Light Reaction – Stage 1 Photosystem – Hundreds of pigments (both chlorophyll and carotenoids) clumped together with proteins in thylakoid membrane. Job – absorb sunlight

Illustration of Photsystem

How it works: Accessory pigments in the photosystems absorb light (photons) The accessory pigments pass on the energy absorbed to a specific chlorophyll a molecule (reaction center) [one in each photosystem]. This causes an electron from chlorophyll a molecule to become “excited” and shot to a higher energy level. The excited electron is transferred to a primary electron acceptor – (the chlorophyll a molecule is oxidized (lose e). The electron is passed down electron transport chain.

Photosystem II Light hits accessory pigments – energy gets transferred to chlorophyll a (reaction center) Excited electrons are lost from chlorophyll a (oxidized) Electrons are picked up by primary electron acceptor (in thylakoid membrane) Electrons are moved to and down electron transport chain – as this occurs the electrons lose energy that is used to bring hydrogens (H+) into thylakoid (from stroma) NOTE: electrons move from PSII to PSI –(replaces “e” lost from photosystem I) causes proteins in thylakoid membrane to allow H+ to enter into the thylakoid from stroma H+ needed to produce ATP

Illustration of ETC

This Rx. is called Photolysis REMEMBER – In photosystem II, chlorophyll a. loses “e” and becomes oxidized. It must get the electrons it lost back some way. Enzyme in PSII that is responsible for splitting 2 waters into 4 (+), 4(-) and O2 released by plant. The “e” are used to replace “e” lost by chloro a. in PSII This Rx. is called Photolysis

Reduction of H2O

Reduction of H2O

Making NADPH Photosystem I, same as Photosystem II (light is absorbed, excite “e” which go to primary “e” acceptor, which are then donated to another ETC. This brings the electrons to the side of the thylakoid closest to stroma. The “e” combine with protons (H+) and NADP+ to produce NADPH (temporary storage of energy for dark Rx.) The NADP + is reduced in the process – gains “e”

Making ATP Chemiosmosis – relies on conc. gradient of protons across thylakoid membrane (through carrier protein) to produce ATP Protons (H (+))come from: A. The splitting of H2O B. Pumping of H(+) from stroma into thylakoid Creates a conc. gradient of H(+) in higher in thylakoid membrane than the stroma. H(+) ions move from inside to outside through ATP synthase – enzyme that uses the energy from protons to convert ADP  ATP (Phosphoralation) Phosphoralation = ADP + PO4 = ATP

Illustration of Chemiosmosis

Final Results of Light Rx. Production of: NADPH from NADP+ + H(+) ATP from ADP Oxygen (released by plant – bi-product) #1 and #2 will be used as energy source for dark reaction (Calvin Cycle)

Review of Light Rx.