photosynthesis: capturing energy photosynthesis: capturing energy chapter 9
TRANSCRIPT
Photosynthesis:Photosynthesis:Capturing EnergyCapturing Energy
Chapter 9
Learning Objective 1Learning Objective 1
• What are the physical properties of light?
• What is the relationship between a wavelength of light and its energy?
Electromagnetic SpectrumElectromagnetic Spectrum
Fig. 9-1, p. 192
One wavelength
Longer wavelength
TV and radio waves Red 700 nmMicro- waves
OrangeInfrared Color
spectrum of visible light
600 nm
YellowVisible
X-rays500 nm
Green
BlueGamma
raysViolet 400 nm
Electromagnetic spectrum Shorter wavelength
760 nm
380 nm
UV
LightLight
• Consists of particles (photons) that move as waves
• Photons with shorter wavelengths have more energy than those with longer wavelengths
SunlightSunlight
Fig. 9-2, p. 192
Sun
Sunlight is a mixture of many wavelengths
Light and EnergyLight and Energy
Fig. 9-3, p. 193
Photon Photon is absorbed by an excitable electron that moves into a higher energy level.
Low energy level
ElectronHigh energy level
Either
Electron acceptor molecule
The electron may return to ground level by emitting a less energetic photon.
The electron may be accepted by an electron acceptor molecule.
Or
KEY CONCEPTSKEY CONCEPTS
• Light energy powers photosynthesis, which is essential to plants and most life on Earth
Learning Objective 2Learning Objective 2
• What is the internal structure of a chloroplast?
• How do its components interact and facilitate the process of photosynthesis?
StructuresStructures
• Photosynthesis in plants• occurs in chloroplasts• located in mesophyll cells• inside the leaf
Leaf StructureLeaf Structure
Fig. 9-4a, p. 194
(a) This leaf cross section reveals that the mesophyll is the photosynthetic tissue. CO2 enters the leaf through tiny pores or stomata, and H2O is carried to the mesophyll in veins.
Palisade mesophyll
Vein
Air space
Stoma
Spongy mesophyll
Fig. 9-4b, p. 194
(b) Notice the numerous chloroplasts in this LM of plant cells.
Mesophyll cell
10 μm
Fig. 9-4c, p. 194
Outer membrane
Inner membrane
Stroma
1 μm
Intermembrane space
Thylakoid membrane
(c) In the chloroplast, pigments necessary for the light-capturing reactions of photosynthesis are part of thylakoid membranes, whereas the enzymes for the synthesis of carbohydrate molecules are in the stroma.
Granum (stack of thylakoids)
Thylakoid lumen
ChloroplastsChloroplasts
• Enclosed by a double membrane • inner membrane encloses stroma and
thylakoids
• Thylakoids • enclose thylakoid lumen• arranged in stacks (grana)
Photosynthetic PigmentsPhotosynthetic Pigments
• In thylakoid membranes• chlorophyll a• chlorophyll b • Carotenoids
Fig. 9-5, p. 195
Hydrocarbonside chain
Porphyrin ring(absorbs light)
in chlorophyll bin chlorophyll a
Learning Objective 3Learning Objective 3
• What happens to an electron in a biological molecule such as chlorophyll when a photon of light energy is absorbed?
Energizing ElectronsEnergizing Electrons
• Photons excite photosynthetic pigments• chlorophyll
• Energized electrons• move to electron acceptor compounds
Active WavelengthsActive Wavelengths
• Combined absorption spectra of chlorophylls a and b • action spectrum for photosynthesis
Absorption SpectraAbsorption Spectra
Fig. 9-6a, p. 195
Chlorophyll b
Chlorophyll a
Est
imat
ed a
bso
rpti
on
(%
)
Wavelength (nm)
(a) Chlorophylls a and b absorb light mainly in the blue (422 to 492 nm) and red (647 to 760 nm) regions.
Fig. 9-6b, p. 195
Rel
ativ
e ra
te o
f p
ho
tosy
nth
esis
Wavelength (nm)
(b) The action spectrum of photosynthesis indicates the effectiveness of various wavelengths of light in powering photosynthesis. Many plant species have action spectra for photosynthesis that resemble the generalized action spectrum shown here.
Action SpectrumAction Spectrum
Fig. 9-7a, p. 196
100 μm(a)
Fig. 9-7b, p. 196
Wavelength of light (nm)
Learning Objective 4Learning Objective 4
• Describe photosynthesis as a redox process
PhotosynthesisPhotosynthesis
• Light energy• is converted to chemical energy
(carbohydrates)
• Hydrogens from water• reduce carbon
• Oxygen from water• is oxidized, forming molecular oxygen
Learning Objective 5Learning Objective 5
• What is the difference between light-dependent reactions and carbon fixation reactions of photosynthesis?
Energy for ReactionsEnergy for Reactions
• Light-dependent reactions• light energizes electrons that generate ATP
and NADPH
• Carbon fixation reactions• use energy of ATP and NADPH to form
carbohydrate
PhotosynthesisPhotosynthesis
Fig. 9-8, p. 197
Light-dependent reactions (in thylakoids)
Carbon fixation reactions (in stroma)
Chloroplast
ATP
Light reactions
ADPCalvin cycle
NADPH
NADP+
H2O O2 CO2Carbohydrates
Stepped Art
Fig. 9-8, p. 197
Light-dependent reactions (in thylakoids)
Chloroplast
Carbon fixation reactions (in stroma)
Light reactions
ATP
ADP Calvin cycleNADPH
NADP+
H2O O2CO2 Carbohydrates
Learning Objective 6Learning Objective 6
• How do electrons flow through photosystems I and II in the noncyclic electron transport pathway?
• What products are produced?
• Contrast this with cyclic electron transport
The PhotosystemsThe Photosystems
• Photosystems I and II • photosynthetic units• include chlorophyll, accessory pigments• organized with pigment-binding proteins
into antenna complexes
A PhotosystemA Photosystem
Fig. 9-10, p. 198
Primary electron acceptor
e-
Chloroplast
Photon
Thylakoid membrane
Photosystem
Reaction CentersReaction Centers
• Reaction center of antenna complex• special pair of chlorophyll a molecules • release energized electrons to acceptor
• P700• reaction center for photosystem I
• P680• reaction center for photosystem II
Noncyclic Electron TransportNoncyclic Electron Transport
• Light-dependent reactions• form ATP and NADPH
Noncyclic SystemsNoncyclic Systems
• Electrons in photosystem I• energized by light• pass through electron transport chain • convert NADP+ to NADPH
• Redox reactions • pass energized electrons along ETC• from photosystem II to photosystem I
Noncyclic SystemsNoncyclic Systems
• Electrons given up by P700 (photosystem I) • replaced by electrons from P680
(photosystem II)
• Electrons given up by P680 (photosystem II)• replaced by electrons from photolysis of
H2O (releasing oxygen)
Cyclic Electron TransportCyclic Electron Transport
• Electrons from photosystem I• return to photosystem I
• ATP produced by chemiosmosis
• No NADPH or oxygen generated
Table 9-1, p. 200
KEY CONCEPTSKEY CONCEPTS
• Photosynthesis, which occurs in chloroplasts, is a redox process
Learning Objective 7Learning Objective 7
• Explain how a proton (H+) gradient is established across the thylakoid membrane and how this gradient functions in ATP synthesis
Proton GradientProton Gradient
Fig. 9-12, p. 200
Stroma
Thylakoid lumen
Thylakoid membrane Protons (H+)
PhotophosphorylationPhotophosphorylation
• Photophosphorylation • synthesis of ATP coupled to transport of
electrons energized by photons
• Electron energy pumps protons across thylakoid membrane • energy gradient generates ATP by
chemiosmosis
ATP SynthesisATP Synthesis
• ATP synthase• enzyme complex in thylakoid membrane• protons diffuse through enzyme• phosphorylate ADP to ATP
Electron Transport and Electron Transport and ChemiosmosisChemiosmosis
Fig. 9-13, p. 201
Thylakoid lumen
Thylakoid membrane
PhotosystemII
Photon
Thylakoidmembrane
Plastocyanin
Plastoquinone
Ferredoxin
Cytochromecomplex
Photosystem I
Photon
Ferredoxin-NADP+
reductase
NADPHNADP+
ADP Pi
ATP
ATPsynthase
KEY CONCEPTSKEY CONCEPTS
• Light-dependent reactions convert light energy to the chemical energy of NADPH and ATP
Learning Objective 8Learning Objective 8
• Summarize the three phases of the Calvin cycle, and the roles of ATP and NADPH
Calvin Cycle (CCalvin Cycle (C33 pathway) pathway)
• Carbon fixation reactions
• 3 phases• CO2 uptake phase• Carbon reduction phase• RuBP regeneration phase
COCO22 Uptake Phase Uptake Phase
• Enzyme rubisco • (ribulose bisphosphate carboxylase/
oxygenase)• combines CO2 with ribulose bisphosphate
(RuBP), a five-carbon sugar• forms 3-carbon phosphoglycerate (PGA)
Carbon Reduction PhaseCarbon Reduction Phase
• Energy of ATP and NADPH • convert PGA molecules to glyceraldehyde-
3-phosphate (G3P)
• For each 6 CO2 fixed• 12 G3P are produced• 2 G3P leave cycle to produce 1 glucose
RuBP Regeneration PhaseRuBP Regeneration Phase
• Remaining G3P molecules are modified to regenerate RuBP
The Calvin CycleThe Calvin Cycle
KEY CONCEPTSKEY CONCEPTS
• Carbon fixation reactions incorporate CO2 into organic molecules
Learning Objective 9Learning Objective 9
• How does photorespiration reduce photosynthetic efficiency?
PhotorespirationPhotorespiration
• C3 plants use O2 and generate CO2 • by degrading Calvin cycle intermediates• but do not produce ATP
• On bright, hot, dry days• plants close stomata, conserving water• prevents passage of CO2 into leaf
Learning Objective 10Learning Objective 10
• Compare the C4 and CAM pathways
CC44 Pathway Pathway
• Takes place in mesophyll cells
• Enzyme PEP carboxylase binds CO2
• CO2 fixed in oxaloacetate• converted to malate
• Malate moves into bundle sheath cell• CO2 is removed
• Released CO2 enters Calvin cycle
CC44 and C and C33 Plants Plants
Fig. 9-15a, p. 205
Upper epidermis
Palisade mesophyll
Bundle sheath cells of veins
Spongy mesophyll
Chloroplasts
(a) In C3 plants, the Calvin cycle takes place in the mesophyll cells and the bundle sheath cells are nonphotosynthetic.
Fig. 9-15b, p. 205
Upper epidermis
Bundle sheath cells of veins
Mesophyll
Chloroplasts
(b) In C4 plants, reactions that fix CO2 into four-carbon compounds take place in the mesophyll cells. The four-carbon compounds are transferred from the mesophyll cells to the photosynthetic bundle sheath cells, where the Calvin cycle takes place.
CC4 4 PathwayPathway
Fig. 9-16, p. 206
CO2
Mesophyll cell
Phosphoenol- pyruvate
Oxaloacetate
ADP NADPH
ATP NADP+
Pyruvate
Pyruvate Bundle sheath cellNADP+
CO2
Glucose NADPH
Vein
(4C)(3C)
(3C)Malate (4C)
Malate(3C) (4C)
(CAM) Pathway(CAM) Pathway
• Crassulacean acid metabolism (CAM)• similar to C4 pathway
• PEP carboxylase fixes carbon at night• in mesophyll cells
• Calvin cycle occurs during the day
A CAM PlantA CAM Plant
Learning Objective 11Learning Objective 11
• How do photoautotrophs and chemoheterotrophs differ with respect to their energy and carbon sources?
Energy SourcesEnergy Sources
• Photoautotrophs • use light as energy source• incorporate atmospheric CO2 into pre-
existing carbon skeletons
• Chemoheterotrophs • obtain energy by oxidizing chemicals• obtain carbon from other organisms
KEY CONCEPTSKEY CONCEPTS
• Most photosynthetic organisms are photoautotrophs
Learning Objective 12Learning Objective 12
• What is the importance of photosynthesis to plants and other organisms?
PhotosynthesisPhotosynthesis
• Ultimate source of all chemical energy and organic molecules• available to plants and other organisms
• Replenishes oxygen in the atmosphere• vital to all aerobic organisms
Summary ReactionsSummary Reactions
• Light-dependent reactions (noncyclic electron transport)
12 H2O + 12 NADP+ + 18 ADP + 18 Pi
→ (light energy, chlorophyll) →
6 O2 + 12 NADPH + 18 ATP
Summary ReactionsSummary Reactions
• Carbon fixation reactions (Calvin cycle)
12 NADPH + 18 ATP + 6 CO2 →
C6H12O6 + 12 NADP+ + 18 ADP
+ 18 Pi + 6 H2O
Summary ReactionsSummary Reactions
• Overall equation for photosynthesis
6 CO2 +12 H2O
→ (light energy, chlorophyll) →
C6H12O6 + 6 O2 + 6 H2O
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Calvin-Benson CycleCalvin-Benson Cycle
Sites of PhotosynthesisSites of Photosynthesis
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Wavelengths of LightWavelengths of Light
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Photosynthesis OverviewPhotosynthesis Overview
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