Light Reactions Takes place in the Thylakoids of chloroplasts in eukaryotes Captures solar energy and converts it to Energy storage molecules ATP and NADPH along with O 2  NADP+=nicotinamide adenine dinucleotide phosphate  NADP + + H + + 2e - --->NADPH  Method of storing high energy electrons

Chlorophyll Contained in Thylakoids Similar to Heme in structure Two main types chlorophyll a and chlorophyll b  Structurally similar, but slight differences Absorbs light at red and blue wavelengths Reflects light at green wavelenghts

Spectral absorption of light Absorbed light is used as energy Chlorophyll a and b have slight differences in absorption Carotenoids can absorb light energy  Red, yellow and orange pigments  Sometimes function in photosynthesis but are usually for “sunscreen”  In fall, chlorophyll production drops and carotnoids remain

Fig (a) Excitation of isolated chlorophyll molecule Heat Excited state (b) Fluorescence Photon Ground state Photon (fluorescence) Energy of electron e–e– Chlorophyll molecule

How solar power works  Excites electrons to a higher energy state  Electrons remain at excited state for very short periods of time  When electron returns to ground state a photon of light is given off  This photon may excite an electron on another chlorophyll molecule Process continues

Photosystems Act as “dish antennae”  Many photon collectors  Only one reaction center Reaction center is adjacent to Primary electron acceptor Electron acceptor captures excited electron from reaction center before it is able to return to ground state

Fig THYLAKOID SPACE (INTERIOR OF THYLAKOID) STROMA e–e– Pigment molecules Photon Transfer of energy Special pair of chlorophyll a molecules Thylakoid membrane Photosystem Primary electron acceptor Reaction-center complex Light-harvesting complexes

Photsystems I and II Photosystem I Discovered first Acts second Has P700 chlorophyll at reaction center Yields NADPH Photosystem II Discovered second Acts first in Light Cycle Has P680 chlorophyll at reaction center Yields ATP + O 2

Noncyclic electron flow

Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 4 Pq Pc Cytochrome complex Electron transport chain 5 ATP Photosystem I (PS I) Light Primary acceptor e–e– P700 6 Fd Electron transport chain NADP + reductase NADP + + H + NADPH 8 7 e–e– e–e– 6 Fig Photosystem II (PS II) Noncyclic electron flow

Noncyclic electron flow 2 Photosystem II collects photon and activated electron is harvested by Primary e acceptor Reaction center chloropyll requires electron  Enzyme degrades H 2 O to obtain 2e -, 2H + and O  Electron is transferred to reaction center Electron is transferred through electron transport chain  Energy levels decrease while released energy is used to make ATP  Plastoquinone, Cytochrome complex and plastocyanin

Noncyclic electron flow 3 Newly grounded electron is used to fill void in P700 following electron loss Electron is excited and captured by Primary acceptor Transferred through another electron transport chain  Ferredoxin, then NADP + reductase NADP + reductase takes 2H + produced earlier plus 2 high energy electrons + NADP + and produces NADPH + H + NADPH stores activated electrons for later use

Noncyclic electron flow summary

Cyclic electron flow Ferredoxin transfers electron back to Cytochrome complex  Electron is recycled and ATP is produced instead of NADPH Results in more ATP than NADPH (required in Calvin)

Fig ATP Photosystem II Photosystem I Primary acceptor Pq Cytochrome complex Fd Pc Primary acceptor Fd NADP + reductase NADPH NADP + + H +