An Overview of Calvin Cycle - Stages Of C3 Cycle

Jul 04, 2023, 16:45 IST

Like every living thing, we are a carbon-based life entity. In other words, the intricate molecules that make up our incredible body are supported by carbon atoms. Although you may already know the carbon basis, have you ever wondered where that carbon originates?

It turns out that the carbon atoms in your body were previously a component of the air molecules of carbon dioxide. Thanks to the Calvin cycle, the second step of photosynthesis, carbon atoms are in you and other living things. (or the light-independent reactions).

To "fix" carbon from CO2 into three-carbon sugars, plants go through a chains of chemical processes known as the Calvin cycle. Plants and animals can later transform these three-carbon substances into amino acids, nucleotides, and more complex carbohydrates like starches. Most new organic matter is produced by "carbon fixation."

Calvin Cycle Introduction

• Carbon dioxide diffuses into the stroma of the chloroplast, the location of the Calvin cycle processes, where sugar is generated, in plants, where it enters the inside of a leaf through holes known as stomata. These are sometimes known as "light-independent reactions" since light is not their energy source.
• In the Calvin cycle, three-carbon sugars are created by fixing carbon atoms from carbon dioxide and incorporating them into organic molecules. ATP and NADPH from the light reactions are used as fuel and are necessary for this process. Calvin cycle reactions occur in the stroma as opposed to the thylakoid membrane, where light reactions occur. (the inner space of chloroplasts).

Calvin Cycle

Since the Calvin cycle is not directly fueled by photons from the Sun, it is also frequently referred to as the "light-independent" process of photosynthesis. Instead, ATP and NADPH, produced by using the energy from photons in the light-dependent processes, drive the Calvin cycle.

Calvin Cycle Steps

Fixation of Carbon

• A CO2 molecule from the atmosphere joins with the five-carbon acceptor molecule ribulose-1,5-bisphosphate during the carbon fixation process. (RuBP).
• The resultant 6-carbon complex is subsequently divided into two molecules of 3-phosphoglyceric acid with three carbons. (3-PGA).
• The RuBP carboxylase/oxygenase enzyme, sometimes called RuBisCO, is the catalyst for this process. RuBisCo is perhaps the most prevalent enzyme on Earth because of its significant role in photosynthesis.

Reduction

• The molecules of glyceraldehyde-3 phosphate(G3P), a simple sugar, is formed from the 3-PGA molecules produced during carbon fixation in the second step of the Calvin cycle.
• ATP and NADPH produced through photosynthesis' light-dependent processes serve as the stage's energy sources. Thus, plants use the Calvin cycle to transform solar energy into compounds that can be stored for a long time, such as sugars. The ATP and NADPH deliver their energy to the carbohydrates.
• The reason this process is known as "reduction" is because NADPH provides electrons to the molecules of 3-phosphoglyceric acid to produce glyceraldehyde-3 phosphate. In chemistry, giving an electron is called "reduction," whereas receiving an electron is called "oxidation."

Regeneration

• While certain molecules of glyceraldehyde-3 phosphate are recycled to create the five-carbon RuBP complex that is utilised to take in fresh carbon molecules, other molecules must be recycled to create glucose.
• ATP is needed for the regeneration process. It is a multi-step, complicated process.
• To create one glucose molecule, this cycle must be carried out six times as it takes six carbon molecules to create one.

Products of Calvin Cycle

• One carbon molecule that can be converted into sugar is "fixed" during each cycle turn of the Calvin process.
• For the Calvin cycle to produce one molecule of glyceraldehyde-3-phosphate, three rotations are necessary.
• After six cycles of the Calvin cycle, two glyceraldehyde-3 phosphate can be joined to create a glucose molecule.
• Reducing (3 electron additions to 3-phosphoglyceric acid to form glyceraldehyde-3 phosphate) and renewing RuBP to allow them to receive a new carbon atom from CO2 from the air consume 3 ATP and 2 NADPH throughout each round of the Calvin cycle.
• This indicates that 18 ATP and 12 NADPH create a single glucose molecule.

Regulation of Calvin Cycle

RuBisCO is the main enzyme employed in the first stage of carbon fixation, and its enzymatic movement is well controlled. Numerous elements, such as ion particles, ATP, ADP, CO2, reduction, oxidation states, and phosphate, influence the movement of the RuBisCO enzyme. The many factors affecting RuBisCO activity directly affect the first phase of the Calvin cycle.

The catalyst aldolase combines G3P and DHAP to create fructose-1,6-bisphosphate in the subsequent stage or phase. One of the two G3P atoms formed is also converted to dihydroxyacetone phosphate (DHAP). The glycolytic catalyst enzyme aldolase is typically described as having the ability to split fructose 1,6-bisphosphate into DHAP and G3P.

The Calvin cycle's third phase, or step, entails the regeneration of RuBP. This stage consists of reactions regulated by different enzymes or catalysts. As follows:

• All Glyceraldehyde 3-phosphate molecules are converted into DHAP by the enzyme trio phosphate isomerase. (dihydroxyacetone phosphate).
• Glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate are converted into fructose-6-phosphate by the enzyme aldolase and fructose 1, 6 bisphosphates.
• To produce erythrose 4-phosphate, the transketolase enzyme removes two carbon atoms from fructose 6-phosphate. (E4P).
• Glyceraldehyde 3-phosphate (G3P) is combined with the two fructose 6-phosphate removed carbons to produce xylulose-5-phosphate. (Xu5P).
• Sedoheptulose-1,7-bisphosphate is produced when the aldolase enzyme converts erythrose 4-phosphate (E4P) and a DHAP (dihydroxyacetone phosphate).
• By removing a phosphate group, sedoheptulose-1,7-bisphosphatase converts sedoheptulose-1,7-bisphosphate into sedoheptulose-7-phosphate (S7P).
• By removing two carbons from S7P and moving two carbons to one of the Glyceraldehyde 3-phosphate (G3P) molecules, the transketolase enzymes produce ribose-5-phosphate (R5P) and another xylulose-5-phosphate.
• The enzyme phosphopentose isomerase converts ribose-5-phosphate (R5P), which is encoded by the RPIA gene, into ribulose-5-phosphate. (Ru5P)
• The RPE gene's phosphopentose epimerase catalyses or switches from xylulose-5-phosphate (Xu5P) to ribulose-5-phosphate. (Ru5P).
• An essential photosynthetic enzyme called phosphoribokinase catalyses the conversion of ribulose-5-phosphate (Ru5P) into ribulose-1,5-bisphosphate.

The Calvin cycle is regarded as finished after this final enzyme has undergone this transformation. The last stage of the Calvin cycle is thought to be the most intricate and tightly controlled.

The Calvin Cycle's Function

The Calvin cycle's primary purpose is to produce three-carbon sugars, which may subsequently be utilised to construct other sugars like glucose, starch, and cellulose, which all plants employ as building blocks.

• The Calvin cycle uses them by directly converting airborne carbon molecules into plant material.
• In most ecosystems, where plants are the primary energy source, the Calvin cycle is essential to survival.
• Without the Calvin cycle, plants could not store energy in a way that herbivores could eat. As a result, herbivores' bodies would not contain any energy that carnivores could access.
• Proteins, nucleic acids, lipids, and all the other components of life are made by plants and animals from the carbon backbones produced during the Calvin cycle.
• Carbon dioxide, a greenhouse gas, is similarly controlled by the Calvin cycle in the Earth's atmosphere.
• Scientists are concerned because humans have decimated around half of Earth's forests, which are crucial in absorbing CO2 from the atmosphere and releasing enormous amounts of CO2 back into the atmosphere through burning coal, oil, and gasoline.

C3 Cycle byproducts

• With each cycle of the Calvin equation, one carbon molecule is fixed.
• In three cycles of the Calvin cycle, glyceraldehyde-3-phosphate is produced in one molecule.
• The union of two glyceraldehyde-3-phosphate molecules creates one glucose molecule.
• The conversion of 3-phosphoglyceric acid to glyceraldehyde-3-phosphate and the regeneration of RuBP both need 3 ATP and 2 NADPH molecules.
• One glucose molecule requires the use of 18 ATP and 12 NADPH.

Calvin Cycle Modifications

Given that the fundamental mechanism has changed little over time, all photosynthetic species have a similar evolutionary history. The method and elements of photosynthesis that use water as an electron donor are generally the same in all forms of photosynthesis, including the enormous tropical leaves in the rainforest and small cyanobacteria. Photosystems employ electron transport chains to transfer energy while absorbing light. With this energy, the Calvin cycle processes put carbohydrate molecules together.

However, like other biochemical processes, different environmental factors result in different adaptations that change the fundamental structure. Dry-climate plants have evolved water-saving adaptations for photosynthesis. Every drop of water and valuable energy must be used to survive in the stifling dry heat.

Such plants have evolved two adaptations. In one way, plants may still photosynthesise even when there is a shortage of CO2, such as when the stomata are closed on hot days. The other adaptation carries out the Calvin cycle's first responses at night because the colder temperatures make opening the stomata more efficient. Additionally, as a drastic defence against prolonged periods of severe aridity, this adaptation has enabled plants to do little photosynthesis without opening stomata.

Frequently Asked Questions(FAQs) Calvin Cycle

Q1. How Important Is Carbon Fixation?

Ans. Carbon is required for the body to produce various nutrients in living things. The structural building blocks for various supplements are framed by carbon. Carbon dioxide will serve as the main source of carbon. Carnivores, omnivores, and heterotrophs cannot directly absorb carbon dioxide in their bodies. For carbon to be present in the natural structure, it must rely on various organic organisms. Plants or other autotrophs can only convert low-energy inorganic carbon dioxide into high-energy natural molecules like glucose, cellulose, and starch. In this sense, the Calvin cycle's most crucial stage is framed by the fixation with carbon.

Q2. Why is the Calvin cycle beneficial to both plants and animals?

Ans.Additionally, plants and animals use the carbon framed during the Calvin cycle to create other nutrients including as proteins, lipids, nucleic acids, and other essential nutrients needed continuously. Direct removal of carbon from the atmosphere through the Calvin cycle results in the creation of plant materials. Most biological systems require the Calvin cycle to function, and plants provide the base of the energy pyramid in these systems. Plants couldn't store energy in a form that herbivores could digest without the Calvin cycle. Thus, carnivores would avoid the energy stored in that part of a herbivore's body.

Q3. What does the Calvin Cycle serve?

Ans. The Calvin Cycle is primarily used to produce three-carbon sugars. Plants use these sugars to build a variety of structures. Carbon is directly collected from the air and converted entirely to plant matter during the Calvin cycle. With the aid of the Calvin cycle, plants may store energy in a form accessible to herbivores. Finally, predators use the energy stored by herbivores for their endurance.

Q4. What does C3 stand for?

Ans. The Calvin-Benson-Bassham (CBB) cycle and the Reductive pentose phosphate cycle are alternate names for the Calvin or C3 cycle. The C3 cycle needs ATP and NADPH provided in the light responses to operate and is to some extent light-sensitive. More carbon is fixed thanks to recovered RuBP in the final step. The given sugars are used as energy reserves or energy storage.

Q5. What is the Calvin cycle's application to people?

Ans. The Calvin cycle also regulates carbon dioxide levels, a gas that depletes ozone, in the atmosphere. Researchers are concerned about the increasing levels of carbon dioxide because humans are cutting down trees and burning fossil fuels more than necessary, reducing the amount of carbon dioxide in the atmosphere.