The “story” of the light reactions of photosynthesis

Overview of the light rxns Absorbs solar/light energy and converts it into chemical energy 1 st step of photosynthesis Main products are oxygen gas, ATP, NADPH Raw materials needed are water, ADP + P, NADP + + H +, light energy Happens across thylakoid membrane

“Cast” of thylakoid membrane (find on F4.9) Photosystems – are clusters of pigments (PSI and PSII) Reaction centers – a part of the PSs made of chlorphyll a to where light energy is transferred that participates directly in e- flow. Electron Transport System (ETS) – groups of molecules that pass electrons from PS II to PS I. ATP synthase (ATPase) – enzyme that helps to synthesize ATP.

Journey of electrons from PSII to PSI Sunlight is absorbed by the photosystems. The light energy is eventually transferred to the reaction center, where the energy gathers. The buildup of energy causes some e- to jump from the reaction center of the PS II to other molecules in the ETS. e- travel from PSII to PSI via the ETS.

e- at PSI e- reach PSI and get an energy boost at the reaction center They now have enough energy to reduce NADP+. Also involved in the reduction are H+ (protons). (NADP + + e- + 2H +  NADPH). The reduced molecule is NADPH and is used in the Calvin Cycle.

PSII is now e- deficient…. PSII replaces the e- lost with e- that result from the oxidation of water. The oxidation of water occurs with the help of an enzyme and forms e-, H +, and O 2 (waste). So, PSII now has a new set of e- to start the flow again.

What about ATP?? As e- are passed along the ETS, some of their energy is used for active transport of protons (H + ) across the membrane from stroma to thylakoid. A gradient now exists (H+ build up inside the thylakoid) and the H+ diffuse out of thylakoid through an enzyme complex called ATPsynthase. As they diffuse, they provide energy to synthesize ATP from ADP and P. –Refer to handout App. 4A picture given – Use book picture 4.9 (~pg 109) to copy that captions onto your picture

Animation animat/metabolism/photosynthesis.swfhttp:// animat/metabolism/photosynthesis.swf

Summing it all up Water is oxidized and PSII is reduced. Excited e- travel from PSII to PSI along the ETS As they do, they provide energy for active transport of H+, which later diffuse through ATP synthase and provide energy for synthesis of ATP e- reach PSI and they, with H+, eventually reduce NADP+ to NADPH So, the light reactions have produced ATP, NADPH, and oxygen gas (waste)