The light-dependent reaction

Involves the capture of light whose energy is used for two purposes:

  • To add an inorganic phosphate (Pi) molecule to ADP, thereby making ATP
  • To split water into H+ ions (protons) and OH- ions. As the splitting is caused by light, it is known as photolysis

Oxidation and reduction:

  • Always take place together

Oxidation -

  • When a substance gains oxygen or loses hydrogen
  • The substance to which oxygen has been added or hydrogen has been lost is said to be oxidised
  • Loses electrons
  • Results in energy being given out

Reduction -

  • When a substance loses oxygen or gains hydrogen
  • Gains electrons
  • Results in energy being taken in

The making of ATP:

  • When a chlorphyll molecule absorbs light energy, it boosts the energy of a pair of electrons within the chlorophyll molecule, raising them to a higher energy level
  • These electrons are said to be in an excited state
  • In fact the electrons becomes so energetic that they leave the chlorophyll molecule all together
  • As a result the chlorophyll molecule becomes ionised and so the process is called photoionisation
  • The electrons that leave the chlorophyll are taken up by an electron carrier
  • Having lost a pair of electrons, the chlorophyll molecule has been oxidised
  • The electron carrier, which has gained electrons, have been reduced
  • The electrons are now passed along a number of electron carriers in a series of oxidation-reduction reactions
  • These electron carriers form a transfer chain that is located in the membranes of the thylakoids
  • Each new carrier is at a slightly lower level energy than the previous one in the chain, and so the electrons lose energy at each stage
  • Some of this energy is used to combine an inorganic phosphate molecule with an ADP molecule in order to make ATP - phosphorylation

The precise mechanism by which ATP is produced can be explained


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