Calvin Cycle: Reactions, Diagram and Photosynthesis

Overview of Calvin Cycle

All organisms on Earth, including humans, are carbon-based life forms. The complex molecules of the amazing human body are built on carbon backbones. There is a chance you already know you are carbon-based, but have you ever thought about where all that carbon comes from?

The carbon atoms in your body were part of carbon dioxide (CO₂) molecules in the air long ago. Carbon atoms enter you and other life forms from the second stage of photosynthesis called the Calvin cycle.

What is a Calvin cycle?

The carbon dioxide in plants passes into the inside of a leaf through pores known as stomata and goes into the chloroplast stroma— the part of the reactions of the Calvin cycle, where sugar is produced. These reactions are also known as light-independent as they are not directly driven by light.

In the Calvin cycle, the atoms of carbon are fixed (integrated into organic molecules) and can build sugars containing three carbon atoms.

This process depends on and is facilitated by ATP and NADPH derived from the light reactions. Unlike the light reactions in the thylakoid membrane, the Calvin cycle reactions occur in the chloroplasts’ stroma or interior (Calvin cycle diagram below).

Reactions of the Calvin cycle

The reactions of the Calvin cycle can be divided into three major stages: 

1. Carbon fixation
2. Carbon Restoration 
3. Restoration of the molecule with which the reaction started.

Here is a general Calvin cycle diagram:

1. Carbon fixation: A molecule of carbon dioxide is combined with a carbon acceptor molecule containing five atoms known as ribulose-1,5-bisphosphate (RuBP). This step gives rise to a compound having six carbon atoms that disintegrate into two molecules of a compound containing three carbons called 3-phosphoglyceric acid (3-PGA). This reaction is catalysed by the enzyme RuBP carboxylase, or rubisco. 
2. Reduction: In the second stage of the Calvin cycle, ATP and NADPH molecules are utilised to change the 3-PGA molecules into a sugar molecule containing three carbon atoms called glyceraldehyde-3-phosphate (G3P). This stage has derived its name from the fact that NADPH donates electrons to a three-carbon intermediate to form G3P. 
3. Regeneration: Some G3P molecules form glucose, while others need to be recycled so that they can regenerate the RuBP acceptor. Regeneration needs ATP and involves a complex series of reactions called the “carbohydrate scramble.” 

For one G3P molecule to exit the cycle and contribute to glucose synthesis, three CO₂​ molecules are required to enter the cycle, giving three new atoms of securely attached carbon. When three molecules of carbon dioxide go into the cycle, six molecules of G3P are formed. One goes out of the cycle and forms a part of glucose, while the other five need to be recycled to restore three molecules of the RuBP acceptor.

Summary of the reactants and products of the Calvin cycle diagram

The Calvin cycle needs to be repeated thrice to produce one G3P molecule that can leave the Calvin cycle and contribute to the formation of glucose. Let us summarise the number of key molecules that go into and come out of the Calvin cycle when one net G3P is produced. In three turns of the Calvin cycle:

1. Carbon: 3 CO₂ atoms combine with 3 RuBP acceptors, forming​ six glyceraldehyde-3-phosphate (G3P) molecules. 1 G3P molecule leaves the cycle and contributes to making glucose. 5 G3P molecules are recycled, restoring three molecules of RuBP acceptor.
2. ATP: 9 ATP are converted into 9 ADP (6 in the fixation step and 3 in the regeneration step).
3. NADPH: During the reduction step, 6 NADPH is converted to 6 NADP+. A G3P molecule has three carbon atoms, so it borrows two G3Ps to form a glucose molecule with six carbon atoms. It would require six repetitions of the cycle, or 6 CO₂, 18 ATP, and 12 NADPH to produce one glucose molecule.

Photosynthesis of Calvin cycle

Photosynthesis Calvin cycle is a combination of biological and chemical processes occurring in all green plants or autotrophs that produce organic molecules from carbon dioxide (CO₂), which contain several carbon-hydrogen (C–H) bonds and are highly reduced than CO₂.

Photosynthesis has two stages:

Light-dependent reactions: As the name depicts, these reactions require light and primarily occur in the daytime.

Reactions independent of light: It is also referred to as the dark reaction or Calvin cycle and occurs both in the presence and absence of sunlight.

Plant cells utilize raw materials given by the light reactions to build organic molecules:

1. Energy: ATP is supplied by cyclic and noncyclic photophosphorylation reactions, and it facilitates endergonic reactions.
2. Reducing power: Photosystem I produces NADPH, the source of hydrogen and the energetic electrons needed to bind them to carbon atoms. A considerable amount of the light energy arrested during photosynthesis turns out to be in the high-energy C—H sugar bonds.

Light energy is stored by plants in the form of carbohydrates, especially starch and sucrose. The carbon and oxygen needed for this process are derived from CO₂, and the energy for carbon fixation is obtained from the ATP, and NADPH generated during the process of photosynthesis.

The process of formation of carbohydrates from CO₂ is known as the Calvin Cycle. It is named after the scientist Melvin Calvin who discovered it. The plants that perform carbon fixation through the Calvin cycle are called C₃ plants.

The shared developmental past of photosynthetic organisms is striking, the fundamental processes having changed to a negligible extent over periods. The process of photosynthesis Calvin cycle remains the same in living beings ranging from the giant tropical rainforest leaves to the minuscule cyanobacteria.

The components of photosynthesis are also the same involving the use of water as an electron donor. The function of the photosystems is to absorb light and utilize electron transport chains to change energy into usable forms. The reactions of the photosynthesis Calvin cycle combine molecules of carbohydrates with this energy.

However, as is the case with all biogeochemical cycles, several conditions result in adaptations that influence the fundamental pattern. In dry-climate plants, the process of photosynthesis has changed with adaptations that preserve water. Every drop of water in harsh scorching heat and energy source must be utilized for survival. In such plants, several adaptations have evolved.

In one form, more sustainable use of CO₂ lets plants carries out photosynthesis even when carbon dioxide supply is low, as, on hot days, stomata are not open. Another adaptation allows the plant to perform preliminary Calvin cycle reactions at night, as the stomata opening at this time saves water due to lower temperatures. Additionally, due to this adaptation, plants can perform lower photosynthesis levels without even opening the stomata, a severe mechanism for facing extremely dry and hot climates.

Calvin Cycle needs the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly referred to as RuBisCO. It gives rise to the triose phosphates, 3-phosphoglycerate (3-PGA), glyceraldehyde-3P (GAP), and dihydroxyacetone phosphate (DHAP) all of which are utilized in the synthesis of the hexose phosphates fructose-1,6-bisphosphate and fructose 6-phosphate.

Products of C3 Cycle

1. Each repetition of the Calvin cycle fixes one carbon molecule.
2. While the Calvin cycle needs to be repeated thrice to create one glyceraldehyde-3 phosphate molecule.
3. One glucose molecule is formed when two glyceraldehyde-3 phosphate molecules combine together.
4. Three molecules of ATP and 2 of NADPH are utilized in the process of 3-phosphoglyceric acid reduction into glyceraldehyde-3 phosphate and in the restoration of RuBP.
5. Eighteen ATP molecules and twelve NADPH molecules are used up in the formation of one glucose molecule.

Main Points of the C₃ Cycle

C₃ cycle means the reaction of photosynthesis that happens in the dark.

It depends on light independently, and the important carriers of energy are products of light-dependent reactions. In the initial stage of the Calvin cycle, the light-independent reactions start, and carbon dioxide is fixed.

In the next stage, 3 PGA is reduced to G3P by ATP and NADPH. ATP and NADPH are then turned into ATP and NADP+. In the last stage, RuBP is restored. This helps to fix more carbon dioxide.

Frequently Asked Questions

1. What is a Calvin Cycle? 

A: The Calvin cycle also referred to as the C₃ cycle, is the name given to the series of biochemical reactions which involve the rooting of the carbon atom from the carbon cycle into sugars. It occurs in the plant cell chloroplast.

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

A: Calvin cycle occurs in three main steps, namely: 

1. Carbon fixation
2. Carbon reduction
3. Restoration or regeneration 

3. What is a Calvin cycle end product?

A: The carbon dioxide is attached to organic intermediates that are stable in the carbon fixation step of the Calvin cycle.

4. What is carbon fixation in the Calvin cycle?

A: The third step of the Calvin cycle is known as regeneration as Ribulose-bisphosphate that commences the cycle and is restored from G₃P.