3.16
細菌の光合成
酸素発生型光合成は、光エネルギーを利用して水の酸化を促進し、分子状酸素（O_2）、アデノシン三リン酸（ATP）、およびニコチンアミドアデニンジヌクレオチドリン酸（NADPH）を生成する基本的な過程です。この過程は地球上の好気性生命を維持するために不可欠であり、主にシアノバクテリア、藻類、そして植物に…
酸素光合成は、光エネルギーを利用して水と二酸化炭素をグルコースと酸素に変換します。
シアノバクテリアや藻類などの酸素性光栄養生物は、原形質膜の折り畳みから生じるチラコイド膜に埋め込まれたクロロフィル色素を使用します。
シアノバクテリアは、これらの色素を光化学系I、P700、および光化学系II、P680の2つの光化学系に編成します。
光化学系IIは水を酸化し、酸素、陽子、電子を放出します。電子はフェオフィチンに移動し、次にプラストキノンを介してシトクロムb6f複合体に移動します。
プラストシアニンは電子を光化学系Iに運び、そこで光がそれらを再活性化します。
励起された電子はフェレドキシンに移動し、次にフェレドキシン-NADP⁺レダクターゼに移動し、それを使用してNADP⁺をNADPHに還元します。
電子輸送は、膜を横切ってプロトンを送り込み、ATPシンターゼを駆動するプロトン勾配を生成します。
場合によっては、光化学系Iからの電子が電子伝達鎖に戻り、NADPHや酸素を生成せずに追加のATPを生成することができます。
View the full transcript and gain access to JoVE Core videos
Q1: What organisms perform oxygenic photosynthesis?
Oxygenic phototrophs, including cyanobacteria, algae, and plants, perform oxygenic photosynthesis. These organisms use chlorophyll pigments embedded in thylakoid membranes to harness light energy and convert water and carbon dioxide into glucose and oxygen. This process is essential for sustaining aerobic life on Earth.
Q2: How do the two photosystems work together in oxygenic photosynthesis?
Photosystem II (P680) initiates electron transport by oxidizing water, releasing oxygen, protons, and electrons. These electrons pass through the electron transport chain components, including pheophytin, plastoquinone, and the cytochrome b6f complex. Plastocyanin then carries electrons to Photosystem I (P700), where light re-energizes them for NADPH production.
Q3: What is the role of the proton gradient in oxygenic photosynthesis?
As electrons traverse the electron transport chain, protons are pumped across the thylakoid membrane, creating a proton gradient. This gradient powers ATP synthase through chemiosmosis and atp synthesis, which phosphorylates ADP to ATP. The ATP provides energy for carbon fixation during the Calvin cycle and other cellular processes.
Q4: How is NADPH produced during oxygenic photosynthesis?
In Photosystem I, light re-energizes electrons that are transferred through ferredoxin to ferredoxin-NADP+ reductase. This enzyme catalyzes the reduction of NADP+ to NADPH, which serves as a reducing agent for biosynthetic pathways and the Calvin cycle. NADPH production is coupled to electron transport and light absorption.
Q5: What is cyclic electron flow and when does it occur?
Cyclic electron flow occurs when electrons from Photosystem I are redirected back to the electron transport chain instead of reducing NADP+. This alternative pathway generates additional ATP without producing NADPH or oxygen, allowing cells to balance their ATP and NADPH ratios according to metabolic demands.
Q6: Where are the photosynthetic pigments located in cyanobacteria?
Photosynthetic pigments in cyanobacteria are embedded in thylakoid membranes, which arise from infolding of the plasma membrane. These membranes organize chlorophyll and other pigments into two photosystems, Photosystem I and Photosystem II, enabling efficient light absorption and energy conversion for oxygenic photosynthesis.
Q7: What are the main products of oxygenic photosynthesis?
Oxygenic photosynthesis produces three main products: glucose (from carbon dioxide fixation), oxygen (from water oxidation), and energy carriers ATP and NADPH. These products support autotrophic growth and sustain aerobic life on Earth by providing both organic compounds and molecular oxygen.