understanding
photosynthesis
the most important process on the planet
John Gray
Department of Plant Sciences
University of Cambridge
Life on earth depends on plants for photosynthetic
CO2 fixation and O2 evolution
Photosynthesis
• a highly efficient energy transduction process
• conversion of light energy into chemical energy
light
6CO2 + 6H2O

6CO2 + 6H2O
 C6H12O6 + 6O2
C6H12O6 + 6O2
energy
respiration
Cross-section of a leaf
100 mm
Mesophyll cells
Thylakoid membrane
• chlorophyll
• light-harvesting
• electron transfer
• O2 evolution
• energy production
Stroma
• Rubisco
• CO2 fixation
• sugar and starch
synthesis
Pea
chloroplast
1 mm
Schematic chloroplast
membrane-enclosed stroma
sealed thylakoid membrane
Photosynthetic processes in the thylakoid membrane
The Light Reactions
Structures of thylakoid membrane complexes
Light absorption by
chlorophylls
• All chlorophyll is associated
with proteins to form
light-harvesting complexes
in the thylakoid membrane
•
There is no free chlorophyll
Structure of LHCII trimer
Kühlbrandt et al. (1994)
Liu et al. (2004)
LHCII trimers in grana stack
Energy transfer in light-harvesting complexes
Light is absorbed by
individual chlorophylls in
the light-harvesting
complexes
Energy is transferred from
one pigment to another via
Resonance Energy Transfer
This transfer funnels the
energy to a reaction centre
where electron transfer
starts
Low resolution structures of photosystem II
• electron microscopy
• membrane preparations
• single particles - negative stain
arrangement in thylakoid membrane
Photosystem II
- at 3.5Å resolution
D1 and D2 polypeptides - the core of PSII
•
•
5 transmembrane spans
similar to purple bacterial reaction centre
D1 is the product of the chloroplast psbA gene
Prosthetic groups of PSII core
OXYGEN EVOLUTION
2H2O  O2 + 4H+ + 4e
by analogy to sulphur bacteria (van Niel 1930)
H2S  S + 2H+ + 2e
1970 Joliot and Kok - measured O2 yield from saturating light flashes
O2 evolution every 4th flash - system for accumulating 4 positive charges
Structure of the
manganese cluster
'Dangler' model
cubane Mn3CaO4 cluster
+ fourth Mn linked via O
Photosynthetic electron transfer
ATP synthesis coupled to electron transfer
Structure of ATP synthase
,  and  subunits
side view
cross section
Mechanism of ATP synthesis
NOBEL PRIZE 1997:
•
•
Paul Boyer (UCLA)
Rotational catalysis
John Walker (Cambridge)
X-ray structure showing
3 different conformations
for 3  subunit dimers
Rotary catalysis by ATP synthase
Models of H+ translocation
proton translocation through a subunit
drives rotation of c subunit ring and
 subunit
b subunits (b and b' in CFo) act as
stator to prevent rotation of 
subunits
Light reactions of photosynthesis
• Light absorption by chlorophylls in light-harvesting
complexes
• Electron transfer initiated at reaction centres in
photosystem II and photosystem I
• Electron transfer from H2O to NADP+
generating O2 and reducing power
• Coupled H+ liberation in thylakoid lumen provides
driving force for ATP synthesis
The dark reactions: capturing CO2
• Light reactions generate ATP and NADPH
• Provide energy for fixing CO2
1000
10
Rubisco appears
800
8
600
CO2 fixation had a
6
massive impact on
4
global
climate
CO2
400
O2
200
2
0
4
3
2
1
0 0.6
Time before present (billion years)
0.4
0.2
0
The dark reactions: capturing CO2
The numbers are
HUGE
• Atmospheric CO2 is 0.035% (and rising!)
• Total CO2 in atmosphere 700 x 109 tonnes
• Photosynthesis fixes ~100 x 109 tonnes per year
• ~15% of total atmospheric CO2 moves into
photosynthetic organisms each year!
Rubisco
• Ribulose 1,5-bisphosphate (RuBP) carboxylase-oxygenase
• catalyses CO2 fixation into C3 compounds
• is the most abundant protein on the planet
QuickTime™ and a
Sorenson Video decompressor
are needed to see this picture.
Active site
Rubisco is made from 8 small and 8 large subunits
Rubisco reaction
CH2OP
CO2 C=O
H-C-OH
H-C-OH
CH2OP
H2O
RuBP
C5
sugar
CH2OP
H-C-OH
COOH
+
COOH
H-C-OH
CH2OP
3-PGA
2 x C3
acid
CH2OP
H-C-OH
NADPH
CHO
ATP
CH2OP
C=O
CH2OH
2 x C3 sugars
6C5
6 ATP
6CO2
sucrose
6C5
12C3
6 ATP
6 NADPH
6 cycles
export
from
chloroplast
12C3
Regeneration
via C4 C5 C6 & C7
sugar phosphates
10C3
C6
2C3
C6
starch
Photosynthesis
• Light-driven electron transfer from H2O to NADP+
generating O2 and reducing power
• Coupled H+ translocation into thylakoid lumen used
to generate ATP
• CO2 fixation into sugars using energy from ATP
and NADPH
• Requires chloroplasts with intact thylakoid membranes
Plant cell stained with DAPI (a DNA fluorochrome)
Chloroplast DNA
• Each chloroplast contains up to 100 copies of chloroplast DNA
• Leaf mesophyll cells contains ~100 chloroplasts
• Leaf mesophyll cells contains ~10000 copies of chloroplast DNA
1 mm
Genes in land plant chloroplast DNA
Rubisco LS
Photosystem II
Cytochrome bf
Photosystem I
ATP synthase
NADH dehydrogenase
rbcL
psb
pet
psa
atp
ndh
Ribosomal RNA
Transfer RNA
Ribosomal proteins
RNA polymerase
Translation initiation factor
rrn
trn
rpl or rps
rpo
infA
Acetyl CoA carboxylase
ATP-dependent protease
Unknown
accD
clpP
ycf
1
13
5
6
6
13
4 (x 2)
~32
19
4
1
1
1
3
44
64
Assembly of photosynthesis complexes
chloroplast
gene product
nuclear
gene product
• All complexes contain at least one nuclear-encoded subunit
• Requires coordination of plastid and nuclear gene expression
nucleus
Coordination of nuclear and
chloroplast gene expression
Nuclear gene products
structural & regulatory
proteins
Plastid signals
Expression of nuclear genes for
chloroplast proteins is regulated
by plastid signals reporting the
functional state of the chloroplasts
chloroplast
STROMULES (stroma-filled tubules)
STROMULES
stroma-filled tubules
interconnecting plastids