The “story” of the light reactions of photosynthesis

Overview of the light rxns• Absorbs solar/light energy and converts it

into chemical energy

• 1st 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 O2 (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

• http://www.stolaf.edu/people/giannini/flashanimat/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)