Name: photosystems in light reactions
Uploaded: 2023-04-30T09:17:16.000Z
Duration: 1 min 32 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Biology

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From Molecules to Organisms- Structures and Processes

LS.Bio.3.2

Photosystems

Photosynthetic organisms capture sunlight through the pigment-protein complexes called photosystems, embedded within the chloroplast&#39;s thylakoid membrane.
These complexes are categorized into photosystem I or PSI and photosystem II or PSII.
Inside the chloroplast, PSI complexes are predominantly located in the unstacked regions, called the stromal lamellae, while PSII complexes are present within the stacked granal lamellae.
Each photosystem is a collection of about 200 chlorophyll and 50 carotenoid pigment molecules, distributed across two different domains of the photosystem&#9472;the core domain called the reaction center and a peripheral domain called the antenna complex.
Although all pigment molecules absorb photons, only a few chlorophyll molecules associated with the reaction center can convert absorbed light energy to chemical energy.
The pigments in the antenna complex only funnel the absorbed energy to the reaction center.
The photosystems also have associated cofactors essential for their functioning.
For instance, PSI has a ferredoxin cofactor, a key junction in the electron transport chain, while PSII contains an oxygen-evolution complex that catalyzes water oxidation, a step crucial for photosynthesis.

photosystems

Photosystems are multiprotein complexes that form the functional units of photosynthesis in plants, algae, and cyanobacteria. They are found embedded in the membrane of tiny sac-like structures called thylakoids placed inside the chloroplast.
Functioning of Photosystems
Photosystems contain many pigment molecules, such as chlorophylls and carotenoids, arranged in a particular organization across two domains &#8212; the antenna complex and the reaction center. The main aim of the pigment molecules distributed in the antenna complex is to absorb light in the form of photons and funnel them to the special chlorophyll pair of the reaction center.
There are two types of photosystems &#8212; photosystem II (PSII) and photosystem I (PSI) that are structurally similar but differ on the basis of the source of the low-energy electron supplier and the acceptor to which they deliver their energized electrons. Both these photosystems work in concert.
The PSII reaction center, also known as P680, absorbs a photon that excites an electron in the chlorophyll. The high-energy electron breaks free and is passed on to the primary electron acceptor, and ultimately to PSI through the electron transport chain. P680&#39;s missing electron is replaced by extracting a low-energy electron from water; thus, water is &#34;split&#34; during this stage of photosynthesis, and PSII is re-reduced after every photoact. Splitting one H2O molecule releases two electrons, two hydrogen atoms, and one atom of oxygen. The oxygen molecules are released into the environment while the hydrogen ions play a critical role in establishing a proton gradient across the thylakoid membrane that is essential for the synthesis of ATP in the chloroplast.
As electrons move through the proteins that reside between PSII and PSI, they lose energy and must be re-energized by PSI; hence, another photon is absorbed by the PSI antenna. This energy is relayed to the PSI reaction center called P700. P700 is oxidized and sends a high-energy electron to NADP+ to form NADPH. Thus, PSII captures the energy to create proton gradients to make ATP, and PSI captures the energy to reduce NADP+ into NADPH.
After the energy from the sun is converted into chemical energy in the form of ATP and NADPH molecules, the cell has the fuel needed to build carbohydrate molecules for long-term energy storage. This is achieved in the second phase of photosynthesis, also known as the light-independent or dark phase of photosynthesis, which occurs in the chloroplast stroma.
This text is adapted from <a href="https://openstax.org/books/biology-2e/pages/8-2-the-light-dependent-reactions-of-photosynthesis">Openstax, Biology 2e, Chapter 8, Section 8.2:The Light-dependent Reactions of Photosynthesis</a>.

Photosystems are multiprotein complexes that form the functional units of photosynthesis in plants, algae, and cyanobacteria. They are found embedded in ...

Education

JoVE Core

Cell Biology

Unit 1

Chloroplasts and Photosynthesis

KEY TERMS AND CONCEPTS

photosystems in light reactions

Photosystems are pigment-rich protein complexes that help drive the light reactions of photosynthesis in plants, algae, and cyanobacteria. They are embedded in the thylakoid membrane inside the chloroplast. Their job is to capture light energy and pass it along for chemical use.

Each photosystem contains many pigment molecules, including chlorophylls and carotenoids. These pigments are arranged in two parts: the antenna complex and the reaction center. The antenna complex absorbs photons and funnels that energy to a special chlorophyll pair in the reaction center.

There are two photosystems, photosystem II (PSII) and photosystem I (PSI). They are structurally similar, but they differ in the low-energy electron source they use and the acceptor that receives their energized electrons. The two systems work together in sequence.

PSII has a reaction center called P680. When P680 absorbs a photon, an electron in chlorophyll becomes excited and leaves the molecule. That electron moves to the primary electron acceptor and then travels through the electron transport chain toward PSI. The missing electron in P680 is replaced by an electron taken from water, so water is split during this stage. Splitting one H2O molecule releases two electrons, two hydrogen atoms, and one oxygen atom. The oxygen is released to the environment, while the hydrogen ions help build a proton gradient across the thylakoid membrane that is needed for ATP synthesis in the chloroplast.

As electrons move through the proteins between PSII and PSI, they lose energy and must be energized again. PSI absorbs another photon through its antenna complex and passes that energy to the PSI reaction center, called P700. P700 then sends a high-energy electron to NADP+ to form NADPH. In this way, PSII helps create the proton gradient for ATP, and PSI helps reduce NADP+ into NADPH.

After sunlight is converted into ATP and NADPH, the cell has the chemical fuel needed to build carbohydrate molecules for long-term energy storage. This happens in the second phase of photosynthesis, also called the light-independent or dark phase, which takes place in the chloroplast stroma.

Photosystems in Light Reactions

what is photosynthesis?

Photosynthesis Powers the Food Chain

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the anatomy of chloroplasts

How Chloroplasts Power Photosynthesis

Chloroplasts are the organelles that power photosynthesis in green algae and plants. They are found in green stems and unripe fruit as well as in leaves. Inside chloroplasts, the two stages of photosynthesis work together to turn sunlight, carbon dioxide, and water into chemical energy. The light-dependent reactions capture sunlight and release oxygen. They also make ATP and NADPH, which are energy-rich molecules. The light-independent reactions then use ATP and NADPH to capture CO2 and build...

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Photorespiration, C4, and CAM Pathways

Photorespiration and the C4 and CAM pathways change how plants handle carbon dioxide during photosynthesis. These processes matter because RuBisCo, the enzyme that starts carbon dioxide fixation, works very slowly. A typical enzyme can process about a thousand molecules per second, but RuBisCo fixes only around three carbon dioxides per second. To make up for this slow rate, photosynthetic cells produce very large amounts of RuBisCo. That is why it is the most abundant single enzyme on Earth. <br...

the photochemical reaction center

Light-Harvesting Antennas in Reaction Centers

Light-harvesting antennas and reaction centers work together in photosynthesis. A reaction center is a pigment-protein complex that starts the conversion of light energy into chemical energy. The term photochemical reaction center is more precise because these complexes carry out the first light-driven chemical step. Robert Emerson and William Arnold gave the first experimental evidence for these centers. They showed that nearly 2,500 chlorophyll molecules were involved in the release of just...

the antenna complex

Light-Harvesting in Photosynthesis

Light-harvesting in photosynthesis depends on the antenna complex, also called the light-harvesting complex (LHC). Plants and other photosynthetic organisms use this system to collect direct sunlight with pigments in both the reaction center and the antenna complex. The reaction center is the main site of photochemical reactions. Under average light conditions, the reaction center pigments absorb light more slowly than the electron transport chain can use it. The reaction center alone cannot...

Photosystems are pigment-rich protein complexes that help drive the light reactions of photosynthesis in plants, algae, and cyanobacteria. They are embedded in the thylakoid membrane inside the chloroplast. Their job is to capture light energy and pass it along for chemical use. Each photosystem contains many pigment molecules, including chlorophylls and carotenoids. These pigments are arranged in two parts: the antenna complex and the reaction center. The antenna complex absorbs photons and...

the z-scheme of electron transport in photosynthesis

the-z-scheme-of-electron-transport-in-photosynthesis

How Electron Flow Powers the Z-Scheme

The Z-scheme explains how electrons move through the light reactions of photosynthesis. In this process, light energy drives electrons from water to NADP+, and oxygen is released when water splits. Because water is hard to oxidize, the reaction needs a strong oxidizing agent. That role is filled by oxidized P680, also called P680+. It is the first product of the light reactions and is known as the most powerful oxidizing agent in biology. After it gains an electron from water-splitting, P680+...

key terms and concepts

Learn how PSII and PSI capture light, split water, build a proton gradient, and produce ATP and NADPH for photosynthesis in chloroplasts.

Photosystems in Light Reactions | JoVE K12

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Photosystems in Light Reactions

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