1. The Light Reactions Chapter 3.3
Photosynthesis
The Light Reactions
Chapter 3.3

2. Photosynthesis Review
light
CO2 + H2O C6H12O6 + O2 photosynthesis – creating sugar using light Only chloroplast organelles and special bacteria have the proteins necessary to carry out photosynthesis.

3. Photosynthesis: Two Major Processes
The Light Reactions
Calvin cycle
harvests light energy to split water, creating O2 , ATP and NADPH
process of producing C6H12O6 , using ATP for energy and producing NADP+

4. Photosynthesis: Two Major Processes

5. Chloroplast Structure: A Review
(thylakoid space)
- compare and contrast mitochondria and chloroplast structure

6. The Light Reactions Absorption of light photons
Photoexcitation
Electron transport
Photophosphorylation (chemiosmosis)
Absorption of light photons
Similar to ETC in mitochondria
ATP synthesis due to electrochemical gradient

7. Photoexcitation e- gain energy when atoms absorb energy.
e- fall back to lowest energy level (ground state) if it isn’t transferred to another molecule
Q: What happens to electrons when light strikes them?
A: Excited, gain energy, higher potential energy level
- think of glow in the dark objects (the light we see is electrons falling back to ground state)

8. Isolated Chlorphyll
If an isolated solution of chlorophyll is illuminated
It will fluoresce, giving off heat and light

9. Photosystems Made up of a variety of proteins
reaction center surrounded by a number of light-harvesting complexes
Contain chlorophyll and other light absorbing pigments
Located in the thylakoid membrane

10. Reaction Centre Contains a primary electron acceptor
contains chlorophyll a molecule which the light energy is focused in a photosystem

11. Two Types of Photosystems
Photosystem I (PS I)
Has P700 chlorophyll a within reaction centre
Best at absorbing nm wavelength (far red part of spectrum)
Phosystem II (PS II)
Has P680 chlorophyll a within reaction centre
Best at absorbing nm wavelength (red)

12. Purposes of Photosystems
Two purposes:
to collect as much light energy as possible
excite chlorophyll a and transfer its electrons to an electron acceptor and through a series of proteins (electron transport)

13. Electron Transport
Electron transport occurs in the thylakoid membrane.
Two mechanisms of electron transport:
Non-cyclic electron flow (the primary pathway)
Cyclic electron flow

14. Non-Cyclic Electron Flow: An Overview
LIGHT
REACTOR
NADP+
ADP
ATP
NADPH
CALVIN
CYCLE
[CH2O] (sugar)
STROMA
(Low H+ concentration)
Photosystem II
H2O
CO2
Cytochrome
complex O2 1
1⁄2 2
Photosystem I
Light
THYLAKOID SPACE
(High H+ concentration)
Thylakoid
membrane
synthase Pq Pc Fd
reductase
+ H+
NADP+ + 2H+ To
Calvin
cycle P 3 H+
2 H+
+2 H+

15. Step 1: PS II
A photon of light strikes a pigment molecule in a light harvesting complex and is relayed to other pigment molecules until it reaches one of the P680 chlorophyll a pigments within the reaction centre.
This excites one of the P680 electrons and is captured by the primary electron acceptor.
PSII splits a water molecule into 2 electrons, 2 hydrogen ions (2 H+) and 1 oxygen atom.
These electrons replace (one by one) the electrons lost to the primary electron acceptor.

16. PS II: The Details STROMA THYLAKOID MEMBRANE THYLAKOID SPACE (LUMEN)
(Low H+ Concentration)
THYLAKOID MEMBRANE
THYLAKOID SPACE (LUMEN)
(High H+ Concentration)

17. Step 2: Pq, Cytochrome Complex, Pc
Each photoexcited electron passes from the primary electron acceptor of PSII to PSI via an ETC (similar to the ETC in cellular respiration)
The ETC between PSII and PSI is made up of:
Pq (plastiquinone) - mobile
Cytochrome Complex
Pc (Plastocyanin) - mobile

18. Pq, CC, Pc: The Details
STROMA
(Low H+ Concentration)
As Plastoquinone (Pq) transfers electrons to the Cytochrome Complex, protons are pumped across the membrane into the thylakoid space (lumen)
This exergonic “fall” of electrons to a lower energy level provides energy for the active transport of H+ ions against its concentration gradient.
Electrons are then transferred to Plastocyanin (Pc), also a moveable component on thylakoid surface in lumen
THYLAKOID MEMBRANE
THYLAKOID SPACE (LUMEN)
(High H+ Concentration)

19. Step 3: PS I
A photon of light strikes a pigment molecule in a light harvesting complex and is relayed to other pigment molecules until it reaches one of the P700 chlorophyll a pigments within the reaction centre.
This excites one of the P700 electrons and is captured by the primary electron acceptor, creating an electron “hole” in the P700.
This hole is filled by an electron that reaches the bottom of the ETC from PS II.

20. PS I: The Details

21. Step 4: Fd and NADP+ Reductase
Electrons are transferred to ferrodoxin (Fd) – moveable component on thylakoid surface in stroma
Electrons are transferred to NADP+ reductase
final electron acceptor is NADP+ that is reduced to NADPH

22. NADH vs. NADPH: A Review

23. Step 5: ATP Synthase
protons pumped into the lumen (from Step 1 and Step 2)pass through ATP synthase by facilitated diffusion
ATP produced in stroma
photophosphorylation – light-dependent formation of ATP by chemiosmosis

24. The spatial organization of chemiosmosis
Differs in chloroplasts and mitochondria
Key
Higher [H+]
Lower [H+]
Mitochondrion
Chloroplast
MITOCHONDRION
STRUCTURE
Intermembrance
space
Membrance
Matrix
Electron
transport
chain H+
Diffusion
Thylakoid
Stroma
ATP P
ADP+
Synthase
CHLOROPLAST

25. Non-Cyclic Electron Flow
LIGHT
REACTOR
NADP+
ADP
ATP
NADPH
CALVIN
CYCLE
[CH2O] (sugar)
STROMA
(Low H+ concentration)
Photosystem II
H2O
CO2
Cytochrome
complex O2 1
1⁄2 2
Photosystem I
Light
THYLAKOID SPACE
(High H+ concentration)
Thylakoid
membrane
synthase Pq Pc Fd
reductase
+ H+
NADP+ + 2H+ To
Calvin
cycle P 3 H+
2 H+
+2 H+

26. Electron transport chain
Photosystem II
(PS II)
Photosystem-I
(PS I)
ATP
NADPH
NADP+
ADP
CALVIN
CYCLE
CO2
H2O O2
[CH2O] (sugar)
LIGHT
REACTIONS
Light
Primary
acceptor Pq
Cytochrome
complex PC e
P680 e– +
2 H+ Fd
reductase
Electron
Transport
chain
Electron transport chain
P700
+ 2 H+
+ H+
Energy Diagram
(Z scheme)
Energy of Electrons
Pq -

28. Non-Cyclic Electron Flow Summary
H2O is split to produce O2 (released from cell) and H+ ions (released into lumen)
enzyme complexes pump protons from stroma to lumen
NADP+ is final electron acceptor and produces NADPH
chemiosmosis to synthesize ATP

29. Light Reaction Animation
watch?v=hj_WKgnL6MI

30. Cyclic Electron Flow
Non-cyclic electron flow produces roughly equal amounts of ATP and NADPH
However, Calvin Cycle uses more ATP than NADPH
Cyclic electron flow makes up the difference in ATP (without producing more NADPH).
H2O
CO2
Light
LIGHT REACTIONS
CALVIN
CYCLE
Chloroplast
[CH2O]
(sugar)
NADPH
NADP 
ADP
+ P O2
ATP

31. Cyclic Electron Flow Primary Fd acceptor Pq NADP+ reductase Cytochrome
complex Pc
NADP+
reductase
NADPH
ATP
Photosystem II
Photosystem I

32. Cyclic Electron Flow Summary
only involves photosystem I (P700)
ferrodoxin returns electrons back to cytochrome complex
Only ATP produced, no NADPH

33. To Do: Section 3.2 Questions (pg. 154-155)
# 1-3, 11
Section 3.3 Questions (pg )
#1-4, 6, 8a(i-iii), 8b