CIE A Level Biology (9700) 2019-2021

Revision Notes

13.1.6 Photophosphorylation

Types of Photophosphorylation

  • The thylakoid membrane is the site of the light-dependent stage of photosynthesis
  • During the light-dependent stage of photosynthesis:
    • Light energy is used to breakdown water (photolysis) to produce hydrogen ions, electrons and oxygen in the thylakoid lumen
    • A proton gradient is formed due to the photolysis of water resulting in a high concentration of hydrogen ions in the thylakoid lumen
    • Electrons travel through an electron transport chain of proteins within the membrane
    • Reduced NADP (NADPH) is produced when hydrogen ions in the stroma and electrons from the electron transport chain combine with the carrier molecule NADP
    • ATP is produced during a process known as photophosphorylation (ADP + Pi → ATP) using the proton gradient between the thylakoid lumen and stroma to drive the enzyme ATP synthase
  • The photophosphorylation of ADP to ATP can be cyclic or non-cyclic, depending on the pattern of electron flow in photosystem I or photosystem II or both
    • In cyclic photophosphorylation, only photosystem I is involved
    • In non-cyclic photophosphorylation, both photosystem I and photosystem II are involved
  • Photosystems are collections of photosynthetic pigments that absorb light energy and transfer the energy onto electrons, each photosystem contains a primary pigment
    • Photosystem II has a primary pigment that absorbs light at a wavelength of 680 nm and is therefore called P680
    • Photosystem II is at the beginning of the electron transport chain and is where the photolysis of water takes place
    • Photosystem I has a primary pigment that absorbs light at a wavelength of 700 nm and is therefore called P700
    • Photosystem I is in the middle of the electron transport chain
  • The energy carried by the ATP is then used during the light-independent reactions of photosynthesis

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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