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

2. 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.

3. 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.

4. 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

5. Link to light reactions sumanas

6. OVERVIEW OF LIGHT RXNS WITHIN THE CHLOROPLAST
Fig
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+

7. (INTERIOR OF THYLAKOID)
Fig
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)

8. Fig
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)

9. Light Reaction Animations Links
Link to Boyer site

10. 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

11. 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

12. 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-

13. 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

14. STEPS OF THE LIGHT REACTIONS
Fig
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)

15. Fig
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)

16. Electron transport chain
Fig
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)

17. Electron transport chain
Fig
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)

18. Electron transport chain
Fig
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)

19. H+ Diffusion Electron transport chain ADP + P
Fig
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+]

20. Link to mitochondria and chloroplasts

21. 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

22. 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

23. CYCLIC PATHWAY – PRODUCES ATP ONLY; NO NADPH
Fig
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

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