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Energy Harvesting Pathways Photosynthesis

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

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Page 1: 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 Harvesting Pathways

Photosynthesis

Page 2: 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

photosynthesis

•reverses the oxidation of glycolysis/respirationC6H12O6 +6 O2 => 6 CO2 +6 H2O + energy

energy +6 CO2 +12 H2O =>6 O2 +C6H12O6

+6 H2O

Page 3: 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

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

Page 4: 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

reactants and products of

photosynthesisFigure 8.1

Page 5: 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

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

Page 6: 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

photosynthesis:

overviewFigure 8.3

Page 7: 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

visible light

occupies a

narrow band of

the electromag

netic spectrum

Figure 8.5

Page 8: 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

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

Page 9: 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

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)

Page 10: 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

green light is

transmitted (and

reflected) as

blue and red

are absorbed

Page 11: 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

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

Page 12: 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

absorption of

a photon excites

a molecul

eFigure 8.4

Page 13: 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

absorption

and action spectra

Figure 8.6

Page 14: 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

Chlorophyll a:√ tetrapyrrole ring √ coordinated Mg√ hydrophobic tail Figure 8.7

Page 15: 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

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

Page 16: 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

photosynthetic

electron transport mutants

fluoresce…

Page 17: 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

photosynthetic light reactions•possible fates of absorbed

energy–loss as heat–loss as fluorescence–intermolecular transfer•direct transfer•electron transport

Page 18: 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

fates of

energy

Figure 8.8

Page 19: 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

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

Page 20: 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

Figure 8.9

Page 21: 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

thylakoids are flat sacks that

reside in the chloroplastFigure 8.11

Page 22: 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

transfers of absorbed energy

Figure 8.11

Page 23: 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

photosynthetic light reactions

•noncyclic electron transport produces ATP and NADPH

•cyclic electron transport produces ATP, but not NADPH

Page 24: 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

Cyclic electron transportFigure 8.10

Page 25: 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

the light reactions of photosynthesisFigure 8.11

Page 26: 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

the light reactions of photosynthesis

•electrons flow from water to NADP+

–NADPH is produced•a proton gradient is formed–ATP is produced

Page 27: 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

light and “dark” reactions

are coupled by ATP & NADPH

Figure 8.3

Page 28: 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

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…

Page 29: 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

Calvin, Benson, et

al.photosynthesis in

Chlorellawith 14CO2

Figure 8.12

Page 30: 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

carbon fixation reactions

•3-phosphoglycerate is the first product of carbon fixation

•other molecules were labeled over time

Page 31: 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

Calvin-Benson Cycle

model of carbon

fixationFigure 8.13

12 3PG

Page 32: 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

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

Page 33: 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

3PG is the first stable productFigure 8.14

Page 34: 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

Calvin-Benson Cycle

model of carbon

fixationFigure 8.13

Page 35: 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

Calvin-Benson Cycle

model of carbon

fixationFigure 8.13

Page 36: 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

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

Page 37: 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

Calvin-Benson Cycle

model of carbon

fixationFigure 8.13

Page 38: 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

Product of Calvin-Benson Cycle

•G3P is the reduced product of the Calvin-Benson cycle

Page 39: 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

Product of Calvin-Benson Cycle

•G3P is the reduced product of the Calvin-Benson cycle

Page 40: 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

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

Page 41: 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

Figure 8.3

Page 42: 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

carbon fixation reactions

•ribulose bisphosphate carboxylase/oxygenase•Rubisco•most abundant protein in the world,

but…

Page 43: 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

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

Page 44: 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

chloroplast, peroxisome, mitochondrionFigure 8.15

Page 45: 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

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

Page 46: 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

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

Page 47: 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

photorespiration - solution

•but…•C4 plants use rubisco, just like C3 plants

•PEP carboxylase and rubisco are separated into different compartments

Page 48: 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

C3 & C4 leaf anatomiesFigure 8.16

Page 49: 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

mesophyll cells and bundle sheath cells

communicate in C4 plantsFigure 8.17

Page 50: 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

photorespiration - solution

•C4 bundle sheath cells are enriched in CO2 relative to O2

•rubisco fixes O much less oftenspatial separation of

initial C fixation and Calvin Benson cycle

Page 51: 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

Table 8.1