BY4.3 Photosynthesis

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green plants using light to synthesise nutrients

  • found in the mesophyll cells and the guard cells
  • energy from sunlight is trapped by the chlorophyll pigment
  • biochemical process divided into 2 parts:
    • light dependent stage
    • light independent stage/ calvin cycle

Light harvesting

  • chlorophylls and accessory pigments are found in the thylakoid membrane
  • grouped into several clusters of hundreds of moleucles - form the antenna complex
  • special proteins assocaited with pigments help to funnel photons of light energy enetering the chlorophylls
    • pigment moleules pass absorbed light energy from one molecule to the next
  • photons of light are passed on until they reach a primary pigemtn molecule known as the reaction centre
    • usually chlorophyll a

light strikes a molecule at the reaction centre and an electron is raised to a higher energy level

it can:

  • move to another atom and form a bond
  • be emitted and go around a ciriuit
  • fall back to a lower energy level and emit light
  • fall back to a lower energy level and pass energy to another substance

there are 2 types of reaction centres :

  • PSI - arranged around a chlorophyll a molecule with a peak absorbance of 700nm (P700)
  • PSII - arranged around a chlorophyll a molecule with a peak absorbance of 680nm (P680)

Light Dependent Stage:

occurs in the thylakoids:

  • water is split by photolysis to form protons and electrons
  • ATP is synthesised through photophosporylation
  • NADP is reduced into NADPH2

ATP can be synthesised through cyclic and non cyclic photophosphorylaton

Non-cyclic photophsophorylation

  • photons of light are absorbed by PSII and passed to chlorophyll a in the reaction centre
  • 2 electrons are rasied to a igher energy level, are displaced and…


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