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

K12 Standards

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Earth and Space Science

Earth's Systems

3.3.9-12.L

The Calvin Cycle

U roślin autotroficznych cykl Calvina rozpoczyna się, gdy atmosferyczny dwutlenek węgla ostatecznie dyfunduje do zrębu chloroplastu.

Tutaj jeden atom węgla z dwutlenku węgla jest dodawany lub wiązany do pięciowęglowej cząsteczki cukru akceptorowego, bisfosforanu rybulozy lub RuBP, w reakcji katalizowanej przez enzym oksygenazę karboksylazy 1,5-bisfosforanowej rybulozy lub w skrócie RuBisCo. Powstała w ten sposób sześciowęglowa cząsteczka jest wysoce niestabilna i rozpada się na dwie trójwęglowe cząsteczki kwasu 3-fosfoglicerynowego, 3-PGA.

Z ATP dostarczającym energię i NADPH dołączającym po jednym wodorze do każdego, trzy łańcuchy PGA są przekształcane w inny trójwęglowy półprodukt zwany gliceraldehydo-3-fosforanem. Jeden G3P następnie wychodzi z cyklu i czeka, aż inny zbuduje glukozę z sześcioma atomami węgla.

Tymczasem pozostałe G3P musi poczekać na kolejne cztery cykle, ponieważ węgiel się gromadzi, a ATP dostarcza więcej energii do regeneracji akceptorów RuBP. Ogólnie rzecz biorąc, sześć zwojów cyklu Calvina wiąże sześć dwutlenku węgla z atmosfery, wykorzystując energię i moc redukującą odpowiednio 18 ATP i 12 NADPH, aby wygenerować jedną cząsteczkę glukozy i odbudować RuBP, aby kontynuować pętlę.

Michelet, Laure, Mirko Zaffagnini, Samuel Morisse, Francesca Sparla, María Esther Pérez-Pérez, Francesco Francia, Antoine Danon i in. "Regulacja redoks cyklu Calvina-Bensona: coś starego, coś nowego". Frontiers in Plant Science 4 (2013). [<a href="https://doi.org/10.3389/fpls.2013.00470" target="__blank">Źródło</a>]

Sharkey, Thomas D. i Sean E. Weise. "Bocznik 6-fosforanowy glukozy wokół cyklu Calvina-Bensona". Journal of Experimental Botany 67, nr 14 (1 lipca 2016): 4067–77. [<a href="https://doi.org/10.1093/jxb/erv484" target="__blank">Źródło</a>]

cykl calvina

<h4>Przegląd</h4>

Fotosynteza tlenowa przekształca rocznie około 200 miliardów ton dwutlenku węgla (CO2) w związki organiczne i wytwarza około 140 miliardów ton tlenu atmosferycznego (O2). Fotosynteza jest podstawą całego zapotrzebowania człowieka na żywność i tlen.

Proces fotosyntezy można podzielić na dwa zestawy reakcji zachodzących w różnych obszarach chloroplastów roślinnych: reakcję zależną od światła i reakcje niezależne od światła lub “ciemne“. Reakcja zależna od światła zachodzi w błonie tylakoidowej chloroplastu. Przekształca energię świetlną w energię chemiczną, przechowywaną w postaci ATP i NADPH. Energia ta jest następnie wykorzystywana w obszarze zrębu chloroplastu do redukcji atmosferycznego dwutlenku węgla do węglowodanów złożonych w drodze niezależnych od światła reakcji cyklu Calvina-Bensona.

<h4>Cykl Calvina-Bensona</h4>

Cykl Calvina-Bensona reprezentuje niezależny od światła zestaw reakcji fotosyntezy. Wykorzystuje trifosforan adenozyny (ATP) i fosforan dinukleotydu nikotynoamidoadeninowego (NADPH) powstałe podczas reakcji zależnych od światła w celu przekształcenia atmosferycznego CO2 w złożone węglowodany. Cykl Calvina-Bensona regeneruje również difosforan adenozyny (ADP) i NADP+ w reakcji zależnej od światła.

Na początku cyklu Calvina-Bensona atmosferyczny CO2 przedostaje się do liścia przez otwory zwane aparatami szparkowymi. W obszarze zrębu chloroplastu enzym karboksylaza/oksygenaza rybulozo-1,5-bisfosforanu (RuBisCO) dodaje jeden atom węgla z CO2 do 5-węglowej (5C) cząsteczki cukru akceptora, rybulozo-1,5-bisfosforanu (RuBP). Powstała cząsteczka 6C jest wysoce niestabilna i rozpada się na dwie cząsteczki kwasu 3-fosfoglicerynowego (3-PGA). Enzym kinaza 3-fosfoglicerynianowa wykorzystuje ATP do fosforylacji cząsteczek 3-PGA z wytworzeniem 1,3-bisfosfoglicerynianu. Dehydrogenaza aldehydu 3-glicerynowego wykorzystuje NADPH do redukcji tych cząsteczek, tworząc 3-fosforan aldehydu glicerynowego (G3P), cukier 3C. Ten produkt końcowy dał początek nazwie wiązanie węgla C3 — aliasowi cyklu Calvina-Bensona.

Aby związać sześć cząsteczek  CO2, cykl Calvina-Bensona redukuje 12 cząsteczek NADPH i 18 cząsteczek ATP. Te źródła energii są uzupełniane przez zależne od światła reakcje fotosyntezy. Sześć CO2 jest przyłączonych do sześciu cząsteczek 5C (RuBP), które rozkładają się na 12 cząsteczek 3C (G3P). Dziesięć z tych cząsteczek G3P regeneruje sześć cząsteczek akceptora RuBP, aby kontynuować cykl. Dwie cząsteczki G3P przekształcają się w jedną glukozę. G3P można również stosować do syntezy innych węglowodanów, aminokwasów i lipidów.

Cykl Calvina

Przegląd

Fotosynteza tlenowa przekształca około 200 miliardów ton dwutlenku węgla (CO2) rocznie w związki organiczne i wytwarza około ...

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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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Carbon Fixation in Photosynthesis

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