Calvin Cycle – Diagram, Stages, Functions and Equations

Calvin Cycle is the biochemical pathway in the plants responsible for synthesizing glucose using carbon dioxide and the energy obtained from sunlight. The Calvin cycle steps include carbon fixation, reduction, and regeneration. The final Calvin Cycle product is glyceraldehyde-3-phosphate (G3P) molecules. The cycle helps plants store energy and form sugar, supporting their growth and survival. This article covers the Calvin Cycle explained – definition, steps, diagram, and products.

What is the Calvin Cycle?

Calvin Cycle Definition: Calvin cycle or C3 cycle is defined as the series of chemical reactions performed by the plants to reduce carbon dioxide and hydrogen-carrying compounds into glucose.

The Calvin Cycle is also known as the Calvin-Benson Cycle or the C3 cycle. It is a metabolic pathway that occurs in the chloroplasts of plants and certain types of bacteria during the process of photosynthesis. It is also known as a light-independent or dark reaction. The Calvin Cycle is found in all photosynthetic eukaryotes as well as numerous photosynthetic bacteria. The Calvin cycle location is in the stroma of the chloroplasts in plant cells.

The Calvin Cycle was discovered by Melvin Calvin and was named after him. The Calvin cycle uses ATP and NADPH to convert carbon dioxide and water into organic compounds that can be used by the organism. Although it is called a dark reaction it does not take place in the dark or the night. It is called so because it requires NADPH, which is short-lived and comes from light-dependent reactions.

Products (ATP and NADPH) of light-dependent reactions are taken by the cycle for further chemical processes. Therefore, the Calvin cycle occurs in the light, independent of the kind of photosynthesis (C3 carbon fixation, C4 carbon fixation, and CAM). This reaction is also called carbon fixation reaction. The enzyme that plays the key role is known as RuBisCO.

Calvin Cycle Diagram

Different stages of C3 cycle are shown below in the diagram; including carbon fixation, reduction, and regeneration.

Steps of Calvin Cycle

The Calvin Cycle that takes place in the stroma of chloroplasts during photosynthesis. It converts carbon dioxide and energy from light-dependent reactions into glucose and other sugars. The Calvin cycle steps are as follows:

Stage 1: Carbon Fixation

• Carbon dioxide (CO₂) from the atmosphere is combined with ribulose-1,5-bisphosphate (RuBP) which is a five-carbon compound.
• The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes this reaction.
• The six-carbon compound splits into two three-carbon molecules, 3-phosphoglycerate (3-PGA).

Stage 2: Reduction

• ATP and NADPH generated in the light-dependent reactions provide energy and electrons.
• A series of chemical reactions convert each molecule of 3-PGA into another three-carbon molecule, glyceraldehyde-3-phosphate (G3P).
• This step involves phosphorylation and reduction processes.

Stage 3: Regeneration of RuBP

• Some of the G3P molecules produced in the previous step are used to regenerate RuBP, the starting molecule of the cycle, and some of the G3P molecules are used to produce glucose.
• This is important to keep the cycle running and continue carbon fixation.

Calvin Cycle Equation

6 CO₂ + 18 ATP + 12 NADPH + 12 H₂O → C₆H₁₂O₆ (glucose) + 18 ADP + 18 Pi+ 12NADP⁺ + 6 H₂O

Calvin Cycle Products

The products of C3 cycle are important molecules that serve as energy compounds for the plant’s growth and survival. The primary end product of the Calvin Cycle is glyceraldehyde-3-phosphate (G3P), a three-carbon sugar molecule. The products of Calvin cycle are as follows:

• One turn of the Calvin cycle produces one molecule of carbon dioxide.
• A glucose molecule is formed by combining two molecules of glyceraldehyde-3 phosphate.
• Three turns of the Calvin cycle produce one molecule of glyceraldehyde-3 phosphate.
• In the reduction of 3-phosphoglyceric acid to glyceraldehyde-3 phosphate and the regeneration of RuBP, 3 ATP and 2 NADPH molecules are used.
• In the production of 1 glucose molecule, 18 ATP and 12 NADPH are consumed.

Regulation of Calvin Cycle

The regulation of the Calvin Cycle takes place in the following way:

• The Calvin Cycle reactions do not occur at night or in the dark. The cycle’s enzymes are regulated by light, especially because the third step requires NADPH.
• Two regulatory systems control when the cycle starts or stops: the thioredoxin/ferredoxin system activates some enzymes, and the RuBisCo enzyme has its activation process involving an activase.
• The thioredoxin/ferredoxin system activates important enzymes in the cycle like glyceraldehyde-3-P dehydrogenase, fructose-1,6-bisphosphatase, and others. This occurs when light is present and ferredoxin, a protein, gets reduced in the thylakoid electron chain
• In the RuBisCo enzyme, a specific lysine amino acid needs to be carbamylated for activation. RuBisCo activase helps this process by enabling the binding of carbon dioxide. Magnesium ion, necessary for RuBisCo’s function, is released from the thylakoid lumen due to proton pumping from the electron flow.
• RuBisCo activase is activated by high ATP concentrations caused by phosphorylation in the stroma.

Functions of Calvin Cycle

The Calvin Cycle performs important functions within the process of photosynthesis, contributing to the overall energy production and growth of plants. Some of the functions of the Calvin Cycle are as follows:

• The primary function of the Calvin Cycle is to fix carbon dioxide from the atmosphere which through a series of chemical reactions is converted to organic molecules like glucose.
• The Calvin Cycle produces glyceraldehyde-3-phosphate (G3P), which can be used to synthesize glucose and other sugars. Glucose is a key source of energy and serves as a storage molecule for later use by the plant.
• The Calvin Cycle is regulated by light-dependent reactions and functions when ATP and NADPH are available. This allows plants to optimize carbon fixation based on the availability of energy from sunlight.
• Organic molecules from the cycle contribute to the synthesis of amino acids, lipids, and other organic molecules required for growth and repair.
• The regeneration of ribulose-1,5-bisphosphate (RuBP) is required for the initial step of carbon fixation, ensuring that the cycle can continue functioning.

Importance of Calvin Cycle

Some of the importance of the Calvin Cycle is as follows:

• Converts carbon dioxide and energy into glucose, which is a fundamental energy source for plants.
• As it is regulated by light-dependent reactions, it optimizes carbon fixation based on sunlight availability.
• It forms the base of the food web or chain by providing energy for plant-eating organisms and higher trophic levels.
• It generates organic molecules that are precursors for amino acids, lipids, and other organic molecules vital for growth and development.
• Stores energy in the form of sugars, providing resources for growth, reproduction, and metabolic processes.

Conclusion: Calvin Cycle

Calvin cycle is also known as C3 cycle. In this cycle, carbon from carbon cycle is fixed into sugars. Calvin cycle steps include: Carbon Fixation, Reduction and Regeneration of RuBP. The Calvin cycle end product is glyceraldehyde-3-phosphate (G3P), a three-carbon sugar molecule. This serve as energy compounds useful in plant growth. The Calvin Cycle diagram is useful in understanding the complex cycle.

FAQs on Calvin Cycle

What is the Calvin Cycle?

The Calvin Cycle is a series of biochemical reactions that occur in the chloroplasts of plants during photosynthesis. It converts carbon dioxide and energy from light-dependent reactions into glucose and other sugars.

Why is the Calvin Cycle Called C3?

The Calvin Cycle is referred to as “C3” because the first stable compound produced during carbon fixation is a three-carbon molecule called 3-phosphoglycerate (3-PGA).

Why is the Calvin Cycle also Called the “Dark Reaction”?

While often referred to as the “dark reaction,” the Calvin Cycle doesn’t directly rely on light but requires the products (ATP and NADPH) generated during the light-dependent reactions.

What are the 4 Steps of the Calvin Cycle Explain?

The 4 steps of the calvin cycle are: Carbon fixation, Reduction phase, Regeneration of RuBP, and Glucose formation.

What is the Full Form of RubisCo?

RubisCo stands for ribulose biphosphate carboxylase oxygenase. RubisCo is an enzyme that catalyzes the process of CO2 fixation during Calvin cycle.

What are the End Product of Calvin Cycle?

The end products of C3 cycle are ADP, NADP, and glucose. ADP and NADP are formed at the end of first stage and glucose is produced in the second stage.

 What are the Main Steps of the Calvin Cycle?

The Calvin cycle involves carbon fixation, reduction phase, and regeneration of the ribulose1,5 bisphosphate (RuBP).

 Where does the Calvin Cycle Take Place?

 The Calvin cycle takes place in the stroma of chloroplasts in plant cells.

 What are the Products of the Calvin Cycle?

The primary product of the Calvin cycle is glyceraldehyde3phosphate (G3P), which can be used to form glucose and other carbohydrates.

How is the Calvin Cycle Diagram used to Explain the Process?

A Calvin cycle diagram visually represents the sequence of reactions, showing how carbon dioxide is assimilated into organic molecules.

What is the Significance of the Calvin Cycle in Photosynthesis?

 The Calvin cycle is crucial for carbon fixation in photosynthesis, converting carbon dioxide and water into glucose, using energy from ATP and NADPH produced in the light-dependent reactions.

What are the Calvin Cycle Reactants and Products?

The Calvin cycle utilizes carbon dioxide, ATP, and NADPH as reactants to produce glucose, which is a key product of photosynthesis.