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The light-dependent reactions

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.

Overview of the light-dependent reactions

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.
  • ATP synthesis. The high-energy electron travels down an electron transport chain, losing energy as it goes. Some of the released energy drives pumping of H+ ions from the stroma into the thylakoid interior, building a gradient. (H+ ions from the splitting of water also add to the gradient.) As H+ ions flow down their gradient and into the stroma, they pass through ATP synthase, driving ATP production in a process known as chemiosmosis.
  • Light absorption in PSI. The electron arrives at photosystem I and joins the P700 special pair of chlorophylls in the reaction center. When light energy is absorbed by pigments and passed inward to the reaction center, the electron in P700 is boosted to a very high energy level and transferred to an acceptor molecule. The special pair's missing electron is replaced by a new electron from PSII (arriving via the electron transport chain).
  • NADPH formation. The high-energy electron travels down a short second leg of the electron transport chain. At the end of the chain, the electron is passed to NADP+ (along with a second electron from the same pathway) to make NADPH.
The net effect of these steps is to convert light energy into chemical energy in the form of ATP and NADPH. The ATP and NADPH from the light-dependent reactions are used to make sugars in the next stage of photosynthesis, the Calvin cycle. In another form of the light reactions, called cyclic photophosphorylation, electrons follow a different, circular path and only ATP (no NADPH) is produced.
It's important to realize that the electron transfers of the light-dependent reactions are driven by, and indeed made possible by, the absorption of energy from light. In other words, the transfers of electrons from PSII to PSI, and from PSI to NADPH, are only energetically "downhill" (energy-releasing, and thus spontaneous) because electrons in P680 and P700 are boosted to very high energy levels by absorption of energy from light.
Image based on, and partially traced from, similar image by R. Gutierrez5
In the rest of this article, we'll look in greater detail at the steps and players involved in the light-dependent reactions.

What is a photosystem?

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.
Image modified from "The Light-Dependent Reactions of Photosynthesis: Figure 7," by OpenStax College, Biology (CC BY 4.0.
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.

Photosystem I vs. photosystem II

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 A07,8.
  • 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.
Image modified from "The Light-Dependent Reactions of Photosynthesis: Figure 7," by OpenStax College, Biology (CC BY 4.0.
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.

Photosystem II

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 the manganese center splits water molecules, it binds two at once, extracting four electrons, releasing four H+ ions, and producing a molecule of O2.9 About 10 percent of the oxygen is used by mitochondria in the leaf to support oxidative phosphorylation. The remainder escapes to the atmosphere where it is used by aerobic organisms (such as us!) to support respiration.

Electron transport chains and photosystem I

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).
Image modified from "The Light-Dependent Reactions of Photosynthesis: Figure 8," by OpenStax College, Biology (CC BY 4.0.
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 other ingredient needed by the Calvin cycle is ATP, and this too is provided by the light reactions. As we saw above, H+ ions build inside the thylakoid interior and make a concentration gradient. Protons "want" to diffuse back down the gradient and into the stroma, and their only route of passage is through the enzyme ATP synthase. ATP synthase harnesses the flow of protons to make ATP from ADP and phosphate (Pi). This process of making ATP using energy stored in a chemical gradient is called chemiosmosis.

Some electrons flow cyclically

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.
Image modified from "The Light-Dependent Reactions of Photosynthesis: Figure 8," by OpenStax College, Biology (CC BY 4.0.
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.

Want to join the conversation?

  • male robot hal style avatar for user Sualeha
    I am still confused whether the hydrogen ions are pumped from lumen to stroma or from stroma to lumen or both?
    (8 votes)
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  • leaf green style avatar for user taracooney24
    Ok so from what I am understanding from this article is that the electrons for the electron transport chain come from the splitting of water, but I am having trouble grasping that? Specifically, are the electrons moving on up and down the chain by themselves...without protons and neutrons?
    (16 votes)
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    • piceratops ultimate style avatar for user Jonathan
      You are correct. When we split the H2O, our 2 protons in the hydrogen (the h+ ions, basically just a proton floating around), the waste product of oxygen, and then our 4 electrons. Since we have these 4 electrons removed, they allow the hydrogen to be positively charged, as the hydrogen now only has a positive charge in it.

      A simpler way to think of it is to relate this to a circuit - only electrons flow through circuitry of say, a light bulb, not entire atoms.
      (17 votes)
  • leaf blue style avatar for user on13021
    How does ATP release energy?
    (8 votes)
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    • piceratops ultimate style avatar for user Ben
      ATP consists of adenosine - itself composed of an adenine ring and a ribose sugar - and three phosphate groups (triphosphate). The phosphoryl groups, starting with the group closest to the ribose, are referred to as the alpha (α), beta (β), and gamma (γ) phosphates. The system of ATP and water under standard conditions and concentrations is extremely rich in chemical energy; the bond between the second and third phosphate groups is loosely said to be particularly high in energy. Strictly speaking, the bond itself is not high in energy (like all chemical bonds it requires energy to break), but energy is produced when the bond is broken and water is allowed to react with the two products. Thus, energy is produced from the new bonds formed between ADP and water, and between phosphate and water.
      The net change in energy at Standard Temperature and Pressure of the decomposition of ATP into hydrated ADP and hydrated inorganic phosphate is -12 kcal / mole in vivo (inside of a living cell) and -7.3 kcal / mole in vitro (in laboratory conditions). This large release in energy makes the decomposition of ATP in water extremely exergonic, and hence useful as a means for chemically storing energy. Again, the energy is actually released as hydrolysis of the phosphate-phosphate bonds is carried out.
      This energy can be used by a variety of enzymes, motor proteins, and transport proteins to carry out the work of the cell. Also, the hydrolysis yields free inorganic Pi and ADP, which can be broken down further to another Pi and AMP.
      I Hope This Helps.
      (24 votes)
  • blobby green style avatar for user Jack Sides
    photosynthesis involves the
    a. oxidizing of carbohydrates
    b produces all the earth oxygen gas
    c reduction of carbohydrates
    d. oxidation of carbohydrates
    (3 votes)
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  • blobby green style avatar for user Gayle Ould
    May I ask about the source of hydrogen ions for reduction of NADP+? I am reading some articles that say that the hydrogen ions derived from the photolysis of water are used to reduce NADP, but in my understanding, photolysis and NADP reduction occur on opposite sides of the thylakoid membrane, photolysis contributes to the proton gradient, and that the uptake of hydrogen ions to form NADPH occurs in the stroma (and thus also indirectly contributes to the size of the proton gradient through consumption of stromal hydrogen ions). Can you make this any clearer for me? Thanks!
    (8 votes)
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  • piceratops tree style avatar for user emma
    if there were an insufficient level of carbon dioxide and the Calvin cycle could not occur any faster, this would affect the supply of reduced hydrogen acceptors and ADP and phosphate. How would this affect the light reactions? would the electron transport chain 'slow down' due to this shortage and speed up if more were available? I'm trying to understand how factors such as carbon dioxide levels affect the rate of photosynthesis when light intensity is already at its maximum, and the light reactions occur at their maximum rate as well.

    Or to rephrase; if there is not enough or very little NADP+ what happens to the electron transport chain? if there is no NADP+ it will not be able to contribute to the production of NADPH, so does it slow down or stop?

    Secondly, and I'm aware that this does not belong to this section and rather is addressed in the next lesson, but what happens to the water produced in the Calvin cycle? is this 'recycled' into the light reactions to supply them with more H+ ions and electrons?
    (7 votes)
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  • leafers ultimate style avatar for user Scout107
    i just thought of this, would it be possible to genetically engineer a plant that can use light in the infared wavelengths (heat) for photosynthesis?
    (6 votes)
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  • aqualine ultimate style avatar for user Olivia
    My textbook says that ATP is made as electrons move along the electron transport chain. It this referring to the contribution of pumping protons across the membrane for chemiosmosis, or is there another method of synthesising ATP entirely?
    (4 votes)
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  • orange juice squid orange style avatar for user Rhys O'Higgins
    In paragraph 13 you say that the ATP and NADPH produced from the light dependent part of photosynthesis are used to fuel the Calvin Cycle. I'm wondering if ALL of the ATP and NADPH get used this way, or if some are used as fuel for other immediate cellular processes. Thanks!
    (4 votes)
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  • leaf green style avatar for user hu205014
    What color is oxygen?
    (3 votes)
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