Energy Harvesting Pathways Photosynthesis. photosynthesis reverses the oxidation of...

Energy Harvesting Pathways

Photosynthesis

photosynthesis

•reverses the oxidation of glycolysis/respirationC6H12O6 +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

reactants and products of

photosynthesisFigure 8.1

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

photosynthesis:

overviewFigure 8.3

visible light

occupies a

narrow band of

the electromag

netic spectrum

Figure 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 is

transmitted (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

absorption of

a photon excites

a molecul

eFigure 8.4

absorption

and action spectra

Figure 8.6

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

electron transport mutants

fluoresce…

photosynthetic light reactions•possible fates of absorbed

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

fates of

energy

Figure 8.8

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

Figure 8.9

thylakoids are flat sacks that

reside in the chloroplastFigure 8.11

transfers of absorbed energy

Figure 8.11

photosynthetic light reactions

•noncyclic electron transport produces ATP and NADPH

•cyclic electron transport produces ATP, but not NADPH

Cyclic electron transportFigure 8.10

the light reactions of photosynthesisFigure 8.11

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 & NADPH

Figure 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 14CO2

Figure 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

3PG is the first stable productFigure 8.14

Calvin-Benson Cycle

model of carbon

fixationFigure 8.13

Calvin-Benson Cycle

model of carbon

fixationFigure 8.13

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

Calvin-Benson Cycle

model of carbon

fixationFigure 8.13

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

Figure 8.3

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

chloroplast, peroxisome, mitochondrionFigure 8.15

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

C3 & C4 leaf anatomiesFigure 8.16

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 oftenspatial separation of

initial C fixation and Calvin Benson cycle

Table 8.1