Calvin Cycle – Photosynthesis in Plants (PART -III)

The term Photosynthesis was first proposed by Prof. Conway MacMillan , when he was working at University of Minnesota in 1893.

In the earlier blog posts we discussed about Introduction to Photosynthesis in green Plants and details of Light Reaction.

As we have seen in previous posts, Photosynthesis can be divided into two different parts : Light reaction – where plant cells convert energy from sunlight into chemical energy in the form of ATP and NADPH along with release of oxygen.

In the next phase, the products of light reaction (ATP and NADPH) are used, along with water for synthesis of food or carbohydrates by fixing Carbon di oxide (CO₂).

Calvin Cycle:

• This Process of converting CO2 , using water and products of light reactions ( ATP and NADPH) into carbohydrates is called Calvin Cycle, named after the scientist who discovered the pathway and because these reactions occur in the form of a cycle.
• It is also called as Calvin-Benson-Bassham cycle, to include names of other discoverers. The other name of this process is “Dark reaction“, because all of its steps are light independent. However the term DARK is misnomer. as it implies these reactions occur only in Dark. It is important to remember that all the steps of Dark reactions also occur during day, similar to light reaction.
• The reactions of Calvin cycle or Dark reactions are dependent on the products (ATP and NADPH) formed during light phase for fixing CO₂.
• ATP and NADPH acts as connecting link between light and dark relation of photosynthesis.
• Calvin cycle reduce carbon atoms from their fully oxidized state as carbon dioxide to the more reduced state as a carbohydrate.
• Calvin cycle takes place in Stroma ( space between chloroplast membrane and Thylakoid membranes ).
• Light reactions occur in thylakoid membrane but ATP and NADPH produced during the reactions, end up in Stroma-the site for Calvin cycle.
• Calvin cycle forms a very important process in plants as it helps in trapping CO₂ ( a gaseous waste product of respiration) and convert it into a form that is useful for many other processes.
• The intermediate products (or their derivatives) of Calvin cycle will become constituents of nucleic acids, proteins, and fats in all living organisms.

The Calvin cycle can be divided into three stages: Carboxylation or Fixation, reduction and regeneration.

Carboxylation of Fixation:

• The starting point in the Calvin cycle and synthesis of sugars in plants is fixing of CO₂ , which happens to be a the gas that is a waste product of respiration in majority of organisms such as microbes, fungi, plants, and animals.
• The products of Calvin cycle brings into living systems the carbon atoms that will form the part of nucleic acids, proteins, and fats.
• CO₂ enters through the stomata in leaves and finally into mesophyll cells and from there it makes it way into stroma of chloroplast – the site for Calvin cycle.
• In Stroma, If 3 molecules of CO₂ interacts with its initial acceptor – six molecules of ribulose bisphosphate (RuBP).
• It is important to remember that only one molecule of CO₂ will be fixed at a given time or in a cycle, by Rubisco. So for 3 molecules of CO₂ fixation, 3 Calvin cycles are required.
• This reaction is catalyzed by an enzyme called ribulose bisphosphate carboxylase (RuBisCO) to form 6 molecules of 3- Phospho Glycerate (3-PGA).
• RuBP is a six carbon compound, which is condensed with CO₂ to produce a six-carbon molecule. This six carbon molecule is immediately fragmented into 2 molecules of 3-PGA. This reaction is also catalyzed by RuBisCO ( for 3 CO₂ molecules – 6 molecules of 3-PGA are formed
• The RuBisCO enzyme which catalyzes one of most vital reactions of converting inorganic carbon into useful biological molecules, is the most abundant protein in chloroplast and also happens to be most common protein on Earth.
• This process of fixation of CO₂ into a stable intermediate is called carboxylation.

Reduction :

• In these steps, the chemical energy synthesized during light reaction in the form of ATP and NADPH will be used in formation of Glucose or sugars.
• 6 molecules of 3-PGA formed during carboxylation step will undergo reduction (in two steps) to form 6 molecules of glyceraldehyde 3-phosphate (G3P). That is a reduction reaction 3-PGA gains electrons during this step from NADPH.
• 12 molecules of ATP and NADPH are utilized during this reduction step.
• After consumption both the molecules (ATP to ADP and NADPH to NADP⁺ ) ADP and NADP+ return to places where light-dependent reactions are taking place, so that they can be reused.
• Out of 6 molecules of G3P formed, 1 molecule leave the cycle and is exported from stroma of chloroplast to cytoplasm, where they are converted to bigger sugars such as glucose. This can be considered as end product of Calvin cycle.

Regeneration :

• The remaining 5 G3P molecules picks up two more carbons to regenerate 5 carbon RuBP in a multi step process, so that it can start a new round of Calvin cycle by interacting with CO₂. This step again requires 6 ATP’s.
• To sum up, if we look at the input and output of the Calvin cycle – for every six molecules of CO₂ and by utilizing 18 ATP’s and 12 NADPH– 1 molecule of Glucose is synthesized along with 18 ADP plus 12 NADP⁺.
• Glucose and Oxygen are two important outcome of photosynthesis. Glucose is very useful molecule which will eventually becomes a component of important cell structures such as plasma membrane, cell wall etc. Many glucose molecules combine to form starch molecule in plants, whenever there is excess of glucose production after photosynthesis. Starch is stored in Chloroplast as granules.
• This Cycle is also called as C3 – Pathway as the first product formed after CO₂ fixation is a three carbon compound ( 3-PGA).
• Melvin Calvin became the sole recipient of Nobel Prize for Chemistry in 1961 for discovering Calvin-Benson cycle or C3 cycle.

Six Calvin cycles are required for making one molecule of Glucose:

Generally enzymes can process a thousand molecules per second, but Rubisco can fix only about three carbon dioxide molecules per second.

Out of 6 molecules of G3P formed after reduction process, 1 leaves the Calvin Cycle to form Glucose.

As G3P exported from the chloroplast has three carbon atoms, it takes three “turns” of the Calvin cycle to fix enough net carbon to export one G3P.

Three CO2 molecules must enter the cycle to provide three new atoms of fixed carbon to the cycle ( as 1 G3P molecules had left the cycle).

A G3P molecule contains three fixed carbon atoms, so it takes two G3Ps to build a six-carbon glucose molecule. Hence it would require six turns of the Calvin cycle to synthesize one molecule of Glucose (each turn fix one CO₂ to give 2 G3P molecules -6 CO₂ will give 12 G3P – Six Calvin cycles for CO₂ fixation will yield 12 G3P (10 G3P used for RuBP regeneration and 2 used for forming 1 Glucose molecule).

In Calvin cycle, for every CO2 molecule to be fixed, 3 ATP molecules and 2 NADPH are required which forms 1/6 of the glucose molecule.

As six CO2 molecules are required to form one glucose molecules with input of 18 ATP and 12 NADPH – so six Calvin cycles are necessary for fixing 6 CO2 molecules.

Links to : Part I of Photosynthesis in Plants

Part II – Light Reaction

Image Credit : Biology Openstax.