1. Photosystems
Photosystem (fig 10.12) = rxn center surrounded by several light-harvesting complexes
Light-harvesting complex = pigment molecules bound to proteins (act as antenna for rxn center)
Rxn center = protein complex that includes 2 special chlorophyll a molecules + primary e- acceptor molecule
First step of light rxns: special chlorophyll a molecule transfers its excited e- to the primary e- acceptor

2. (INTERIOR OF THYLAKOID)
A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes
A photosystem
Is composed of a reaction center surrounded by a number of light-harvesting complexes
Thylakoid
Photon
Photosystem
STROMA
Light-harvesting
complexes
Reaction
center
Primary election
acceptor
Thylakoid membrane e–
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
Figure 10.12

3. Light-harvesting Complexes and Reaction Centers
The light-harvesting complexes consist of pigment molecules bound to particular protein
They funnel the energy from photons of light to the reaction center
When a reaction-center chlorophyll a molecule absorbs energy, one of its electrons gets bumped up to a primary electron acceptor

4. Photosystems
Two types of photosystems embedded in the thylakoid membranes of land plants (fig 10.13)
1. Photosystem I (PS I)
Rxn center chlorophyll a = P700
Cyclic and noncyclic e- flow
2. Photosystem II (PS II)
Rxn center chlorophyll a = P680
Noncyclic e- flow
Noncyclic e- flow (fig 10.13)
Uses PS II & PS I
Excited e- from PS II  goes through ETC  produces ATP
Excited e- from PS I  ETC  used to reduce NADP+
Electrons ultimately supplied from splitting water  releases O2 and H+
Cyclic e- flow (fig 10.15)
Uses only PS I
Only generates ATP
Excited e- from PS I cycle back from 1st ETC
No O2 release & no NADPH made

5. Two Photosystems The thylakoid membrane
Is populated by two types of photosystems, I and II

6. Noncyclic Electron Flow – Involves both Photosystems
Produces NADPH, ATP, and oxygen, and is the primary pathway of energy transformation in the light rxns.
Figure 10.13
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+ 7 4 2 8 3 5 1 6

7. A mechanical analogy for the light reactions
Mill
makes
ATP e–
Photon
Photosystem II
Photosystem I
NADPH
Figure 10.14 

8. Cyclic Electron Flow Under certain conditions
Photoexcited electrons take an alternative path
Uses Photosystem I only

9. In cyclic electron flow
Electrons cycle back to the first ETC
Only ATP is produced
Primary
acceptor Pq Fd
Cytochrome
complex Pc
NADP+
reductase
NADPH
ATP
Figure 10.15
Photosystem II
Photosystem I

10. Light Reactions and Chemiosmosis
The light reactions and chemiosmosis: (fig 10.17)
As e- are brought down their E gradient through the ETCs embedded in the thylakoid membranes  that E is being used to pump H+ into the thylakoid space
ATP synthase is the only place H+ can flow along its concentration gradient  E from this flow is used to join ADP + Pi  ATP
NADPH is made by NADP+ reductase
ATP and NADPH  used to power Calvin cycle

11. Light reactions and chemiosmosis: organization of the thylakoid membrane
As e- are brought down their E gradient through the ETCs embedded in the thylakoid membranes  that E is being used to pump H+ into the thylakoid space
ATP synthase is the only place H+ can flow along its concentration gradient  E from this flow is used to join ADP + Pi  ATP
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+
Figure 10.17

12. Light Reactions and Chemiosmosis
Comparison of chemiosmosis in mitochondria & chloroplasts (fig 10.16):
ATP generation basically the same (ETC + ATP synthase)
Mitochondria use high-E e- extracted from organic molecules (glucose)
Chloroplasts use light E to drive e- to top of the ETC

13. Comparison of Chemiosmosis in Chloroplasts and Mitochondria
Generate ATP by the same basic mechanism: chemiosmosis
But use different sources of energy to accomplish this. Chloroplasts use light energy and mitochondria use the chemical energy in organic molecules.

14. 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
Figure 10.16

15. Chemiosmosis: Chloroplasts vs. Mitochondria
In both organelles
Redox reactions of electron transport chains generate a H+ gradient across a membrane
ATP synthase
Uses this proton-motive force to make ATP

16. The Calvin Cycle
The Calvin cycle uses ATP + NADPH to convert CO2  sugar
Calvin cycle (fig 10.18) = anabolic process that builds carbohydrates from smaller molecules
Consumes E (ATP)
Uses NADPH as reducing agent for adding high E e- to make sugar
Phase 1: Carbon fixation
CO2 incorporated by attaching to 5-C sugar (ribulose bisphosphate = RuBP)
Catalyzed by Rubisco (ribulose bisphosphate carboxylase/oxygenase) = most abundant protein on Earth!
Unstable 6-C intermediate  2 molecules of 3-phosphoglycerate (3-C sugar)
Phase 2: Reduction
3-phosphoglycerate has phosphate added  NADPH reduces intermediate  G3P (glyceraldehyde-3-phosphate)
One G3P exits the cycle to be used by the plant cell
Other 5 recycled to regenerate RuBP
Phase 3: Regeneration of CO2 acceptor (RuBP)
Requires ATP
Five G3P (3-C)  Three 5-C molecules of RuBP

17. Calvin cycle uses ATP & NADPH to convert CO2 to sugar
The Calvin cycle
Is similar to the citric acid cycle
Occurs in the stroma

18. The Calvin Cycle
Calvin cycle (fig 10.18) = anabolic process that builds carbohydrates from smaller molecules (CO2 and H2O
Consumes E (ATP) from the light reactions
Uses NADPH (from the light reactions) as the reducing agent for adding high energy e- to make sugar

19. The Calvin cycle The Calvin cycle Figure 10.18 Input Light 3 CO2
(G3P)
Input
(Entering one
at a time)
CO2 3
Rubisco
Short-lived intermediate
3 P P
Ribulose bisphosphate (RuBP)
3-Phosphoglycerate
6 P 6
1,3-Bisphoglycerate
6 NADPH
6 NADPH+
Glyceraldehyde-3-phosphate
6 ATP
ATP
3 ADP
CALVIN
CYCLE 5 1
G3P (a sugar) Output
Light
H2O
LIGHT REACTION
ATP
NADPH
NADP+
ADP
[CH2O] (sugar)
CALVIN CYCLE
Figure 10.18 O2
6 ADP
Glucose and other organic compounds
Phase 1: Carbon fixation
Phase 3: Regeneration of the CO2 acceptor (RuBP)
Phase 2: Reduction

20. The Calvin cycle has three phases
Carbon fixation
Reduction
Regeneration of the CO2 acceptor

21. The Calvin cycle Phase 1: Carbon fixation
CO2 incorporated by attaching to 5-C sugar (ribulose bisphosphate = RuBP)
Catalyzed by Rubisco (ribulose bisphosphate carboxylase/oxygenase) = most abundant protein on Earth!
Unstable 6-C intermediate  2 molecules of 3-phosphoglycerate (3-C sugar)
(G3P)
Input
(Entering one
at a time)
CO2 3
Rubisco
Short-lived intermediate
3 P P
Ribulose bisphosphate (RuBP)
3-Phosphoglycerate
6 P 6
1,3-Bisphoglycerate
6 NADPH
6 NADPH+
Glyceraldehyde-3-phosphate
6 ATP
ATP
3 ADP
CALVIN
CYCLE 5 1
G3P (a sugar) Output
Light
H2O
LIGHT REACTION
ATP
NADPH
NADP+
ADP
[CH2O] (sugar)
CALVIN CYCLE O2
6 ADP
Glucose and other organic compounds
Phase 1: Carbon fixation
Phase 3: Regeneration of the CO2 acceptor (RuBP)
Phase 2: Reduction

22. The Calvin cycle Phase 2: Reduction
(G3P)
Input
(Entering one
at a time)
CO2 3
Rubisco
Short-lived intermediate
3 P P
Ribulose bisphosphate (RuBP)
3-Phosphoglycerate
6 P 6
1,3-Bisphosphoglycerate
6 NADPH
6 NADPH+
Glyceraldehyde-3-phosphate
6 ATP
ATP
3 ADP
CALVIN
CYCLE 5 1
G3P (a sugar) Output
Light
H2O
LIGHT REACTION
ATP
NADPH
NADP+
ADP
[CH2O] (sugar)
CALVIN CYCLE O2
6 ADP
Glucose and other organic compounds
Phase 2: Reduction
3-phosphoglycerate has phosphate added  NADPH reduces intermediate  G3P (glyceraldehyde-3-phosphate)
One G3P exits the cycle to be used by the plant cell
Other 5 recycled to regenerate RuBP
Phase 1: Carbon fixation
Phase 3: Regeneration of the CO2 acceptor (RuBP)
Phase 2: Reduction

23. The Calvin cycle Phase 3: Regeneration of CO2 acceptor (RuBP)
Requires ATP
Five G3P (3-C)  Three 5-C molecules of RuBP
(G3P)
Input
(Entering one
at a time)
CO2 3
Rubisco
Short-lived intermediate
3 P P
Ribulose bisphosphate (RuBP)
3-Phosphoglycerate
6 P 6
1,3-Bisphoglycerate
6 NADPH
6 NADPH+
Glyceraldehyde-3-phosphate
6 ATP
ATP
3 ADP
CALVIN
CYCLE 5 1
G3P (a sugar) Output
Light
H2O
LIGHT REACTION
ATP
NADPH
NADP+
ADP
[CH2O] (sugar)
CALVIN CYCLE O2
6 ADP
Glucose and other organic compounds
Phase 1: Carbon fixation
Phase 3: Regeneration of the CO2 acceptor (RuBP)
Phase 2: Reduction

24. Balancing gas exchange against water loss
Terrestrial plants have to balance gas exchange (CO2 + O2) w/ H2O loss
Happens at the stomata on leaves
If a plant closes its stomata during hottest part of day  accumulation of O2 & no CO2 uptake
Photorespiration = process that uses O2 instead of CO2  only generates 1/2 the amount of 3-phosphoglycerate  decreases Ps output by siphoning organic material from Calvin cycle (up to 50%)
Rubisco can act on both CO2 & O2 (not discriminate)
C3 plants
Most plants
Use only Calvin cycle to fix CO2 in mesophyll
Limited by photorespiration

25. On hot, dry days, plants close their stomata
Conserving water but limiting access to CO2
Causing oxygen to build up

26. Photorespiration: An Evolutionary Relic?
In photorespiration
O2 substitutes for CO2 in the active site of the enzyme rubisco
The process consumes oxygen and releases CO2
The photosynthetic rate is reduced

27. Adaptations to Hot, Arid Climates
Alternative mechanisms of carbon fixation have evolved in hot, arid climates
C4 plants separate initial carbon fixation from the Calvin cycle in space
CAM plants separate initial carbon fixation from the Calvin cycle in time

28. Adaptations to Hot, Arid Climates
C4 plants (fig 10.21a) - exhibit a spatial separation of C-fixation
Preface Calvin cycle with alternate mode of C-fixation that forms a 4-C compound (occurs in mesophyll cells) – Ex. Corn and sugarcane
Associated w/ unique leaf anatomy (Kranz anatomy, fig 10.19)
PEP carboxylase adds CO2 to PEP (phosphoenolpyruvate)  4-C oxaloacetate  converted to malate  transported to bundle sheath cells  broken down to CO2 (Calvin cycle) + pyruvate (3-C goes back to mesophyll to regenerate PEP)
PEP carboxylase has a much higher affinity for CO2 than rubisco does, and no affinity for O2
CAM plants (fig 10.21b) Ex. Succulents, cacti, pineapples, etc.
Crassulacean Acid Metabolism plants adapted to arid environments
Temporal separation of C-fixation: open stomata at night and close during day
Take up CO2 at night  oxaloacetate  malate  malic acid stored in vacuole during the day
ATP + NADPH synthesized during day  malic acid transported out of vacuole  malate  CO2 + pyruvate at night

29. Adaptations to Hot, Arid Climates
C4 plants (fig 10.21a)
CAM plants (fig 10.21b)
These types of plants separate initial carbon fixation from the Calvin Cycle either in space or time.
This allows them to keep their stomata partially closed during the day to minimize water loss.

30. C4 Plants C4 plants minimize the cost of photorespiration
By incorporating CO2 into four carbon compounds in mesophyll cells
These four carbon compounds
Are exported to bundle sheath cells, where they release CO2 used in the Calvin cycle

31. C4 leaf anatomy and the C4 pathway
Separates initial carbon fixation from the Calvin cycle in space
CO2
Mesophyll cell
Bundle-
sheath
cell
Vein
(vascular tissue)
Photosynthetic
cells of C4 plant
leaf
Stoma
Mesophyll
C4 leaf anatomy
PEP carboxylase
Oxaloacetate (4 C)
PEP (3 C)
Malate (4 C)
ADP
ATP
Sheath
Pyruate (3 C)
CALVIN
CYCLE
Sugar
Vascular
tissue
Figure 10.19

32. CAM Plants
CAM plants
Open their stomata at night, incorporating CO2 into organic acids
During the day, the stomata close
And the CO2 is released from the organic acids for use in the Calvin cycle
So they separate initial carbon fixation from the Calvin cycle in time.

33. CAM pathway is similar to the C4 pathway
Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different
types of cells.
(a)
Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times.
(b)
Pineapple
Sugarcane
Bundle-
sheath cell
Mesophyll Cell
Organic acid
CALVIN CYCLE
Sugar
CO2 C4
CAM
CO2 incorporated
into four-carbon
organic acids
(carbon fixation)
Night
Day 1 2
Organic acids
release CO2 to
Calvin cycle
Figure 10.20

34. Importance of Photosynthesis
About 50% of the organic material made by Ps is consumed as fuel for respiration in the plant body
Not all plant cells make their own food  have to be supplied by Ps cells
Two most important products we derive from plants come directly from Ps (what are they?)
Fig = nice overview of Ps

35. The Importance of Photosynthesis: A Review
A review of photosynthesis
Light reactions:
• Are carried out by molecules in the
thylakoid membranes
• Convert light energy to the chemical
energy of ATP and NADPH
• Split H2O and release O2 to the
atmosphere
Calvin cycle reactions:
• Take place in the stroma
• Use ATP and NADPH to convert
CO2 to the sugar G3P
• Return ADP, inorganic phosphate, and NADP+ to the light reactions O2
CO2
H2O
Light
Light reaction
Calvin cycle
NADP+
ADP
ATP
NADPH
+ P 1
RuBP
3-Phosphoglycerate
Amino acids
Fatty acids
Starch
(storage)
Sucrose (export)
G3P
Photosystem II
Electron transport chain
Photosystem I
Chloroplast
Figure 10.21

36. Two most important products of photosynthesis
Organic compounds produced by photosynthesis
Provide the energy and building material for ecosystems
Oxygen produced by photosynthesis
provides an aerobic environment that allows for cellular respiration