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FCH 532 Lecture 32
Chapter 31: Photosynthesis
Quiz on Friday(4/20): Ribonucleotide reductase
mechanism
Friday (4/20): extra credit seminar, Dr. Jimmy
Hougland,
145 Baker, 3-4PM.
ACS exam has been moved to Monday (4/30)
Quiz on Final is scheduled for May 4, 12:45PM2:45PM, in 111 Marshall
Biosynthesis of of
NAD and NADP+
Page 1099
Produced from vitamin
precursors Nicotinate
and Nicotinamide and
from quinolinate, a Trp
degradation product
Biosynthesis
of FMN and
FAD from
riboflavin
Page 1100
FAD is synthesized from
riboflavin in a tworeaction pathway.
Flavokinase
phosphorylates the 5’OH
group to give FMN
FAD pyrophosphorylase
catalyzes the next step
(coupling of FMN to
ADP).
Page 1101
Biosynthesis of CoA
from pantothenate
Photosynthesis
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Life on Earth depends on solar energy.
Plants and cyanobacteria use photosynthesis to fix CO2 to carbohydrates
General reaction for oxygenic photosynthesis:
Light
CO2 + H2O
(CH2O) + O2
General reaction for anoxygenic photosynthesis:
Light
CO2 + 2H2S
(CH2O) + 2S + H2O
Page 872
Figure 24-1 Chloroplast
from corn.
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Figure 24-2
Electron micrograph of a section through the
purple photosynthetic bacterium Rhodobacter sphaeroides.
Choroplasts
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Site for photosynthesis in eukaryotes
Part of organelles found in plants (plastids).
Similar to mitochondria
Highly permeable outer membrane
Nearly impremeable inner membrane
Inner membrane encloses stroma-contains
enzymes, DNA RNA and ribosomes similar to mitochondrial matrix.
Stroma encloses a third membrane
component - thylakoids
Thylakoids is a single, highly folded vesicle
that appears as a stack of discs called
grana.
The grana are interconnected by stromal
lammelae.
Each chloroplast has about 10-100 grana.
stroma
chloroplast
thylakoid
membrane
DNA for chloroplast proteins can be in the nucleus or
chloroplast genome
Buchannan et al. Fig. 4.4
Import of proteins
into chloroplasts
Buchannan et al. Fig. 4.6
Biochemistry inside plastids
• Photosynthesis – reduction of C, N, and S
• Amino acids, essential amino acid synthesis restricted
to plastids
– Phenylpropanoid amino acids and secondary compounds start
in the plastids (shikimic acid pathway)
– Site of action of several herbicides, including glyphosate
– Branched-chain amino acids
– Sulfur amino acids
• Fatty acids – all fatty acids in plants made in plastids
Lipids of the thylakoid membrane
6
HO
4
OH
5
OH
O O
1
2
*
O
CH2
HC
O
C
3
O
OH
Page 1093
R1
HC
O
digalactosyl diacylglycerol
C
R2
•Only ~10%
phospholipids
•~80% are monoand digalactosyl
diacylglycerols
•~10% are
sulfoquinovosyl
diacylglycerol
Biochemistry inside plastids
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Carotenoids – source of vitamin A
Thiamin and pyridoxal, B vitamins
Ascorbic acid – vitamin C
Tocopherol – vitamin E
Phylloquinone (an electron accepttor in
PS I – vitamin K)
Light and dark reactions
Photosynthesis happens in 2 distinct phases:
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Light reactions use light energy to generate NADPH and ATP.
Dark reactions (light independent reactions), use NADPH and ATP to make
carbohydrate from CO2 and H2O.
Light reactions take place in the thylakoid membrane
Light reactions similar to electron transport in mitochondria and oxidative
phosphorylation.
Takes place in plasma of the inner membrane or invaginated structures called
chromatophores.
In eukaryotes, dark reactions occur in the stroma
Synthesizing carbohydrates from CO2 and water
presents a formidable thermodynamic problem:
6 CO2 + 6 H2O
C6H12O6 + 6 O2
DGo = +679 kcal/mol (+2480 kJ/mol). Keq = 10-496
Photosynthetic organisms use the energy of light to drive
carbohydrate synthesis against this enormous gradient.
The energy of red light (700 nm) is E = Nhn = 41 kcal/einstein* (172 kJ/einstein)
6 CO2 + 6 H2O + 48 hn
C6H12O6 + 6 O2
DGo = -1290 kcal/mol (-5398 kJ/mol). Keq = 10942 !
*An einstein is a mol of photons. N = Avogadro’s number (6x1023); h =
Planck’s constant (6.63x10-34 J/s); n = frequency (s-1).
Absorption of light
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1.
2.
3.
4.
Main photoreceptor for photosynthesis is chlorophyll.
Cyclic tetrapyrolle, like heme groups of cytochromes and
globins.
Differs from these molecules in 4 ways
Central metal ion is Mg2+ not Fe(II) or Fe(III).
Has cyclopentenone ring, (Ring V), fused to pyrrole Ring III
Pyrolle Ring IV is partially reduced in chlorophyll a (Chl a)
and chlorophyll b (Chl b). In bacteriochlorophyll Rings II and
IV are partially reduced.
Propionyl side chain of Ring IV is esterified to tetraisoprenoid
alcohol. In Chl a and b and Bchlb it is phytol.
The photochemically reactive pigments are chlorins or
bacteriochlorins, which are structurally related to hemes
O
N
N
Fe
N
N
N
N
N
Mg
N
N
N
O
Mg
N
N
Hemes
Chlorophylls
Bacteriochlorophylls
symmetrical
p systems;
asymmetrical
p systems;
more asymmetrical p
systems;
absorb blue
light
absorb blue
& red light
absorb blue, orange
& near-IR light
O
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Page 874
Absorption of light
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Molecules have numerous electronic quantum states of
differing energies.
Absorption of light by a molecule promotes an electron from
its ground (lowest energy)state molecular orbital to one of
higher energy.
A given molecule can only absorb photons of certain
wavelengths-conservation of energy.
The energy difference between the two states must exactly
match that of the absorbed photon.
Absorption of light
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Amount of light absorbed by a substance at a given wavelength is described by the BeerLambert Law:
A = log
I0
I
A = absorbance
I0 = intensity of incident light
I = intensity of transmitted light
c = molar concentration of sample
l = length of the light path
= molar extinction coefficient
= cl
The electromagnetic spectrum
PAR = photosynthetically available radiation
Different pigments absorb light
differently
Page 875
Figure 24-5
Absorption spectra of various photosynthetic
pigments.
Different pigments absorb light
differently
Increasing Energy
When light raises a molecule to an excited electronic state, the
molecule becomes a stronger reductant
LUMO
LUMO
HOMO
LIGHT
HOMO
electrons
A
B
A*
B
Absorption of light
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Peak molar extinction coefficient of chlorophylls >105 M-1 cm-1
Small chemical differences (structure) affect their abs specta
The electronically excited molecule can dissipate excitation
energy in a number of ways.
Figure 24-4
Energy diagram indicating the electronic states of
chlorophyll and their most important modes of interconversion.
Short wavelength abs
Page 875
Long wavelength abs
Dissipation of excitation energy
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Internal conversion-electronic energy is converted to heat
(molecular motion). Occurs very rapidly (<10-11s) and
molecules returned to ground state.
Excitation energy of a chlorophyll molecule that abs a shortwavelength band (2nd excited state) is no different than if
photon was absorbed in its less energetic long-wavelength
band (1st excited state).
Figure 24-4
Energy diagram indicating the electronic states of
chlorophyll and their most important modes of interconversion.
Short wavelength abs
Page 875
Long wavelength abs
Dissipation of excitation energy
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Fluorescence-electronic energy is reduced to ground state
by emitting a photon Occurs slower than internal conversion
(~10-8s).
Emitted photon has a longer wavelength (lower energy) than
the initially absorbed photon.
Accounts for 3-6% of light energy absorbed-usually causes
red fluorescence.
Figure 24-4
Energy diagram indicating the electronic states of
chlorophyll and their most important modes of interconversion.
Short wavelength abs
Page 875
Long wavelength abs
Dissipation of excitation energy
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Exciton transfer (resonance energy transfer)-electronic
energy is directly transferred to nearby unexcited molecules
with similar electronic properties
Funnels the light to photosynthetic reaction centers
Photooxidation-light-excited donor molecule is oxidized by
transferring an electron to an acceptor molecule.
Most of the pigments in photosynthetic cells do not participate in the
electron-transfer reactions of photosynthesis. Instead, they serve as
an antenna that increases the absorption of light.
R. Emerson & W. Arnold
measured the amount of O2
formed when they excited algae
with short flashes of light.
At high light intensity, the
maximum O2 released per flash
was about 1 O2 per 2400 Chls.
0.0004
O2/Chl
0.0002
At low light intensity, 1 O2 is
formed for ~each 8 photons
absorbed (yellow dashed line).
0
0.004
0.008
0
Light absorbed (photons/Chl)
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Light absorbed is transferred to
Photosynthetic Reaction Centers
Large excess of chlorophyll molecules don’t all participate in
photochemical reactions.
Most chlorophyll act as light harvesting antennas (antenna
chlorophyll).
They pass their energy until they reach a reaction center.
Transfer occurs <10-10 s with an efficiency of 90%.
RC intercepts only 1 photon per second.
Important as light harvesting complexes (LHCs)
Page 877
Figure 24-7a Flow of energy through a photosynthetic antenna
complex. (a) Diagram of random photon migration by exciton
transfer.
Page 877
Figure 24-7b Flow of energy through a photosynthetic antenna
complex. (b) The excitation is trapped by the RC chlorophyll.
When the antenna is excited with light, excitations are
transferred to the reaction center within ~40 ps
0.1 - 0.2 ps
Smaller “LH2” antenna
complexes transfer
energy rapidly to LH1
LH2
LH2
LH2
35 ps
view normal to
the membrane
1 ps = 10-12 s
RC
1.2 ps
LH1
antenna BChls are
green and blue in
this figure.