Table of Contents

What is Calvin-Benson Cycle?

Let’s begin the post with some basic questions related to the C3 cycle.

What is Calvin-Benson Cycle? Why Most of the Plants are C3 Plants? What are the examples of C3 plants? Why C3 cycle is so-called? How many steps are involved in C3 cycle?

Also read the post about C4 cycle and C4 plants

Also read about kranz anatomy in C4 plants

Calvin cycle is a cyclic process in the light-independent or dark phase of photosynthesis, in which CO2 (carbon dioxide gas) is reduced in the absence of sunlight, and glucose molecule (CO2 reduced into Glucose in the absence of sunlight) is formed.

The complete reaction of this cycle was first discovered by Calvin and Benson. All the reactions of this entire pathway were carried out by Calvin, Benson, and their colleagues using radioactive carbon (14C) in the form of carbon dioxide gas.
For this, he used two algae. which are namely, Chlorella and Scenendesmus.

Calvin and his colleagues observed that radioactivity appeared just after three seconds in first stable intermediate 3 carbons compound called Phosphoglyceric Acid.

Phosphoglyceric acid is the first stable intermediate and which is three carbons organic compound of the Calvin cycle, that is it contains 3-carbons, hence this cycle is also known as the C3 cycle.

The Callvin cycle is a universal pathway of carbon dioxide fixation in all types of plants on the earth, it means any plants, whether it is C3-plants or C4-plants they use C3 pathaway for CO2 fixation and formation of glucose.

It is also called Photosynthetic Carbon Reduction-PCR Cycle because in this carbon dioxide gas (CO2 gas reduced into glucose) is reduced into glucose.

This whole process takes place in that conditions of photosynthesis where there is no direct need of sunlight instead the product of light reactions are used in C3 cycle. The primary CO2 acceptor in this process is a 5-carbons compound called RuBP-ribulose 1,5 bisphosphate.

Phases of Calvin Cycle

The entire Calvin cycle is divided into three phases, which are namely as  –
(1) Carboxylation- fixation of CO2
(2) Reduction- conversion of CO2 into sugar
(3) Regeneration- recreation of RuBP-ribulose 1,5 bisphosphate

Lets understand each of these steps.


In this step, CO2 gas is fixed in a stable organic intermediate chemical.

Carboxylation is an important step in the Calvin cycle. where CO2 is used in the carboxylation of RuBP-ribulose 1,5 bisphosphate.

Enzyme RuBisCO (Ribulose 1,5 bisphosphate Carboxylase Oxygenase) is required to complete this step.

It is the most abundant enzyme or protein on earth.

Carboxylation of RuBP-ribulose 1,5 bisphosphate leads to the formation of two molecules phosphoglycerate (3 PGA), which have 3 carbons. That’s why Calvin Cycle is also called C3-cycle.


This is a reaction of several steps, in which Phosphoglycerate changes into glucose.

Here 2 molecules of ATP and 2 molecules of NADPH (Per CO2 gas fixation needs 2 molecules of ATP & 2 molecules of NADPH-Nicotinamide adenine dinucleotide phosphate) are needed to fix one carbon dioxide.

Therefore total of 6 Calvin cycles runs to make one glucose (6 turns of Calvin cycle required to fix 6 CO2 molecules).


Regeneration of RuBP-ribulose 1,5 bisphosphate, takes place in this step so that the Calvin cycle continues without any uninterruptions. A molecule ATP is used in the regeneration of RuBP-ribulose 1,5 bisphosphate.


1 – Who discovered the C3 cycle?
It was discovered by Calvin & Benson.

2 – Why is the Kelvin cycle also called the C3-cycle?

Phosphoglyceric acid is the first stable intermediate three carbons organic compound of the Kelvin cycle, which contains 3-carbons, hence this cycle is also known as the C3 cycle.

3 – What is the other name of the Kelvin Cycle?
The full name of the Calvin cycle is also the C3 cycle.

4 – Who is the primary CO2 acceptor in the Calvin cycle?
The primary CO2 acceptor in the Calvin cycle is the 5-carbon RuBP.

5 – How many steps are there mainly in Calvin Cycle?   

There are three main steps in Calvin Cycle, namely – Carboxylation, Reduction and Regeneration

6 – How much ATP is used to fix one CO2?
It takes 2-ATP to fix one CO2.

7 – How many times the Calvin cycle is runs to make one glucose?
six turns of Calvin cycle goes on to make glucose.

8-Where does the Calvin cycle take place?
The Calvin cycle takes place in the stroma of the chloroplast.

9 – How many ATP is used to regenerate one RUBP?

It takes 1-ATP to regenerate one RUBP.


We have tried to cover almost all the points related to Calvin Cycle, and have also shared important questions at the end.

But still any kind of suggestion or update and if you see any mistake, then you must tell us.

We will try our best to update your suggestion or any mistake which happened somewhere in the post.

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