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Photosynthesis Light reactions

Photosynthesis Light reactions. Pigments : substances that absorb light The following pigments help drive the process of photosynthesis due to their absorption of light: Chlorophyll a (reflects blue/green) Chlorophyll b – accessory pigment (reflect yellow/green)

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Photosynthesis Light reactions

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  1. PhotosynthesisLight reactions

  2. Pigments: substances that absorb light • The following pigments help drive the process of photosynthesis due to their absorption of light: • Chlorophyll a (reflects blue/green) • Chlorophyll b – accessory pigment (reflect yellow/green) • Carotenoid- accessory pigment (reflect yellow/orange-protective role)

  3. Photons: light that has a fixed quantity of energy • Photoexcitation of chlorophyll: absorption of a photon causes electrons to jump from their outer most shell (when in a grounded/stable state) into an orbit of higher energy (excited state)

  4. Photosystems: Light Harvesting Complexes of the Thylakoid Membrane • Photosystems: small organic molecules/bundles that contain chlorophyll and proteins - have a light gathering antennae complex which consists of chlorophyll a, chlorophyll b, and carotenoid molecules

  5. Pathway of energy: Photon (light)  to pigments (chlorophyll b and carotenoid)  chlorophyll a  reaction centre  primary electron acceptor

  6. Chlorophyll a loses an electron to the primary electron acceptor (due to electron being in its excited state/higher orbit) and energy is trapped

  7. Thylakoid Membrane has Two Types of Photosystems • Photosystem I: reaction centre P700 (absorbs wavelength 700nm-red) • Photosystem II: reaction centre P680 (absorbs wavelength 680nm-red) **P700 and P680 are identical chlorophyll a molecules but contain different proteins

  8. Remember • Light reactions drive the synthesis of NADPH and ATP due to energy transformation (electron flow) in the thylakoid membrane

  9. Two Routes Electrons can Flow • Cyclic Electron Flow • Noncyclic Electron Flow (predominant) • Noncyclic Electron Flow: i.) Photosystem II absorbs light • Electron goes into excited state and at the reaction centre chlorophyll P680 the electron is captured by the primary electron acceptor • Chlorophyll P680 wants an electron back

  10. Enzyme extracts electrons from H2O and supplies P680 electrons • This splits H2O molecule (2H+ ions and ½ O O2) • Each photoexcited electron moves from the primary electron acceptor of Photosystem II to Photosystem I via an electron transport chain • ATP is synthesized by photophosphorylation (ATP made by light) and chemiosmosis

  11. v.) electrons travel down ETC and fills in the “hole” (missing electron) because the P700 chlorophyll a was excited from light and lost an electron to primary electron acceptor in Photosystem I

  12. vi.) primary electron acceptor of Photosystem I passes the photoexcited electrons to a second ETC

  13. ** Photosystem II produces equal amounts of ATP and NADPH • Cyclic Electron Flow • Photosystem I (P700) • The electron is replaced by the electrons in the ETC from P680 • Does notproduce O2 • Does not produce NADPH • Does produce ATP

  14. light reaction summary

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