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

In autotrophic plants, the Calvin Cycle starts when atmospheric carbon dioxide eventually diffuses into the stroma of the chloroplast.

Here, one carbon atom from the carbon dioxide is added or fixed to a five-carbon acceptor sugar molecule, ribulose bisphosphate, or RuBP, in a reaction catalyzed by the enzyme Ribulose 1,5-bisphosphate-carboxylase-oxygenase, or RuBisCo for short. The resulting six-carbon molecule is highly unstable and splits into two three-carbon molecules of 3-phosphoglyceric acid, 3-PGA.

With ATP providing the energy, and NADPH affixing one hydrogen to each, the three PGA chains are converted into another three-carbon intermediate called glyceraldehyde-3-phosphate. One G3P then exits the cycle and waits for another one to build glucose with six carbon atoms.

Meanwhile, the remaining G3P must wait for four more cycles as carbons accumulate and ATP provides more energy to regenerate the RuBP acceptors. Overall, six turns of the Calvin Cycle fix six carbon dioxides from the atmosphere using the energy and reducing power of 18 ATPs and 12 NADPHs, respectively, to generate one molecule of glucose and rebuild RuBP to continue the loop.

the calvin cycle

OverviewOxygenic photosynthesis plays a central role in the global carbon and oxygen cycles. The carbohydrates produced support nearly all food webs, while the oxygen by&#8209;product enables aerobic life.Light&#8209;dependent and light&#8209;independent reactionsPhotosynthesis occurs in two main stages, each in a different part of the chloroplast: light&#8209;dependent reactions and light&#8209;independent reactions, also called the Calvin&#8209;Benson cycle or simply the Calvin cycle.Light&#8209;dependent reactions take place in the thylakoid membranes. Chlorophyll absorbs light energy, which is used to split water molecules, releasing O&#8322; and generating ATP and NADPH. These energy carriers then move to the stroma, where the Calvin&#8209;Benson cycle occurs.The Calvin&#8209;Benson cycle uses ATP and NADPH to reduce atmospheric CO&#8322; into carbohydrates. ADP and NADP&#8314;, produced as by&#8209;products, return to the thylakoids to be recycled in the light reactions.Calvin&#8209;Benson CycleThe Calvin&#8209;Benson cycle consists of three phases: carbon fixation, reduction, and regeneration of the CO&#8322; acceptor (RuBP).<ul><li>Carbon fixation: CO&#8322; enters the leaf through stomata. In the stroma, the enzyme RuBisCO attaches each CO&#8322; molecule to ribulose&#8209;1,5&#8209;bisphosphate (RuBP), forming an unstable six&#8209;carbon intermediate that immediately splits into two molecules of 3&#8209;phosphoglycerate (3&#8209;PGA).</li><li>Reduction: Each 3&#8209;PGA molecule is phosphorylated by ATP to form 1,3&#8209;bisphosphoglycerate, which is then reduced by NADPH to glyceraldehyde&#8209;3&#8209;phosphate (G3P), a three&#8209;carbon sugar. For every six CO&#8322; molecules fixed, two G3P molecules exit the cycle. These G3P molecules help form glucose and other organic compounds.</li><li>Regeneration: The remaining G3P molecules undergo a series of reactions that regenerate RuBP, using additional ATP. This regeneration step allows the cycle to continue fixing CO&#8322;.</li></ul>

Overview

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

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KEY TERMS AND CONCEPTS

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...

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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Carbon Fixation in Photosynthesis

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