Chapter 21 Photosynthesis
chloro1. Green
2. Chlorine (Cl)
-plast
living substance,
organelle, cell
Figure 21.1 Electron micrograph of a chloroplast.
21.1
Outline
• What are the general properties of photosynthesis?
• How is solar energy captured by chlorophyll?
• What kinds of photosystems are used to capture light
energy?
• What is the molecular architecture of photosynthetic
reaction centers?
• What is the quantum yield of photosynthesis?
• How does light drive the synthesis of ATP?
• How is carbon dioxide used to make organic molecules?
• How does photorespiration limit CO2 fixation?
21.2
The Sun - Ultimate Energy
1.5 x 1022 kJ of sunlight energy falls on the earth each day
• 1% is absorbed by photosynthetic organisms and
transformed into chemical energy
• 6CO2 + 6H2O  C6H12O6 + 6O2
• 1011 tons of CO2 are fixed globally per year
• Formation of sugar from CO2 and water requires energy
• Sunlight is the energy source!
Figure 17.1 The flow of energy in
the biosphere is coupled to the
carbon and oxygen cycles.
21.3
21.1 What Are the General Properties of
Photosynthesis?
['θailәˏkɔid]
葉綠層; 層狀體; 類囊膜
Grana
葉綠餅
基質
['grænәm]
[lә'melә]
薄層、片板
Figure 21.2 Schematic diagram of an idealized chloroplast.
Chloroplasts possess DNA, RNA and ribosomes.
21.4
Photosynthesis Consists of Both Light
Reactions and Dark Reactions
thylakoid membranes
reducing potential
(NADPH) and ATP
stroma
The Calvin Cycle
Hexose sugar
Figure 21.4 The light-dependent and light-independent
reactions of photosynthesis.
21.5
Water is the Ultimate e- Donor for
Photosynthetic NADP+ Reduction
The reaction sequence for photosynthesis in green plants:
2H2O + 2NADP+ + xADP + xPi →
O2 + 2NADPH + 2H+ + xATP + xH2O
A More Generalized Equation for Photosynthesis:
CO2 + 2H2A
→
Hydrogen Hydrogen
acceptor donor
(CH2O) + 2A
+
Reduced Oxidized
acceptor donor
H2O
In photosynthetic bacteria,H2A can be H2S or other oxidizable substrates,
like isopropanol:
Photosynthesis provides the oxygen on which we depend
21.6
21.2 How Is Solar Energy Captured by
Chlorophyll?
Figure 21.5 Structures (a)
and absorption spectra (b)
of chlorophyll a and b. The
phytyl side chain of ring IV
provides a hydrophobic tail
to anchor the chlorophyll in
membrane protein
complexes.
['klorә,fiI] 葉綠素
Since the absorption
spectra for a and b differ,
plants that possess both
can harvest a wider
spectrum of incident
energy.
21.7
Accessory Light-harvesting Pigments
Increase the Possibility for Absorption of
Light
藻藍膽素
[ˏfaikәu'saiәnɔbilin]
Figure 21.6 Structures of
representative accessory
light-harvesting pigments in
photosynthetic cells. (a) βCarotene, an accessory lightharvesting pigment in leaves.
(b) Phycocyanobilin, a blue
pigment found in
cyanobacteria.
藍綠色的
21.8
The Light Energy Absorbed by
Photosynthetic Pigments Has Several
Possible Fates
Figure 21.7 Possible
fates of the quantum of
light energy absorbed
by photosynthetic
pigments.
Loss as heat
Loss as light
changes the
reduction
potential of
the pigment
21.9
The Z scheme of photosynthesis
Photosystem II
splits water,
producing O2,
use quinones as
terminal
electron
acceptors.
Cyt b6f
complex
is a
proton
pump
plastoquinone
['plæ stәukwinәun]
質體醌
Photosystem I provides
reducing power, use photophosphorylation
ferredoxins (Fd) as
terminal electron
acceptors.
plastocyanin
Fd: ferredoxins
[ˏplæstә'saiәnin]
鐵氧化還原蛋白
質體藍素
Fp: flavoprotein,
copper-containing
ferredoxin-NADP+ reductase
protein
Figure 21.11 (b) The functional relationships among PSI, PSII, the cyt bf
complex, and the ATP synthase in the thylakoid membrane.
21.10
The Z Scheme of Photosynthesis
Ao: chlorophyll a
A1: PSI quinone
Figure 21.11 (a) The electron carriers are arranged according to their
standard reduction potentials
21.11
Photosynthesis on Photosystems I (PSI) and
PSII
PSII splits water, producing O2,
use quinones as terminal
electron acceptors.
醌類
PSI provides reducing power,
use ferredoxins as terminal
electron acceptors. 鐵氧化還原蛋白
electrons flow from H2O to NADP+
Figure 21.10 Roles of the two photosystems, PSI and PSII.
21.12
Electrons Flow From Pheophytin in the Z
Scheme Via Plastoquinones
Figure 21.13 The structures of
plastoquinone A and its reduced
form, plastohydroquinone (or
plastoquinol). Plastoquinone A
has nine isoprene units and is
the most abundant
plastoquinone in plants and
algae.
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.
21.13
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
• Cytochrome b6f complex is homologous to the
cytochrome bc1 complex of mitochondria
• The purpose of this complex is to mediate the transfer of
electrons from PSII to PSI and to pump protons across the
thylakoid membrane
• Plastocyanin (PC)
• 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
21.14
21.4 What Is the Molecular
Architecture of Photosynthetic
Reaction Centers?
玫瑰紅色
Figure 21.14 The
Rhodopseudomonas
viridis reaction center
(RC) is an integral
membrane protein.
(a) Diagram of RC
showing light
activation and path of
e- transfer.
Pheo: bacteriopheophytin
QA: ubiquione A
21.15
Photosynthetic Electron Transfer by the
R. viridis Center Leads 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.
21.16
How Does PSII Generate O2 From H2O?
Figure 21.17 Structure of
the PSII oxygen-evolving
complex (OEC). Four Mn
atoms (red, A-D) and a Ca
atom (green) form the
water-splitting metal
cluster of the OEC.
Bridging O atoms are
purple.
21.17
The Architecture of PSI Resembles the R.
viridis Center and also PSII Architecture
Figure 21.18 The
architecture of PSI. (a)
Subunit organization. (b) A
model for PSI. PsaA is
orange; PgaB is magenta;
PsaC is yellow; iron-sulfur
clusters are red.
21.18
The Architecture of PSI Resembles the R.
viridis Center and also PSII Architecture
• PSI from S. 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 S. elongatus quinone is phylloquinone
[ˏfilokwә`non]
葉綠醌, Vitamin K1, 凝血維生素一
21.19
How Do Green Plants Carry Out
Photosynthesis?
• Light-harvesting complex 1 (LHC1) is a supercomplex 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 PSILHC1 supercomplex, from the stromal
side of the membrane.
Chlorophylls are green, lipids are red.
21.20
21.6 How Does Light Drive the Synthesis of
ATP?
• The transduction of the electrochemical gradient into the
chemical energy of ATP is carried out by the chloroplast
CF1CF0-ATP synthase
• 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
21.21
Cyclic Photophosphorylation Generates
ATP but Not NADPH or O2
• 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 rereduce P700+
Figure 21.21 The
pathway of cyclic
photophosphorylation
by PSI.
21.22
Photosynthetic Electron Transport and
ATP Synthesis
Biology / Medicine Animations HD
https://www.youtube.com/watch?v=LtecIPc30nM
21.23
Dark Reactions of Photosynthesis
thylakoid membranes
reducing potential
(NADPH) and ATP
stroma
The Calvin Cycle
Hexose sugar
Figure 21.4 The light-dependent and light-independent
reactions of photosynthesis.
21.24
21.7 How Is Carbon Dioxide Used to Make
Organic Molecules?
Fixation of CO2 is a unique ability of plants, algae,
etc.
• Melvin Calvin at Berkeley in 1945 showed that
Chlorella could take up 14CO2 and produce 3phosphoglycerate
• What was actually happening was that CO2 was
combining with a 5-C sugar to form a 6-C
intermediate
• This breaks down to two 3-P-glycerates
• Ribulose-1,5-bisphosphate is the CO2 acceptor in
CO2 Fixation
21.25
Ribulose-1,5-Bisphosphate is the CO2
Acceptor in CO2 Fixation
RuBP carboxylase
二磷酸核酮糖羧化酶
(Rubisco)
Ribulose-1,5Bisphosphate
(RuBP)
Figure 21.23 The ribulose bisphosphate carboxylase reaction.
Mg2+ at the active site aids in stabilizing the 2,3-enediol
transition state (I) for CO2 addition and in facilitating the
carbon-carbon bond cleavage that leads to product formation.
21.26
Ribulose-1,5-Bisphosphate is the CO2
Acceptor in CO2 Fixation
3-P-glycerates
Figure 21.23 The ribulose bisphosphate carboxylase reaction.
Mg2+ at the active site aids in stabilizing the 2,3-enediol
transition state (I) for CO2 addition and in facilitating the
carbon-carbon bond cleavage that leads to product formation.
21.27
The Calvin-Benson Cycle (The Calvin Cycle )
• The set of reactions that transform 3-P-glycerate
into hexose sugar
• The principal CO2 fixation pathway in nature
• A large part of this pathway is really a disguised
gluconeogenesis pathway!
• With some pentose phosphate pathway reactions
thrown in
21.28
CO2 Fixation into Carbohydrate Proceeds Via
the Calvin-Benson Cycle
Net: 6 CO2 + 18 ATP + 12 NADPH + 12 H+ + 12 H2O -->
glucose + 18 ADP + 18 Pi + 12 NADP+
21.29
CO2 Fixation into Carbohydrate
Proceeds Via the Calvin-Benson Cycle
(Rubisco)
21.30
CO2 Fixation into Carbohydrate
Proceeds Via the Calvin-Benson Cycle
21.31
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
• Light induces pH changes in chloroplast compartments
• Rubisco, rubisco activase, and several Calvin cycle
enzymes are more active at alkaline pH
• Light energy generates reducing power
• Reduced ferredoxin and NADPH
• Light induces movement of Mg2+ ions from the thylakoid
vesicles into the stroma
21.32
The Carbon Dioxide Fixation Pathway Is
Indirectly Activated by Light
Figure 21.25 Light regulation of CO2
fixation prevents a substrate cycle
between cellular respiration and
hexose synthesis by CO2 fixation.
21.33
The Carbon Dioxide Fixation Pathway Is
Indirectly Activated by Light
Figure 21.26 Light-induced pH changes in chloroplast
compartments. These pH changes modulate the activity of key
Calvin cycle enzymes.
21.34
21.9 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 3phosphoglycolate and phosphoglycolate
• Dephosphorylation and oxidation convert phosphoglycolate to
glyoxylate
• Transamination yields glycine
21.35
Photosynthesis (Dark Reactions, Calvin Cycles)
Biology / Medicine Animations HD
https://www.youtube.com/watch?v=joZ1EsA5_NY
21.36
The Hatch-Slack Pathway:
The C-4 Pathway for CO2 Fixation
葉肉細胞
束鞘細胞
Figure 21.29 Essential
features of the
compartmentation and
biochemistry of the
Hatch-Slack pathway of
carbon dioxide uptake in
C4 plants.
21.37