Photosynthesis Light Reaction
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Questions and Answers

What is the main purpose of the light reaction?

Provides energy for the dark reactions in the form of ATP and NADPH

What are the key components of the light reaction?

  • Two photosystems
  • Thylakoid membrane
  • Electron transport chain
  • All of the above (correct)

    • Photosystem I is located at the inner surface of the grana thylakoid membrane.

    False (B)

    Photosystem II is associated with the photolysis of water.

    <p>True (A)</p> Signup and view all the answers

    Which of the following statements is true about Photosystem II?

    <p>It is associated with the photolysis of water. (C)</p> Signup and view all the answers

    What is the role of the electron transport chain in the light reaction?

    <p>The electron transport chain moves electrons from photosystem II to photosystem I using a series of protein complexes and electron carriers. This movement of electrons releases energy that is used to pump protons across the thylakoid membrane, creating a proton gradient.</p> Signup and view all the answers

    What is chemiosmosis and how is it related to the light reaction?

    <p>Chemiosmosis is the process by which ATP is produced using the proton gradient created by the electron transport chain. This proton gradient is generated across the thylakoid membrane during the light reaction, and it is then used by ATP synthase to produce ATP.</p> Signup and view all the answers

    Explain the process of cyclic photophosphorylation.

    <p>Cyclic photophosphorylation is a process that only generates ATP. In this process, electrons from photosystem I are cycled back through the electron transport chain without being passed to NADP+.</p> Signup and view all the answers

    Explain the role of the enzyme ATP synthase in the light reaction.

    <p>ATP synthase is an enzyme that utilizes the proton gradient generated across the thylakoid membrane during the light reaction to produce ATP.</p> Signup and view all the answers

    What is the role of Pheophytin in Photosystem II?

    <p>Pheophytin is a pigment molecule in Photosystem II that accepts an electron from chlorophyll a after the light energy has been captured. This process is crucial for the movement of electrons in the electron transport chain.</p> Signup and view all the answers

    What is the role of plastoquinone in the light reaction?

    <p>Plastoquinone is a mobile electron carrier that accepts electrons from the primary electron acceptor of Photosystem II and transports them to the cytochrome b6f complex.</p> Signup and view all the answers

    The oxygen produced in the light reaction comes from carbon dioxide.

    <p>False (B)</p> Signup and view all the answers

    Flashcards

    Light Reaction

    The phase of photosynthesis that converts light energy into chemical energy as ATP and NADPH.

    ATP

    A high-energy molecule generated during the light reactions, used for energy transfer.

    NADPH

    An electron carrier produced in the light reactions, crucial for fueling the dark reactions.

    Photosystems

    Two protein-pigment complexes (PSI and PSII) involved in capturing light energy.

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    Thylakoid Membrane

    The site in chloroplasts where the light reactions take place.

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    Electron Transport Chain

    A series of protein complexes in the thylakoid membrane that transfer electrons and pump protons.

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    Chemiosmosis

    The process that uses the proton gradient created by the electron transport chain to synthesize ATP.

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    Cyclic Photophosphorylation

    A process within the light reaction that produces ATP without NADPH, using only Photosystem I.

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    Water-Plastoquinone Reductase

    An enzyme complex associated with Photosystem II that helps convert water into electrons.

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    Dark Reactions

    The phase of photosynthesis that uses ATP and NADPH from light reactions to fix carbon dioxide into glucose.

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    Study Notes

    Photosynthesis: Light Reaction

    • The main purpose of the light reaction is to provide energy for the dark reactions in the form of ATP and NADPH.
    • Two photosystems (PS II and PS I) and an electron transport chain are involved.
    • This process occurs in the thylakoid membrane.
    • Photosystem II (PS II) is located at the inner surface of the grana thylakoid membrane and contains a reaction center known as P680.
    • Photosystem I (PS I) is positioned at the outer surface of the grana thylakoid membrane and contains a reaction center called P700.
    • PS II is associated with water photolysis (splitting water into oxygen, protons, and electrons). This process produces ATP and NADPH.
    • PS I participates in cyclic and non-cyclic photophosphorylation. However, it is primarily involved in non-cyclic reactions.
    • The electron transport chain carries electrons from PS II to PS I, creating a proton gradient across the thylakoid membrane.
    • ATP synthase uses the proton gradient to produce ATP from ADP and inorganic phosphate.
    • NADP+ reductase uses electrons from PS I to reduce NADP+ into NADPH.
    • The electron transport chain between PS II and PS I includes plastoquinone (PQ), cytochrome b6f complex, and plastocyanin (Pc).
    • Water is oxidized to oxygen (O2) during the light-dependent reaction.
    • Chemiosmosis is the process where the proton gradient drives ATP synthesis.

    Photosystem I and Photosystem II (PS I vs PS II)

    • PS I is located on the outer surface of the thylakoid membrane (non-appressed granal regions and stroma lamellae). 
    • PS II is positioned on the inner surface of the grana thylakoid membrane (appressed granal regions).
    • The reaction centre of PS I is P700, and PS II is P680.
    • The core complex of PS I includes a smaller number of proteins (~15 subunits) compared to the PSII core complex (~25-30 subunits).
    • Pigments in PS I absorb longer wavelengths of light (>680 nm), while pigments in PS II absorb shorter wavelengths of light (<680 nm).
    • PS I is rich in chlorophyll a and chlorophyll b.  PS II is also rich in chlorophyll but chlorophyll b is more abundant.
    • PS I is involved in both cyclic and noncyclic photophosphorylation, whereas PS II is only involved in noncyclic photophosphorylation.
    • PS II is associated with water oxidation and is the source of electrons for the electron transport chain.
    • PS II catalyzes water oxidation/photolysis.

    Photosystem II (water-plastoquinone reductase)

    • Water is split into oxygen, protons, and electrons in the lumen.
    • Electrons are passed from water to plastoquinone (PQ).
    • Plastoquinone is reduced to plastoquinol, taking up two protons.
    • Plastoquinol carries electrons to the cytochrome b6f complex.

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    Related Documents

    Photosynthesis: Light Reaction PDF

    Description

    Explore the intricate details of the light reactions in photosynthesis, focusing on the roles of photosystems II and I. Understand the processes of ATP and NADPH production, as well as the electron transport chain and how they contribute to plant energy generation.

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