Foundation of life in the entire planet
Photosynthesis
Purpose of Photosynthesis
• Photosynthetic organisms carry out this reaction
C 6 H 1 2 O6 + 6 O 2
Glucose
6 CO 2 + 6 H 2 O
• The equation represents two processes
•light reactions: NADPH and ATP are produced
H 2 O + N AD P+
ADP + Pi
N AD PH + H + + 1 / 2 O 2
A TP + H2 O
•dark reactions: ATP and NADPH provide the energy
and reducing power for the fixation CO2
Factory of Photosynthesis
Where Photosynthesis execute ?
• Site of photosynthesis
•prokaryotes: in granules bonded to the plasma
membrane
•eukaryotes: in chloroplasts
• Chloroplast
•inner, outer, and thylakoid membranes
•grana, which consist of stacks of thylakoid disks
•trapping of light and production of O2 take place in
thylakoid disks
•light reactions take place in thylakoid disks
•dark reactions take place in the stroma
Antenna of light absorption
Figure 19.1 Membrane structure in chloroplasts
1
Comparison of light antenna
Chlorophylls (Chl) a & b
• Absorb red (600 - 700 nm) and blue (400 - 500
nm) light
•accessory pigments absorb in other portions of the
visible spectrum and transfer energy to chlorophylls
• Chlorophylls arranged in photosynthetic units
•antennae chlorophylls gather light
•harvested light energy passed to specialized Chl
molecules at a reaction center
•generally several hundred light-harvesting antennae
Chl for each Chl at a reaction center
•chemical reactions of photosynthesis begin at reaction
centers
Figure 19.2 Chlorophylls a and b, and bacteriochlorophyll a
Wavelength and Energy
• The energy of light is proportional to its
frequency ()
E = h
•Wavelength () of light is related to its frequency
= c/
•Combining these equations gives
E = h= hc/
•As can be seen from the last equation
the shorter the wavelength, the higher the energy
the longer the wavelength, the lower the energy
Photosynthesis
Strong
red. agent
Light
(< 700 nm)
Light
(< 680 nm)
Photosystem I
Photosystem II
The Light Reactions
• In the light reactions of photosynthesis, H2O is
oxidized to O2 and NADP+ is reduced to NADPH
• This series of redox reactions is coupled to the
phosphorylation of ADP to ATP in a process
called photophosphorylation
N AD P+ + H 2 O
N AD PH + H+ + 1 / 2 O 2
A DP + Pi
A TP + H2 O
•the light reactions are accomplished by two distinct
photosystems; photosystem I and photosystem II
Photosystems I and II
N AD P+ + H +
N AD PH
(Insert Fig 19.5)
Biosynthesis
of glucose in
the dark
Mild
ox. agent
A TP
Mild
red. agent
A DP + Pi
Strong
ox. agent
O 2 + 4 H+
2 H2 O
Figure 19.5 Electron flow in photosystems I and II
2
The Light Reactions
Photosystems I and II
•Photosystem I (PSI): the reduction of NADP + to NADPH
•Photosystem II (PSII): the oxidation of H2O to O2
N AD P+ + H+ + 2 e -
The net reaction of photosystems I and II is
2 H2 O + 2 N AD P+
O 2 + 2 NADPH + 2 H+
N AD PH
1 / 2 O 2 + 2 H+ + 2 e -
H2 O
N AD P+ + H2 O
N AD PH + H+ + 1 / 2 O 2
G 0' = +220 kJol
-1
•this reaction is endergonic; it is driven by the light
energy absorbed by the chlorophylls of the two
photosystems
Cyclic Electron Transport
Structure of a Photosystem
Figure 19.8 Cyclic electron flow coupled to photophosphorylation
• Rhodopseudomonas viridis
•a reaction center contains a pair of bacteriochlorophyll
molecules embedded in a protein complex that is, in
turn, an integral part of the photosynthetic membrane
•absorption of light raises it to a higher energy level
•an excited electron is passed to pheophytin, then to
menaquinone, and then to ubiquinone (next screen)
•a cytochrome molecule transfers an electron to the
reaction center; the cytochrome molecule now has a
positive charge
•the excited electron is passed to menaquinone and
then to ubiquinone
•the charge separation represents stored energy
Quinone e- Acceptors
Photophosphorylation
• A proton gradient across the inner mitochondrial
membrane drives the phosphorylation of ADP
O
O
CH3
H 3 CO
H
8
O
CH3
Menaquinone (Q A )
H 3 CO
CH3
n
O
CH3
Ubiquinone
(Coenzyme Q, Q B)
H
• The proton gradient is created
•by the splitting of H2O which releases H+ into the
thylakoid space
•by electron transport from Photosystem II to
Photosystem I
•when Photosystem I reduces NADP+ by using H+ in the
stroma to produce NADPH
• The flow of H+ back to the stroma through ATP
synthase provides the energy for the synthesis
of ATP from ADP and Pi
3
Photophosphorylation
Figure 10.10 Chloroplasts can phosphorylate ADP in the
dark if they are provided with a pH gradient, ADP, and P i
Photophosphorylation
Figure 19.12 Components of the electron transport chain
of the thylakoid membrane
Photosynthesis and O2
• Photosynthetic prokaryotes other than
cyanobacteria have only one photosystem and
do not produce oxygen
•anaerobic photosynthesis is not as efficient as
photosynthesis linked to oxygen
•it appears to be an evolutionary way station
•the ultimate source of electrons for these organisms is
not H2O, but some more easily oxidized substance,one
of which is H2S
CO 2 + 2 H2 S
H-acceptor H-donor
Photophosphorylation
Figure 19.11 Electron transport chain & thylakoid membrane
Photosynthesis
Figure 19.13 Two possible electron transfer pathways in a
photosynthetic anaerobe
The Dark Reaction
• CO2 fixation takes place in the stroma
6 CO 2 + 1 2 NA D PH + 18ATP
enzymes
C6 H1 2 O6 + 1 2 NA D P+ + 18AD P + 18Pi
• The overall reaction is called the Calvin cycle
after Melvin Calvin, Nobel Prize for Chemistry in
1961
( CH2 O) n + 2 S + H 2 O
Carbohydrate
•H-acceptor may also be NO2- or NO3 - --> NH3
4
The Dark Reaction
Figure 19.14 The main features of the Calvin cycle
The Calvin Cycle
• The first reaction is the carboxylation of six
molecules of ribulose-1,5-bisphosphate
•this carboxylation is the actual fixation step
•each carboxylation product splits to give two
molecules of 3-phosphoglycerate (twelve total)
•two 3-phosphoglycerates are converted to glucose
•ten 3-phosphoglycerates are used to regenerate six
molecules of ribulose-1,5-bisphosphate
Fig. 10.17.1
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 10.17.2
The Calvin Cycle
• For the net synthesis of one G3P molecule,
the Calvin recycle consumes nine ATP and six
NAPDH.
•It “costs”three ATP and two NADPH per CO2.
• The G3P from the Calvin cycle is the starting
material for metabolic pathways that
synthesize other organic compounds,
including glucose and other carbohydrates.
Fig. 10.17.3
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
5
RibuloseRibulose-1,51,5-bisphosphate
CH2 OPO 3 2 -
CH2 OPO 3 2 -
C= O
-
CO 2
OOC C-OH
CHOH
H2 O
C= O
CHOH
CHOH
CH2 OPO 3
2-
CH2 OPO 3
Ribulose
1,5-bisphosphate
2-
O
O
COPO3 2 CO ATP
CHOH
CHOH
CH 2 OPO 3 2 CH2 OPO 3 2 ADP
3-Phospho1,3-Bisphosphoglycerate
glycerate
2-Carboxy-3-ketoribitol
1,5-bisphosphate
CH 2 OPO 3
CHOH
+
CH2 OPO 3 2 -
3-Phosphoglycerate
Fructose-6-phosphate
H
HO
H
H
NADP + + P i
CH 2 OH
C= O
CH 2 OPO 3 2 Dihydroxyacetone
phosphate
RibuloseRibulose-1,51,5-bisphosphate
CH2 OPO 3 2 C= O
H2 O
HO H
H
OH
Pi
H
OH
CH2 OPO 3 2 Fructose-1,6-bisphosphate
CH2 OH
C= O
HO H
H
OH
H
OH
CH2 OPO 3 2 -
NADPH
3-Phosphoglycerate
The Path to Glucose
3-Phosphoglyceraldehyde
+
Dihydroxyacetone
phosphate
CHO
CHOH
CH 2 OPO 3 2 3-Phosphoglyceraldehyde
COO-
2-
CHOH
COO-
CHO
OH H2 O
H
OH
Pi
OH
CH2 OPO 3 2 -
Glucose-6-phosphate
• Can be divided into four stages: preparation,
reshuffling, isomerization, and phosphorylation
H
HO
H
H
CHO
OH
H
OH
OH
CH2 OH
Glucose
C6 + C3 -> C4 + C5
•preparation: production of glyceraldehyde-3phosphate and dihydroxyacetone phosphate
•reshuffling: many reshuffling reactions are like those
of the pentose phosphate pathway and involve
transaldolase and transketolase reactions
C6 + C3
C4 + C5
C4 + C3
C7
Net
+ C3
C 6 + 3 C3
C7
C5 + C5
3 C5
C3 + C4 -> C7
CH 2 OPO 3 2 -
CH2 OH
C= O
HO H
H OH
H OH
The Path to Glucose
+
CH2 OPO 3 2 Fructose6-phosphate
C= O
CH 2 OH
CHO
OH
CH 2 OPO 3 2 Glyceraldehyde3-phosphate
H
H
H
CHO
OH
+
OH
2CH 2 OPO 3
Erythrose4-phosphate
CH2 OH
C= O
HO H
H OH
CH2 OPO 3 2 Xylulose-5-phosphate
Dihydroxyacetone
phosphate
+
CHO
H OH
H OH
CH 2 OPO 3 2 Erythrose4-phosphate
CH2 OPO 3 2 C= O
HO H
H OH
H OH
H OH
CH2 OPO 3 2 Sedoheptulose1,7-bisphosphate
6
C7 + C3 -> C5 + C5
RibuloseRibulose-1,51,5-bisphosphate
•isomerization followed by phosphorylation
CH2 OH
C= O
H
+
OH
OH
OH
CH2 OPO 3 2 Sedoheptulose7-phosphate
HO
H
H
H
CH 2 OH
C= O
HO H
H OH
CH 2 OPO 3 2 -
CHO
H
OH
CH 2 OPO 3 2 Glyceraldehyde3-phosphate
Xylulose5-phosphate
CH 2 OH
CHO
OH
C= O
OH
HO H
+
OH
H OH
CH 2 OPO 3 2 CH 2 OPO 3 2 Ribose-5-phosphate
Xylulose-5-phosphate
CHO
OH
OH
OH
CH 2 OPO 3 2 Ribose5-phosphate
H
H
H
H
H
H
Ribulose 1,51,5-bisphosphate
•the net reaction for shuffling, isomerization, and
phosphorylation is:
CH2 OH
C= O
H OH
H OH
CH2 OPO 3 2 Ribulose5-phosphate
ATP
ADP
CH 2 OPO 3 2 C= O
H OH
H OH
CH 2 OPO 3 2 Ribulose1,5-bisphosphate
Calvin Cycle - overview
Figure 19.20 The complete Calvin cycle
3ATP
CHO
3ADP + 2P i
OH
CH 2 OPO 3 2 Glyceraldehyde 3-phosphate
CH2 OPO 3 2 (5 x 3 = 15 carbons)
C= O
H OH
3
H OH
CH2 OPO 3 2 Ribulose 1,5-bisphosphate
(3 x 5 = 15 carbons)
5H
Photosynthesis
• Net equation for the path of carbon is
6 CO 2 +
12
18
12
12
N AD PH
A TP
H+
H2 O
The HatchHatch-Slack Pathway
• An alternative pathway for CO2 fixation in tropical
plants; also called a C4 pathway
Glucose
+
•CO2 enters the outer (mesophyll) cells and reacts with
phosphoenolpyruvate to give oxaloacetate and P i
1 2 N AD P+
1 8 A DP
1 8 Pi
PEP
OPO 3 2 CH 2 = C-COO - + CO 2 carboxylase
•G°’per mole of CO2 reduced to glucose is +478 kJ
•energy of light of = 600 nm is 1593 kJ •mol-1
•efficiency of photosynthesis is approximately
478 x 100 = 30%
1593
PEP
-
O
OOC-CH 2 - C-COO - + Pi
Oxaloacetate
•oxaloacetate is reduced malate
-
malate
O
dehydrogenase
OOC-CH 2 - C-COO - + N AD PH + H +
Oxaloacetate
-
OH
OOC-CH 2 - CH-COO - + N AD P+
L-Malate
7
The HatchHatch-Slack Pathway
•malate is transported to inner (bundle-sheath) cells
where it is oxidized and decarboxylated to pyruvate
-
OH
OOC-CH 2 - CH-COO - + N AD P+
L-Malate
The Hatch-Slack Pathway
Figure 19.21 The C4 pathway
malic
enzyme
O
CH 3 -C- COO - + CO 2 + N AD PH + H +
Pyruvate
•CO2 is then passed to the Calvin cycle where it reacts
with ribulose-1,5-bisphosphate etc.
Absorption of light
Figure 19.3(a) Absorption of light by chlorophylls a and b
Absorption of light
Figure 19.3b Absorption of light by accessory pigments
Photosynthetic unit
Figure 19.4 A photosynthetic unit
8