chapter 10 photosynthesis text haas
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Campbell AP BiologyTRANSCRIPT
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Chapter 10Chapter 10
Photosynthesis
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• Overview: The Process That Feeds the Biosphere
• Photosynthesis
– Is the process that converts solar energy into chemical energy
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• Plants and other autotrophs
– Are the producers of the biosphere
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• Plants are photoautotrophs
– They use the energy of sunlight to make organic molecules from water and carbon dioxide
Figure 10.1
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• 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 dioxideand (in most cases) water. They feed not onlythemselves, but the entire living world. (a) Onland, plants are the predominant producers offood. In aquatic environments, photosyntheticorganisms include (b) multicellular algae, suchas this kelp; (c) some unicellular protists, suchas Euglena; (d) the prokaryotes calledcyanobacteria; and (e) other photosyntheticprokaryotes, such as these purple sulfurbacteria, which produce sulfur (sphericalglobules) (c, d, e: LMs).
(a) Plants
(b) Multicellular algae
(c) Unicellular protist 10 m
40 m(d) Cyanobacteria
1.5 m(e) Pruple sulfurbacteria
Figure 10.2
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• Heterotrophs
– Obtain their organic material from other organisms
– Are the consumers of the biosphere
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• 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
Vein
Leaf cross section
Figure 10.3
Mesophyll
CO2 O2Stomata
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• Chloroplasts
– Are the organelles in which photosynthesis occurs
– Contain thylakoids and grana
Chloroplast
Mesophyll
5 µm
Outermembrane
Intermembranespace
Innermembrane
Thylakoidspace
ThylakoidGranumStroma
1 µm
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Tracking Atoms Through Photosynthesis: Scientific Inquiry
• Photosynthesis is summarized as
6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O
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The Splitting of Water
• Chloroplasts split water into
– Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules
6 CO2 12 H2OReactants:
Products: C6H12O66 H2O 6 O2
Figure 10.4
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Photosynthesis as a Redox Process
• Photosynthesis is a redox process
– Water is oxidized, carbon dioxide is reduced
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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
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• The Calvin cycle
– Occurs in the stroma
– Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power
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• An overview of photosynthesis
H2O CO2
Light
LIGHT REACTIONS
CALVINCYCLE
Chloroplast
[CH2O](sugar)
NADPH
NADP
ADP
+ P
O2Figure 10.5
ATP
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• Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH
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The Nature of Sunlight
• Light
– Is a form of electromagnetic energy, which travels in waves
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• Wavelength
– Is the distance between the crests of waves
– Determines the type of electromagnetic energy
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• The electromagnetic spectrum
– Is the entire range of electromagnetic energy, or radiation
Gammarays X-rays UV Infrared
Micro-waves
Radiowaves
10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m
106 nm 103 m
380 450 500 550 600 650 700 750 nm
Visible light
Shorter wavelength
Higher energy
Longer wavelength
Lower energyFigure 10.6
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• The visible light spectrum
– Includes the colors of light we can see
– Includes the wavelengths that drive photosynthesis
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Photosynthetic Pigments: The Light Receptors
• Pigments
– Are substances that absorb visible light
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– Reflect light, which include the colors we see
Light
ReflectedLight
Chloroplast
Absorbedlight
Granum
Transmittedlight
Figure 10.7
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• The spectrophotometer
– Is a machine that sends light through pigments and measures the fraction of light transmitted at each wavelength
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• An absorption spectrum
– Is a graph plotting light absorption versus wavelength
Figure 10.8
Whitelight
Refractingprism
Chlorophyllsolution
Photoelectrictube
Galvanometer
Slit moves topass lightof selectedwavelength
Greenlight
The high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.
The low transmittance(high absorption) readingchlorophyll absorbs most blue light.
Bluelight
1
2 3
40 100
0 100
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• The absorption spectra of chloroplast pigments
– Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis
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• 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
RESULTSA
bso
rptio
n o
f lig
ht
by
chlo
rop
last
pig
me
nts
Chlorophyll a
(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.
Wavelength of light (nm)
Chlorophyll b
Carotenoids
Figure 10.9
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• The action spectrum of a pigment
– Profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis
Rat
e o
f ph
otos
ynth
esis
(mea
sure
d by
O2 r
elea
se)
Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids.
(b)
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• The action spectrum for photosynthesis
– Was first demonstrated by Theodor W. Engelmann
400 500 600 700
Aerobic bacteria
Filamentof alga
Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most.Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b.
(c)
Light in the violet-blue and red portions of the spectrum are most effective in driving
photosynthesis.
CONCLUSION
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• Chlorophyll a
– Is the main photosynthetic pigment
• Chlorophyll b
– Is an accessory pigment C
CH
CH2
CC
CC
C
CNNC
H3C
C
CC
C C
C
C
C
N
CC
C
C N
MgH
H3C
H
C CH2CH3
H
CH3C
HHCH2
CH2
CH2
H CH3
C O
O
O
O
O
CH3
CH3
CHO
in chlorophyll a
in chlorophyll b
Porphyrin ring:Light-absorbing“head” of moleculenote magnesiumatom at center
Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshown
Figure 10.10
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• Other accessory pigments
– Absorb different wavelengths of light and pass the energy to chlorophyll a
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Excitation of Chlorophyll by Light
• When a pigment absorbs light
– It goes from a ground state to an excited state, which is unstable
Excitedstate
Ene
rgy
of e
lect
ion
Heat
Photon(fluorescence)
Chlorophyllmolecule
GroundstatePhoton
e–
Figure 10.11 A
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• If an isolated solution of chlorophyll is illuminated
– It will fluoresce, giving off light and heat
Figure 10.11 B
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A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes
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• A photosystem
– Is composed of a reaction center surrounded by a number of light-harvesting complexes
Primary electionacceptor
Photon
Thylakoid
Light-harvestingcomplexes
Reactioncenter
Photosystem
STROMA
Thy
lako
id m
embr
ane
Transferof energy
Specialchlorophyll amolecules
Pigmentmolecules
THYLAKOID SPACE(INTERIOR OF THYLAKOID)Figure 10.12
e–
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• The light-harvesting complexes
– Consist of pigment molecules bound to particular proteins
– Funnel the energy of photons of light to the reaction center
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• When a reaction-center chlorophyll molecule absorbs energy
– One of its electrons gets bumped up to a primary electron acceptor
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• The thylakoid membrane
– Is populated by two types of photosystems, I and II
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Noncyclic Electron Flow
• Noncyclic electron flow
– Is the primary pathway of energy transformation in the light reactions
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• Produces NADPH, ATP, and oxygen
Figure 10.13Photosystem II
(PS II)
Photosystem-I(PS I)
ATP
NADPH
NADP+
ADP
CALVINCYCLE
CO2H2O
O2 [CH2O] (sugar)
LIGHTREACTIONS
Light
Primaryacceptor
Pq
Cytochromecomplex
PC
e
P680
e–
e–
O2
+
H2O2 H+
Light
ATP
Primaryacceptor
Fd
ee–
NADP+
reductase
ElectronTransportchain
Electron transport chain
P700
Light
NADPH
NADP+
+ 2 H+
+ H+
1
5
7
2
3
4
6
8
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• A mechanical analogy for the light reactions
MillmakesATP
ATP
e–
e–e–
e–
e–
Pho
ton
Photosystem II Photosystem I
e–
e–
NADPH
Pho
ton
Figure 10.14
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Cyclic Electron Flow
• Under certain conditions
– Photoexcited electrons take an alternative path
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• In cyclic electron flow
– Only photosystem I is used
– Only ATP is produced
Primaryacceptor
Pq
Fd
Cytochromecomplex
Pc
Primaryacceptor
Fd
NADP+
reductaseNADPH
ATPFigure 10.15
Photosystem II Photosystem I
NADP+
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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
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• The spatial organization of chemiosmosis
– Differs in chloroplasts and mitochondria
Key
Higher [H+]Lower [H+]
Mitochondrion Chloroplast
MITOCHONDRIONSTRUCTURE
Intermembrancespace
Membrance
Matrix
Electrontransport
chain
H+ DiffusionThylakoidspace
Stroma
ATPH+
PADP+
ATPSynthase
CHLOROPLASTSTRUCTURE
Figure 10.16
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• 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
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• The light reactions and chemiosmosis: the organization of the thylakoid membrane
LIGHTREACTOR
NADP+
ADP
ATP
NADPH
CALVINCYCLE
[CH2O] (sugar)STROMA(Low H+ concentration)
Photosystem II
LIGHT
H2O CO2
Cytochromecomplex
O2
H2O O21
1⁄2
2
Photosystem ILight
THYLAKOID SPACE(High H+ concentration)
STROMA(Low H+ concentration)
Thylakoidmembrane
ATPsynthase
PqPc
Fd
NADP+
reductase
NADPH + H+
NADP+ + 2H+
ToCalvincycle
ADP
PATP
3
H+
2 H++2 H+
2 H+
Figure 10.17
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• 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
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• The Calvin cycle has three phases
– Carbon fixation
– Reduction
– Regeneration of the CO2 acceptor
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• The Calvin cycle
(G3P)
Input(Entering one
at a time)CO2
3
Rubisco
Short-livedintermediate
3 P P
3 P P
Ribulose bisphosphate(RuBP)
P
3-Phosphoglycerate
P6 P
6
1,3-Bisphoglycerate
6 NADPH
6 NADPH+
6 P
P6
Glyceraldehyde-3-phosphate(G3P)
6 ATP
3 ATP
3 ADP CALVINCYCLE
P5
P1
G3P(a sugar)Output
LightH2O CO2
LIGHTREACTION
ATP
NADPH
NADP+
ADP
[CH2O] (sugar)
CALVINCYCLE
Figure 10.18
O2
6 ADP
Glucose andother organiccompounds
Phase 1: Carbon fixation
Phase 2:Reduction
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
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• 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
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C4 Plants
• C4 plants minimize the cost of photorespiration
– By incorporating CO2 into four carbon compounds in mesophyll cells
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• These four carbon compounds
– Are exported to bundle sheath cells, where they release CO2 used in the Calvin cycle
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• C4 leaf anatomy and the C4 pathway
CO2
Mesophyll cell
Bundle-sheathcell
Vein(vascular tissue)
Photosyntheticcells of C4 plantleaf
Stoma
Mesophyllcell
C4 leaf anatomy
PEP carboxylase
Oxaloacetate (4 C) PEP (3 C)
Malate (4 C)
ADP
ATP
Bundle-Sheathcell CO2
Pyruate (3 C)
CALVINCYCLE
Sugar
Vasculartissue
Figure 10.19
CO2
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CAM Plants
• CAM plants
– Open their stomata at night, incorporating CO2 into organic acids
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• 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
Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in differenttypes of cells.
(a) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cellsat different times.
(b)
PineappleSugarcane
Bundle-sheath cell
Mesophyll Cell
Organic acid
CALVINCYCLE
Sugar
CO2 CO2
Organic acid
CALVINCYCLE
Sugar
C4 CAM
CO2 incorporatedinto four-carbonorganic acids(carbon fixation)
Night
Day
1
2 Organic acidsrelease CO2 toCalvin cycle
Figure 10.20
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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
CO2H2O
Light
Light reaction Calvin cycle
NADP+
ADP
ATP
NADPH
+ P 1
RuBP 3-Phosphoglycerate
Amino acidsFatty acids
Starch(storage)
Sucrose (export)
G3P
Photosystem IIElectron transport chain
Photosystem I
Chloroplast
Figure 10.21
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• Organic compounds produced by photosynthesis
– Provide the energy and building material for ecosystems