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Energy Harvesting Pathways. Photosynthesis. photosynthesis. reverses the oxidation of glycolysis/respiration C 6 H 12 O 6 +6 O 2 => 6 CO 2 +6 H 2 O + energy energy +6 CO 2 +12 H 2 O =>6 O 2 +C 6 H 12 O 6 +6 H 2 O. photosynthesis. reverses the oxidation of glycolysis/respiration
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Energy Harvesting Pathways Photosynthesis
photosynthesis • reverses the oxidation of glycolysis/respiration C6H12O6 +6 O2 => 6 CO2 +6 H2O + energy energy +6 CO2 +12 H2O =>6 O2 +C6H12O6 +6 H2O
photosynthesis • reverses the oxidation of glycolysis/respiration • reduces highly oxidized carbon • stores energy in hydrocarbon bonds • utilizes “free” resources • water from soil reservoir • CO2 from atmospheric reservoir • energy from diurnal light source • releases O2 as a byproduct
photosynthesis • occurs in chloroplasts • “light reactions” on thylakoid membranes • “dark reactions” in aqueous stroma • two interconnected pathways • light-driven electron transport generates reductant & energy • Calvin-Benson cycle reduces CO2 & assembles carbohydrates
visible light occupies a narrow band of the electromagnetic spectrumFigure 8.5
photosynthetic light reactions • light is the visible portion of the electromagnetic radiation spectrum • between ultraviolet and infrared • light travels in wave-like fashion • wavelength & frequency are inversely related • shorter wavelength : higher frequency • longer wavelength : lower frequency
photosynthetic light reactions • light energy occurs in discrete units: photons • energy of a photon is inversely proportional to wavelength • shorter wavelength : higher energy • longer wavelength : lower energy • intensity measures number of photons striking a unit area per unit time (e.g. µE·m-2·s-1)
green light istransmitted (and reflected) as blue and red are absorbed
photosynthetic light reactions • molecules absorb electromagnetic radiation • pigments absorb visible light of certain wavelengths • photon-pigment interactions • reflection • transmission • absorption - pigment is excited by photon • excited state - ground state = energy of photon
Chlorophyll a:√ tetrapyrrole ring √ coordinated Mg√ hydrophobic tail Figure 8.7
photosynthetic light reactions • molecules absorb electromagnetic radiation • a pigment absorbs only certain wavelengths • an absorption spectrum is a molecular fingerprint • an action spectrum plots effectiveness vs. wavelength • eukaryotic photosynthesis uses chlorophyll a as the central pigment • accessory pigments transfer energy to Chl a • in plants: Chl b, carotenoids
photosynthetic light reactions • possible fates of absorbed energy • loss as heat • loss as fluorescence • intermolecular transfer • direct transfer • electron transport
photosynthetic light reactions • excited reaction center chlorophyll a is a good reducing agent • PSII Chl a* drives electron transport through carriers in the thylakoid membrane • PSI reaction center chlorophyll is reduced by electrons transported from PSII • PSI Chl a reduces NADP+ => NADPH • PSII Chl a+is reduced with e- from H2O • O2 is released as a byproduct
thylakoids are flat sacks that reside in the chloroplastFigure 8.11
photosynthetic light reactions • noncyclic electron transport produces ATP and NADPH • cyclic electron transport produces ATP, but not NADPH
the light reactions of photosynthesis • electrons flow from water to NADP+ • NADPH is produced • a proton gradient is formed • ATP is produced
light and “dark” reactions are coupled by ATP & NADPHFigure 8.3
carbon fixation reactions • How does the plant incorporate CO2 into the existing “carbon pool”? • CO2 must be attached to one or more existing molecules - which one(s)? • …feed a plant CO2 and watch where it goes…
Calvin, Benson, et al.photosynthesis in Chlorellawith 14CO2Figure 8.12
carbon fixation reactions • 3-phosphoglycerate is the first product of carbon fixation • other molecules were labeled over time
Calvin-Benson Cycle model of carbon fixationFigure 8.13 12 3PG
carbon fixation reactions • the acceptor is not a 2-carbon molecule • it’s ribulose 1,5-bisphosphate • a 5-C sugar • the first product is not 3PG • it’s an unstable 6-C intermediate
carbon fixation reactions • Calvin-Benson cycle accomplishes three tasks • carbon fixation - by rubisco • reduction of fixed C into carbohydrate • 3-phosphoglyceric acid => • glyceraldehyde 3-phosphate • requires reductant & energy • formation of more RuBP (hence, cycle) • requires multiple enzymes & ATP
Product of Calvin-Benson Cycle • G3P is the reduced product of the Calvin-Benson cycle
Product of Calvin-Benson Cycle • G3P is the reduced product of the Calvin-Benson cycle
Product of Calvin-Benson Cycle • G3P is the reduced product of the Calvin-Benson cycle • 1/6 of G3P is product; 5/6 are reaction intermediates • “excess” G3P is used to make monosaccharides • 1/3 of G3P is stored in the chloroplast as starch • 2/3 of G3P is transported elsewhere as sucrose
carbon fixation reactions • ribulose bisphosphate carboxylase/oxygenase • Rubisco • most abundant protein in the world, but…
photorespiration • ribulose bisphosphate carboxylase/oxygenase is very ineffective • rubisco adds CO2 to RuBP or adds O2 to RuBP • 5C + 1C => 2 · 3C • 5C + 0C => 2C + 3C • costs ATPs to regenerate RuBP
photorespiration • ribulose bisphosphate carboxylase/oxygenase • carboxylase & oxygenase activities compete • rubisco CO2 affinity is low • stomata must be open for efficient PS • easy access for 20% O2 & 0.035% CO2 • up to 30% of fixed carbon is lost to photorespiration in important crops plants • some plants don’t suffer so much from photorespiration
photorespiration - solution • C3 plants and C4 plants • 3PG is the first detectable product of C fixation in C3 plants • C4 plants produce a 4-C product first PEP + CO2 ======> oxaloacetate 3C 1C PEP C’ase 4C • PEP carboxylase • has high CO2 affinity • is never an oxygenase
photorespiration - solution • but… • C4 plants use rubisco, just like C3 plants • PEP carboxylase and rubisco are separated into different compartments
mesophyll cells and bundle sheath cells communicate in C4 plantsFigure 8.17
photorespiration - solution • C4 bundle sheath cells are enriched in CO2 relative to O2 • rubisco fixes O much less often spatial separation of initial C fixation and Calvin Benson cycle