Chapters
Like all organisms on Earth, all of us, are carbon-based life forms. It implies that carbon is the backbone of the complex molecules present in our bodies. We already understand that we are the carbon-based life form, however, have you ever wondered from where we get all this carbon?
Well, here is the answer. The carbon atoms in our bodies were once the constituent of carbon dioxide molecules in the air. Atoms of carbon are present inside us and in other organisms due to the second stage of photosynthesis which is called the Calvin cycle (or light-independent reactions).
In the next section of the article, we will discuss the Calvin cycle in detail.
The Calvin Cycle
The Calvin cycle is an energy consuming process that builds carbohydrates from smaller molecules. It employs ATP as a source of energy and NADPH as a reducing agent to add high-energy electrons for the synthesis of sugar. Carbon enters the cycle in the form of carbon dioxide and exits the cycle in a sugar molecule. To create a single molecule of sugar (G3P), the cycle is completed three times. It means that it fixes three molecules of carbon from carbon dioxide.
Now, the question arises that how the carbon dioxide enters the plant? Well, the plants have specialized pores known as stomata which allow the carbon dioxide to enter them through diffusion across these pores. It reaches the mesophyll cells by diffusing through intracellular spaces. Once the carbon dioxide is in the cell, it reaches the stoma of the chloroplasts.
In the next section of the article, we will discuss the three stages of the Calvin cycle in detail.
Stages of the Calvin Cycle
- Energy from ATP and hydrogen from reduced NADP are transferred from the light-independent stage to the light-dependent stage of photosynthesis. Remember that there are two stages of photosynthesis: light-independent and light-dependent. The light-independent stage occurs in the presence or absence of light, whereas the light-dependent stage requires light.
- The energy and hydrogen are employed during the light-independent reactions (these reactions are collectively called the Calvin cycle) to generate complex organic molecules which include carbohydrates like:
- Starch for storage
- Cellulose which is used in the composition of cell walls
- Sucrose which is employed for translocation around the plant
- We know that the light-independent stage of photosynthesis does not require light and it can occur in light as well as dark. However, because it needs inputs of ATP and reduced NADP from the light-dependent stage, therefore, the Calvin cycle is unable to proceed indefinitely in the dark because these inputs will run out and it will eventually stop.
- The light-independent reactions that take place during the Calvin cycle can be divided into the following three stages:
- Carbon Fixation: Rubisco catalyzes (speeds up) the carbon dioxide fixation by combining with a molecule of ribulose bisphosphate (RuBP), a 5C compound, to generate two molecules of glycerate 3-phosphate (GP), a 3C compound
- Reduction: In a reaction that includes reduced NADP and ATP, the reduction of GP to triose phosphate (TP) takes place
- Regeneration: In reactions that utilize ATP, RuGP is regenerated from TP.
Now, let us discuss these three stages in more detail.
Carbon Fixation
- Carbon dioxide combines with ribulose bisphosphate which is a five-carbon (5C) sugar
- An enzyme known as rubisco (ribulose bisphosphate carboxylase) catalyzes, i.e. speeds up this reaction
- The resultant six-carbon (6C) compound is unstable and divides into two
- This yields two molecules of three carbon (3C) compounds called glycerate 3-phosphate (GP)
- Now, the carbon dioxide has been fixed which means that it has been eliminated from the external environment and it now the part of the plant cell
- Glycerate 3-phosphate (GP) is not a carbohydrate, however, the subsequent stage of the Calvin cycle transforms it into a carbohydrate.
Glycerate 3-phosphate Reduction
- Energy from ATP and hydrogen from reduced NADP both generated during the light-dependent stage of the photosynthesis are employed to reduce glycerate 3-phosphate (GP) to phosphorylated three-carbon (3C) sugar, referred to as triose phosphate (TP)
- One-sixth of the triose phosphate (TP) molecules are utilized to yield important organic molecules for the plant such as:
- Sub arrows) Triose phosphate (TP) is able to condense to become hexose phosphates (6C) which can be employed to produce carbohydrates such as starch, cellulose, and sucrose
- Triose phosphates can be transformed into glycerol, whereas the glycerate 3-phosphates can be converted into fatty acids that combine to form lipids for cell membranes
- Triose phosphates can be employed to produce amino acids for the synthesis of protein
Regeneration of Ribulose Bisphosphate
- Five-sixths of the triose phosphate (TP) molecules are employed in the regeneration of ribulose bisphosphate (RuBP)
- This process needs ATP
In the next section of the article, we will discuss that Calvin cycle intermediates are employed to produce other molecules.
Calvin Cycle Intermediates
- Intermediate molecules of the Calvin cycle, for example, glycerate 3-phosphate and triose phosphate) are utilized in the production of other molecules
- Glycerate 3-phosphate (GP) is employed in the production of other molecules
- Triose phosphate (TP) is employed to produce the following:
- Hexose phosphates (6C) are employed to produce carbohydrates such as starch, cellulose, and sucrose
- Amino acids for the synthesis of proteins
- Lipids for cell membranes
Summary of the Calvin Cycle
- The Calvin cycle employs carbon dioxide for the synthesis of sugar
- Carbon dioxide enters the plants through tiny pores known as stomata in the leaves
- Carbon dioxide is integrated into a 5 carbon sugar, RuBP, which results in the immediate formation of two molecules of 3-PGA which is catalyzed by rubisco
- 3-PGA is transformed into G3P, with 1,3-bisphosphoglycerate as an intermediate, needing ATP and NADPH
- Some of the G3P is utilized in the synthesis of sugar, whereas the remaining is employed in the regeneration of RuBP
- Photorespiration is a wasteful process that takes place due to the build-up of oxygen within the cell.
- Three turns of the Calvin cycle are required to make a single G3P molecule that can leave the cycle and go towards creating glucose.
Did you like this article? Rate it!
Loading...
Keep on teaching us,you are excellent teachers
This is great
Thanks a lot for this book,it really helped me a lot
It’s useful to me
Thanks a lot for your Better book!
It’s a perfect article, go ahead