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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 eacceptor molecule
First step of light rxns: special chlorophyll a
molecule transfers its excited e- to the primary eacceptor
A Photosystem: A Reaction Center Associated with
Light-Harvesting Complexes
• A photosystem
Thylakoid
Photosystem
Photon
Light-harvesting
complexes
Thylakoid membrane
– Is composed of a
reaction center
surrounded by a number
of light-harvesting
complexes
Reaction
center
STROMA
Primary election
acceptor
e–
Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
Figure 10.12
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
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
Two Photosystems
• The thylakoid membrane
– Is populated by two types of photosystems, I
and II
Noncyclic Electron Flow – Involves both Photosystems
•
Produces NADPH, ATP, and oxygen, and is the primary pathway of energy
transformation in the light rxns.
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
e
H2O
2 H+
+
O2
2
Fd
Pq
8
e
e–
Cytochrome
complex
3
NADP+
reductase
5
e–
PC
+ H+
P700
P680
Light
6
ATP
Figure 10.13
NADP+
+ 2 H+
NADPH
e–
Light
1
7
Primary
acceptor
4
Photosystem II
(PS II)
Photosystem-I
(PS I)
A mechanical analogy for the light reactions
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Figure 10.14
Photosystem II
Photosystem I
Cyclic Electron Flow
• Under certain conditions
– Photoexcited electrons take an alternative path
– Uses Photosystem I only
In cyclic electron flow
• In cyclic electron flow
– Electrons cycle back to the first ETC
– Only ATP is produced
Primary
acceptor
Primary
acceptor
Fd
Fd
Pq
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Figure 10.15
Photosystem II
ATP
NADP+
Photosystem I
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
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
H2O
CO2
LIGHT
NADP+
ADP
LIGHT
REACTOR
CALVIN
CYCLE
ATP
NADPH
STROMA
(Low H+ concentration)
O2
[CH2O] (sugar)
Cytochrome
Photosystem II
complex
Photosystem I
NADP+
reductase
Light
2 H+
Fd
3
NADP+ + 2H+
NADPH + H+
Pq
Pc
2
H2O
THYLAKOID SPACE
1
(High H+ concentration)
1⁄
2
O2
+2 H+
2 H+
To
Calvin
cycle
STROMA
(Low H+ concentration)
Thylakoid
membrane
ATP
synthase
ADP
ATP
P
Figure 10.17
H+
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
Comparison of Chemiosmosis in Chloroplasts and Mitochondria
• 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.
The spatial organization of chemiosmosis
• Differs in chloroplasts and mitochondria
Key
Higher [H+]
Lower [H+]
Chloroplast
Mitochondrion
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
Intermembrance
space
Membrance
Matrix
Figure 10.16
H+ Diffusion
Electron
transport
chain
ATP
Synthase
ADP+
Thylakoid
space
Stroma
P
H+
ATP
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
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
Calvin cycle uses ATP & NADPH to convert CO2 to sugar
• The Calvin cycle
– Is similar to the citric acid cycle
– Occurs in the stroma
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
The Calvin cycle
• The Calvin cycle
Light
H2 O
CO2
Input
3 (Entering one
CO2 at a time)
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTION
ATP
Phase 1: Carbon fixation
NADPH
O2
Rubisco
[CH2O] (sugar)
3 P
3 P
P
Short-lived
intermediate
P
Ribulose bisphosphate
(RuBP)
P
6
3-Phosphoglycerate
6
ATP
6 ADP
CALVIN
CYCLE
3 ADP
3
ATP
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 P
6 NADPH
6 NADPH+
6 P
P
5
(G3P)
6
P
Glyceraldehyde-3-phosphate
(G3P)
P
1
Figure 10.18
P
1,3-Bisphoglycerate
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
The Calvin cycle has three phases
• The Calvin cycle has three phases
– Carbon fixation
– Reduction
– Regeneration of the CO2 acceptor
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 3phosphoglycerate (3-C
sugar)
Light
H2 O
CO2
Input
3 (Entering one
CO2at a time)
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTION
ATP
Phase 1: Carbon fixation
NADPH
O2
Rubisco
[CH2O] (sugar)
3 P
3 P
P
Short-lived
intermediate 6
P
3-Phosphoglycerate
P
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
CALVIN
CYCLE
3 ADP
3
ATP
6 P
P
1,3-Bisphoglycerate
6 NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADPH+
6 P
P
5
(G3P)
6
P
Glyceraldehyde-3-phosphate Phase 2:
(G3P)
Reduction
1
P
G3P
(a sugar)
Output
Glucose and
other organic
compounds
The Calvin cycle
• Phase 2: Reduction
• 3-phosphoglycerate
has phosphate added
 NADPH reduces
intermediate  G3P
(glyceraldehyde-3phosphate)
Light
H2 O
–
Other 5 recycled
to regenerate
RuBP
Input
3 (Entering one
CO2at a time)
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTION
ATP
Phase 1: Carbon fixation
NADPH
O2
Rubisco
[CH2O] (sugar)
3 P
3 P
P
Short-lived
intermediate 6
P
3-Phosphoglycerate
P
Ribulose bisphosphate
(RuBP)
6
ATP
6 ADP
CALVIN
CYCLE
3 ADP
3
• One G3P exits the
cycle to be used by
the plant cell
CO2
ATP
6 P
P
1,3-Bisphosphoglycerate
6 NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADPH+
6 P
P
5
(G3P)
6
P
Glyceraldehyde-3-phosphate Phase 2:
(G3P)
Reduction
1
P
G3P
(a sugar)
Output
Glucose and
other organic
compounds
The Calvin cycle
• Phase 3:
Regeneration of
CO2 acceptor
(RuBP)
Light
H2 O
CO2
Input
3 (Entering one
CO2at a time)
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTION
ATP
Phase 1: Carbon fixation
NADPH
O2
Rubisco
[CH2O] (sugar)
3 P
3 P
P
Ribulose bisphosphate
(RuBP)
• Requires ATP
6
ATP
6 ADP
CALVIN
CYCLE
3 ADP
• Five G3P (3-C) 
Three 5-C
molecules of
RuBP
P
Short-lived
intermediate 6
P
3-Phosphoglycerate
3
ATP
6 P
P
1,3-Bisphoglycerate
6 NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADPH+
6 P
P
5
(G3P)
6
P
Glyceraldehyde-3-phosphate Phase 2:
(G3P)
Reduction
1
P
G3P
(a sugar)
Output
Glucose and
other organic
compounds
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
On hot, dry days, plants close their stomata
• Conserving water but limiting access to CO2
• Causing oxygen to build up
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
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
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
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.
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
C4 leaf anatomy and the C4 pathway
• Separates initial carbon fixation from the Calvin cycle in space
Mesophyll
cell
Mesophyll cell
Photosynthetic
cells of C4 plant
leaf
CO
CO
2 2
PEP carboxylase
Bundlesheath
cell
PEP (3 C)
ADP
Oxaloacetate (4 C)
Vein
(vascular tissue)
Malate (4 C)
ATP
C4 leaf anatomy
BundleSheath
cell
Pyruate (3 C)
CO2
Stoma
CALVIN
CYCLE
Sugar
Vascular
tissue
Figure 10.19
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.
CAM pathway is similar to the C4 pathway
Pineapple
Sugarcane
C4
Mesophyll Cell
Organic acid
Bundlesheath
cell
(a) Spatial separation
of steps. In C4
plants, carbon fixation
and the Calvin cycle
occur in different
Figure 10.20 types of cells.
CALVIN
CYCLE
Sugar
CAM
CO2
CO2
1 CO2 incorporated Organic acid
into four-carbon
organic acids
(carbon fixation)
2 Organic acids
release CO2 to
Calvin cycle
CALVIN
CYCLE
Sugar
Night
Day
(b) Temporal separation
of steps. In CAM
plants, carbon fixation
and the Calvin cycle
occur in the same cells
at different times.
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 10.21 = nice overview of Ps
The Importance of Photosynthesis: A Review
• A review of photosynthesis
Light reaction
Calvin cycle
H2O
CO2
Light
NADP+
ADP
+P1
RuBP
3-Phosphoglycerate
Photosystem II
Electron transport chain
Photosystem I
ATP
NADPH
G3P
Starch
(storage)
Amino acids
Fatty acids
Chloroplast
Figure 10.21
O2
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
Sucrose (export)
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
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