6-bisphosphatase is inhibited by fructose-2,6-bisphosphate, which accumulates when sugar levels are high. This mechanism helps prevent overproduction of sugars and maintains metabolic balance.

41. What would happen to the Calvin cycle if there was a sudden increase in oxygen concentration?

A sudden increase in oxygen concentration would likely increase the rate of photorespiration. RuBisCO can fix both CO2 and O2, but fixing O2 leads to photorespiration, which reduces the efficiency of the Calvin cycle. With more O2 available, RuBisCO would fix O2 more frequently, producing 2-carbon compounds that can't be used in the cycle. This would decrease the overall rate of carbon fixation and sugar production.

42. How does the Calvin cycle in CAM plants differ from that in C3 plants?

In CAM (Crassulacean Acid Metabolism) plants, the Calvin cycle operates similarly to C3 plants, but with a temporal separation. CAM plants open their stomata at night to collect CO2, which is stored as malate. During the day, when stomata are closed to conserve water, the stored CO2 is released and used in the Calvin cycle. This adaptation allows CAM plants to minimize water loss while still fixing carbon efficiently.

43. What is the role of the enzyme triose phosphate isomerase in the Calvin cycle?

Triose phosphate isomerase plays a crucial role in the Calvin cycle by catalyzing the interconversion of dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P). This isomerization is important because only G3P can be used for glucose synthesis or to regenerate ribulose bisphosphate. By allowing the interconversion of these three-carbon sugars, the enzyme helps maintain the balance of metabolites in the cycle.