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

独立栄養植物では、大気中の二酸化炭素がまんべんなく葉緑体ストロマに拡散すると カルビン回路が始まります。ここで、リブロースビスリン酸カルボキシラーゼまたは 略してRuBisCoによる触媒反応において、二酸化炭素からの1個の炭素原子は、5個の炭素受容体糖分子、リブロース二リン酸、またはRuBPに追加または固定されます。得られた6個の炭素分子は非常に不安定で、2、3個の炭素分子に分かれます。それはエネルギーを提供するATPおよび それぞれに水素を1つ付加するNADPHをもつ 3-ホスホグリセリン酸 です。3-PGA鎖はグリセルアルデヒド-3-リン酸と呼ばれる 別の3炭素中間体に変換されます。その後、1つのG3Pが回路を終了し、別のG3Pが6つの炭素原子を持つグルコースを生成するのを待機します。一方で、炭素が蓄積し、ATPがより多くのエネルギーを供給し RuBP受容体を再生する必要があるため、残りのG3Pはさらに4サイクル待機する必要があります。全体として、カルビン回路の6回転で、それぞれ18個のATPおよび12個のNADPHのエネルギーおよび 還元力を使って 大気から6個の二酸化炭素を固定し、グルコースを1分子生成し、RuBPを再構築してループを続けます。

Michelet, Laure, Mirko Zaffagnini, Samuel Morisse, Francesca Sparla, María Esther Pérez-Pérez, Francesco Francia, Antoine Danon, et al. "Redox Regulation of the Calvin–Benson Cycle: Something Old, Something New." Frontiers in Plant Science 4 (2013).[<a href="https://doi.org/10.3389/fpls.2013.00470" target="__blank">ソース</a>]

「カルビン・ベンソンサイクル周辺のグルコース6-リン酸シャント」 Journal of Experimental Botany 67, no. 14 (2016年7月1日): 4067–77.[<a href="https://doi.org/10.1093/jxb/erv484" target="__blank">ソース</a>]

カルビン回路

<h4>概要</h4>
 
 酸素発生型光合成は、年間約2,000億トンの二酸化炭素（CO2）を有機化合物に変換し、約1,400億トンの大気中の酸素（O2）を生成しています。光合成は、人間が必要とするすべての食物と酸素の基となっています。
 
 光合成のプロセスは、植物の葉緑体の異なる領域で行われる2つの反応に分けることが出来ます。光依存性反応と光非依存性反応です。光依存性反応は、葉緑体のチラコイド膜で行われます。光エネルギーを化学エネルギーに変換し、ATPとNADPHとして蓄えます。このエネルギーは、葉緑体のストロマ領域で利用され、カルビン・ベンソン回路の光非依存性反応により、大気中の二酸化炭素を複合炭水化物に還元します。
 
 <h4>カルビン・ベンソン回路</h4>
 
 カルビン・ベンソン回路は光合成反応のうち、光非依存性反応の代表です。光依存性反応で生成されたアデノシン三リン酸(ATP)とニコチンアミド-アデニンジヌクレオチドリン酸(NADPH)を用いて、大気中のCO2を複合炭水化物に変換します。また、カルビン・ベンソン回路では、光依存性反応に必要なアデノシン二リン酸（ADP）とNADP+を再生します。
 
 カルビン・ベンソン回路の開始時には、大気中のCO2が気孔と呼ばれる開口部から葉の中に入ります。葉緑体のストロマ領域では、リブロース-1,5-ビスリン酸カルボキシラーゼ/オキシゲナーゼ（RuBisCO）という酵素が、CO2から炭素数5（5C）のアクセプター糖分子であるリブロース-1,5-ビスリン酸（RuBP）に炭素原子を1つ加えます。その結果、6C分子は非常に不安定となり、2分子の3-ホスホグリセリン酸（3-PGA）に分裂します。3-ホスホグリセリン酸キナーゼという酵素は、ATPを使ってこの3-PGA分子をリン酸化し、1,3-ビスホスホグリセリン酸を生成します。グリセルアルデヒド3リン酸デヒドロゲナーゼは、NADPHを用いてこれらの分子を還元し、3C糖であるグリセルアルデヒド3リン酸（G3P）を生成します。この最終生成物から、カルビン・ベンソン回路の別名、C3炭素固定という名前が生じました。
 
 カルビン・ベンソンサイクルは、6個のCO2分子を固定するため、12個のNADPHと18個のATP分子を還元します。これらのエネルギー源は、光合成の光依存性反応によって補充されます。6個のCO2は6個の5C分子（RuBP）に結合し、12個の3C分子（G3P）に分解されます。このG3Pのうち10分子は、RuBPアクセプターの6分子を再生し、サイクルを続けます。2分子のG3Pが1つのグルコースに変換されます。G3Pは、他の炭水化物、アミノ酸、脂質の合成にも使用されます。

概要

酸素光合成は、年間約2,000億トンの二酸化炭素(CO2)を有機化合物に変換し、約...

Education

JoVE Core

Biology

生物学

Cellular Processes

細胞プロセス

Photosynthesis

光合成

KEY TERMS AND CONCEPTS

主要用語と概念

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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