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All biological energy ultimately comes from solar or geothermal energy harnessed by
autotrophic organisms
Chemoautotrophs
Photoautotrophs
Photosynthesis occurs in 5 of the 9 phylogenetic divisions of eubacteria
Bacteriorhodopsin, the simplest form of phototrophy
All trans retinal 13-cis retinal
CO2 + 2H2S ---> (CH2O) + 2S + H2O
Photosynthetic green sulfur bacteria
Hypothesis
CO2 + 2H2A ---> (CH2O) + 2A + H2A
h
h
A = oxygen in cyanobacteria and plants and anoxic sulfur in green and purple
sulfur bacteria
CO2 + 2H2A ---> (CH2O) + 2A + H2A
2 half reactions
2H2A ---> 2A + 4[H•]
4[H•] + CO2 ---> (CH2O) + H2O
Light reactions
h
Dark reactions
Electron micrograph of a section through the purple photosynthetic bacterium Rhodobacter
sphaeroides.
Amount needed to make ATP = 30-30 kJ
E = h = hc/
The nature of light
h = 6.626 x 10-34 J•s
c = 2.998 x 108 m•s-1
Amount of light absorbed is a funtion of the physical properties of the absorbing medium
A = log I0/I = cl
= molar extinction coefficient M-1cm-1)c = concentration (M)l = pathlength (cm)
for chlorophylls are among the highest for organic molecules ≈ 105 M-1cm-1
Absorbed energy can be dissipated in several ways
Internal conversion: very fast < 10-11 s
Electronic energy is converted to kinetic energy
Fluorescence: very fast ≈ 108 sAbsorbed energy is re-emitted
generally at a lower energy/longer wavelength
Absorbed energy can be dissipated in several ways
Excition transfer: slowEnergy is passed from molecule to molecule
Photoxidation: slow
Energy is transferred to a photosynthetic reaction system
The amount of O2 evolved by Chlorella algae versus the intensity of light flashes.
300 chlorophyll/Reaction center
Since there is an excess of chlorophyll it is unlikely that all of them function as reaction
centers
Most are act as antennae to harvest light from a variety of wavelengths
and transfer it to a single reaction center
What is the nature of the antennae
Take the example of purple non-sulfur bacteria
-proteobacteria
Light harvesting complex 2
Green = bacteriochlorophyll aYellow = lycopene
Light harvesting complex 1 surrounds the reaction center
Its chlorophyll is slightly lower in energy to facilitate exciton transfer
Ultimately all the photons harvested make their way to the reaction center
Cyanobacteria
PE = phycoerythrinPC = phycocyaninAP = allophyocyanin
Light harvesting complexes in plants are much more complex and have a wide array of pigment molecules
-Carotene
HN
O
R
HN
R
HN
R
HN
O
R
Phycocyanobilin
LH2 from pea
Green = chlorophyll aRed = chlorophyll bYellow = lutein
Alpha proteobacteria
(Chl)2 + 1 exciton ---> (Chl)*2
(Chl)*2 + Pheo ---> •(Chl)+2 + •Pheo-
2 •Pheo- + 2H+ + QB ---> 2Pheo + QBH2
∆E’º = +0.95V !!!!
Coenzyme Q UbiquinoneCoQQ
Redox loops pumps out four protons!
Photosynthetic electron-transport system of purple photosynthetic bacteria.
Related to complex III
Oxidation of sulfur
Electrons taken from reaction center to reduce NAD+ are replaced by the
oxidation of H2S to S0 and SO4
2-
Plants and cyanobacteria
FeS type
Pheo type
H3CO CH3
O
O
(CH2H3CO CH
C
CH3
CH2)nH
H3CO CH3
OH
OH
(CH2H3CO CH
C
CH3
CH2)nH
H3C H
O
O
(CH2H3C CH
C
CH3
CH2)nH
H3C H
OH
OH
(CH2H3C CH
C
CH3
CH2)nH
Ubiquinone Plastoquinone
Net reaction
2H2O + 2NADP+ + 8 photons ---> O2 + 2NADPH + 2H+
4P680 + 4H+ + 2PQB + 4photons ---> 4P680+ + 2PQBH2
Oxygen evolvingComplex
In cyanobacteria plastocyanin is be replaced by a small cytochrome c like protein
Cyt c6 can perform both roles in this bacterium
Photosystem I is related to bacterial FeS type photosystem
2Fdred + 2H+ + NADP+ ---> 2Fdox + NADPH + H+
During Cu deficiency plastocyanin can be replaced with a cytochrome c like molecule
About 3ATP are made per O2 produced
2H2O + 8 photons + 2NADP+ + 3ADP + 3Pi --->O2 + 3ATP + 2NADPH
Cyclic pathway does not generate NADPH
Photosystem I and II are spatially separated to prevent exciton transfer and loss of
proton gradient
Photosystem I in unstacked stroma lamellae
Photosystem II in closely stacked grana
The Calvin cycle.
3CO2 -----> GAP
9 ATP and 6 NADPH
3C53C1
6C3
6C3
1C3
C6
C3+C3
C3+C4
C6+C3
C5
C4
C7+C3
C7
C5
C3 + C3 ---> C6
C3 + C6 ---> C4 + C5
C3 + C4 ---> C7
C3 + C7 ---> C5 + C5
Overal reaction = 5C3 ---> 3C5
1 GAP molecule is made from 3CO2
3CO2 + 9ATP + 6NADPH ---> GAP + 9ADP + 8Pi + 6NADP+
GAP is converted to glucose by gluconeogenesis
aldolasetransketolase
aldolasetransketolase
3C5 + 3C1 ---> 6C3
Photorespiration
Dissipates ATP and NADPH
What is the purpose?
To protect from photo oxidation in the absence
of CO2?
On a hot bright day CO2 may be depleted and O2 may accumulate
Under these conditions photorespiration may take over
This may prevent the photooxidation of reaction centers
By decreasing photorespiration plants save water because they do not have to
have their pores open to acquire CO2
C4 plants (such as grasses) reduce photorespiration by
physically separating CO2 and O2 acquisition from rubisco
These plants assimilateCO2 in mesophyll cells as
malate and transporting this to the site of rubisco in
bundle-sheath cells
C4 plants outgrow C3 plants on hot days
It uses more ATP to make sugars
Another type of plants called CAM plants use a variant of the C4 cycle
In this case CO2 acquisition is temporally separated from rubisco
At night when the air is cool and moist CAM plants open their pores and let CO2 in. The CO2 is
incorporated into malate and stored in the vacuole.
During the day the CO2 is released from malate and there is a steady supply of CO2 to prevent
photorespiration.
Control of the Calvin Cycle
PhosphoribulokinaseRubiscoPhosphoglycerate kinase/GAPDHFructose bisphosphataseSedoheptulose bisphosphatase
Regulation of enzymes by light
PhosphoribulokinaseGlyceraldehyde-3-phosphate dehydrogenase
Fructose bisphosphataseSedoheptulose bisphosphatase
What about Rubisco?
Responds to light dependent factors
pH of stroma increases by 1 unit when photosynthesis is on. Rubisco has a pH optimum at pH 8.0
Rubisco is activated by Mg2+, light induced influx of H+ into lumen is accompanied by Mg2+ efflux into
stroma