Photosynthesis
Chapter 10
A.P. Biology
Liberty Senior High School
Mr. Knowles
Photosynthesis
• Conversion of unusable light energy
into usable chemical energy.
C6H12O6
The Players
Prokaryotic Photosynthesizers
• Purple Sulfur Bacteria
• Cyanobacteria
Eukaryotic Photosynthesizers
• Protists (Algae)
• Plants (single-celled and
multicellular)
Photosynthesis
Two Step Process:
• Light-Dependent Reactions (Light
Reactions)-produce ATP and
NADPH.
• Light-Independent Reactions
(Calvin Cycle)-fix CO2 into
sugars.
• Light Reactions
–Occur in the grana.
–Split water, release oxygen,
produce ATP, and form
NADPH
• The Calvin Cycle
–Occurs in the stroma
–Forms sugar from carbon
dioxide, using ATP for energy
and NADPH for reducing
power
• An overview of photosynthesis
H2 O
CO2
Light
NADP 
ADP
+ P
LIGHT REACTIONS
CALVIN
CYCLE
ATP
NADPH
Chloroplast
Figure 10.5
O2
[CH2O]
(sugar)
Light Dependent
Reactions
• Pigments absorb the light energy.
• Different pigments are used in
different organisms.
• Each with different absorption
spectra.
• Why?
• A photosystem
– Is composed of a reaction center surrounded by a number of
light-harvesting complexes
Thylakoid
Photosystem
Photon
STROMA
Thylakoid membrane
Light-harvesting
complexes
Primary election
acceptor
e–
Transfer
of energy
Figure 10.12
Reaction
center
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
• Light-harvesting complexes
–Consist of pigment molecules bound to
particular proteins.
–Funnel the energy of photons of light to
the reaction center.
Light-Dependent Reactions
• Photon captured by pigments arranged in
a network with proteins embedded in a
membrane- Photosystem (I and II).
• In bacteria, this membrane is the cell
membrane.
• In protists (algae) and plants, this
membrane is the thylakoid of the
chloroplasts.
Photosystems
• In plants, the reaction center
molecule is a type of
chlorophyll a called P700 for
photosystem l.
• In the sulfur bacteria, the
reaction center molecule is
P870 for photosystem l.
The First Photosynthesizers
• The purple sulfur bacteria more than 3
billion years ago.
• Used P870 in photosystem l.
• The excited electron is ejected from the
pigment and travels in a circular path.
Used to power a proton pump to make
ATP- chemiosmosis.
• This circular movement is called - Cyclic
Photophosphorylation.
A 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
• The spatial organization of chemiosmosis
– Differs in chloroplasts and mitochondria
Key
Higher [H+]
Lower [H+]
Chloroplast
Mitochondrion
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
H+
Diffusion
Thylakoid
space
Intermembrance
space
Membrance
Electron
transport
chain
ATP
Synthase
Matrix
Figure 10.16
ADP+
Stroma
P
H+
ATP
• 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
Cyclic
Photophosphorylation.
• Produces ATP from light energy.
• Major limitation: no biosynthesis
(no carbohydrates made from CO2.
Therefore, no long term storage of
energy.
• These bacteria must find other sources
of hydrogen to reduce CO2.
• Inefficient.
How do you make a
better photosynthesizer?
The Evolutionary Process
Continues!
Make another Photosystem-The
Advent of Noncyclic
Photosysthesis
Noncyclic Electron Flow
• Noncyclic electron flow
–Is the primary pathway of energy
transformation in the light reactions
• Produces NADPH, ATP, and oxygen
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
H2O
e
2
2 H+
e
3
NADPH
5
Light
+ H+
P700
P680
Light
6
ATP
Figure 10.13
NADP+
+ 2 H+
PC
e–
e–
1
e–
8
NADP+
reductase
Cytochrome
complex
+
O2
7
Fd
4
Pq
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
Enter the Competition!
Cyanobacteria
• Another photosystem was created.
• Photosystem II uses a slightly different form
of chlorophyll a called P680 (absorbs shorter
wavelengths of light, more energy).
• Excited electron enters the electron transport
chain and powers proton pumps.
• This leads to chemiosmosis and the synthesis
of ATP.
Photosystem II
• The excited electron is eventually
passed to the P700 molecule in
Photosystem I.
• Next, Photosystem I can absorb
another photon and excites an
electron.
• This electron provides the reducing
power in the form of NADPH.
Photosystems II and I
• Together, these are called Noncyclic
Photophosphorylation.
• Why does II come before I?
• Why does noncyclic photo. Produce
NADPH instead of NADH?
• How is the excited electron replaced
in the P680 of Photosystem II?
Making Oxygen Gas!
• The P680 is a strong oxidizer and it
removes an e- from the Z protein.
• The Z protein obtains an e from H2O.
Z enzyme
+
• H2 O
H + OH
• OH- reassembled into O2 and H2O.
+
• H remain in the thylakoid space.
In summary…
• The Light Reactions:
Photosystem II makes ATP
from light energy.
Photosystem I makes
NADPH from light energy.
Both occur in the thylakoid
spaces of chloroplasts.
Cyclic Electron Flow
• Under certain conditions
-Photoexcited electrons take an
alternative path.
-Calvin cycle requires more ATP and
NADPH.
-This path makes up the difference.
• In cyclic electron flow
– Only photosystem I is used
– Only ATP is produced
Primary
acceptor
Primary
acceptor
Fd
Fd
NADP+
Pq
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Figure 10.15
Photosystem II
ATP
Photosystem I
• The light reactions and chemiosmosis: the
organization of the thylakoid membrane
H2O
CO2
LIGHT
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTOR
ATP
NADPH
STROMA
(Low H+ concentration)
O2
[CH2O] (sugar)
Cytochrome
complex
Photosystem II
Photosystem I
NADP+
reductase
Light
2 H+
3
NADP+ + 2H+
Fd
NADPH
+ H+
Pq
Pc
2
H2O
THYLAKOID SPACE
(High H+ concentration)
1⁄
2
1
O2
+2 H+
2 H+
To
Calvin
cycle
STROMA
(Low H+ concentration)
Thylakoid
membrane
ATP
synthase
ADP
ATP
P
Figure 10.17
H+
World’s Largest Organism!
The Dark Reactions
• Use the ATP and the NADPH made
in the light reactions and build sugar
molecules from the CO2 in the
atmosphere.
• Needs a source of energy (from ATP)
and the H to reduce the CO2 (from
NADPH).
• The Calvin Cycle, Fig. 10.12 and
10.13.
• Concept 10.3: The Calvin cycle uses ATP and
NADPH to convert CO2 to sugar
• The Calvin cycle
– Is similar to the citric acid cycle
– Occurs in the stroma
• The Calvin Cycle has three
phases
– Carbon fixation
– Reduction
– Regeneration of the CO2 acceptor
The Calvin Cycle
H2O
CO2
Input
(Entering one
3
at a time)
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTION
ATP
Phase 1: Carbon fixation
NADPH
Rubisco
O2
[CH2O] (sugar)
3 P
P
Short-lived
intermediate
3 P
P
Ribulose bisphosphate
(RuBP)
P
6
3-Phosphoglycerate
6 ATP
6 ADP
CALVIN
CYCLE
3 ADP
3
6 P
ATP
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
(G3P)
P
1
Figure 10.18
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
The Calvin Cycle
• Also called the C3 cycle because of
a 3-carbon molecule (PGA).
• It is the reverse of glycolysis.
• Taking C atoms from atmospheric
CO2 and making sugars-Carbon
Fixation.
• Occurs in the stroma.
The Calvin Cycle
• First step is dependent on RuBP
carboxylase enzyme (RUBISCO)
which adds C from CO2.
• When the temperature >28° C or the
concentration of CO2 falls,
RUBISCO will also oxidize instead
of adding C’s- Photorespiration- the
opposite of the Calvin Cycle.
RUBISCO Activity
>28° C
Photorespiration
• Loses 1/4 to 1/2 of all fixed C
that enters the Calvin Cycle.
• The C3 plants would not
efficiently photosynthesize in
tropical climates.
• Enter the competition!
• Concept 10.4: Alternative mechanisms
of carbon fixation have evolved in hot,
arid climates
• On hot, dry days, plants close their stomata
– Conserving water but limiting access to CO2
– Causing O2 to build up
Photorespiration: An
Evolutionary Relic?
• In photorespiration
–O2 substitutes for CO2 in the
active site of the enzyme rubisco.
–The photosynthetic rate is
reduced.
C3 Leaf Structure
C4 Leaf Structure
C4 Plants
• C4 plants minimize the cost of
photorespiration
–By incorporating CO2 into four
carbon compounds in mesophyll
cells
–Are exported to bundle sheath
cells, where they release CO2 used
in the Calvin cycle
C4 leaf anatomy and the C4 pathway
Mesophyll
cell
Mesophyll cell
Photosynthetic
cells of C4 plant
leaf
CO
CO
2 2
PEP carboxylase
Bundlesheath
cell
PEP (3 C)
Oxaloacetate (4 C)
ADP
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
Comparsion of C3 and C4
Plants
C3 Plants
• Are temperate •
plants.
• Perform light and •
dark reactions in
the same cellmesophyll cells.
C4 Plants
Are tropical plants.
(Corn and sugar cane).
Perform light
reactions in
mesophyll cells but
the Calvin cycle in
bundle sheath cells.
The Cost to C4 Plants
• It costs 2 ATP to transport each CO2
into a bundle sheath cell.
• The energetic cost of C4 Photosynthesis
is twice that of C3 Photosynthesis.
• Photosynthesis is advantegous in hot
climates.
• C4 plants outcompete C3 plants in
tropical climates.
C3 Leaf Structure
C4 Leaf Structure
What about plants in
extremely hot climates?
Must conserve water!
Transpiration
• Water loss from the leaf
tissue through the
stomata. (Analogy:
Sweating.)
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
CAM Plants
• Succulent plants must conserve water
loss from stomata. Cacti and pineapple
use another strategy.
• Crassulacean Acid Metabolism
(CAM) Plants.
• The stomata are closed during the day
to prevent water loss and reduce
photorespiration by preventing CO2
from leaving the leaf. Open the stomata
at night.
CAM Plants
• Perform C3 and C4 pathways in the same
cell (mesophyll) but at different times.
• The C4 pathway is used at night when the
stomata are open. Prevent CO2 losses
• The C3 pathway is used during the day when
the stomata are closed and there is a need to
reduce water loss. The CO2 for making
sugars during the day come from organic
molecules made during the previous night,
none from atmosphere.
The 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
types of cells.
Figure 10.20
CAM
CO2
CO2
CALVIN
CYCLE
Sugar
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.
The Importance of Photosynthesis: A
Review
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