Download Properties of Light λ

I. Light Reactions
Photosynthesis
The Nature of Light
Outline
Light Absorption by pigments
Absorption and Action Spectra
Photosystems I & II
Light-Harvesting Complexes and Reaction Centers
Z-Scheme electron carriers
Proton Transport and ATP synthesis
Repair and regulation of photosynthetic machinery
Electromagnetic Spectrum, Wavelength & Frequency
C = Speed of a light wave (3.0 X 108 m s-1)
(Greek letter lambda)
C = λv λ = Wavelength (nm)
-1
V = frequency (s ) (Greek letter nu)
II. Carbon Reactions
Calvin cycle: carboxylation, reduction & regeneration
Sucrose and starch synthesis
C2 Oxidative Photosynthetic Carbon Cycle
CO2 concentrating mechanisms
CO2 pumps, CAM, C4 Photosynthesis
III. Physiological and Ecological Considerations
Concepts and Units in the Measurement of Light
Leaf anatomy and light absorption
Photosynthetic responses to light, CO2 & Temperature
Leaf Energy Budget: Dissipation of Excess Energy
Properties of Light
1. Light has a dual nature
Light behaves like a wave
wavelength = distance between successive crests
frequency = number of wave crests in a given time
The Nature of Light – Solar Spectrum
Solar constant = 1366 watts per square meter
= one two-billionth of solar output!
Irradiance = Watts m-2
Irradiance = power m-2
Irradiance = energy time-1 area-1
Light behaves like a particle
Photon = light particle
Quantum = energy in one photon
450
2. Sunlight = photon rain
Different wavelengths
Different frequencies
Different energies
Physics of Light Energy
E=
(h ∗ c)
λ
E = energy per photon
h = Planck’s constant
c = speed of light
λ = wavelength
3x1017 speed of light (nm/sec)
Light Absorption by Chlorophyll
Fates of an excited electron
1. Return to Ground State
A. Heat
B. Fluorescence 1 X 10-8 s
6.63x10-34 Plancks constant
450 wavelength (nm)
4.42x10-19 Energy of light per photon in joules
252 Energy (kJ
2. Resonance Transfer
1 X 10-12 sec
mole-1)
3x1017 speed of light (nm/sec)
6.63x10-34 Planck’s constant
3. Photochemistry =
Excited Electron Transfer
660 wavelength (nm)
3.01x10-19 Energy of light per photon in joules
181 Energy (kJ mole-1)
1
Photosynthetically Active Radiation
PAR
Photosynthesis Equation
= Photosynthetically Active Radiation
= photons of light from 400-700nm
= unit is microEinsteins Æ uE m-2 s-1
Einstein = 1 mole of photons
microEinstein = 10-6 mole photons
microEinstein = 6.02 x 1017 photons
Light + 6CO2 + 6H2O Æ C6H12O6 + 6O2
Values for PAR
2000 uE m-2 s-1 Full, direct sunlight high noon summer.
<100 uE m-2 s-1 Shade
15 - 85 uE m-2 s-1 = Light Compensation Point
2000 uE m-2 s-1 ≈ 0.5 kJ energy ≈ 0.5kw sec
Pigments of Photosynthesis
Chlorophylls
Carotenoids
Photosynthesis Equation
Light + CO2 + H2O ÆC6H12O6 + O2
1. Nearly impossible for products to form
Keq = 10-500
Porphyrin ring
2. About 26% of absorbed light energy is used:
For CO2 + H2O Æ(CH2O) + O2 ΔGo’ = +460 kJ mol-1
Light @ 680nm = 1760 J/mole O2
460/1760 = 26%
Energy conversion efficiency ≈ 26%
…or 74% photon energy is lost as heat.
Chlorophylls a & b
Hydrocarbon
tail
Photosynthesis Overview
Chloroplast Structure
H20
Thylakoid
Inner membrane
Outer membrane
Granum
O2
Light
Harvesting
Thylakoid
Reactions
Stroma
ATP
Sugar
Building
NADPH
Stroma
Reactions
Carbohydrate
CO2
2
7.9 Schematic diagram of the action spectrum measurements by T. W. Engelmann
7.8 Action spectrum compared with an absorption spectrum
7.11 Relationship of oxygen production to flash energy
7.10 Basic concept of energy transfer during photosynthesis
Evidence for an antenna complex (Robert Emerson & William Arnold 1932)
Light harvesting antenna complex
Reaction center complex
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Light Harvesting Complex & Reaction Centers
Fig. 10.6a (TEArt)
Chlorophyll molecules,
Carotenoids & proteins
in Light-Absorbing
Antenna System
Sun
Evidence for two photosystems
Quantum yield of PS = oxygen produced/light quanta absorbed
= 1 oxygen/10 photons
Red drop
RED DROP at
λ > 680nm, i.e.
Far red light
had little effect
on O2
production
Thylakoid
membrane
Chlorophyll
still absorbs
at 680 nm
Reaction Center Chlorophyll
P700 in PSI P680 I PSII
3
Z Scheme of Photosynthesis
Evidence for two photosystems
The Emerson enhancement effect
700nm
680nm
2. Two photosystems operate in series
A. Photosystem I = PSI Æ Produces a strong reductant (reducing agent)
& weak oxidizing agent
B. Photosystem II = PSII Æ Produces a strong oxidant (oxidizing agent)
& weaker reducing agent than in PSI
Strong
reducing
agent
Redox
potential
Enhancement
effect of
photosynthesis
1. Two Photosystems: each chemically & physically distinct
Separate antenna pigments
Separate photochemical reaction centers
Linked by an electron transport chain
680
+
720nm
Electron transport
Strong
oxidizing
agent
Organization of Four Major Protein Complexes of Thylakoid Membrane
Energy Transfer in Light-Absorbing Antenna
PSII – in stacked regions of thylakoid
PSI – adjacent to stromal regions of thylakoid
PSI Æ in Granal Thylakoids
PSII Æ in Stromal Thylakoids
PSII/PSI ≈ 1.5:1
Evenly
distributed
Adjacent to
stromal regions
7.14 Z scheme of photosynthesis
Z-Scheme of Photosynthesis
Photosystem II
4
Energy & Electron Flow in Photosystem II
Structure of LHCII Antenna Complex Protein and Pigments
Energy and electron flow from antenna pigments to Cytb6f complex.
Protein
3 α-helical regions
1. Light energy picked up by antenna pigments (carotenoid or chlor b)
Carotenoids
Four X-shaped molecules
2. FRET energy transfer from carotenoid Æ chlor b Æ chlor a
3. FRET energy transfer from chlor a to Rx Center P700.
Chlorophyll a & b
14 bound to protein
4. P700 energy transfer to Pheophytin (chlor a –Mg)
5. One e- transfer from Pheo Æ Plastoquinone A then same e- Æ PQB
6. PQB receives two electrons Æ PQB-2
7. PQB-2 receives 2H+ from stroma Æ PQBH2
8. PQBH2 detaches from the RX center protein Æ enters membrane
hydrocarbon layer
Electrons enter Cytb6f complex.
Protons enter thylakoid lumen.
Structure of PSII Reaction Center Protein and Pigments
“View from Lumen”
“Side View”
Antenna Complex
Extrinsic
Oxygen-evolving
complex
Reaction Centers
PSII Reaction Center & Antenna Complex
1. Multisubunit pigment-protein complex
Barrels = Protein Helixes
Colors = Different proteins
2. D1 & D2 = Core Reaction Center Proteins
P680 = Primary Rx. Center Chlorophyll
Carotenoids
Pheophytin (chlorophyll without Mg+)
Plastoquinones (2 e- acceptors)
3. Antenna complex (LHCII) proteins and chlorophylls
∼200-300 chlorophylls
4. Oxygen Evolution proteins
Bound to D1 & D2
Primary donor chlorophyll
More chlorophylls
Carotenoids
Pheophytins (Mg replaced by 2H)
Plastoquinones (2e- acceptors)
Antenna Complex
Electron Transfer from Reaction Center
Water Oxidation in PSII
2 H 2O → O2 + 4 H + + 4e −
Pheophytin Æ 2 electrons (sequentially) Æ QA Æ QB + 2H+
S0 Æ S4 = different oxidation states of Manganese
1. Oxygen
produced
PQH2
Plastohydroquinone
= Mobile electron & proton carrier
+
2 H 2O → O2 + 4 H + 4e
−
2. Electrons
released
Diffuses in core of membrane bilayer
Picked up by a
tyrosine on D1
protein
3. Protons
released
5
Electron Transfer between PSII & PSI
Cytochrome b6f Complex
1
Plastoquinone
1. PQ picks up two
electrons from Pheo
e-
from PQ flows linearly to PC
PC reduces P700 in PSI
2nd e- from PQ flows in a cyclic process
back to PQ
This is a proton pump transferring protons
from stroma to lumen of thylakoid.
2. PQ picks up 2 protons
from Stroma.
3. PQ Shuttles electrons
through nonpolar lipid
hydrocarbons to
Cytochrome b6f
releasing H+ into Lumen
Plastocyanin
1. small Cu-containing
protein
2. H2O soluble & mobile
3. Found in lumen
4. Transfer e- from Cyt b6f
to PSI.
Z-Scheme of Photosynthesis
Photosystem I
PSI Reaction Center
2 Core proteins
1. Large multisubunit complex with
major core proteins PSaA & PsaB
2Reaction Center Complex
Major Proteins: Psa A & Psa B
P700
Core antenna of 100 chlorophylls
3. Electron Transfer Cofactors centered
inside core antenna pigments
Ao = Chlorophyll ?
A1 = quinone ?
Iron-sulfur proteins
FeSx, FeSA, FeSB
4. Ferredoxin (Fd) soluble protein
Reduces NADP+
PSI & Cyclic Electron Flow
Chemiosmotic ATP Synthesis
1. Protons from: Water oxidation & Pumping at Cyt b6f
e-
2. Proton gradient drives ATP Synthase
4 protons ≈ 1 ATP
3. Molecular Rotation of Stalk and CFo drives ATP synthesis
6
Herbicide Effect on Photosynthesis
Light Harvesting
Light-Harvesting
Component
Photosystem I
Photosystem II
Cytochrome b6f
Complex
ATP Synthase
Function(s)
Reduce NADP+ to NADPH
Cyclic electron flow Æ ATP synthesis
Electron Transfer to PSI
Water oxidation
Protons pumped into thylakoid lumen
Transfers e- between PSII & PSI
Proton Pumping
Chemiosmotic ATP Synthesis
O2
superoxides
Photosystem II Responses to High Light Intensity
Protection of the LightHarvesting Apparatus
Defense Mechanisms
Antenna
System
2) Nonphotochemical quenching
by carotenoids = Xanthophyll cycle
conversion of energy to heat
Controlled by
pH of thylakoid lumen
in peripheral antenna complex
Excess
Energy
delivery
1) Photochemical quenching by
carotenoids that quench excited
state of chlorophyll
1. Suppression
2. Scavenge
3. Repair
Toxic photoproducts
O2 ÆSinglet oxygen
formed when excited chlor
reacts with oxygen
1. Superoxide
dismutase
2. Ascorbate
peroxidase
Damage to Rx Center
= Photoinhibition
Damage to D1 protein
by excess light
Xanthophyll Cycle
PSII Reaction
Center
1) Remove & resynthesize D1
protein
2) Recycle other Rx center
components
Diurnal changes in xanthophyll content as a function of irradiance
Low
Light
Energy
Light
energy
increases
Zeaxanthin binds to
light-harvesting
antenna proteins…
leading to
… quenching
… heat dissipation
Light
energy
increases
High
Light
Energy
7
Protection of the Light-Harvesting Apparatus
1. Problem Æ Excited chlorophyll forms highly reactive substances
singlet oxygen 1O2*
superoxide
O2peroxide
END Photosynthesis
H2O2
2. Defense mechanisms
Antenna System
A. Photochemical Quenching by chlorophyll excitation energy transfer
to carotenoids
… carotenoids collect excitation energy
… energy reradiated as heat
END
Light Harve
sting
B. Nonphotochemical quenching by xanthophylls
… Xanthophylls use the Xanthophyll Cycle
… Zeaxanthin (Zea) synthesis under high light
… Zea binds to antenna proteins & energy dissipated as heat
Reaction Center Damage & Repair
A. Photoinhibition
… Damage to PSII reaction center D1 protein
… Removal and synthesis of new D1 protein
8