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

- [Narratore] Nelle piante autotrofe

il ciclo di Calvin inizia

quando l'anidride carbonica atmosferica

finisce col diffondere nello
stroma dei cloroplasti.

FISSAZIONE DEL CARBONIO

Qui un atomo di carbonio
dell'anidride carbonica

viene aggiunto, o fissato,

a una molecola di zucchero accettore

a cinque atomi di carbonio, il
ribulosio bifosfato, o RuBP,

in una reazione catalizzata dall'enzima

ribulosio-1,5-bifosfato
carbossilasi/ossigenasi,

abbreviato in RuBisCo.

La molecola risultante
a sei atomi di carbonio

è fortemente instabile

e si divide in due molecole
da tre atomi di carbonio,

o acido 3-fosfoglicerico, 3-PGA.

RIDUZIONE

Con l'ATP che fornisce l'energia

e l'NADPH che fissa un
idrogeno su ciascuno,

le catene di 3-PGA sono convertite

in un altro intermedio
a tre atomi di carbonio

chiamato gliceraldeide-3-fosfato, G3P.

Una G3P quindi esce dal
ciclo e ne aspetta un'altra

per costruire una molecola di glucosio

a sei atomi di carbonio.

Nel frattempo la seconda G3P

deve attendere per altri quattro cicli

mentre il carbonio si accumula
e l'ATP fornisce più energia

per rigenerare l'accettore RuBP.

In totale, sei giri del ciclo di Calvin

fissano sei molecole di
anidride carbonica atmosferica,

usando l'energia e il potere riducente

rispettivamente di 18 ATP e 12 NADPH

per generare una molecola di glucosio

e ricostituire il RuBP
per continuare il ciclo.

Michelet, Laure, Mirko Zaffagnini, Samuel Morisse, Francesca Sparla, María Esther Pérez-Pérez, Francesco Francia, Antoine Danon, et al. "Regolazione redox del ciclo di Calvin-Benson: qualcosa di vecchio, qualcosa di nuovo". Frontiere nella scienza delle piante 4 (2013). [<a href="https://doi.org/10.3389/fpls.2013.00470" target="__blank">Sorgente</a>]

Sharkey, Thomas D. e Sean E. Weise. "Lo shunt del glucosio 6-fosfato attorno al ciclo di Calvin-Benson." Journal of Experimental Botany 67, n. 14 (1 luglio 2016): 4067–77. [<a href="https://doi.org/10.1093/jxb/erv484" target="__blank">Sorgente</a>]

il ciclo di calvin

<h4>Panoramica</h4>

La fotosintesi ossigenaria converte circa 200 miliardi di tonnellate di anidride carbonica (CO2) all'annoin composti organici e produce circa 140 miliardi di tonnellate di ossigeno atmosferico (O2). La fotosintesi è alla base di tutto il fabbisogno di cibo e ossigeno umano.

Il processo fotosintetico può essere diviso in due serie di reazioni che si verificano in diverse regioni di cloroplasti vegetali: la reazione dipendente dalla luce e le reazioni indipendenti dalla luce o "oscure". La reazione dipendente dalla luce avviene nella membrana tilacoide del cloroplasto. Converte l'energia luminosa in energia chimica, immagazzinata come ATP e NADPH. Questa energia viene quindi utilizzata nella regione stroma del cloroplasto, per ridurre l'anidride carbonica atmosferica in carboidrati complessi attraverso le reazioni indipendenti dalla luce del ciclo Calvin-Benson.

<h4>Il ciclo Calvin-Benson</h4>

Il ciclo Calvin-Benson rappresenta l'insieme di reazioni fotosintetiche indipendenti dalla luce. Utilizza il triposfato adenosina (ATP) e il fosfato dinucleotide da nicotinamide (NADPH) generato durante le reazioni dipendenti dalla luce per convertire laCO 2 atmosferica in carboidrati complessi. Il ciclo Calvin-Benson rigenera anche il difosfato di adenosina (ADP) e ilNADP per la reazione dipendente dalla luce.

All'inizio del ciclo Calvin-Benson, l'atmosfera CO2 entra nella foglia attraverso aperture chiamate stomi. Nella regione stroma del cloroplasto, l'enzima ribulosio-1,5-bisfosfato carbossilasi/ossigenasi (RuBisCO) aggiunge un atomo di carbonio da CO2 a una molecola di zucchero accettante a 5 carbonio (5C), il ribulosio-1,5- bisfosfato (RuBP). La molecola 6C risultante è altamente instabile e si divide in due molecole di acido 3-fosforico (3-PGA). L'enzima 3-fosfoglicerato chinasi utilizza ATP per fosforo queste molecole 3-PGA per formare 1,3-bisfosfoglicetrato. Gliceraldeide 3-fosfato deidrogenasi utilizza NADPH per ridurre queste molecole per formare gliceraldeide 3 fosfato (G3P), uno zucchero 3C. Questo prodotto finale dà origine al nome C3 fissazione del carbonio, un alias per il ciclo Calvin-Benson.

Per fissare sei molecole di CO2, il ciclo Calvin-Benson riduce 12 molecole di NADPH e 18 ATP. Queste fonti di energia sono reintegrate dalle reazioni dipendenti dalla luce della fotosintesi. Le sei CO2 sono collegate a sei molecole 5C (RuBP) che si rompono in 12 molecole 3C (G3P). Dieci di queste molecole G3P rigenerano sei molecole dell'accettatore RuBP, per continuare il ciclo. Due molecole di G3P vengono convertite in un glucosio. G3P può anche essere utilizzato per sintetizzare altri carboidrati, aminoacidi, e lipidi.

Il ciclo di Calvin

Overview

Oxygenic photosynthesis converts approximately 200 billion tons of carbon dioxide (CO2) annually to organic compounds and produces approximately ...

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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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