The Calvin cycle
How the products of the light reactions, ATP and NADPH, are used to fix carbon into sugars in the second stage of photosynthesis.
You, like all organisms on Earth, are a carbon-based life form. In other words, the complex molecules of your amazing body are built on carbon backbones. You might already know that you’re carbon-based, but have you ever wondered where all of that carbon comes from?
As it turns out, the atoms of carbon in your body were once part of carbon dioxide () molecules in the air. Carbon atoms end up in you, and in other life forms, thanks to the second stage of photosynthesis, known as the Calvin cycle (or the light-independent reactions).
Overview of the Calvin cycle
In plants, carbon dioxide () enters the interior of a leaf via pores called stomata and diffuses into the stroma of the chloroplast—the site of the Calvin cycle reactions, where sugar is synthesized. These reactions are also called the light-independent reactions because they are not directly driven by light.
In the Calvin cycle, carbon atoms from are fixed (incorporated into organic molecules) and used to build three-carbon sugars. This process is fueled by, and dependent on, ATP and NADPH from the light reactions. Unlike the light reactions, which take place in the thylakoid membrane, the reactions of the Calvin cycle take place in the stroma (the inner space of chloroplasts).
This illustration shows that ATP and NADPH produced in the light reactions are used in the Calvin cycle to make sugar.
Reactions of the Calvin cycle
The Calvin cycle reactions can be divided into three main stages: carbon fixation, reduction, and regeneration of the starting molecule.
Here is a general diagram of the cycle:
Diagram of the Calvin cycle, illustrating how the fixation of three carbon dioxide molecules allows one net G3P molecule to be produced (that is, allows one G3P molecule to leave the cycle).
3 molecules combine with three molecules of the five-carbon acceptor molecule (RuBP), yielding three molecules of an unstable six-carbon compound that splits to form six molecules of a three-carbon compound (3-PGA). This reaction is catalyzed by the enzyme rubisco.
In the second stage, six ATP and six NADPH are used to convert the six 3-PGA molecules into six molecules of a three-carbon sugar (G3P). This reaction is considered a reduction because NADPH must donate its electrons to a three-carbon intermediate to make G3P.
- Regeneration. One G3P molecule leaves the cycle and will go towards making glucose, while five G3Ps must be recycled to regenerate the RuBP acceptor. Regeneration involves a complex series of reactions and requires ATP.
- Carbon fixation. A molecule combines with a five-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP). This step makes a six-carbon compound that splits into two molecules of a three-carbon compound, 3-phosphoglyceric acid (3-PGA). This reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, or rubisco.
- Reduction. In the second stage, ATP and NADPH are used to convert the 3-PGA molecules into molecules of a three-carbon sugar, glyceraldehyde-3-phosphate (G3P). This stage gets its name because NADPH donates electrons to, or reduces, a three-carbon intermediate to make G3P.
- Regeneration. Some G3P molecules go to make glucose, while others must be recycled to regenerate the RuBP acceptor. Regeneration requires ATP and involves a complex network of reactions, which my college bio professor liked to call the "carbohydrate scramble."
In order for one G3P to exit the cycle (and go towards glucose synthesis), three molecules must enter the cycle, providing three new atoms of fixed carbon. When three molecules enter the cycle, six G3P molecules are made. One exits the cycle and is used to make glucose, while the other five must be recycled to regenerate three molecules of the RuBP acceptor.
Summary of Calvin cycle reactants and products
Three turns of the Calvin cycle are needed to make one G3P molecule that can exit the cycle and go towards making glucose. Let’s summarize the quantities of key molecules that enter and exit the Calvin cycle as one net G3P is made. In three turns of the Calvin cycle:
- Carbon. combine with RuBP acceptors, making molecules of glyceraldehyde-3-phosphate (G3P).
- G3P molecule exits the cycle and goes towards making glucose.
- G3P molecules are recycled, regenerating RuBP acceptor molecules.
- ATP. ATP are converted to ADP ( during the fixation step, during the regeneration step).
- NADPH. NADPH are converted to NADP (during the reduction step).
A G3P molecule contains three fixed carbon atoms, so it takes two G3Ps to build a six-carbon glucose molecule. It would take six turns of the cycle, or , ATP, and NADPH, to produce one molecule of glucose.
Want to join the conversation?
- Yes, they can. For example, morning glories and Venus flytraps move. Some other plants also have reflexive movement, as in a response to being touched.(46 votes)
- if a plant is in drought conditions i.e. wilting does chloroplast activity slow down?(13 votes)
- Yes. Because the plant can no longer absorb more H2O through its root structure, the plant will use what water it has stored. The wilting occurs because it is undergoing plasmolysis which reduces the turgor pressure on the plant's cell wall.(29 votes)
- Why is the O2 not counted in the calvin cycle? In light reaction plant takes H2O and uses the H but releases the O. In calvin cycle plant takes CO2 and uses the carbon but my gues is that the O2 is not lost. Is it also released to the atmosphere?(12 votes)
- It gets added to the glucose molecule( C.6 H.12 O.6 )(15 votes)
- To clarify, one cycle of the Calvin Cycle would produce 1/6 of a glucose molecule, hypothetically speaking?(5 votes)
- Not quite. It is true that you need to fix six CO2 molecules for each glucose molecule you produce, and you need 6 ribulose-1,5-bisphosphate to do so, however the reactions need 3 of each to produce phosphoglyceraldehyde. ie. the cycle does not produce 1/6th of a glucose molecule 6 times and join the units together. Instead, it takes 3 of each reactant to produce PGAL, which happens two times to lead to production of 1 glucose molecule. This is why it takes 2 cycles to produce one glucose molecule.(29 votes)
- Does calvin cycle produces oxygen as a by product too?(4 votes)
- No it does not.
All the oxygen released comes from "splitting" of water by photosystem II during the light-dependent reactions.
The "extra" oxygen in CO₂ gets used during the hydrolysis of ATP during the Calvin Cycle.(14 votes)
- Where did the Oxygen removed from 1st Carbon in 3PGA went?
In the reduction process 3PGA(Carboxylic acid)--> G3P(aldehyde), 1 O is missing? Where did it went?(8 votes)
- actually the O is in between of C-1 and P. when 2H are added by NADPH2 , the bond between C and P breaks. 1H is used to reduce C. other H togather with O combines with P forming phosphate group. hope you understand(5 votes)
- Why does it take 6 turns of the Calvin Cycle to form 1 molecule of glucose?(4 votes)
- because three carbons bond with 3 RuBp to make 3 molecules. This splits into 6PG or 6 phosphoglycerate. 6 phosphoglycerate is changed into 6 biphsophoglycerate, which is changed to 6G3P. 5 of those goes back into the cycle to make RuBp so that it can do it all over again, and one is put aside to make glucose. But you need 6 of these to make glucose, so it will take you 6 turns. Hope this helps!(7 votes)
- Where does the sugar go after being produced from the compounds leaving the Calvin Cycle?(5 votes)
- Sugar goes into plastids to be stored or being actively utilized for plant growth, flowering and seeds.(2 votes)
- in the last line why does it say that it takes 6 cycles to make one molecule of glucose? is only one CO2 and one RuBP used in each cycle?(2 votes)
- One cycle takes in one CO2. Three cycles, after many smaller steps, creates six G3P (three-carbon); here, five goes back into the cycle, and only one is used for glucose. Glucose is a six-carbon molecule, so two G3P are needed. The math, then, is 3X2=6.(7 votes)
- what is RuBP acceptor?I think it's same as RuBP right??(3 votes)
- This article is worded weirdly, but yes they just mean RuBP, which is acting as an acceptor for CO₂.(3 votes)