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

- [Instructeur] In autotrofe planten, begint de Calvin-cyclus wanneer atmosferisch kooldioxide uiteindelijk diffundeert in het stroma van de chloroplast. Hier, wordt één koolstofatoom van de koolstofdioxide toegevoegd, of gefixeerd op een vijf koolstofatomen, acceptor-suikermolecuul ribulose bifosfaat of RuBP. In een reactie gekatalyseerd door het enzym ribulose 1,5 bifosfaat- carboxylase-oxygenase, of kortweg RuBisCo.

Het resulterende zes-koolstof molecuul is zeer onstabiel en splitst zich in drie-koolstof moleculen van drie fosfoglycerinezuur, 3-PGA. Met ATP die de energie levert en NADPH die elk een waterstof aanbrengt, worden de 3-PGA-ketens omgezet in een ander tussenproduct met drie koolstofatomen glyceraldehyde-3-fosfaat genoemd. Eén G3P verlaat vervolgens de cyclus en wacht op een andere om glucose met zes koolstofatomen op te bouwen.

Ondertussen moet de resterende G3P nog vier cycli wachten, omdat koolstoffen zich ophopen en ATP meer energie levert om de RuBP-acceptoren te regenereren. In totaal fixeren zes omwentelingen van de Calvin-cyclus zes koolstofdioxiden uit de omgeving. Met behulp van de energie en het reducerend vermogen van respectievelijk 18 ATP's en 12 NADPH's, om één glucose-molecule te genereren en RuBP opnieuw op te bouwen om de lus voort te zetten.

de calvijncyclus

<h4>Overzicht</h4>

Zuurstoffotosynthese zet jaarlijks ongeveer 200 miljard ton kooldioxide (CO2) om in organische verbindingen en produceert ongeveer 140 miljard ton atmosferische zuurstof (O2). Fotosynthese vormt de basis van alle menselijke voedsel- en zuurstofbehoeften.

Het fotosyntheseproces kan worden onderverdeeld in twee reeksen reacties die plaatsvinden in verschillende delen van de chloroplasten van planten: de lichtafhankelijke reactie en de lichtonafhankelijke of "donkere" reactie. De lichtafhankelijke reactie vindt plaats in het thylakoïdmembraan van de chloroplast. Deze zet lichtenergie om in chemische energie, die wordt opgeslagen als ATP en NADPH. Deze energie wordt vervolgens gebruikt in het stromagebied van de chloroplast om kooldioxide uit de atmosfeer te reduceren tot complexe koolhydraten via de lichtonafhankelijke reacties van de Calvin-Benson-cyclus.

<h4>De Calvin-Benson-cyclus</h4>

De Calvin-Benson-cyclus vertegenwoordigt de lichtonafhankelijke reeks fotosynthesereacties. Het maakt gebruik van het adenosinetrifosfaat (ATP) en nicotinamide-adenine-dinucleotidefosfaat (NADPH) dat wordt gegenereerd tijdens de lichtafhankelijke reacties om atmosferisch CO2 om te zetten in complexe koolhydraten. De Calvin-Benson-cyclus regenereert ook adenosinedifosfaat (ADP) en NADP+ voor de lichtafhankelijke reactie.

Aan het begin van de Calvin-Benson-cyclus komt atmosferisch CO2 het blad binnen via huidmondjes. In het stromagebied van de chloroplast voegt het enzym ribulose-1,5-bisfosfaatcarboxylase/oxygenase (RuBisCO) één koolstofatoom uit CO2 toe aan een acceptorsuikermolecuul met vijf koolstofatomen (5C), genaamd ribulose-1,5-bisfosfaat (RuBP). Het resulterende 6C-molecuul is zeer onstabiel en splitst zich in twee moleculen 3-fosfoglycerinezuur (3-PGA). Het enzym 3-fosfoglyceraatkinase gebruikt ATP om deze 3-PGA-moleculen te fosforyleren en 1,3-bisfosfoglyceraat te vormen. Glyceraldehyde-3-fosfaatdehydrogenase gebruikt NADPH om deze moleculen te reduceren tot glyceraldehyde-3-fosfaat (G3P), een 3C-suiker. Dit eindproduct geeft aanleiding tot de naam C3-koolstoffixatie, een alias voor de Calvin-Benson-cyclus.

Om zes CO2-moleculen te fixeren, reduceert de Calvin-Benson-cyclus 12 NADPH- en 18 ATP-moleculen. Deze energiebronnen worden aangevuld door de lichtafhankelijke reacties van fotosynthese. De zes CO2-moleculen worden gebonden aan zes 5C-moleculen (RuBP), die uiteenvallen in 12 3C-moleculen (G3P). Tien van deze G3P-moleculen regenereren zes moleculen van de RuBP-acceptor om de cyclus voort te zetten. Twee G3P-moleculen worden omgezet in één glucosemolecuul. G3P kan ook worden gebruikt om andere koolhydraten, aminozuren en lipiden te synthetiseren.

De Calvijncyclus

Overview

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

Education

JoVE Core

Biology

Biologie

Cellular Processes

Cellulaire processen

Photosynthesis

Fotosynthese

KEY TERMS AND CONCEPTS

BELANGRIJKE TERMEN EN CONCEPTEN

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.

Carbon Fixation in Photosynthesis | JoVE K12

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3.3.9-12.N

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Carbon Fixation in Photosynthesis

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