Light Reactions n n Takes place in the

Light Reactions n n 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++ 2 e- --->NADPH ¨ Method of storing high energy electrons ¨

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

Spectral absorption of light n n n 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. 10 -11 Energy of electron e– Excited state Heat Photon (fluorescence) Photon Chlorophyll molecule Ground state (a) Excitation of isolated chlorophyll molecule (b) Fluorescence

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 ¨ n Process continues

Photosystems n Act as “dish antennae” Many photon collectors ¨ Only one reaction center ¨ n n 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. 10 -12 Photosystem STROMA Light-harvesting Reaction-center complexes Primary electron acceptor Thylakoid membrane Photon e– Transfer of energy Special pair of chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)

Photsystems I and II n n n Photosystem I Discovered first Acts second Has P 700 chlorophyll at reaction center Yields NADPH n n n Photosystem II Discovered second Acts first in Light Cycle Has P 680 chlorophyll at reaction center Yields ATP + O 2

Noncyclic electron flow

Fig. 10 -13 -5 Ele c Primary acceptor 2 H+ + 1/ O 2 2 H 2 O e– 2 tron Pq Primary acceptor 4 tran spo rt c hai e– n Cytochrome complex 3 E tra lect n ro ch spo n ain rt 7 Fd e– e– 8 NADP+ reductase Pc e– e– P 700 5 P 680 Light 1 Light 6 ATP Pigment molecules Photosystem II (PS II) Photosystem I (PS I) Noncyclic electron flow NADP+ + H+ NADPH

Noncyclic electron flow 2 n n 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 2 e-, 2 H+ and O ¨ Electron is transferred to reaction center ¨ n 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 n n n Newly grounded electron is used to fill void in P 700 following electron loss Electron is excited and captured by Primary acceptor Transferred through another electron transport chain ¨ n n Ferredoxin, then NADP+ reductase takes 2 H+ 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 n Ferredoxin transfers electron back to Cytochrome complex ¨ n Electron is recycled and ATP is produced instead of NADPH Results in more ATP than NADPH (required in Calvin)

Fig. 10 -15 Primary acceptor Fd Fd Pq NADP+ reductase Cytochrome complex NADPH Pc Photosystem II ATP NADP+ + H+