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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
Photosynthetic green sulfur bacteria
h
CO2 + 2H2S ---> (CH2O) + 2S + H2O
Hypothesis
h
CO2 + 2H2A ---> (CH2O) + 2A + H2A
A = oxygen in cyanobacteria and plants
and anoxic sulfur in green and purple
sulfur bacteria
CO2 + 2H2A ---> (CH2O) + 2A + H2A
2 half reactions
Light reactions
2H2A h
---> 2A + 4[H•]
Dark reactions
4[H•] + CO2 ---> (CH2O) + H2O
Electron micrograph of a section through the
purple photosynthetic bacterium Rhodobacter
sphaeroides.
The nature of light
h = 6.626 x 10-34 J•s
E = h = hc/
c = 2.998 x 108 m•s-1
Amount needed to make ATP = 30-30 kJ
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 s
Absorbed energy is re-emitted
generally at a lower
energy/longer wavelength
Absorbed energy can be dissipated in
several ways
Excition transfer: slow
Energy is passed from molecule to molecule
Photoxidation: slow
Energy is transferred to a photosynthetic
reaction system
300 chlorophyll/
Reaction center
The amount of O2 evolved by Chlorella algae versus
the intensity of light flashes.
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 a
Yellow = 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 = phycoerythrin
PC = phycocyanin
AP = allophyocyanin
Light harvesting complexes in plants are
much more complex and have a wide array
of pigment molecules
b-Carotene
H
N
H
N
H
N
H
N
O
O
R
R
R
Phycocyanobilin
R
LH2
from
pea
Green = chlorophyll a
Red = chlorophyll b
Yellow = 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
Ubiquinone
CoQ
Q
Redox loops pumps out
four protons!
Related to
complex III
Photosynthetic electron-transport system of purple
photosynthetic bacteria.
Electrons taken from
reaction center to
reduce NAD+ are
replaced by the
oxidation of H2S to S0
and SO42-
Oxidation of
sulfur
Plants and cyanobacteria
FeS type
Pheo type
O
O
H3CO
CH3
H3C
H
CH3
CH3
H3CO
(CH2 C
H
C
CH2)nH
H3C
(CH2 C
H
C
CH2)nH
O
O
OH
OH
H3CO
CH3
H3C
H
CH3
CH3
H3CO
(CH2 C
H
C
CH2)nH
H3C
(CH2 C
H
C
OH
OH
Ubiquinone
Plastoquinone
CH2)nH
Net reaction
2H2O + 2NADP+ + 8 photons ---> O2 + 2NADPH + 2H+
4P680 + 4H+ + 2PQB + 4photons ---> 4P680+ + 2PQBH2
Oxygen evolving
Complex
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
During Cu deficiency
plastocyanin can be
replaced with a
cytochrome c like
molecule
2Fdred + 2H+ + NADP+ ---> 2Fdox + NADPH + H+
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
6C3
6C3
3C5
3C1
1C3
C3+C3
C6
C4
C3+C4
C7
C5
C5
C6+C3
C7+C3
3C5 + 3C1 ---> 6C3
C3 + C3 ---> C6
C3 + C6 ---> C4 + C5
C3 + C4 ---> C7
C3 + C7 ---> C5 + C5
aldolase
transketolase
aldolase
transketolase
Overal reaction = 5C3 ---> 3C5
3CO2 + 9ATP + 6NADPH ---> GAP + 9ADP + 8Pi + 6NADP+
1 GAP molecule is made from 3CO2
GAP is converted to glucose by gluconeogenesis
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 assimilate
CO2 in mesophyll cells as
malate and transporting this
to the site of rubisco in
bundle-sheath cells
It uses more ATP to make sugars
C4 plants outgrow C3 plants
on hot days
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
Phosphoribulokinase
Rubisco
Phosphoglycerate kinase/GAPDH
Fructose bisphosphatase
Sedoheptulose bisphosphatase
Regulation of enzymes by light
Phosphoribulokinase
Glyceraldehyde-3-phosphate dehydrogenase
Fructose bisphosphatase
Sedoheptulose 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