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) Carotenoid- accessory pigment (reflect yellow/orange-protective role)

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)

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

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

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

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

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

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

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

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

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

** 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 not produce O2 Does not produce NADPH Does produce ATP

light reaction summary