Name: carbon fixation in photosynthesis
Uploaded: 2025-08-11T09:30:18.000Z
Duration: 1 min 40 s
Channel: JoVE

K12 Standards

Pennsylvania State Academic Standards

9-12

Earth and Space Science

Earth's Systems

3.3.9-12.L

The Calvin Cycle

Chez les plantes autotrophes,le cycle de Calvin commencelorsque le dioxyde de carbone atmosphériquefinit par diffuser dans le stroma du chloroplaste. 
Ici, un atome de carbone du dioxyde de carbone est ajoutéou fixé à une molécule de sucre acceptant 5 carbones,le biphosphate de ribulose, ou RuBP,dans une réaction catalysée par l'enzymeRibulose 1-5-biphosphate-carboxylase-oxygénase,ou RuBisCo pour faire court. 
La molécule de carbone qui en résulte est très instable,et se divise en deux molécules à 3 carbonesd'acide 3-phosphoglycérique, 3-PGA,avec l'ATP fournissant l'énergie et l'apposition du NADPHà un atome d'hydrogène par chaîne, les chaînes 3-PGAsont converties en un autre intermédiaire à 3 carbonesappelé Glycéraldéhyde-3-phosphate. 
Un G3P quitte alors le cycle et en attend un autrepour construire du glucose avec 6 atomes de carbone. 
Entre temps, le G3P restant doit attendre encore 4 cycles,que les carbones s'accumulentet que l'ATP fournisse plus d'énergiepour régénérer les accepteurs de RuBP. 
Dans l'ensemble, 6 tours du cycle de Calvinfixent 6 dioxydes de carbone de l'atmosphère,en utilisant l'énergie et la puissance réductrice de 18 ATPet 12 NADPH, respectivement, pour générer des émissions deune molécule de glucose et reconstruire le RuBPpour continuer la boucle.

Michelet, Laure, Mirko Zaffagnini, Samuel Morisse, Francesca Sparla, María Esther Pérez-Pérez, Francesco Francia, Antoine Danon, et al. « Régulation redox du cycle de Calvin-Benson : quelque chose de vieux, quelque chose de nouveau ». Frontiers in Plant Science 4 (2013). [<a href="https://doi.org/10.3389/fpls.2013.00470 » target="__blank">Source</a>]

Sharkey, Thomas D. et Sean E. Weise. « Le shunt de glucose 6-phosphate autour du cycle de Calvin-Benson. » Journal of Experimental Botany 67, n° 14 (1er juillet 2016) : 4067-77. [<a href="https://doi.org/10.1093/jxb/erv484 » target="__blank">Source</a>]

le cycle de calvin

<h4>Aperçu</h4>
 
 La photosynthèse oxygénique transforme environ 200 milliards de tonnes de dioxyde de carbone (CO2) par an en composés organiques et produit environ 140 milliards de tonnes d’oxygène atmosphérique (O2). La photosynthèse est à la base de tous les besoins humains en nourriture et en oxygène.
 
 Le processus photosynthétique peut être divisé en deux séries de réactions qui ont lieu dans différentes régions des chloroplastes végétaux : la réaction dépendante de la lumière et les réactions indépendantes de la lumière ou “ sombres ”. La réaction dépendante de la lumière a lieu dans la membrane thylakoïdienne du chloroplaste. Elle convertit l’énergie lumineuse en énergie chimique, stockée sous forme d’ATP et de NADPH. Cette énergie est ensuite utilisée dans la région du stroma du chloroplaste, pour réduire le dioxyde de carbone atmosphérique en glucides complexes grâce aux réactions indépendantes de la lumière du cycle de Calvin-Benson.
 
 <h4>Le cycle de Calvin-Benson</h4>
 
 Le cycle de Calvin-Benson représente l’ensemble des réactions photosynthétiques indépendantes de la lumière. Il utilise l’adénosine triphosphate (ATP) et le nicotinamide-adénine dinucléotide phosphate (NADPH) générés pendant les réactions dépendantes de la lumière pour transformer le CO2 atmosphérique en glucides complexes. Le cycle de Calvin-Benson régénère également l’adénosine diphosphate (ADP) et le NADP+ pour la réaction dépendante de la lumière.
 
 Au début du cycle de Calvin-Benson, le CO2 atmosphérique pénètre dans la feuille par des ouvertures s’appelant des stomates. Dans la région du stroma du chloroplaste, l’enzyme ribulose-1,5-bisphosphate carboxylase/oxygénase (RuBisCO) ajoute un atome de carbone du CO2 à une molécule acceptrice de sucre à 5 carbones (5C), le ribulose-1,5- bisphosphate (RuBP). La molécule 6C qui en résulte est très instable et se divise en deux molécules d’acide 3-phosphoglycérique (3-PGA). L’enzyme 3-phosphoglycérate kinase utilise l’ATP pour phosphoryler ces molécules de 3-PGA pour former le 1,3-bisphosphoglycérate. La glycéraldéhyde 3-phosphate déshydrogénase utilise le NADPH pour réduire ces molécules afin de former le glycéraldéhyde 3-phosphate (G3P), un sucre 3C. Ce produit final donne naissance au nom de fixation du carbone en C3 : un alias pour le cycle de Calvin-Benson.
 
 Pour fixer six molécules de CO2, le cycle de Calvin-Benson réduit 12 molécules de NADPH et 18 molécules d’ATP. Ces sources d’énergie sont réapprovisionnées par les réactions dépendantes de la lumière de la photosynthèse. Les six CO2 sont fixés à six molécules 5C (RuBP) qui se divisent en 12 molécules 3C (G3P). Dix de ces molécules de G3P régénèrent six molécules de l’accepteur RuBP, pour poursuivre le cycle. Deux molécules de G3P sont transformées en un seul glucose. Le G3P peut également être utilisé pour synthétiser d’autres glucides, acides aminés et lipides.

Le cycle de Calvin

Aperçu

La photosynthèse oxygénée convertit environ 200 milliards de tonnes de dioxyde de carbone (CO2) par an en composés organiques et produit environ ...

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carbon fixation in photosynthesis

Photosynthesis uses light energy to turn carbon dioxide into carbohydrates. It also releases oxygen as a by-product. These products help support most food webs and make aerobic life possible.

Photosynthesis has two main stages, and they happen in different parts of the chloroplast. The light-dependent reactions take place in the thylakoid membranes. Chlorophyll absorbs light energy there and helps split water molecules. This releases O2 and makes ATP and NADPH.

The light-independent reactions happen in the stroma, where the Calvin-Benson cycle operates. This cycle uses ATP and NADPH to reduce atmospheric CO2 into carbohydrates. After they are used, ADP and NADP+ return to the thylakoids so the light reactions can recycle them.

The Calvin-Benson cycle has three phases: carbon fixation, reduction, and regeneration of the CO2 acceptor, RuBP. CO2 enters the leaf through stomata. In the stroma, the enzyme RuBisCO attaches each CO2 molecule to ribulose-1,5-bisphosphate, or RuBP. This forms an unstable six-carbon intermediate that quickly splits into two molecules of 3-phosphoglycerate, or 3-PGA.

During reduction, each 3-PGA molecule is phosphorylated by ATP to form 1,3-bisphosphoglycerate. NADPH then reduces it to glyceraldehyde-3-phosphate, or G3P, a three-carbon sugar. For every six CO2 molecules fixed, two G3P molecules leave the cycle. These molecules help form glucose and other organic compounds.

In the final regeneration phase, the remaining G3P molecules are rearranged to rebuild RuBP. This step uses additional ATP. Regenerating RuBP lets the cycle continue fixing CO2.

Carbon Fixation in Photosynthesis

what is photosynthesis?

Photosynthesis in Chloroplasts and Leaves

Photosynthesis happens in plants and in some bacteria. It is the process that captures carbon dioxide and solar energy to make glucose. Glucose stores chemical energy as carbohydrates. The overall biochemical equation is 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2. Photosynthesis also releases oxygen into the atmosphere and helps maintain Earth’s atmospheric oxygen content. Photosynthetic reactions take place in chloroplasts, which are specialized membrane-enclosed parts of the plant cell.

light as energy

How Plant Pigments Capture Light

Photosynthesis uses light energy from the sun, which is a form of electromagnetic radiation. The full range of possible wavelengths is called the electromagnetic spectrum. Light is made of photons, which are small packets of energy. Photons act like waves and are described by frequency, wavelength, and amplitude. Waves with higher frequencies carry more energy and have shorter wavelengths. Waves with lower frequencies carry less energy and have longer wavelengths. For most organisms,...

anatomy of chloroplasts

Chloroplasts and Photosynthesis in Plant Cells

Chloroplasts are the organelles where photosynthesis takes place in green algae and plants. They are also found in green stems and unripe fruit. In plants, mesophyll cells in the leaf contain the highest density of chloroplasts. Each chloroplast has a double membrane. The outer membrane faces the plant cell cytoplasm on one side and the chloroplast intermembrane space on the other. The inner membrane separates that narrow space from the stroma, which is the watery interior of the chloroplast.

photosystem ii

Photosystem II and Water Splitting

Photosystem II is a multi-protein complex that captures light energy and begins the flow of electrons in photosynthesis. It uses bound pigments to harvest photons and send that energy to its reaction center. From there, the energy continues through the electron transport chain and on to photosystem I. The pigments in photosystem II include chlorophyll a, chlorophyll b, and carotenoids. They are arranged in two parts of the photosystem: the antenna complex and the reaction center. The antenna...

photosystem i

Photosystem I and NADPH Formation

Photosystem I helps convert light energy into NADPH during photosynthesis. It works with photosystem II in the thylakoid membrane of the chloroplast, but it uses a different electron supplier and a different electron acceptor. Electrons excited in photosystem II move to photosystem I through an electron transport chain. That chain includes plastoquinone, the dual-protein cytochrome complex, and plastocyanin. As the electrons travel between the two photosystems, they lose energy and must be...

the calvin cycle

Photosynthesis uses light energy to turn carbon dioxide into carbohydrates. It also releases oxygen as a by-product. These products help support most food webs and make aerobic life possible. Photosynthesis has two main stages, and they happen in different parts of the chloroplast. The light-dependent reactions take place in the thylakoid membranes. Chlorophyll absorbs light energy there and helps split water molecules. This releases O2 and makes ATP and NADPH. The light-independent reactions...

c4 pathway and cam

Plant Carbon Fixation in Hot Climates

Plant carbon fixation in hot climates can follow the C4 pathway or the CAM pathway. These alternative pathways help some plants reduce energy loss from photorespiration. Photorespiration happens when oxygen levels are high and rubisco, the enzyme in the Calvin cycle, binds O2 instead of CO2. When that happens, photosynthesis slows and energy is used up. The C4 pathway is used by plants such as corn and sugarcane. In mesophyll cells, CO2 first joins with PEP, or phosphoenolpyruvate, through the...

key terms and concepts

Explore how the Calvin-Benson cycle fixes carbon dioxide into sugars, using ATP and NADPH in the chloroplast stroma during photosynthesis.

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3.3.9-12.N

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Carbon Fixation in Photosynthesis

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