Chapter 21
Photosynthesis
Biochemistry
by
Reginald Garrett and Charles Grisham
Essential Question
1. How is solar energy captured and
transformed into metabolically useful
chemical energy?
2. How is the chemical energy produced by
photosynthesis used to create organic
molecules from carbon dioxide?
Outline
1. What are the general properties of
photosynthesis?
2. How is solar energy captured by chlorophyll?
3. What kinds of photosystems are used to capture
light energy?
4. What is the molecular architecture of
photosynthetic reaction centers?
5. What is the quantum yield of photosynthesis?
6. How does light drive the synthesis of ATP?
7. How is carbon dioxide used to make organic
molecules?
8. How does photorespiration limit CO2 fixation?
The Sun - Ultimate Energy
1.5 × 1022 kJ of sunlight energy falls on the earth
each day
• 1% is absorbed by photosynthetic organisms
and transformed into chemical energy
– CO2 fixation:
6CO2 + 6H2O → C6H12O6 + 6O2
– 1011 tons of CO2 are fixed globally per year
– Formation of sugar from CO2 and water requires
energy
• The remaining 99%, 2/3 is absorbed by earth
and ocean; 1/3 is lost as light reflected back
21.1 What Are the General Properties of
Photosynthesis?
• Photosynthesis occurs in thylakoid membranes
of chloroplasts
– Structures involving paired folds (lamellae) that
stack to form "grana"
– The soluble portion of the chloroplast is the
"stroma"
– The interior of the thylakoid vesicles is the
"thylakoid space" or "thylakoid lumen"
• Chloroplasts possess DNA, RNA and
ribosomes
Photosynthesis Consists of Both Light
Reactions and Dark Reactions
• The light reactions
– Associated with the thylakoid membranes
– Capture light energy and convert it to chemical
energy in the form of reducing potential (NADPH)
and ATP, with evolution of oxygen
• The dark reactions (CO2 fixation)
– Located in the stroma
– Use NADPH and ATP to drive the endergonic
process of hexose sugar formation from CO2 in a
series of reactions in the stroma
Photosynthesis Consists of Both Light Reactions
and Dark Reactions
Figure 21.4 The light-dependent and light-independent reactions of
photosynthesis.
Water is the Ultimate e- Donor for Photosynthetic
NADP+ Reduction
The reaction sequence for photosynthesis in green plants:
2 H2O + 2 NADP+ + x ADP + x Pi →
O2 + 2 NADPH + 2 H+ + x ATP + x H2O
The stoichiometry of the metabolic pathway of CO2
fixation
12 NADPH + 12 H+ + 18 ATP + 6 CO2 + 12 H2O →
C6H12O6 + 12 NADP+ + 18 ADP + 18 Pi
A More Generalized Equation for Photosynthesis:
CO2
+ 2H2A
Hydrogen
Hydrogen
acceptor
donor
→
(CH2O) +
2A
+
H2 O
Reduced Oxidized
acceptor
donor
In photosynthetic bacteria, H2A can be H2S or other
oxidizable substrates, like isopropanol:
In higher plants, H2A can be H2O and 2 A is O2
21.2 How Is Solar Energy Captured by
Chlorophyll?
• Chlorophyll is a photoreactive, isoprene-based
pigment
– A planar, conjugated ring system—similar to porphyrins
– Mg2+ is coordinated in the center of the planar
conjugated ring structure
– A long chain alcohol, phytol, group confers membrane
solubility
– Aromaticity makes chlorophyll an efficient absorber of
light
• Light absorption promotes an electron to a higher
orbital, enhancing the potential for transfer of this
electron to a suitable acceptor
Since the absorption spectra for a
and b differ, plants that possess
both can harvest a wider spectrum
of incident energy.
Accessory light-harvesting pigments increase the
possibility for absorption of light
β-Carotene, an accessory
light-harvesting pigment in
leaves.
Phycocyanobilin, a blue
pigment found in cyanobacteria.
The Light Energy Absorbed by Photosynthetic
Pigments Has Several Possible Fates
• Each photon represents a quantum of energy
• Each quantum of light energy has 4 possible fates:
1. Loss as heat. Energy can be dissipated as heat through
redistribution into atomic vibrations within the pigment
molecule.
2. Loss as light. A light-excited electron can return to a
lower orbital, emitting a photon of fluorescence.
3. Resonance energy transfer. Excitation energy can be
transferred to a neighboring molecule.
4. Energy transduction. The excitation energy changes the
reduction potential of the pigment, making it an effective
electron donor.
Figure 21.7 Possible fates of the quantum of light
energy absorbed by photosynthetic pigments.
Transduction of Light Energy into Chemical
Energy Involves Oxidation-Reduction
• The basis of photosynthesis is transduction of light
energy into chemical energy (of oxidation-reduction).
• Photon absorption raises chlorophyll (Chl) to Chl*
• Electron transfer from Chl* to an adjacent molecule A,
producing oxidized Chl (Chl•+) and reduced A (A-)
• Oxidation of A- eventually culminates in reduction of
NADP+ to NADPH
• The system is restored to its original state once NADPH
is formed and water is oxidized
Figure 21.8 Model for light absorption
by chlorophyll and transduction of light
energy into an oxidation-reduction
reaction. The process culminates in
production of NADPH and oxidation of
water.
Photosynthetic Units Consist of Many Chlorophyll
Molecules but Only a Single Reaction Center
• The photosynthetic unit consists of (Fig 21.9)
– Several hundred light-capturing chlorophylls
– A pair of special chlorophylls in the reaction center
• Light is captured by one of the "antenna
chlorophylls" and routed from one to the other
until it reaches the reaction center chlorophyll
dimer that is photochemically active
• Oxidation of chlorophyll leaves a cationic free
radical, Chl•+, whose properties as an electron
acceptor are important to photosynthesis
Figure 21.9 Schematic diagram of a photosynthetic unit.
21.3 What Kinds of Photosystems Are Used
to Capture Light Energy?
Oxygenic phototrophs have two distinct
photosystems: PSI (P700) and PSII (P680)
• PSI has a maximal red light absorption at 700 nm and
use ferredoxins as terminal electron acceptors
• PSII has a maximal red absorption at 680 nm and use
quinones as terminal electron acceptors
• Chloroplasts given light at 680 and 700 nm
simultaneously yield more O2 than the sum of amounts
when each is used alone
• All chlorophyll is protein-bound—as part of either PSI
or PSII or light-harvesting complexes (LHCs)
PSI and PSII Participate in the Overall Process
of Photosynthesis
• PSII splits water (photolysis), producing O2, and feeds
the electrons released into an electron transport chain
that couples PSII to PSI via cytochrome b6f
• PSI provides reducing power in the form of NADPH
• Electron transfer between PSII and PSI pumps protons
for chemiosmotic ATP synthesis
• Essentially, electrons flow from H2O to NADP+, driven
by light energy absorbed at the reaction centers
• Light-driven phosphorylation of ADP to make ATP is
termed photophosphorylation
Figure 21.10 Roles of the two photosystems, PSI and PSII.
The Pathway of Photosynthetic Electron
Transfer Is Called the Z Scheme
• PSI and PSII contain unique complements of
electron carriers, and these carriers mediate the
stepwise of electrons from water to NADP+
• According to their standard reduction potential,
the zigzag result resembles the letter “Z”, Thus
the pathway name – the Z scheme (Fig 21.11a)
• Overall potosynthetic electrons is accomplished
by three membrane-spanning supramoleculear
complex (Fig 21.11b)
1. PSII
2. Cytochrome b6f complex
3. PSI
•
•
•
•
•
Pheo= pheophytin
PQ = plastoquinone pool
PC = plastocyanin
"F"s = ferredoxins
Ao = a special
chlorophyll a
• A1 = a special PSI
quinone
• Cytochrome
b6/cytochrome f complex
is a proton pump
Figure 21.11 The Z
scheme of
photosynthesis.
Oxygen Evolution Requires the Accumulation of
Four Oxidizing Equivalents in PSII (Fig 21.12)
• When isolated chloroplasts that have been held in
the dark are illuminated with very brief flashes of
light, O2 evolution reaches a peak on the third flash
and every fourth flash thereafter
• The interpretation of this observation: PSII (P680)
cycles through five oxidation states
• 1 e- and 1 proton are removed in each step
• Oxygen (O2) is released in the fifth step and two
more molecules of H2O are bound
• The net of this cycle: 2H2O oxidized to O2 + 4H+
3
7
11
15
19
Figure 21.12 Oxygen evolution requires the accumulation of
four oxidizing equivalents in PSII
Electrons Flow From Pheophytin in the Z
Scheme Via Plastoquinones
Plastoquinone (PQ) exists as a pool
within the chloroplast membrane.
Because of its lipid nature,
plastoquinone is mobile within the
membrane and shuttles electrons
from PSII to the cytochrome b6f
complex.
Plastoquinone A has nine isoprene
units and is the most abundant
plastoquinone in plants and algae.
Note the similarity to Coenzyme Q.
Electrons from PSII Are Transferred to PSI via
the Cytochrome b6f Complex
• The cytochrome b6f complex
– is a large multimeric protein possessing 26
transmembrane α-helices
– This complex is homologous to the cytochrome
bc1 complex of mitochondria
– Mediate the transfer of electrons from PSII to PSI
and to pump protons across the thylakoid
membrane
• Plastocyanin (PC in the Z scheme, Figure
21.11) is a small copper-containing protein
that carries electrons from cytochrome b6f to
PSI
• The copper in PC cycles between the reduced
Cu+ and oxidized Cu2+ states in this transfer
21.4 What Is the Molecular Architecture of
Photosynthetic Reaction Centers?
• Rhodopseudomonas viridis is a photosynthetic
prokaryote with a single photosystem that
resembles PSII
– Lack an OEC and the capacity to oxidize water
• The reaction center (P870) of R. viridis
photosystem is located in the plasma membrane
– Is composed of four peptides: L, M, H and
cytochrome (Figures 21.14)
– L and M each bear two bacteriochlorophyll
molecules and one bacteriopheophytin
– L also has a bound quinone molecule, QA
The R. viridis Photosynthetic
Reaction Center is an
Integral Membrane Protein
Cytochrome subunit
Figure 21.14 The R.
viridis reaction center
(RC). (a) Diagram of
RC showing light
activation and path of etransfer.
Photosynthetic Electron Transfer by the R.
viridis Center Leads to ATP Synthesis
• The prosthetic groups of the R. viridis reaction center
(P870, BChl, Bpheo, and the bound quinones) are fixed
in a spatially to facilitate photosynthetic e- transfer (Fig
21.15)
• Photoexcitation of P870 leads to e- loss via electron
transfer to the nearby bacteriochlorophyll (BChl)
• The e- is then transferred via the L protein to QA (on L)
and then to QB (on M)
• The reduced quinone formed at QB diffuses to a
neighboring cytochrome bc1 membrane complex
• Oxidation of the quinone at bc1 is coupled to proton
transport, and thus eventually to ATP synthesis
Figure 21.15 Photophosphorylation. Photoexcitation of R. viridis
RC leads to reduction of a quinone, Q, to form QH2. Oxidation of
QH2 by the bc1 complex leads to H+ translocation for ATP synthesis.
The Molecular Architecture of PSII Resembles
the R.viridis Reaction Center
• Type II photosystems in higher plants, green algae and
cyanobacteria contain more than 20 subunits but
resemble the R. viridis center in several respects
• T. elongatus PSII is a homodimeric structure, with each
monomer having 23 different protein subunits
– Each “monomer” has at least 34 transmembrane α-helical
segments
– The D1 and D2 subunits of PSII are similar to the L and M
subunits of the R. viridis reaction center
– Electron transfer from the QA site on D2 to the QB site on D1
is analogous to the electron transfer seen in R. viridis
Figure 21.16 The molecular architecture of the T. elongatus PSII
dimer. (a) the arrows show the direction of electron transfer.
How Does PSII Generate O2 From H2O?
• The oxidation of H2O to O2 is chemically difficult
• The oxygen-evolving complex (OEC) is a large globular
protein domain on the lumenal side of the D1 subunit of
PSII (Figure 21.16)
• The active site of the OEC contains a cube-like metal
cluster of four Mn ions, one Ca ion, and five O atoms
bridging the Mn atoms.
• This cluster is held in place by Glu, Asp, and His
residues of D1 and a Glu from CP43, a chlorophyllcontaining inner subunit of PSII (Figure 21.17)
• Electron removal from H2O molecules at the cluster (one
from each Mn) facilitates O2 formation
Figure 21.17 Structure of the PSII oxygenevolving complex (OEC).
The Architecture of PSI Resembles the R. viridis
Center and also PSII Architecture
• PSI from T. elongatus is a cloverleaf-shaped trimer
• Each monomer consists of 12 subunits and 127 cofactors
• All of the prosthetic groups essential to function are
located in just 3 of the subunits: PsaA, PsaB, PsaC
• PsaA and PsaB form the reaction center heterodimer
• PsaC interacts with the stromal face of the PsaA-PsaB
dimer
• The electron-transfer system of PSI consists of three pairs
of chlorophyll molecules that mediate electron transfer to
the quinone acceptor
• The T. elongatus quinone is phylloquinone
Figure 21.18 The architecture of PSI.
(a) Subunit organization.
How Do Green Plants Carry Out
Photosynthesis?
• The structure of a PSI-light-harvesting complex
(LHC) supercomplex has been determined
• LHC1 is a complex of 16 distinct protein subunits and
200 prosthetic groups
• Four LHC1 subunits form an arc around one side of the
PSI reaction center
• A second LHC (LHC2) binds to another side
• The many Chl molecules and other light-harvesting
molecules of the supercomplex form an integrated
network for highly efficient transfer of light energy
into P700
Figure 21.19 View of the plant PSI-LHC1 supercomplex, from the
stromal side of the membrane. Chlorophylls are green, lipids are
red.
21.5 What Is the Quantum Yield of
Photosynthesis?
• The quantum yield of photosynthesis is defined as the
amount of product formed per equivalent of light input
i.e., the amount of O2 evolved per photon absorbed
• Four photons per reaction center: 8 quanta total drive the
evolution of 1 O2, reduction of 2 NADP+, and the
translocation of 14 H+
• Chloroplast ATP synthase has 14 c-subunits, so 3 ATPs
are formed for every 14 H+ translocated
• So 3 ATP should be synthesized from an input of 8
photons of light
21.6 How Does Light Drive the Synthesis
of ATP?
Photophosphorylation
• The transduction of the electrochemical gradient into
the chemical energy of ATP is carried out by the
chloroplast ATP synthase, more properly called the
CF1CF0-ATP synthase
• Electron transfer through the proteins of the Z scheme
drives the generation of a proton gradient across the
thylakoid membrane
• Protons pumped into the lumen of the thylakoids flow
back out, driving the synthesis of ATP
• CF1-CFo ATP synthase is similar to the mitochondrial
ATP synthase
Figure 21.20 Mechanism of photophosphorylation. Photosynthetic
electron transport establishes a proton gradient that is tapped by the
CF1-CF0 ATP synthase to drive ATP synthesis.
c-ring of CF1-CF0-ATP synthase
Figure 21.21 The
c-subunit assembly
of spinach
chloroplast ATP
synthase consists
of 14 identical
subunits.
Cyclic Photophosphorylation Generates ATP
but Not NADPH or O2
• Noncyclic photophosphorylation has been
described in Figure 21.20
• In cyclic photophosphorylation, the photoexcited electron removed from P700 returns to
P700 in a pathway (Figure 21.22)
– Cyclic photophosphorylation depends only on PSI,
not on PSII
– It diverts the activated electron lost from PSI back
through the PQ pool, the cytochrome b6f complex,
and plastocyanin to re-reduce P700+
Figure 21.22 The pathway of cyclic photophosphorylation by PSI.
21.7 How Is Carbon Dioxide Used to
Make Organic Molecules?
Fixation of CO2 is a common (though not universal)
ability in plants, algae, etc.
• Melvin Calvin at Berkeley in 1945 showed that
Chlorella could take up 14CO2 and produce 3phosphoglycerate
• CO2 was combining with a 5-C sugar to form a 6C intermediate
• The 6-C intermediate breaks down to two 3-Pglycerates
• Ribulose-1,5-bisphosphate is the CO2 acceptor in
CO2 fixation
Ribulose-1,5-Bisphosphate Carboxylase
Fixation is accomplished by ribulose bisphosphate
carboxylase (oxygenase), rubisco
• Probably the world's most abundant protein
• Rubisco exists in 3 forms
– E: inactivative
– EC: carbamylated inactivative(CO2 added to Lys201)
– ECM: carbamylated and has Mg2+; active
• RuBP (substrate!) is inhibitor and must be
released from inactive rubisco by rubisco
activase. Carbamylation and Mg2+ then activate.
E-RuBP activase E
EC
ECM
Ribulose-1,5-Bisphosphate
2-Carboxy-3keto-arabinitol-1,5-Bisphosphate
3-phosphoglycerate
The Calvin-Benson Cycle
• The set of reactions that transform 3-P-glycerate
into hexose sugar
• Serve three metabolic purpose
1. The principal CO2 fixation pathway in nature
2. Accomplish the reduction of 3-phosphoglycaerate
to glyceraldehyde-3-P, so carbohydrate synthesis
becomes feasible
3. Transform 3-C compounds into 4-, 5, 6, and 7-C
compounds
• A large part of this pathway is really a disguised
gluconeogenesis pathway!
• With some pentose phosphate pathway reactions
thrown in (see Chapter 22 for gluconeogenesis
and the pentose-P pathway).
CO2 Fixation into Carbohydrate Proceeds Via the
Calvin-Benson Cycle
Figure 21.25
The CalvinBenson cycle
CO2 Fixation into Carbohydrate Proceeds Via
the Calvin-Benson Cycle
The Carbon Dioxide Fixation Pathway Is
Indirectly Activated by Light
• The activities of key Calvin cycle enzymes are
coordinated with the output of photosynthesis
• In effect, these enzymes respond indirectly to
light activation
1. Light induces pH changes in
chloroplast compartments
2. Light energy generates
reducing power
3. Light induces movement of
Mg2+ ions from the thylakoid
vesicles into the stroma
The Carbon Dioxide Fixation Pathway Is
Indirectly Activated by Light
1. Light induces pH changes in chloroplast
compartments
– Rubisco, rubisco activase, and several Calvin
cycle enzymes are more active at alkaline pH
2. Light energy generates reducing power
– Reduced ferredoxin and NADPH
3. Light induces movement of Mg2+ ions from
the thylakoid vesicles into the stroma
Figure 21.27 Light-induced pH changes in chloroplast
compartments. These pH changes modulate the activity of key
Calvin cycle enzymes.
Figure 21.28 The pathway for light-induced reduction of Calvin
cycle enzymes. Light-generated reducing power (Fdred =
reduced ferredoxin) provides e- for reduction of thioredoxin (T)
by FTR.
21.8 How Does Photorespiration Limit
CO2 Fixation?
Ribulose bisphosphate carboxylase/oxygenase
catalyzes an alternative reaction in which O2
replaces CO2 as the substrate added to RuBP
• This oxygenase reaction diminishes plant productivity
because it leads to loss of RuBP (the CO2 acceptor)
• The products of the oxygenase reaction are 3phosphoglycerate and phosphoglycolate
• Dephosphorylation converts phosphoglycolate to
glyoxylate
• Transamination yields glycine
Figure 21.29 The
oxygenase reaction of
rubisco. (a) The reaction
of ribulose bisphosphate
carboxylase with O2 and
ribulose bisphosphate
yields 3-phosphoglycerate
and phosphoglycolate. (b)
Conversion of two
phosphoglycolates to
serine + CO2.
The C-4 Pathway for CO2 Fixation
The Hatch-Slack Pathway
• Not an alternative to Calvin cycle, nor even a net
CO2 fixation pathway
• It is a CO2 delivery system
– Carries CO2 from the O2-rich leaf surface to interior
cells where O2 won't compete in the rubisco reaction
– Oxaloacetate and malate are the CO2 transporters
• Read about Crassulacean acid metabolism, an
alternative to the C4 pathway used by succulent
plants
• Tropical Grasses Use the Hatch-Slack Pathway
to Capture Carbon in CO2 Fixation
Figure 21.30 Essential
features of the
compartmentation and
biochemistry of the
Hatch-Slack pathway of
carbon dioxide uptake
in C4 plants.