Two parts to PhotosynthesisLight reactions: Convert light energy to chemical energy as ATP and NADPHLight-independent reactions: Use ATP and NADPH (from the light reactions) plus CO2 to produce carbohydrates
Light ChemistryLight is a form of electromagnetic radiation (travels as a wave and a particle)Light is propagated as waves: - the energy of light is inversely proportional to its wavelength.Light also behaves as particles: - called photons.
Light ChemistryPigments: Molecules that absorb specific wavelengths in the visible range of the spectrum are called pigments.When a photon meets a molecule (pigment) it can be:Scattered: photon bounces off the moleculeTransmitted: photon is passed through the moleculeAbsorbed: molecule acquires the energy of the photon. The molecule goes from ground state to excited state
Antennae SystemsPigments are arranged in antenna systems, or light-harvesting complexes.A photosystem consists of multiple antenna systems and their pigments and surrounds a reaction center.Pigments are packed together on thylakoid membrane proteins.Excitation energy passes from pigments that absorb short wavelengths to those that absorb longer wavelengths, and ends up in the reaction center pigment.
Reaction CentersThe reaction center converts light energy into chemical energy.The excited chlorophyll a molecule (Chl*) is a reducing agent (electron donor).A is an acceptor molecule (oxidizing agent). + − Chl + A → Chl + A * * + −
Electron TransportTwo systems of electron transport:Noncyclic electron transport: - produces NADPH and ATPCyclic electron transport: - produces ATP only
Noncyclic Electron TransportLight energy is used to oxidize water → O2, H+, and electrons.Two photosystems required - Photosystem I (P700) - Photosystem II (P680)
Noncyclic Electron TransportPhotosystem II Photosystem I• Light energy • Light energy oxidizes water → O2, reduces NADP+ to H+, and electrons. NADPH• Reaction center has • Reaction center has chlorophyll a chlorophyll a molecules P680— molecules: P700— absorb at 680nm. absorb in the 700nm range
Noncyclic Electron TransportThe “Z scheme” model of noncyclic electron transport:• Extracts electrons from water and transfers them to NADPH, using energy from photosystems I and II and resulting in ATP synthesis• Yields NADPH, ATP and O2
PhotophosphorylationLight-driven production of ATPType of Chemiosmosis:H+ is transported via electron carriers across the thylakoid membrane into the lumen (creating an electrochemical gradient.)
Cyclic Electron TransportOnly makes ATPAn electron from an excited chlorophyll molecule cycles back to the same chlorophyll molecule.Cyclic electron transport begins and ends in photosystem I.Released energy is stored and can be used to form ATP.
Light-Independent Rections (Stroma)CO2 fixation: CO2 is reduced to carbohydrates.Enzymes in the stroma use the energy in ATP and NADPH to reduce CO2.Production of ATP and NADPH is light- dependent; therefore CO2 fixation must also take place in the light.
Calvin CycleThe enzyme catalyzing the intermediate formation is rubisco—ribulose bisphoshate carboxylase/oxygenase— the most abundant protein in the world.CO2 is first added to an acceptor molecule—5-C RuBP; the 6-C compound immediately breaks down into two molecules of 3PG.
Calvin CycleConsists of 3 processes:• Fixation of CO2• Reduction of 3PG to G3P• Regeneration of RuBP
Carbon FixationRubisco:ribulose bisphoshate carboxylase/oxygenase6RuBP + 6CO2 12 (3PG)Grabs CO2 gas from the air “fixation”
Reduction and Sugar ProductionATP and NADPH from the light reaction are used to covert 12x(3PG) to 12xG3P2 G3P used to make sugarRest used to regenerate RuBP
Regenerate RuBPUses more ATP to convert 10xsG3P back to 6xRuBPStarts over the cycle..
Summary of Calvin CycleIt take 6 turns to make 1 Light indirectly provides theglucose molecule substrates needed for the Requires 18 ATP Calvin Cycle 12 NADPH 2 G3P go towards glucose 10 G3P replenish RuBP½ glucose used to make starch2/3 converted to a disaccharidecalled sucrose (mobile)
PhotorespirationRubisco is an oxygenase Consumes O2, releases as well as a CO2, and takes place in carboxylase. light.It can add O2 to RuBP Opposite of the Calvin instead of CO2; Cycle reducing the amount of CO2 converted to carbohydrates may limit plant growth.Uses ATP and NADPH
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