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The energetics of life on Earth
Lower energy
Higher energy
Lower energy
Photosynthesis
Aerobic respiration
CH 2OH
Chloroplast
H
H
OH
CO 2 + H 2O
Mitochondrion
O H
OH
H
ATP
CO 2 + H 2O
H
HO
OH
Carbohydrate
(contains high energy electrons)
e–
e–
NADPH
e–
e–
e–
e–
e–
e–
e–
e–
e–
e–
NADH
ADP
NADP +
O2
NAD+
+
chemical energy
(ATP)
O2
Sun
Light
energy
H 2O
e–
e–
e–
–
e
e–
e– e – e –
e–
e– e–
e–
(contains low energy electrons)
e–
e–
e–
e–
e–
e–
e–
e–
e–
e–
e–
e–
H 2O
e–
e–
e–
–
e
e–
e – e– e –
e–
e– e –
e–
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Photosynthesis in cyanobacteria and plants
First photosynthetic organisms were bacteria that utilized
minerals (mainly SH2) as the source of electrons to produce
reducing power and ATP. The Earth had a very reducing
environment then, with very little free oxygen.
The cyanobacteria first became capable of using water as the
electron donor.
They produced molecular oxygen as a by-product, which
accumulated rapidly in the atmosphere killing many
anaerobic microorganisms.
Higher plants also use water as a source of electrons and
evolve oxygen. ATP and NADPH are used to reduce
atmospheric CO2 and to synthesize carbohydrates.
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6CO2 + 12 H2O
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light
C6H12O6 + 6H2O + 6O2
One mole of photons of red light contains 45 kcal of energy
O
Light is absorbed by a mixture of
photosynthetic pigments.
In bacteriochlorophyll a
C CH3
C
CH3
H
C
C
Porphyrin Ring
H3C
H3 C
C
C
N
N
C
C
N
N
C
H
C
C
C
C
CH2
HC
C O
CH2
C O
CH3
CH2
chlorophyll
CH
H3C
C
CH2
H2C
CH2
H3C
HC
CH2
H2C
CH2
H3C
HC
CH2
H3C
CH2
H3C
C
C
O
CH2 CH3
CH
C O O
Phytol Tail
C
Mg
C
H
C
C
C
HC
The main photosynthetic pigment of
cyanobacteria and higher plants is:
In chlorophyll b
CHO
CH2
HC
CH3
Chlorophyll a
CH3
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In algae and higher plants, photosynthesis occurs within
highly specialized organelles called chloroplasts.
The chloroplast is the largest organelle of any eukaryotic cell
(as large as a red blood cell).
It is bound by a double outer membrane that surrounds an
internal space called stroma.
Within the stroma are found abundant membrane sacs called
thylakoids. Thylakoids occur in stacks known as grana, and
single sacs. The grana contain most of the proteins that make
Photosystems I and II, while the single sacs are rich in ATP
synthase.
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The efficiency of light
collection is increased by
Photon
a light harvesting antenna
complex comprising
~300 molecules of
chlorophyll. When any of
the pigments in the
antenna is struck by a
photon of light, the
resulting high energy
Reaction center
electrons are channeled to
a reaction center pigment molecule that transfers them to an
acceptor molecule.
Antenna pigment molecules
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During the light
reactions, low
energy electrons
are pumped up
the energy scale
through two
photosystems,
until they reach a
level high enough
to reduce NADP+
to NADPH,
which provides
the reducing power necessary to reduce CO2 to (CHOH)n.
The sequence of reactions is summarized by the Z-scheme.
Electron acceptor
–400
e–
e–
NADP + + H+
e–
e–
NADPH
Electrontransport
system
–200
Electron acceptor
0
Electrontransport
system
e–
e–
Energy content
of electrons (mV)
e–
e–
+200
e–
e–
e–
e–
e–
e–
Antenna
molecules
+400
Reaction center (P700)
Photolysis
+600
2H+
1/2 O2
e–
e–
Incoming
photon
e–
e–
Antenna
molecules
Photosystem I
Light
Reaction center (P680)
+800
H 2O
Incoming
photon
Photosystem II
Light
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The absorption of light photons by the Photosystem II
reaction center P680 initiates (and maintains) the net transfer
of electrons from water to the acceptor molecule
plastoquinone. The splitting of water is accomplished via a
cluster of four manganese (Mn) atoms that become ionized to
Mn+ after transferring their electrons to P680+ (the positively
charged form of the Photosystem II reaction center).
Figure 6.13 summarizes the sequence of electron transfer
reactions that takes place at Photosystem II. The end result of
this chain is the formation of one molecule of reduced
plastoquinone (PQH2), which diffuses throughout the lipid
bilayer of the thylakoid membrane.
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The electrons from PQH2 are transferred to the cytochrome
b6f complex, and the two protons are released into the
thylakoid lumen.
Cyt b6f in turn transfers the electrons to the copper-protein
plastocyanin, a water diffusible electron carrier that resides
within the lumen of the thylakoid.
Plastocyanin donates its electrons to P700+ the positively
charged Photosystem I reaction center. A second electron
transport chain carries electrons ejected from P700 to the
iron-containing protein ferredoxin.
The enzyme ferredoxin-NADP+ reductase catalyzes the
reduction and protonation of NADP+ to NADPH.
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Figure 6.18 summarizes the series of electron transfers and
proton translocations that take place during the light reactions
of photosynthesis. The net balance of all these reactions is:
A total of at least eight photons are absorbed by PSI + PSII
A difference of ten H+ is created between the thylakoid
lumen and the chloroplast stroma
Two molecules of NADPH are produced
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Photophosphorylation
The pH gradient established across the thylakoid membrane
drives the synthesis of ATP from ADP and Pi.
The enzyme that catalyzes this reaction, ATP synthase,
comprises a base piece CF0 that forms a proton channel
across the membrane, and a headpiece CF1, that contains the
catalytic subunit.
Chloroplast ATP synthase is made of multiple protein
subunits which are homologous to ATP synthases from
bacteria and mitochondria.
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In cyclic photophosphorylation, the electrons transferred
from Photosystem I to Ferredoxin can be passed back to the
Cytb6f (instead of being used to reduce NADP+), allowing
the cytochrome to translocate H+ across the membrane. This
light-driven proton translocation system is sufficient to
sustain ATP synthesis in the absence of NADPH formation.
2
H+
Fd
1
3
cytb6 f
PSI
PC
H+
4
Light
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Dark reactions and the Calvin-Benson cycle
The ATP and NADPH made during the light reactions, are
used to drive the reduction of CO2 and the carboxylation of
the intermediate ribulose 1,5-biphosphate (RuBP), a key
reaction catalyzed by RuBP carboxylase (RUBISCO).
RUBISCO is the most abundant protein in plant leaves, and
possibly the most abundant protein on Earth.
The pathway was elucidated by feeding radioactive C14O2 to
plant and algal cells, and tracing the distribution of C14 into
different compounds as a function of time.
Figure 6.22