Name: plant carbon fixation in hot climates
Uploaded: 2019-03-11T09:21:13.000Z
Duration: 1 min 27 s
Channel: JoVE

K12 Standards

Louisiana Student Standards for Science

High School

Environmental Science

Earth’s Systems

HS-ESS2-6

C4 Pathway and CAM

トウモロコシのような一部の植物は，暑くて乾燥した環境で より効率的に炭素を固定するための方法をとっています。一つは、C4経路です。最初の炭素固定の間に、二酸化炭素CO2が中温菌細胞に入り、酵素PEPカルボキシラーゼを介し 3炭素化合物PEPと結合して 4炭素化合物オキサロアセテートを形成し、別の有機酸に変換されます。この場合はリンゴ酸塩です。リンゴ酸は酸素を分解されて 葉の深部にある維管束鞘細胞に運ばれ、CO2を放出し、カルビン回路を経て ルビスコと相互作用して 最終的に糖を生成します。パイナップルのような植物は 炭素を固定するために ベンケイソウ型有機酸代謝(CAM)を使います。CAM植物は，日中の水分損失を防ぐために 夜間にのみ気孔を開きます。C4経路と同様に、CO2はまずPEPカルボキシラーゼによってオキサロ酢酸に固定され、オキサロ酢酸はリンゴ酸に変換されます。リンゴ酸を葉の他の部分に輸送するかわりに，CAM植物はこの化合物を中温性細胞液胞に貯蔵して 日中に放出し、カルビンサイクルが光合成における光反応と 同時進行できるようになります。

<ol>
	<li>エドワーズ、EJ進化の軌跡、アクセシビリティ、その他の比喩:C4とCAM光合成の場合。 新しい植物学者。223(4)、1742-1755(2019)。</li>
</ol>

c４経路とcam経路

<h4>概要</h4>
 
 サトウキビやトウモロコシのように、高温下で生育する植物は、C4経路と呼ばれる別のプロセスで炭素を固定します。この回路では、大気中のCO2が葉肉細胞に入り、ホスホエノールピルビン酸（PEP）からオキサロ酢酸（炭素数4の分子）を生成することから始まります。オキサロ酢酸はリンゴ酸に変換され、酸素濃度の低い維管束細胞に輸送されます。そこで、リンゴ酸からCO2が放出され、カルビン回路に入り、糖に変換されます。CAM経路は、サボテンのように日中も水分を節約する必要のある植物で行われます。CAM植物は、夜間にCO2を葉に取り込み、リンゴ酸を生成して液胞に蓄え、翌日まで保存します。それから、リンゴ酸は液胞から放出され、カルビン回路で処理されます。C4経路は異なるプロセスを局所的に分離する一方で、CAM経路は時系列的に分離します。
 
 <h4>C4経路</h4>
 
 トウモロコシやサトウキビなどの植物は、高温で乾燥した環境での水分損失を避けて炭素を固定する別の方法を進化させてきました。そのような方法の一つがC4経路です。まず、CO2が葉肉細胞に入り、ホスホエノールピルビン酸（PEP）カルボキシラーゼという酵素が、炭素数3の化合物であるPEPにCO2を加え、炭素数4の化合物、オキサロ酢酸を生成します。それから、オキサロ酢酸はリンゴ酸という有機酸に変換されます。
 
 続いて、リンゴ酸は酸素濃度の低い葉の奥にある維管束細胞に運ばれます。リンゴ酸は分解され、CO2分子を放出してカルビン回路に入り、ルビスコという酵素によって糖に変換されます。C4経路では、植物が水分を節約するために気孔を閉じるので、高温で乾燥した環境では有利です。その結果、植物は酸素濃度を低く保つことができ、O2よりもCO2がルビスコとの結合に有利になります。酸素濃度が高くなると、ルビスコはCO2ではなくO2と結合するようになり、光呼吸と呼ばれるプロセスで光合成を停止させ、エネルギーを消費します。
 
 <h4>CAM経路</h4>
 
 サボテンやパイナップルのように、炭素を固定するためにCAM（Crassulacean Acid代謝）経路を使う植物もあります。CAM植物は、主に夜間に気孔を開き、暑い日中の水分損失を防ぎます。夜に、CO2は葉肉細胞に入り、PEPと結合してオキサロ酢酸となり、最終的にリンゴ酸を生成します。その後、リンゴ酸は液胞に蓄えられ、翌日に液胞から放出されてカルビン回路に入ります。光合成の最初の段階は光依存的であるため日中に行われ、光非依存的なカルビン回路の反応は夜間に行われます。このようにCAM植物は、1日の異なる時間帯を利用することで、CO2の固定と糖の合成を分離しています。

C４経路とCAM経路

ほとんどの植物は、炭素固定にC3経路を使用しています。ただし、サトウキビ、トウモロコシ、サボテンなど、暑い条件で育つ植物の中には、代替のものを使用しているものもあります...

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主要用語と概念

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 action of PEP carboxylase. This forms the 4-carbon compound oxaloacetate, which is then changed into organic acids, mostly malate.

Malate moves into the bundle sheath cells deep in the leaf, where oxygen levels are low. There, malate is broken down and releases CO2. That CO2 enters the Calvin cycle and is turned into sugar with the help of RuBisCo. In hot, dry conditions, this separation of carbon fixing and sugar production helps the plant close its stomata and save water while keeping oxygen low.

Other plants, such as cacti and pineapple, use the CAM pathway, or crassulacean acid metabolism. CAM plants open their stomata at night to limit water loss during the day. CO2 enters the mesophyll cells at night and combines with PEP to form oxaloacetate and then malate.

The malate is stored in vacuoles until the next day. During the day, it is released so the Calvin cycle can begin. In CAM plants, carbon fixation happens at night, while sugar synthesis follows during the day with energy from the light-dependent reactions. This time-based separation helps these plants survive hot, dry environments.

Plant Carbon Fixation in Hot Climates

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.

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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

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the calvin cycle

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...

c4 pathway and cam

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

Learn how C4 and CAM pathways help plants like corn, sugarcane, cacti, and pineapple reduce photorespiration and save water in hot climates.

Plant Carbon Fixation in Hot Climates | JoVE K12

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Plant Carbon Fixation in Hot Climates

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