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• Overview: The Process That Feeds the
Biosphere
• Photosynthesis
– Is the process that converts solar energy into
chemical energy
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Plants and other autotrophs
– Are the producers of the biosphere
• Plants are photoautotrophs
– They use the energy of sunlight to make
organic molecules from water and carbon
dioxide
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Photosynthesis
– Occurs in plants, algae, certain other protists,
and some prokaryotes
These organisms use light energy to drive the
synthesis of organic molecules from carbon dioxide
and (in most cases) water. They feed not only
themselves, but the entire living world. (a) On
land, plants are the predominant producers of
food. In aquatic environments, photosynthetic
organisms include (b) multicellular algae, such
as this kelp; (c) some unicellular protists, such
as Euglena; (d) the prokaryotes called
cyanobacteria; and (e) other photosynthetic
prokaryotes, such as these purple sulfur
(a) Plants
bacteria, which produce sulfur (spherical
globules) (c, d, e: LMs).
(c) Unicellular protist 10 m
(e) Pruple sulfur
bacteria
Figure 10.2
(b) Multicellular algae
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(d) Cyanobacteria
40 m
1.5 m
• Heterotrophs
– Obtain their organic material from other
organisms
– Are the consumers of the biosphere
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Concept 10.1: Photosynthesis converts light
energy to the chemical energy of food
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Chloroplasts: The Sites of Photosynthesis in Plants
• The leaves of plants
– Are the major sites of photosynthesis
Leaf cross section
Vein
Mesophyll
Stomata
Figure 10.3
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
CO2
O2
• Chloroplasts
–
–
Mesophyll
Are the
organelles
in which
photosynth
esis occurs
Chloroplast
5 µm
Contain
thylakoids
and grana
Outer
membrane
Thylakoid
Stroma
Granum
Intermembrane
space
Thylakoid
space
Inner
membrane
1 µm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Tracking Atoms Through Photosynthesis: Scientific
Inquiry
• Photosynthesis is summarized as
6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Photosynthesis as a Redox Process
• Photosynthesis is a redox process
– Water is oxidized, carbon dioxide is reduced
– This is the reverse flow of electrons as in
cellular respiration
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The Two Stages of Photosynthesis: A Preview
• Photosynthesis consists of two processes
– The light reactions
– The Calvin cycle
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• The light reactions
– Occur in the grana
– Split water, release oxygen, produce ATP, and
form NADPH
• The ATP is produced by photophosphorylation
– generating ATP using chemiosmosis to power
the addition of phosphate to ADP
– The third way of generating ATP we’ve seen
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The Calvin cycle
– Occurs in the stroma
– Forms sugar from carbon dioxide, using ATP
for energy and NADPH for reducing power
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• An overview of photosynthesis
H2O
CO2
Light
NADP 
ADP
+ P
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
Chloroplast
Figure 10.5
O2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
[CH2O]
(sugar)
• Concept 10.2: The light reactions convert solar
energy to the chemical energy of ATP and
NADPH
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The absorption spectra of chloroplast pigments
– Provide clues to the relative effectiveness of
different wavelengths for driving
photosynthesis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The absorption spectra of three types of pigments
in chloroplasts
Three different experiments helped reveal which wavelengths of light are photosynthetically
important. The results are shown below.
EXPERIMENT
RESULTS
Absorption of light by
chloroplast pigments
Chlorophyll a
Chlorophyll b
Carotenoids
Wavelength of light (nm)
(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by
three types of chloroplast pigments.
Figure 10.9
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Chlorophyll a
–
Is the main photosynthetic pigment
• Chlorophyll b
in chlorophyll a
CHO
in chlorophyll b
CH2
CH
–
CH3
Is an accessory pigment
C
H3C
C
H
C
C
C
C
C
N
C
N
C
Mg
N
C
C
C
H
C
N
C
H3C
CH3
H
CH2
H
H
C
C
C
O
C
H
C
CH3
CH3
Porphyrin ring:
Light-absorbing
“head” of molecule
note magnesium
atom at center
C
O
O
CH2
C
C
CH2
C
O
O
CH3
CH2
Figure 10.10
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts: H atoms not
shown
• Other accessory pigments
– Absorb different wavelengths of light and pass
the energy to chlorophyll a
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Excitation of Chlorophyll by Light
• When a pigment absorbs light
– It goes from a ground state to an excited state,
which is unstable
e–
Excited
state
Heat
Photon
(fluorescence)
Photon
Chlorophyll
molecule
Figure 10.11 A
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Ground
state
A Photosystem: A Reaction Center Associated with
Light-Harvesting Complexes
• A photosystem
Thylakoid
Photosystem
Photon
STROMA
Light-harvesting
complexes
Thylakoid membrane
– Is composed
of a reaction
center
surrounded
by a number
of lightharvesting
complexes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Primary election
acceptor
e–
Transfer
of energy
Figure 10.12
Reaction
center
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
• The light-harvesting complexes
– Consist of pigment molecules bound to
particular proteins
– Act as an antenna for the reaction center
– Funnel the energy of photons of light to the
reaction center
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Reaction center –
– Is a protein complex, including two special
chlorophyll molecules and the primary electron
acceptor
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
First step of light reactions
• When a reaction-center chlorophyll molecule
absorbs energy
– One of its electrons gets bumped up to a
primary electron acceptor
– This is a redox rxn
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The thylakoid membrane
– Is populated by two types of photosystems, I
and II
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Noncyclic Electron Flow
• Noncyclic electron flow
– Is the primary pathway of energy
transformation in the light reactions
– Produces ATP and NADPH which will provide
chemical energy and reducing power to the
sugar making Calvin cycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Produces NADPH, ATP, and oxygen
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
Fd
2
2 H+
+
O2
Pq
e
H2O
e
NADP+
NADP+
+ 2 H+
reductase
3
NADPH
PC
e–
5
+ H+
P700
P680
Light
6
ATP
Figure 10.13
8
e–
Cytochrome
complex
e–
Light
1
7
4
Photosystem II
(PS II)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Photosystem-I
(PS I)
Cyclic Electron Flow
• Under certain conditions
– Photoexcited electrons take an alternative path
– This is because the Calvin cycle needs more
ATP than NADPH.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• In cyclic electron flow
– Only photosystem I is used
– Only ATP is produced
Primary
acceptor
Primary
acceptor
Fd
Fd
Pq
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Figure 10.15
Photosystem II
ATP
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
NADP+
Photosystem I
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The light reactions and chemiosmosis: the
organization of the thylakoid membrane
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
H+
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The Calvin cycle has three phases
– Carbon fixation
– Reduction
– Regeneration of the CO2 acceptor
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
P
1,3-Bisphoglycerate
6 NADPH
6 NADPH+
6 P
P
5
(G3P)
6
P
Glyceraldehyde-3-phosphate
(G3P)
P
1
Figure 10.18
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
• Concept 10.4: Alternative mechanisms of
carbon fixation have evolved in hot, arid
climates
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• On hot, dry days, plants close their stomata
– Conserving water but limiting access to CO2
– Causing oxygen to build up
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Photorespiration: An Evolutionary Relic?
• In photorespiration
– O2 substitutes for CO2 in the active site of the
enzyme rubisco
– The photosynthetic rate is reduced
– No sugar is made, ATP is used, not made
– Hold over from earlier time
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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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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)
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
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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
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• 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
Figure 10.20 types of cells.
CAM
CO2
CALVIN
CYCLE
CO2
1 CO2 incorporated Organic acid
into four-carbon
organic acids
(carbon fixation)
2 Organic acids
release CO2 to
Calvin cycle
Sugar
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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
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
• Organic compounds produced by
photosynthesis
– Provide the energy and building material for
ecosystems
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings