2.18 Light Reactions

What is a photosystem?

Collection of photosynthetic

Light reactions occur in 3 steps:

  1. Pigments embedded in thylakoid membrane absorb energy from light and release high energy electrons (ionized). Antenna pigments transfer electrons to reaction center. Electrons passed to acceptor molecule in ETC

  2. Movement of electrons and H+ ions leads to production of NADPH and ATP via chemiosmosis

  3. Electrons released by pigments are replaced by splitting of H2O (photolysis) producing O2

Overall result is formation of ATP +NADPH - used in Calvin cycle to construct glucose

The Specifics

  • Light energy of a certain wavelength (680 nm) excites chlorophyll a in photosystem II causing released of electrons

  • Excited electrons are transferred to carrier molecules within the thylakoid membrane

  • P680 goes with photosystem 2

  • Electrons released by photosystem II are transferrer through series of proteins in thylakoid membrane (ETC) to photosystem I

  • As electrons move they loose energy and H+ ions move from stroma to thylakoid space

  • H+ gradient creates proton motive force

  • Chemiosmosis through ATP Synthase creates ATP via phosphorylation

  • Light energy (700 nm) wxcites chlorophyll a in photosystem I causing electrons to move into second series of membrane proteins

  • NADP+ is final electron acceptor. It is reduced to NADPH

  • NADP+ is final electron acceptor. It is reduced to NADPH

  • Photolysis (splistting of water by light) provides electrons to replace ones lost by P680, O2, and H+ ions