Chapter 10

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