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- The light-dependent reactions begin in photosystem II. In PSII, energy from sunlight is used to split water, which releases two electrons, two hydrogen atoms, and one oxygen atom. When a chlorophyll a molecule within the reaction center of PSII absorbs a photon, the electron in this molecule attains a higher energy level.
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The two photosystems absorb light energy through proteins containing pigments, such as chlorophyll. The light-dependent reactions begin in photosystem II. In PSII, energy from sunlight is used to split water, which releases two electrons, two hydrogen atoms, and one oxygen atom.
- 8.2: The Light-Dependent Reactions of Photosynthesis
The pigments of the first part of photosynthesis, the...
- 2.5.3: The Light-Dependent Reactions of Photosynthesis
The actual step that converts light energy into chemical...
- 8.2: The Light-Dependent Reactions of Photosynthesis
The pigments of the first part of photosynthesis, the light-dependent reactions, absorb energy from sunlight. A photon strikes the antenna pigments of photosystem II to initiate photosynthesis. The energy travels to the reaction center that contains chlorophyll a to the electron transport chain, which pumps hydrogen ions into the thylakoid ...
The actual step that converts light energy into chemical energy takes place in a multiprotein complex called a photosystem, two types of which are found embedded in the thylakoid membrane: photosystem II (PSII) and photosystem I (PSI) (Figure 8.17). The two complexes differ on the basis of what they oxidize (that is, the source of the low ...
- Overview
- Introduction
- Overview of the light-dependent reactions
- What is a photosystem?
- Photosystem I vs. photosystem II
- Photosystem II
- Electron transport chains and photosystem I
- Some electrons flow cyclically
How light energy is used to make ATP and NADPH. Photosystems I and II. Reaction center chlorophylls P700 and P680.
Plants and other photosynthetic organisms are experts at collecting solar energy, thanks to the light-absorbing pigment molecules in their leaves. But what happens to the light energy that is absorbed? We don’t see plant leaves glowing like light bulbs, but we also know that energy can't just disappear (thanks to the First Law of Thermodynamics).
As it turns out, some of the light energy absorbed by pigments in leaves is converted to a different form: chemical energy. Light energy is converted to chemical energy during the first stage of photosynthesis, which involves a series of chemical reactions known as the light-dependent reactions.
In this article, we'll explore the light-dependent reactions as they take place during photosynthesis in plants. We'll trace how light energy is absorbed by pigment molecules, how reaction center pigments pass excited electrons to an electron transport chain, and how the energetically "downhill" flow of electrons leads to synthesis of ATP and NADPH. These molecules store energy for use in the next stage of photosynthesis: the Calvin cycle.
[What about non-plant photosynthesis?]
Before we get into the details of the light-dependent reactions, let's step back and get an overview of this remarkable energy-transforming process.
The light-dependent reactions use light energy to make two molecules needed for the next stage of photosynthesis: the energy storage molecule ATP and the reduced electron carrier NADPH. In plants, the light reactions take place in the thylakoid membranes of organelles called chloroplasts.
Photosystems, large complexes of proteins and pigments (light-absorbing molecules) that are optimized to harvest light, play a key role in the light reactions. There are two types of photosystems: photosystem I (PSI) and photosystem II (PSII).
Both photosystems contain many pigments that help collect light energy, as well as a special pair of chlorophyll molecules found at the core (reaction center) of the photosystem. The special pair of photosystem I is called P700, while the special pair of photosystem II is called P680.
In a process called non-cyclic photophosphorylation (the "standard" form of the light-dependent reactions), electrons are removed from water and passed through PSII and PSI before ending up in NADPH. This process requires light to be absorbed twice, once in each photosystem, and it makes ATP . In fact, it's called photophosphorylation because it involves using light energy (photo) to make ATP from ADP (phosphorylation). Here are the basic steps:
•Light absorption in PSII. When light is absorbed by one of the many pigments in photosystem II, energy is passed inward from pigment to pigment until it reaches the reaction center. There, energy is transferred to P680, boosting an electron to a high energy level. The high-energy electron is passed to an acceptor molecule and replaced with an electron from water. This splitting of water releases the O2 we breathe.
Photosynthetic pigments, such as chlorophyll a, chlorophyll b, and carotenoids, are light-harvesting molecules found in the thylakoid membranes of chloroplasts. As mentioned above, pigments are organized along with proteins into complexes called photosystems. Each photosystem has light-harvesting complexes that contain proteins, 300 -400 chlorophylls, and other pigments. When a pigment absorbs a photon, it is raised to an excited state, meaning that one of its electrons is boosted to a higher-energy orbital.
Most of the pigments in a photosystem act as an energy funnel, passing energy inward to a main reaction center. When one of these pigments is excited by light, it transfers energy to a neighboring pigment through direct electromagnetic interactions in a process called resonance energy transfer. The neighbor pigment, in turn, can transfer energy to one of its own neighbors, with the process repeating multiple times. In these transfers, the receiving molecule cannot require more energy for excitation than the donor, but may require less energy (i.e., may absorb light of a longer wavelength)6 .
Collectively, the pigment molecules collect energy and transfer it towards a central part of the photosystem called the reaction center.
The reaction center of a photosystem contains a unique pair of chlorophyll a molecules, often called special pair (actual scientific name—that's how special it is!). Once energy reaches the special pair, it will no longer be passed on to other pigments through resonance energy transfer. Instead, the special pair can actually lose an electron when excited, passing it to another molecule in the complex called the primary electron acceptor. With this transfer, the electron will begin its journey through an electron transport chain.
There are two types of photosystems in the light-dependent reactions, photosystem II (PSII) and photosystem I (PSI). PSII comes first in the path of electron flow, but it is named as second because it was discovered after PSI. (Thank you, historical order of discovery, for yet another confusing name!)
Here are some of the key differences between the photosystems:
•Special pairs. The chlorophyll a special pairs of the two photosystems absorb different wavelengths of light. The PSII special pair absorbs best at 680 nm, while the PSI special absorbs best at 700 nm. Because of this, the special pairs are called P680 and P700, respectively.
•Primary acceptor. The special pair of each photosystem passes electrons to a different primary acceptor. The primary electron acceptor of PSII is pheophytin, an organic molecule that resembles chlorophyll, while the primary electron acceptor of PSI is a chlorophyll called A0 7,8 .
•Source of electrons. Once an electron is lost, each photosystem is replenished by electrons from a different source. The PSII reaction center gets electrons from water, while the PSI reaction center is replenished by electrons that flow down an electron transport chain from PSII.
During the light-dependent reactions, an electron that's excited in PSII is passed down an electron transport chain to PSI (losing energy along the way). In PSI, the electron is excited again and passed down the second leg of the electron transport chain to a final electron acceptor. Let’s trace the path of electrons in more detail, starting when they're excited by light energy in PSII.
When the P680 special pair of photosystem II absorbs energy, it enters an excited (high-energy) state. Excited P680 is a good electron donor and can transfer its excited electron to the primary electron acceptor, pheophytin. The electron will be passed on through the first leg of the photosynthetic electron transport chain in a series of redox, or electron transfer, reactions.
After the special pair gives up its electron, it has a positive charge and needs a new electron. This electron is provided through the splitting of water molecules, a process carried out by a portion of PSII called the manganese center9 . The positively charged P680 can pull electrons off of water (which doesn't give them up easily) because it's extremely "electron-hungry."
When an electron leaves PSII, it is transferred first to a small organic molecule (plastoquinone, Pq), then to a cytochrome complex (Cyt), and finally to a copper-containing protein called plastocyanin (Pc). As the electron moves through this electron transport chain, it goes from a higher to a lower energy level, releasing energy. Some of the energy is used to pump protons (H+ ) from the stroma (outside of the thylakoid) into the thylakoid interior.
This transfer of H+ , along with the release of H+ from the splitting of water, forms a proton gradient that will be used to make ATP (as we'll see shortly).
Once an electron has gone down the first leg of the electron transport chain, it arrives at PSI, where it joins the chlorophyll a special pair called P700. Because electrons have lost energy prior to their arrival at PSI, they must be re-energized through absorption of another photon.
Excited P700 is a very good electron donor, and it sends its electron down a short electron transport chain. In this series of reactions, the electron is first passed to a protein called ferredoxin (Fd), then transferred to an enzyme called NADP+ reductase. NADP+ reductase transfers electrons to the electron carrier NADP+ to make NADPH. NADPH will travel to the Calvin cycle, where its electrons are used to build sugars from carbon dioxide.
The pathway above is sometimes called linear photophosphorylation. That's because electrons travel in a line from water through PSII and PSI to NADPH. (Photophosphorylation = light-driven synthesis of ATP.)
In some cases, electrons break this pattern and instead loop back to the first part of the electron transport chain, repeatedly cycling through PSI instead of ending up in NADPH. This is called cyclic photophosphorylation.
After leaving PSI, cyclically flowing electrons travel back to the cytochrome complex (Cyt) or plastoquinone (Pq) in the first leg of the electron transport chain10,11 . The electrons then flow down the chain to PSI as usual, driving proton pumping and the production of ATP. The cyclic pathway does not make NADPH, since electrons are routed away from NADP+ reductase.
Why does the cyclic pathway exist? At least in some cases, chloroplasts seem to switch from linear to cyclic electron flow when the ratio of NADPH to NADP+ is too high (when too little NADP+ is available to accept electrons)12 . In addition, cyclic electron flow may be common in photosynthetic cell types with especially high ATP needs (such as the sugar-synthesizing bundle-sheath cells of plants that carry out C4 photosynthesis)13 . Finally, cyclic electron flow may play a photoprotective role, preventing excess light from damaging photosystem proteins and promoting repair of light-induced damage14 .
The light-dependent reactions begin in a grouping of pigment molecules and proteins called a photosystem. Photosystems exist in the membranes of thylakoids. A pigment molecule in the photosystem absorbs one photon, a quantity or “packet” of light energy, at a time. A photon of light energy travels until it reaches a molecule of chlorophyll.
Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants , algae , and cyanobacteria .
Electrons and Energy. Removing electrons from a molecule (oxidation) results in a decrease in potential energy in the oxidized compound. The electron does not remain unbonded; it is shifted to a second compound, reducing the second compound.
