Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (Layer 3)

How can you explain the shape of these two graphs that describe the rate of photosynthesis as a function of temperature and light intensity? Answers: Light intensity – this is a saturation curve graph. Initially increases light increases the production of key reaction intermediates in photosynthesis, however eventually the enzymes become saturated and the rate plateaus. Temperature- This is a bell curve. There is an optimum temperature at which the activity of the enzymes that catalyze the reactions of photosynthesis operate most efficiently.

Light Reactions of Photosynthesis How can light energy be used to drive endergonic chemical rxns? ANS: When light is absorbed by an electron in the atom, the electron is raised to an excited state farther from the nucleus of the atom. When the electron is farther from the nucleus, its attraction to the nucleus is weaker, and it can therefore by attracted away to a different atom.

Figure 10.13 A mechanical analogy for the light reactions NOTICE THAT PHOTOSYTEM II RECEIVES PHOTONS BEFORE PHOTOSYSTEM I P OT ENT I A L E N R G Y

Link to light reactions sumanas

OVERVIEW OF LIGHT RXNS WITHIN THE CHLOROPLAST Fig. 10-17 OVERVIEW OF LIGHT RXNS WITHIN THE CHLOROPLAST STROMA (low H+ concentration) Cytochrome complex Photosystem II Photosystem I 4 H+ Light NADP+ reductase Light Fd 3 NADP+ + H+ Pq NADPH e– Pc e– 2 H2O 1 1/2 O2 THYLAKOID SPACE (high H+ concentration) +2 H+ 4 H+ To Calvin Cycle Thylakoid membrane ATP synthase STROMA (low H+ concentration) ADP + ATP P i H+

(INTERIOR OF THYLAKOID) Fig. 10-12 Photosystem STROMA Photon ANTANNAE COMPLEX PIGMENT Primary electron acceptor Light-harvesting complexes Reaction-center complex e– Thylakoid membrane Pigment molecules Transfer of energy Special pair of chlorophyll a molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)

Fig. 10-13-1 ANTENNAE COMPLEX –PIGMENT MOLECULES HELP CAPTURE LIGHT ENERGY AND FUNNEL IT TO THE REACTION CENTER CHLOROPHYLL VIA EXCITATION TRANSFER Primary acceptor 2 e– THE TRANSFER OF ELECTRONS FROM CHLOROPHYLL TO THE ELECTRON ACCEPTOR IN THE ELECTRON TRANSPORT CHAINS OCCURS ONLY AT SPECIFIC LOCATIONS (RXN CENTERS) WITHIN THE CHLOROPLAST P680 1 Light THE CHANCES OF A PHOTON STRIKING THE RXN CENTER DIRECTLY AND EXCITING ITS ELECTRON IS EXTREMELY SMALL Pigment molecules Photosystem II (PS II)

Light Reaction Animations Links Link to Boyer site

3 POSSIBLE OUTCOMES WHEN A MOLECULE ABSORBS LIGHT ENERGY 1) Excited electron returns to ground state emitting a photon of light (e.g., isolated chlorophyll molecule) 2) Energy transfer by excitation transfer – Energy from excited electron is transferred to adjacent pigment molecule, exciting its electrons 3) Electron transfer – excited electron is captured by another molecule

Excited Electron Returning to Ground State by emitting photon of light Chlorophyll pigment with no electron acceptor nearby; excited electrons return to ground state emitting red light

Energy Transfer by Excitation Transfer Antennae pigments funneling light energy EXCITATION TRANSFER Molecule #1 with excited e- Molecule #2 with ground state e- Molecule #1 with ground state e- Molecule #2 with excited e-

Electron transfer between excited state of one molecule to another molecule Rxn Center Rxn ELECTRON TRANSFER Molecule #1 Exited State Molecule #2 Ground State Molecule #1 Ground State

STEPS OF THE LIGHT REACTIONS Fig. 10-13-1 STEPS OF THE LIGHT REACTIONS Primary acceptor ANTENNAE COMPLEX –PIGMENT MOLECULES HELP CAPTURE LIGHT ENERGY AND FUNNEL IT TO THE REACTION CENTER CHLOROPHYLL VIA EXCITATION TRANSFER 2 e– P680 1 Light Pigment molecules Photosystem II (PS II)

Fig. 10-13-2 Electrons from water replace electrons from chlorophyll transferred to primary acceptor Primary acceptor When light energy reaches rxn center P680 Chlorophyll molecules, excited electron is transferred to primary acceptor. 2 H2O e– 2 H+ + 3 1/2 O2 e– e– P680 1 Light Pigment molecules Photosystem II (PS II)

Electron transport chain Fig. 10-13-3 Primary acceptor 4 Electron transport chain Pq 2 H2O e– Cytochrome complex 2 H+ + O2 3 1/2 Pc e– e– 5 P680 1 Light ATP Pigment molecules Photosystem II (PS II)

Electron transport chain Fig. 10-13-4 Primary acceptor Primary acceptor 4 Electron transport chain Pq e– 2 H2O e– Cytochrome complex 2 H+ + 3 1/2 O2 Pc e– e– P700 5 P680 Light 1 Light 6 ATP Pigment molecules Photosystem I (PS I) Photosystem II (PS II)

Electron transport chain Fig. 10-13-5 Electron transport chain Primary acceptor Primary acceptor 4 7 Electron transport chain Fd Pq e– 2 e– 8 e– H2O e– NADP+ + H+ Cytochrome complex 2 H+ NADP+ reductase + 1/2 O2 3 NADPH Pc e– e– P700 5 P680 Light 1 Light 6 6 ATP Pigment molecules Photosystem I (PS I) Photosystem II (PS II)

H+ Diffusion Electron transport chain ADP + P Fig. 10-16 Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE H+ Diffusion Intermembrane space Thylakoid space Electron transport chain Inner membrane Thylakoid membrane ATP synthase Matrix Stroma Key ADP + P i ATP Higher [H+] H+ Lower [H+]

Link to mitochondria and chloroplasts

Cyclic vs. Noncyclic Light Rxns Noncyclic – “normal” pathway; produces ATP and NADPH; O2 is continuously produced Cyclic – pathway that occurs only when [NADPH] in cell is very high; produces ATP only; O2 not produced because recycling e- instead of introducing new electrons

Electron transport chain Electron transport chain NADP+ + H+ Fig. 10-UN1 H2O CO2 NONCYCLIC PATHWAY (AKA “Z” SCHEME) Primary acceptor Electron transport chain Primary acceptor Electron transport chain Fd NADP+ + H+ H2O Pq NADP+ reductase O2 Cytochrome complex NADPH Pc Photosystem I ATP Photosystem II O2

CYCLIC PATHWAY – PRODUCES ATP ONLY; NO NADPH Fig. 10-15 CYCLIC PATHWAY – PRODUCES ATP ONLY; NO NADPH ALSO NOTE THAT NO O2 IS PRODUCED Primary acceptor Primary acceptor Fd Fd NADP+ + H+ Pq NADP+ reductase Cytochrome complex NADPH Pc Photosystem I Photosystem II ATP

Light Reaction Animations Links Link to light reaction energy changes Link to light reaction structures