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.

if a plant is in drought conditions i.e. wilting does chloroplast activity slow down?

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.

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?

It gets added to the glucose molecule( C.6 H.12 O.6 )

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?

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

To clarify, one cycle of the Calvin Cycle would produce 1/6 of a glucose molecule, hypothetically speaking?

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.

Where does the sugar go after being produced from the compounds leaving the Calvin Cycle?

Sugar goes into plastids to be stored or being actively utilized for plant growth, flowering and seeds.

Does calvin cycle produces oxygen as a by product too?

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.

Why does it take 6 turns of the Calvin Cycle to form 1 molecule of glucose?

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!

What's the formula of the calvin cycle?

3 CO2 + 6 NADPH + 6 H+ + 9 ATP + 5 H2O → glyceraldehyde-3-phosphate (G3P) + 6 NADP+ + 9 ADP + 8 Pi (Pi = inorganic phosphate) Taken from wikipedia article on it

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?

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.

Main content

Course: AP®︎/College Biology > Unit 3

Lesson 4: Photosynthesis

The Calvin cycle

Google Classroom

How the products of the light reactions, ATP and NADPH, are used to fix carbon into sugars in the second stage of photosynthesis.

Introduction

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 (

{CO}_{2}

) 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 (

{CO}_{2}

) 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

{CO}_{2}

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).

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:

Image modified from "Using light energy to make organic molecules: Figure 1," by OpenStax College, CC BY 4.0

Carbon fixation. A ${CO}_{2}$ ‍ 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.
The first stage of the Calvin cycle incorporates carbon from ${CO}_{2}$ ‍ into an organic molecule, a process called carbon fixation. In plants, atmospheric ${CO}_{2}$ ‍ enters the mesophyll layer of leaves by passing through pores on the leaf surface called stomata. It can then diffuse into mesophyll cells, and into the stroma of chloroplasts, where the Calvin cycle takes place.
Simplified diagram (showing carbon atoms but not full molecular structures) illustrating the reaction catalyzed by rubisco. Rubisco attaches a carbon dioxide molecule to an RuBP molecule, and the six-carbon intermediate thus produced breaks down into two 3-phosphoglycerate (3-PGA) molecules.
In the first step of the cycle, an enzyme nicknamed rubisco (RuBP carboxylase-oxygenase) catalyzes attachment of ${CO}_{2}$ ‍ to a five-carbon sugar called ribulose bisphosphate (RuBP). The resulting 6-carbon molecule is unstable, however, and quickly splits into two molecules of a three-carbon compound called 3-phosphoglycerate (3-PGA). Thus, for each ${CO}_{2}$ ‍ that enters the cycle, two 3-PGA molecules are produced.
The actual molecular structures are show below:
Diagram showing the molecular structures of RuBP and carbon dioxide, the unstable six-carbon intermediate formed when they combine, and the two 3-PGA molecules produced by the intermediate's breakdown.
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.
The reduction stage of the Calvin cycle, which requires ATP and NADPH, converts 3-PGA (from the fixation stage) into a three-carbon sugar. This process occurs in two major steps:
Simplified diagram of the reduction stage of the Calvin cycle, showing carbon atoms but not full molecular structures. A molecule of 3-PGA first receives a second phosphate group from ATP (generating ADP). Then, the doubly phosphorylated molecule receives electrons from NADPH and is reduced to form glyceraldehyde-3-phosphate. This reaction generates NADP+ and also releases an inorganic phosphate.
First, each molecule of 3-PGA receives a phosphate group from ATP, turning into a doubly phosphorylated molecule called 1,3-bisphosphoglycerate (and leaving behind ADP as a by-product).
Second, the 1,3-bisphosphoglycerate molecules are reduced (gain electrons). Each molecule receives two electrons from NADPH and loses one of its phosphate groups, turning into a three-carbon sugar called glyceraldehyde 3-phosphate (G3P). This step produces NADP $^{+}$ ‍ and phosphate ( $P_{i}$ ‍) as by-products.
The actual chemical structures and reactions are:
Reactions of the reduction stage of the Calvin cycle, showing the molecular structures of the molecules involved.
The ATP and NADPH used in these steps are both products of the light-dependent reactions (the first stage of photosynthesis). That is, the chemical energy of ATP and the reducing power of NADPH, both of which are generated using light energy, keep the Calvin cycle running. Reciprocally, the Calvin cycle regenerates ADP and NADP $^{+}$ ‍, providing the substrates needed by the light-dependent reactions.
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." $^{1}$ ‍

In order for one G3P to exit the cycle (and go towards glucose synthesis), three

{CO}_{2}

molecules must enter the cycle, providing three new atoms of fixed carbon. When three

{CO}_{2}

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. $3$ ‍ ${CO}_{2}$ ‍ combine with $3$ ‍ RuBP acceptors, making $6$ ‍ molecules of glyceraldehyde-3-phosphate (G3P).
- $1$ ‍ G3P molecule exits the cycle and goes towards making glucose.
- $5$ ‍ G3P molecules are recycled, regenerating $3$ ‍ RuBP acceptor molecules.
ATP. $9$ ‍ ATP are converted to $9$ ‍ ADP ( $6$ ‍ during the reduction step, $3$ ‍ during the regeneration step).
NADPH. $6$ ‍ NADPH are converted to $6$ ‍ 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

6

{CO}_{2}

18

ATP, and

12

NADPH, to produce one molecule of glucose.

Attribution:

This article is a modified derivative of the following articles:

"The Calvin cycle," by OpenStax College, Concepts of Biology, CC BY 4.0. Download the original article for free at http://cnx.org/contents/b3c1e1d2-839c-42b0-a314-e119a8aafbdd@9.10.
"Using light energy to make organic molecules," by OpenStax College, Biology, CC BY 4.0. Download the original article for free at http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.53.

The modified article is licensed under a CC BY-NC-SA 4.0 license.

Works cited:

Koning, R. E. (1994). Calvin cycle. In Plant physiology information website. Retrieved from http://plantphys.info/plant_physiology/calvincycle.shtml.

References:

Berg, J. M., Tymoczko, J. L., and Stryer, L. (2002). The Calvin cycle synthesizes hexoses from carbon dioxide and water. In Biochemistry (5th ed., section 20.1). New York, NY: W. H. Freeman. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK22344/.

Koning, R. E. (1994). Calvin cycle. In Plant physiology information website. Retrieved from http://plantphys.info/plant_physiology/calvincycle.shtml.

Openstax College, Biology. (2015, March 3). Using light energy to make organic molecules. In OpenStax CNX. Retrieved from http://cnx.org/contents/36b3469d-e561-44ff-ac32-99f4d18830e2/Using-Light-Energy-to-Make-Org.

Purves, W.K., Sadava, D., Orians, G.H., and Heller, H.C. (2004). Photosynthesis: energy from the sun. In Life: the science of biology (7th ed., pp. 154-156). Sunderland, MA: Sinauer Associates, Inc.

Raven, P. H., Johnson, G. B., Mason, K. A., Losos, J. B., and Singer, S. R. (2014). Photosynthesi. In Biology (10th ed., AP ed., pp. 147-167). New York, NY: McGraw-Hill.

Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). Photosynthesis. In Campbell biology (10th ed., pp. 199-200). San Francisco, CA: Pearson.

Soderberg, T. (n.d.). Carboxylation and decarboxylation reactions. In Organic chemistry with a biological emphasis (section 13.5). Retrieved October 31, 2015 from UC Davis ChemWiki: http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter_13%3A_Reactions_with_stabilized_carbanion_intermediates_I/Section_13.5%3A_Carboxylation_and_decarboxylation_reactions.

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if a plant is in drought conditions i.e. wilting does chloroplast activity slow down?
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- Matthew Demetrious
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To clarify, one cycle of the Calvin Cycle would produce 1/6 of a glucose molecule, hypothetically speaking?
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- Andrew Persic
  Posted 7 years ago. Direct link to Andrew Persic's post “Not quite. It is true tha...”
  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.
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Heidi Yliharju
Posted 8 years ago. Direct link to Heidi Yliharju's post “Why is the O2 not counted...”
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?
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- Retsn J C
  Posted 8 years ago. Direct link to Retsn J C's post “It gets added to the gluc...”
  It gets added to the glucose molecule( C.6 H.12 O.6 )
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Does calvin cycle produces oxygen as a by product too?
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- tyersome
  Posted 6 years ago. Direct link to tyersome's post “No it does not. All the ...”
  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.
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shreypatel0101
Posted 8 years ago. Direct link to shreypatel0101's post “Where did the Oxygen remo...”
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?
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- muhammad hamza
  Posted 7 years ago. Direct link to muhammad hamza's post “actually the O is in betw...”
  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
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Why does it take 6 turns of the Calvin Cycle to form 1 molecule of glucose?
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- Lau Sky
  Posted 7 years ago. Direct link to Lau Sky's post “because three carbons bon...”
  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!
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David Li
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What's the formula of the calvin cycle?
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- Forever Learner
  Posted a year ago. Direct link to Forever Learner's post “3 CO2 + 6 NADPH + 6 H+ + ...”
  3 CO2 + 6 NADPH + 6 H+ + 9 ATP + 5 H2O → glyceraldehyde-3-phosphate (G3P) + 6 NADP+ + 9 ADP + 8 Pi (Pi = inorganic phosphate)
  Taken from wikipedia article on it
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- Ivana - Science trainee
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  Sugar goes into plastids to be stored or being actively utilized for plant growth, flowering and seeds.
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Prakriti Marwah
Posted 8 years ago. Direct link to Prakriti Marwah's post “in the last line why does...”
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?
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- Taesun Shim
  Posted 7 years ago. Direct link to Taesun Shim's post “One cycle takes in one CO...”
  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.
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