Light-independent reaction

The process of photosynthesis is a biological procedure in which plants produce oxygen and energy (sugar) by using light from the sun, water, and carbon dioxide (CO₂). The photosynthesis cycle is made up of two stages.

1. Light Dependent Reaction
2. Calvin Cycle or Light Independent Reaction (dark reactions)

Which set of reactions uses H₂O and produces O₂?
As the name indicates, light-dependent reactions use sunlight energy and change this into chemical energy. The energy is saved in the form of NADPH and ATP. Therefore in photosynthesis, NADPH is used for storing energy. The light-dependent reaction of photosynthesis utilizes water (H₂O) and produces oxygen (O₂). The light reactions of photosynthesis supply the Calvin cycle with carbon dioxide to produce sugars.

Light-independent Reaction Definition

What are light-independent reactions in photosynthesis?
The light-independent reaction uses the electrons from the light-dependent reactions to energize the process, particularly during the conversion of inorganic compounds into organic compounds, such as carbohydrates. They do not require light or energy from the sun to initiate the reaction. Light-independent reactions are also identified as the Calvin cycle for the reason that the process is cyclic.

Where do light-independent reactions occur?
The Calvin cycle or light-independent reaction of photosynthesis happens in the stroma of the chloroplast. It has certain enzymes that work with NADPH and ATP. They form carbon-containing molecules (such as glucose molecules) from carbon dioxide. The chloroplast has its genetic makeup, which is separate from the cell. This genetic material is also stored in the stroma.

What is the Calvin Cycle?
In light-dependent reactions, solar energy is used to produce NADPH and ATP. This is the fuel that is further used by light-independent reactions to form carbohydrate molecules. The carbohydrate molecules require carbon atoms. These carbon atoms are provided by carbon dioxide. The carbon dioxide diffuses into the leaves through stomata. The Calvin cycle uses the energy of light-dependent reactions for the manufacturing of carbohydrates and glucose.

Overview of the Calvin Cycle

What is the Calvin cycle or Calvin cycle definition in biology?
In green plants, leaves have small openings known as stomata. Through these stomata, CO₂ enters the plant. From stomata, it reaches into mesophyll cells through the intercellular spaces. When carbon dioxide gets into the mesophyll cells, it circulates into the stroma of the chloroplast. Stroma is the spot where the light-independent reaction occurs. The light-independent reaction is also recognized as the Calvin-Benson cycle, Calvin Cycle, and dark reaction. The name dark reaction creates confusion as the name shows the reaction takes place in darkness but it’s not true. This name is given to this because this reaction does not require sunlight for proceeding.

 

Calvin cycle location/ where does the Calvin cycle take place?
The Calvin cycle takes place in the stroma (the inner space of chloroplasts). See Figure 2.

Light-independent Reactions: Calvin Cycle Steps

Following are the three steps of Calvin Cycle Reaction:

1. Fixation
2. Reduction
3. Regeneration

 

Step 1: Fixation

Where does carbon fixation occur?
In the stroma of chloroplast, carbon fixation takes place. Here, along with carbon dioxide, some other components are also present that help in the reaction. One of them is the ribulose bisphosphate carboxylase (RuBisCO) and the other is ribulose bisphosphate (RuBP).

• RuBisCo is an enzyme that plays a main part in the first step of the Calvin cycle.
• RuBP has 5 atoms of carbon and 2 phosphates at the edges. It is also an enzyme. It helps in catalyzing the reaction. This enzyme is an organic substance that in plants it is the main CO₂ acceptor. It is catalyzed by the RuBisCo.

In the fixation stage, one molecule of RuBP reacts with CO₂. As a result, 2 molecules of 3-phosphoglyceric acid (3-PGA) are formed. There is one phosphate and 3 carbons in each 3-PGA. During the reaction, the number of carbon atoms remains the same despite the formation of new bonds.

15 atoms from 3RuBP + 3 atoms from 3CO₂ = 18 atoms in 3 atoms of 3-PGA

As in this step, carbon dioxide is fixed from an inorganic molecule into an organic molecule — that’s why it is known as carbon fixation.

Step 2: Reduction

For the conversion of 6 molecules of 3-PGA, NADPH and ATP are used. 3-PGA gets converted into G3P- glyceraldehyde 3-phosphate. G3P is an aldotriose phosphate that is the 3-phospho derivative of glyceraldehyde. 3-PGA gains electrons; that’s why it is a reduction reaction.

All the 6 molecules of NADPH and ATP get used. ATP converts into ADP due to the loss of phosphate atoms and energy is released. NADP+ is formed from NADPH due to the loss of H+ and energy. Both of these molecules can be reenergized and reused.

Is NADPH a reducing agent?
The source of energy is ATP but NADPH is the reducing agent because it adds high-energy electrons for the formation of sugar.

Step 3: Regeneration

In this step, for the formation of some other compounds that the plants require, a molecule of G3P goes into the cytoplasm from the Calvin cycle.

G3P comes from the chloroplast and has 3-C atoms. It takes three rounds of Calvin cycle for exporting one G3P. There is the formation of two G3Ps in each round. So, 6-G3Ps form in three rounds.

One of the G3P is exported and the remaining 5-GEPs are used for RuBP regeneration.

RuBP allows the system to get more CO₂ fixation.

In the regeneration reaction further 3 more molecules of ATP are used.

For the process of photosynthesis: Plant Metabolism – Photosynthesis Tutorial

Summary of Calvin cycle Reactants and Products

What does the Calvin cycle produce?

2 G3P, 2 NADP+, and 3 ADP molecules are formed in one round of the Calvin cycle. ADP and NADP+ are not actual products. They are further utilized by light-dependent reaction

A molecule of G3P consists of 3 carbon atoms.

For the Calvin cycle to continue to work, RuBP needs to be regenerated. For this, 5 carbon atoms from 6 of 2 G3P molecules are used. So in each round, there is only net production of 1 carbon.

G3P to Glucose

For the formation of 1 extra G3P, 3 carbon atoms are needed and consequently 3 turns of the Calvin cycle. Therefore, 6 rounds of the Calvin cycle are needed for the formation of one molecule of glucose. The extra G3P can be utilized for the formation of carbohydrates like sucrose, cellulose, and starch.

Products of light-independent reactions

These are the products of the light-independent reactions:

Purpose	Utilize stored chemical energy for carbon dioxide fixation and form glucose as a product
Site of Reaction	Stroma of Chloroplast
Input	ATP, NADPH, CO2
Output	G3P, ADP, NADP+

 

The primary functions of the Calvin Cycle

1. The purpose of the Calvin cycle is the formation of structural building blocks like cellulose, glucose, and starch for plants. For this, the Calvin cycle picks the carbon molecules from the environment and converts them into material needed for plants.
2. As the plants are the base of energy in the environment so the Calvin cycle is very necessary for the stability of an ecosystem. Without this cycle, plants cannot store the energy in such form needed for herbivores, and ultimately no energy will be transferred to carnivores and thus the stability of the ecosystem gets disturbed.
3. The carbon molecules which are created as a backbone in the Calvin cycle are very essential. These molecules are utilized by animals and plants for the formation of lipids, proteins, and nucleic acids, etc.
4. The level of carbon dioxide, which is a greenhouse gas, is also regulated by the Calvin cycle. Humans cut down the forests and due to the large-scale burning of gas, coal, and oil, the level of CO₂ is increasing in the environment day by day which is a major concern.
5. Generally, the chief function of the Calvin cycle is to utilize the ATP and NADPH, which are produce by light-dependent reactions, and convert them into organic material which the plant can utilize for its growth. The organic products are protein, lipids, and glucose.

C4 Photosynthesis

Most of the plants use C3 photosynthesis. In C3, the first carbon compound which is produced has 3 carbon atoms.

But some plants have evolved another form of photosynthesis which is C4. They have adapted this to minimize their loss in dry and hot environments. Due to C4 photosynthesis, the leaves which are formed have a different and unique structure. This structure causes the CO₂ to concentrate in ‘bundle sheath’ cells around Rubisco. There is direct delivery of CO₂ to Rubisco. Due to this the requirement of photorespiration is not required. Because of this adaptability, the stomata remain close and there is less loss of water.

Sorghum, maize, and sugarcane are few examples of plants having C4 photosynthesis.

 

 

Try to answer the quiz below to check what you have learned so far about light-independent reaction.

References

• The Two Parts of Photosynthesis. (2021, January 4). Retrieved August 6, 2021, from https://bio.libretexts.org/@go/page/8997
• The Light-Independent Reactions of Photosynthesis. (11/19/2019). Retrieved August 6, 2021, from https://courses.lumenlearning.com/boundless-biology/chapter/the-light-independent-reactions-of-photosynthesis/
• Photosynthesis. (1996 – 2021). Retrieved August 6, 2021, from https://www.nationalgeographic.org/encyclopedia/photosynthesis/#:~:text=Photosynthesis%20is%20the%20process%20by,in%20the%20form%20of%20sugar.
• The Light Independent Reactions. (2021). Retrieved August 6, 2021, from https://openoregon.pressbooks.pub/mhccmajorsbio/chapter/calvin-cycle/
  Primary Function of the Calvin Cycle. (November 01, 2019).Retrieved August 6, 2021, from https://www.thoughtco.com/the-purpose-of-the-calvin-cycle-608904
• The difference between C3 and C4 plants. (March 18, 2020).Retrieved August 6, 2021, from https://ripe.illinois.edu/blog/difference-between-c3-and-c4-plants.
• Photosynthesis Reactions. (2021).Retrieved August 6, 2021, from https://www.visiblebody.com/learn/biology/photosynthesis/reactions

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