Light independent reaction

When you think of light independent reaction what is the first thing that comes to mind ? The reaction which doesn't need light and the second thing that comes to your mind is Calvin cycle. Ever wondered why you end up thinking of Calvin cycle ?

Light independent reaction is linked with Calvin cycle. It's okay we all don't know in the start so in this article we will be going through what is light independent reaction ? Calvin cycle and the end products of this reaction. 

What is light independent reaction ?

The reaction which does not require light is known as light independent reaction. This reaction is also known as dark reaction because it does not require any light. This reaction takes place in the stroma of the chloroplast

This is the second phase of photosynthesis. The first phase of photosynthesis is light dependent reaction which requires light. The main aim of this reaction is to reduce CO2 to form carbohydrates with the help of the energy that is provided by the assimilatory power. The assimilatory power which is used in this reaction is formed in the light reaction, so you can tell that sun is being used indirectly.

You must be thinking what is assimilatory power ? Let me tell you, the power which is in plants in the form of ATP and NADPH which helps in obtaining food in the form of carbohydrates which is done by reducing CO2 during photosynthesis. 

When CO2 is reduced it is known as CO2 assimilation, CO2 fixation or carbon reactions. To understand in simple form of what light independent reaction means, you can simply remember that in this reaction CO2 is fixed or reduced to glucose. For this reaction the products that are formed in the light dependent reaction such as ATP and NADPH are used. 

The process of carbon fixation is different in for different plants. In total there are three types of fixations which are as follows:
  1. Calvin cycle 
  2. Hatch - Slack cycle 
  3. CAM cycle 
Our main focus is going to be on Calvin cycle because the rest are just modifications of Calvin cycle which are used by different plants.

Calvin cycle:

The cycle in which RuBP is generated in order to form PGA is called Calvin Cycle, didn't understand a thing right ? Let me tell you in detail now. 

Dr. Melvin Calvin was the one to discover the Calvin cycle in the year 1954, now you know why this is called as Calvin cycle. Calvin cycle is a basic mechanism by which CO2 is fixed in order to form carbohydrates.

According to Calvin, he showed the correct "path of carbon" in photosynthesis along with his co-researchers A.A. Benson, J. Bashman, etc. hence this is also known as Calvin-Benson cycle. The first stable compound formed in this cycle is a 3C compound hence Calvin called it as C3 cycle or C3 Pathway and the plants that use C3 cycle are known as C3 plants.
Calvin conducted experiments on unicellular green algae such as Chlorella and Scenedesmus. For this experiment he used a radioactive isotope of carbon C14 as tracer. He experimentally showed and proved that the biochemical reactions which leads to glucose synthesis takes place in a cyclic manner. 

Giving and overview of the cycle, there is a substance present in the stroma of the chloroplast which is known as the initial acceptor that accepts CO2 from the atmosphere which further form the first product of photosynthesis. The substance undergoes many changes and then the final product is formed i.e. glucose. This process takes 90 seconds to form glucose. RuBP is the initial acceptor. After glucose is formed the initial acceptor keeps regenerating to keep the process going on.

This was just an overview of how the reaction occurs. Now we will be looking at the steps in which this reaction occurs.

Steps involved in Calvin cycle:

The reactions in Calvin cycle take place in four major steps:
  1. Carboxylation of RuBP 
  2. Reduction of PGA 
  3. Sugar formation/Synthesis of glucose
  4. Regeneration of RuBP 
Sounds like a piece of cake right ? Actually, it's not. These steps are very confusing if you go to learn them at once the reason why I'll be explaining each and every step as slow as I can. 

1. Carboxylation of RuBP:

As we know that there is a substance present in the stroma that accepts CO2 from the atmosphere. That CO2 is accepted by a 5-carbon compound called Ribulose-1-5-di-phosphate i.e. RuDP or Ribulose-1-5diphosphate i.e RuBP. CO2 is accepted in the presence of RuDP carboxlyase or RuBP carboxylase (RuBisCo) which later forms a 6-carbon unstable compound

Related article: Dual nature of RuBisCO

This 6-carbon unstable compound is then split by hydrolysis into two molecules of 3-carbon compound i.e Phosphoglyceric acid which is our first stable compound of the Calvin cycle and this compound is formed in the presence of same enzymes. 

The process in these steps might be a little hard to understand the reason why I have prepared a flowchart for you to refer to for a quick glance of Carboxylation of RuBP. 

 Carboxylation of RuBP flowchart
Carboxylation of RuBP flowchart

2. Reduction of PGA:

Since PGA is formed it has to be reduced in order to move the cycle ahead. For that PGA has to be phosphorylated. Phosphorylation means to add a phosphate group to an organic compound. PGA is phosphorylated using ATP to produce 1, 3di-phosphoglyceric acid

This 1, 3di-phosphoglyceric acid is then reduced to produce phospho-glyceraldehyde (PGAL) using NAPDH2 further releasing inorganic phosphate. 

Some of the 3PGAL is converted to its isomer Dihydroxyacetone phosphate (DHAP) in the presence of the enzyme triose-phosphate isomerase

Here is a flowchart of reduction of PGA for quick glance:

 Reduction of PGA flowchart
Reduction of PGA flowchart

3. Sugar formation/Synthesis of glucose:

Six turns of Calvin cycle or six molecules of RuBP and six molecules of CO2 are required for every one molecule of glucose. In total there are 12 molecules of PGAL, out of those 12 only 2 are used for the synthesis of glucose i.e. 1/6 part of PGAL. 

By combining one molecule of PGAL and one molecule of DHAP, one molecule of fructose, 1 6-diphosphate is obtained. Dephosphorylation takes place formed fructose-6-phosphate. As phosphorylation means adding of one phosphate group, dephosphorylation has a totally opposite meaning. It means that there is removal of a phosphate group. 

After dephosphorylation takes place you get fructose-6-phosphate and then it isomerizes into glucose-6-phosphate. Again there is dephosphorylation taking place and glucose-6-phosphate loses a phosphate group to produce glucose

The glucose that is produced at is stored as starch or utilized. 

Here is a flowchart of sugar formation/synthesis of glucose for quick glance:

 Sugar formation/Synthesis of glucose flowchart
Sugar formation/Synthesis of glucose flowchart

4. Regeneration of RuBP:

You know regeneration of RuBP is required in order to keep the cycle on going, for that RuBP is regenerated in sever biochemical reactions which are called sugar phosphate interconversions. That means all the compounds or the intermediates that are formed during this reaction are sugar phosphates. 

There were 12 molecules from that we have already used 2 for the synthesis glucose and the remaining 10 molecules are going to be used to regenerate 6 molecules of ribulose mono-phosphate (RuMP) i.e 5/6 part of PGAL which was remaining is now used. 

After RuMP is formed, it is phosphorylated to RuBP by using ATP. To produce 6 RuBP, 6 ATPs are required

Here is a flowchart of regeneration of RuBP for quick glance:

 Regeneration of RuBP flowchart
Regeneration of RuBP flowchart

How many NADPH2 and ATPs are required for the formation of one glucose molecule ?

12 NADPH2 and 18 ATPs are required for the formation of one glucose molecule. 

C3 plants:

The plants that use Calvin cycle to fix CO2 are called C3 plants. It is known that about 85% of plant species are C3 plants. These plants are found in wet and cool climate. 

Sunflower, wheat, rice, cotton, spinach, beans, barley, oats, etc are some of the examples of C3 plants. 

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