Edexcel A (SNAB) A Level Biology:复习笔记5.2.4 Light-dependent Reactions

Light-dependent Reactions

  • Photosynthesis takes place in two distinct stages
    • The light-dependent reactions, which rely on light directly
    • The light-independent reactions, which do not use light directly, though do rely on the products of the light-dependent reactions
  • Both these sets of reactions take place within the chloroplast
    • The light-dependent reactions take place across the thylakoid membrane
    • The light-independent reactions take place in the stroma
  • Light energy in the light-dependent reactions enables the splitting of water molecules in a reaction known as photolysis
    • Photolysis of one molecule of water, or H2O, produces
      • 2 hydrogen ions (2H+), also known as protons
      • 2 electrons (2e-)
      • One atom of oxygen (O)
    • The hydrogen ions and electrons are used during the light-dependent reactions while the oxygen is given off as a waste product
  • During the light-dependent reactions light energy is converted into chemical energy in the form of ATP and reduced NADP
    • NADP is a type of molecule called a coenzyme; its role is to transfer hydrogen from one molecule to another
    • When NADP gains hydrogen, it is reduced and can be known as either reduced NADP or NADPH
    • Remember that
      • Reduction is gain of electrons, gain of hydrogen, or loss of oxygen
      • Oxidation is loss of electrons, loss of hydrogen, or gain of oxygen
    • Reduced NADP can reduce other molecules by giving away hydrogen
    • NADP can oxidise other molecules by receiving hydrogen
  • The useful products of the light-dependent reactions, ATP and NADPH, are transferred to the light-independent reactions within the chloroplast

The products of the light-dependent reaction are ATP, NADPH, and oxygen. Oxygen is given off as a waste product while ATP and NADPH pass to the light-independent reactions. The ADP and NADP produced during the light-independent reaction can pass back to the light-dependent reactions to allow more ATP and NADPH to be produced.

Production of ATP and NADPH

  • ATP and NADPH are produced during the light-dependent reactions as a result of a series of events that occur on the thylakoid membrane known as photophosphorylation
    • Photo = light
    • Phosphorylation = the addition of phosphate; in this case to ADP to form ATP
  • Two types of photophosphorylation take place
    • Non-cyclic photophosphorylation
      • This produces both ATP and NADPH
    • Cyclic photophosphorylation
      • This produces ATP only
  • Both cyclic and non-cyclic photophosphorylation involve
    • A series of membrane proteins which together make up the electron transport chain
      • Electrons pass from one protein to another along the electron transport chain, releasing energy as they do so
    • Chemiosmosis
      • The energy released as electrons pass down the electron transport chain is used to produce ATP

Non-cyclic photophosphorylation

  • Light energy hits photosystem II in the thylakoid membrane
    • It is slightly confusing that photosystem II comes first in this sequence; the numbers simply reflect the order in which the photosystems were discovered
  • Two electrons gain energy and are said to be excited to a higher energy level
  • The excited electrons leave the photosystem and pass to the first protein in the electron transport chain
    • As the excited electrons leave photosystem II they are replaced by electrons from the photolysis of water
  • The electrons pass down the chain of electron carriers known as an electron transport chain
  • Energy is released as the electrons pass down the electron transport chain which enables chemiosmosis to occur
    • H­­+ ions are actively pumped from a low concentration in the stroma to a high concentration in the thylakoid space, generating a concentration gradient across the thylakoid membrane
    • H­­+ ions diffuse back across the thylakoid membrane into the stroma via ATP synthase enzymes embedded in the membrane
    • The movement of H­­+ ions causes the ATP synthase enzyme to catalyse the production of ATP
  • At the end of the electron transport chain the electrons from photosystem II are passed to photosystem I
  • Light energy also hits photosystem I, exciting another pair of electrons which leave the photosystem
  • The excited electrons from photosystem I also pass along an electron transport chain
  • These electrons combine with hydrogen ions from the photolysis of water and the coenzyme NADP to form reduced NADP

H+ + 2e- + NADP+ → NADPH

  • The reduced NADP and the ATP pass to the light-independent reactions

Non-cyclic photophosphorylation involves photosystems I and II and produces both ATP and NADPH

Cyclic photophosphorylation

  • Light hits photosystem I
  • Electrons are excited to a higher energy level and leave the photosystem
  • The excited electrons pass along the electron transport chain, releasing energy as they do so
  • The energy released as the electrons pass down the electron transport chain provides energy to drive the process of chemiosmosis
    • H­­+ ions are actively pumped from a low concentration in the stroma to a high concentration in the thylakoid space, generating a concentration gradient across the thylakoid membrane
    • H­­+ ions diffuse back across the thylakoid membrane into the stroma via ATP synthase enzymes embedded in the membrane
    • The movement of H­­+ ions cause the ATP synthase enzyme to catalyse the production of ATP
  • At the end of the electron transport chain the electrons rejoin photosystem I in a complete cycle; hence the term cyclic photophosphorylation
  • The ATP produced enters the light-independent reaction

Cyclic photophosphorylation involves Photosystem I and produces ATP

 

 

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