Light-dependent reaction

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6 Terms

1
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Occurs in the thylakoid membrane inside chloroplasts. Thylakoids have several key adaptations for this reaction:

  • large surface area: so they can contain more chlorophyll and electron carriers

  • ATP synthase channels: this allows the synthesis of ATP

  • selectively permeable: this allows a proton gradient to be set up


2
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The light dependent reaction uses water and light energy. It produces ATP, reduces NADP, and oxygen as a by product.

It is sometimes called phosphorylation, as it is the process by which ATP is formed using light energy

3
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Stages of the light - dependent reaction:

  1. Absorption of light energy and photoionisation

  2. Electron transfer along the electron transport chain

  3. Photolysis of water

  4. Chemiosmosis


4
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Absorption, photoionisation, and the electron transport chain:

  1. light energy is absorbed by pigments

  2. photoionisation occurs, which is when these electrons are excited and lost from the pigments (the pigments are oxidised)

  3. electrons are transferred to an electron carrier molecule (the electron carrier is reduced)

  4. electrons are passed along the electron transport chain, releasing energy as they go


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Photolysis of water:

  1. Light is used to split water into electrons, protons and oxygen: 2H2O→4H+ + 4e- + O2

  2. The electrons replace those lost from pigments during photoionisation

  3. The protons are used for ATP production and combine with electrons to reduce NADP

  4. Oxygen gas is released as a by product


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Chemiosmosis generating ATP and reduced NADP

  1. The energy lost by electrons along the electron transport chain is used to pump protons across the thylakoid membrane into the thylakoid space

  2. this produces a proton gradient, where protons are in a higher concentration in the thylakoid space

  3. the protons then diffuse through ATP synthase in the stroma

  4. this movement powers ATP synthase to produce ATP from ADP and Pi

  5. NADP takes up protons and electrons in the stroma and is reduced

  6. Reduced NADP is carried into the light independent reaction