chapter 10
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Chapter 10 Chapter 10
Photosynthesis Photosynthesis
PhotosynthesisPhotosynthesis
““Synthesis from light”Synthesis from light”
Plants take in COPlants take in CO22, produce , produce
carbohydrates, and release water and Ocarbohydrates, and release water and O22
Light is requiredLight is required
2612622 666 OOHCOHCO
PhotosynthesisPhotosynthesis
Stomata
Two parts to PhotosynthesisTwo parts to Photosynthesis
Light reactionsLight reactions: : Convert light energy Convert light energy to chemical energy to chemical energy as ATP and NADPHas ATP and NADPH
Light-independent Light-independent reactionsreactions: Use ATP : Use ATP and NADPH and NADPH (from (from the light reactions)the light reactions) plus COplus CO22 to produce to produce
carbohydrates carbohydrates
Light ChemistryLight Chemistry
Light is a form of Light is a form of electromagnetic electromagnetic radiationradiation (travels as a wave and a (travels as a wave and a particle)particle)
Light is propagated as wavesLight is propagated as waves::
- the energy of light is inversely - the energy of light is inversely proportional to its wavelength.proportional to its wavelength.
Light also behaves as particlesLight also behaves as particles: :
- called photons.- called photons.
Examples of Wavelengths Examples of Wavelengths
Light ChemistryLight ChemistryPigmentsPigments: Molecules that absorb specific : Molecules that absorb specific
wavelengths in the visible range of the wavelengths in the visible range of the spectrum are called pigments.spectrum are called pigments.
When a photon meets a molecule (pigment) it can be:When a photon meets a molecule (pigment) it can be:
ScatteredScattered:: photon bounces off the moleculephoton bounces off the molecule
TransmittedTransmitted:: photon isphoton is passed through the moleculepassed through the molecule
AbsorbedAbsorbed:: molecule acquires the energy of the photon. molecule acquires the energy of the photon. The molecule goes from ground state to The molecule goes from ground state to excited excited statestate
Light ChemistryLight Chemistry
PigmentsPigments
Absorption spectrumAbsorption spectrum: : Plot of wavelengths Plot of wavelengths absorbed by a absorbed by a pigmentpigment
Action spectrumAction spectrum: Plot : Plot of of biological activitybiological activity as a function of as a function of exposure to varied exposure to varied wavelengths of lightwavelengths of light
Types of PigmentsTypes of Pigments
Chlorophylls Chlorophylls aa and and bb
Accessory pigmentsAccessory pigments: Absorb in red and : Absorb in red and blue regions, transfer the energy to blue regions, transfer the energy to chlorophylls.chlorophylls.
Examples: carotenoids and phycobilinsExamples: carotenoids and phycobilins
Antennae ComplexAntennae Complex
Antennae SystemsAntennae Systems
Pigments are arranged in antenna systems, or Pigments are arranged in antenna systems, or light-harvesting complexeslight-harvesting complexes..
A A photosystemphotosystem consists of multiple antenna consists of multiple antenna systems and their pigments and surrounds a systems and their pigments and surrounds a reaction centerreaction center..
Pigments are packed together on thylakoid Pigments are packed together on thylakoid membrane proteins.membrane proteins.
Excitation energy passes from pigments that Excitation energy passes from pigments that absorb short wavelengths to those that absorb absorb short wavelengths to those that absorb longer wavelengths, and ends up in the longer wavelengths, and ends up in the reaction center pigment.reaction center pigment.
Antennae SystemsAntennae Systems
Reaction CentersReaction Centers
The reaction center converts light energy The reaction center converts light energy into chemical energy.into chemical energy.
The excited chlorophyll The excited chlorophyll aa molecule (Chl*) molecule (Chl*) is a reducing agent (electron donor).is a reducing agent (electron donor).
A is an acceptor molecule (oxidizing A is an acceptor molecule (oxidizing agent).agent).
AChlAChl* AChlAChl*
Electron TransportElectron Transport
Two systems of electron transport:Two systems of electron transport:
Noncyclic electron Noncyclic electron transport: transport:
- produces NADPH and ATP- produces NADPH and ATP
Cyclic electron Cyclic electron transport:transport:
- produces ATP only- produces ATP only
Noncyclic Electron TransportNoncyclic Electron Transport
Light energy is used to oxidize water Light energy is used to oxidize water → → OO22, H, H++, and electrons., and electrons.
Two photosystems requiredTwo photosystems required
- Photosystem I (P700)- Photosystem I (P700)
- Photosystem II (P680)- Photosystem II (P680)
Noncyclic Electron TransportNoncyclic Electron Transport
Photosystem IIPhotosystem II
• Light energy Light energy oxidizes water → Ooxidizes water → O22, ,
HH++, and electrons., and electrons.• Reaction center has Reaction center has
chlorophyll chlorophyll aa molecules Pmolecules P680680——
absorb at 680nm.absorb at 680nm.
Photosystem IPhotosystem I
• Light energy Light energy reduces NADPreduces NADP++ to to NADPHNADPH
• Reaction center has Reaction center has chlorophyll chlorophyll aa molecules: Pmolecules: P700700——
absorb in the 700nm absorb in the 700nm rangerange
Noncyclic Electron TransportNoncyclic Electron Transport
The “Z scheme” model of noncyclic The “Z scheme” model of noncyclic electron transport: electron transport:
• Extracts electrons from water Extracts electrons from water and and transfers them to NADPH, using energy transfers them to NADPH, using energy from photosystems I and II and from photosystems I and II and resulting in ATP synthesisresulting in ATP synthesis
• Yields NADPH, ATP and OYields NADPH, ATP and O22
Noncyclic Electron TransportNoncyclic Electron Transport
How Do We Make ATPHow Do We Make ATP
Photophosphorylation Photophosphorylation
Light-driven production of ATPLight-driven production of ATP
Type of Chemiosmosis:Type of Chemiosmosis:
HH++ is transported via electron carriers is transported via electron carriers across the thylakoid membrane into the across the thylakoid membrane into the lumen (creating an electrochemical lumen (creating an electrochemical gradient.)gradient.)
Cyclic Electron TransportCyclic Electron Transport
Only makes ATPOnly makes ATP
An electron from an excited chlorophyll An electron from an excited chlorophyll molecule cycles back to the same molecule cycles back to the same chlorophyll molecule.chlorophyll molecule.
Cyclic electron transport begins and Cyclic electron transport begins and ends in photosystem I.ends in photosystem I.
Released energy is stored and can be Released energy is stored and can be used to form ATP.used to form ATP.
Cyclic Electron TransportCyclic Electron Transport
Light-Independent Rections Light-Independent Rections (Stroma)(Stroma)
COCO22 fixation: CO fixation: CO22 is reduced to is reduced to
carbohydrates.carbohydrates.
Enzymes in the stroma use the energy in Enzymes in the stroma use the energy in ATP and NADPH to reduce COATP and NADPH to reduce CO22. .
Production of ATP and NADPH is light-Production of ATP and NADPH is light-dependent; therefore COdependent; therefore CO22 fixation must fixation must
also take place in the light.also take place in the light.
Calvin CycleCalvin Cycle
The enzyme catalyzing the intermediate The enzyme catalyzing the intermediate formation is rubisco—ribulose formation is rubisco—ribulose bisphoshate carboxylase/oxygenase—bisphoshate carboxylase/oxygenase—the most abundant protein in the world.the most abundant protein in the world.
COCO22 is first added to an acceptor is first added to an acceptor molecule—5-C RuBP; the 6-C molecule—5-C RuBP; the 6-C compound immediately breaks down compound immediately breaks down into two molecules of 3PG.into two molecules of 3PG.
Calvin CycleCalvin Cycle
Consists of 3 processes:Consists of 3 processes:
• FixationFixation of CO of CO22
• ReductionReduction of 3PG to of 3PG to G3PG3P
• Regeneration Regeneration of RuBPof RuBP
Carbon FixationCarbon Fixation
Rubisco:Rubisco:
ribulose bisphoshate ribulose bisphoshate carboxylase/oxygenasecarboxylase/oxygenase
6RuBP + 6CO6RuBP + 6CO2212 (3PG)12 (3PG)
Grabs COGrabs CO2 2 gas from the airgas from the air
““fixation”fixation”
Reduction and Sugar Reduction and Sugar ProductionProduction
ATP and NADPH from ATP and NADPH from the light reaction are the light reaction are used to covert used to covert 12x(3PG) to 12xG3P12x(3PG) to 12xG3P
2 G3P used to make 2 G3P used to make sugar sugar
Rest used to Rest used to regenerate RuBPregenerate RuBP
Regenerate RuBPRegenerate RuBP
Uses more ATP to Uses more ATP to convert 10xsG3P convert 10xsG3P back to 6xRuBPback to 6xRuBP
Starts over the cycle..Starts over the cycle..
Summary of Calvin CycleSummary of Calvin Cycle
It take 6 turns to make 1 It take 6 turns to make 1 glucose moleculeglucose molecule
Requires 18 ATPRequires 18 ATP
12 NADPH 12 NADPH
2 G3P go towards glucose 2 G3P go towards glucose
10 G3P replenish RuBP10 G3P replenish RuBP
½ glucose used to make starch ½ glucose used to make starch
2/3 converted to a disaccharide 2/3 converted to a disaccharide called sucrose (mobile)called sucrose (mobile)
Light indirectly provides the Light indirectly provides the substrates needed for the substrates needed for the Calvin CycleCalvin Cycle
PhotorespirationPhotorespirationRubisco is an oxygenase as Rubisco is an oxygenase as
well as a carboxylase.well as a carboxylase.
It can add OIt can add O22 to RuBP to RuBP
instead of COinstead of CO22; reducing ; reducing
the amount of COthe amount of CO22
converted to converted to carbohydrates may limit carbohydrates may limit plant growth.plant growth.
Uses ATP and NADPHUses ATP and NADPH
Consumes OConsumes O22, releases , releases
COCO22,, and takes place in and takes place in
light.light.
Opposite of the Calvin Opposite of the Calvin CycleCycle