THE LIGHT
DEPENDENT
REACTION
OXIDATION AND REDUCTION


Oxidation Is a Loss of electrons (OIL)
Reduction Is a Gain of electrons (RIG)
© 2010 Paul Billiet ODWS
Natural Electron ACCEPTORS
Nicotinamide Adenine Dinucleotide Phosphate
(NADP) used in photosynthesis in chloroplasts
NADP+
+
2H+
+
2e-
Reduction
NADPH + H+
Oxidation
Ferredoxin the most difficult to reduce (and most easily
oxidised)
Cytochromes Conjugate proteins which contain a haem
group.
The iron atom undergoes redox reactions
Reduction
Fe3+ + e-
Fe2+
Oxidation
NB The iron atom in the haem group of haemoglobin does not go
through a redox reaction
Haemoglobin is oxygenated or deoxygenated
© 2010 Paul Billiet ODWS
CLASSIFYING ORGANISMS ACCORDING TO THEIR
CARBON SOURCE AND ENERGY SUPPLIES
Type of Organism
Carbon
Source
Photolithotrophs
CO2
Photoorganotrophs
Organic
compounds
Chemolithotrophs
CO2
Energy
Source
Light
Light
Examples
Green plants,
Inorganic compounds photosynthetic protoctists,
blue-greens,
(H2O, H2S, S)
photosynthetic bacteria
Organic compounds
Non-sulphur purple
bacteria
Redox
Inorganic compounds Hydrogen, sulphur, iron
reactions (H2, S, H2S, Fe2+, NH3) and denitrif ying bacteria
Organic
Chemoorganotrophs compounds Redox
reactions
© 2010 Paul Billiet ODWS
Electron Donors
Organic compounds
(e.g. Glucose)
Animals, fungi, nonphotosynthetic protoctists,
saprophytic and parasitic
bacteria
The origin of oxygen in photosynthesis
CO2 or H2O?
Van Neil 1932 Comparing the biochemistry
of autotrophs
 Photosynthetic sulphur bacteria use H2S as
their source of hydrogen
CO2 + 2H2S 
(CH2O) + H2O + 2S
 This suggested that in green plants the
oxygen originates from the water molecule
CO2 + 2H2O

(CH2O) + O2 + H2O
Ruben 1941 Confirmed this hypothesis using
the heavy isotope 18O and mass spectrometry
© 2010 Paul Billiet ODWS
Using chloroplasts in vitro
Hill 1937 Studying redox reactions in
photosynthesis using artificial electron acceptors
CO2 absent
LIGHT
Reduced
electron acceptor
Oxidised electron
acceptor
H2O
CHLOROPLAST
O2
produced
No (CH2O) produced
© 2010 Paul Billiet ODWS
Using chloroplasts in vitro
Oxidised electron
acceptor
H2O
© 2010 Paul Billiet ODWS
DARK
CHLOROPLAST
No reduction
of electron
acceptor
The Hill reaction using natural electron
acceptors Arnon 1954
CO2 absent
LIGHT
ADP +Pi
NADP
H2O
© 2010 Paul Billiet ODWS
No (CH2O) produced
ATP
NADPH + H+
CHLOROPLAST
O2
produced
Then ……
DARK
ATP
NADPH+H+
Add CO2
ADP + Pi
NADP
CHLOROPLAST
(CH2O)
produced
Arnon had effectively separated the light dependent
reaction, which produces ATP, NADPH + H+ and oxygen,
from the light independent reaction, which produces
sugars
© 2010 Paul Billiet ODWS
CHLOROPHYLL AND PHOTOSYNTHESIS
Pigments in the leaves of green plants and algae
PIGMENT
COLOUR
ABSORPTION PEAK / nm
Chlorophyll a
Blue-green
430 and 660
Chlorophyll b
Yellow-Green
455 and 640
Phycocyanins
Blue-Grey
560 to 660
Phycoerythrins
Red
550 to 570
Carotenoids
YellowOrange
430 to 570
© 2010 Paul Billiet ODWS
Pigments underwater
Light received from
the sun
Space
200 to 4000 nm
Atmosphere
Light used by green
plants
Ground
300 to 1000 nm
Photosynthesis
400 to 700 nm
Underwater blue light penetrates the
deepest as it has most energy. Green
light next finally red light penetrates least.
The distribution of algae with different
photosynthetic pigments is related to this.
© 2010 Paul Billiet ODWS
Green
algae
Brown
algae
Red
algae
The fluorescence of chlorophyll


Pure chlorophyll +
light  Red
fluorescence
Chlorophyll in
chloroplasts + light
 Splits water,
synthesises ATP and
NADPH + H+
© 2010 Paul Billiet ODWS
Fluorescence
The excitement of an electron to a high
energy level by the action of light energy
 Followed by the release of that energy as
light again as the electron falls back to its
former low energy level

© 2010 Paul Billiet ODWS
Chlorophylls
Absorption spectra of the
main photosynthetic
pigments
Chlorophyll a
molecule
OXIDATION AND REDUCTION
Something must be happening in the chloroplast
to capture these electrons and use their energy
 Free electrons can lead to OXIDATION AND
REDUCTION reactions
Remember
 Oxidation Is a Loss of electrons (OIL)
 Reduction Is a Gain of electrons (RIG)

© 2010 Paul Billiet ODWS
Oxidation & reduction in
photosynthesis




When compounds are oxidised energy is
released
If this release of energy is COUPLED to
biological reactions then WORK can be done
Similarly when compounds are reduced energy
has to be put into the system
In photosynthesis the source of electrons for
reducing CO2  CH2O is water and the source
of energy is light
© 2010 Paul Billiet ODWS
The chloroplast
Grana
Thylakoid
membrane
Frets
outer
membrane
Chloroplast
envelope
inner
membrane
Stroma
© 2010 Paul Billiet ODWS
Starch grains
X 33 300 Open University S Hurry (1965) Murray
X 22 000 Open University S Hurry (1965) Murray
X 80 000 Open University S Hurry (1965) Murray
CHLOROPHYLL IN THE CHLOROPLAST





Pigment molecules are located on the thylakoid
membranes
The pigment molecules are arranged in an antenna
complex
Light strikes the antenna complex and it is channelled
towards the reaction centre
The electrons are excited by the light energy in the
reaction centre
The electrons are picked up by electron acceptors
(1 photon of light = 1 electron released)
© 2010 Paul Billiet ODWS
Photolysis
The electrons that are lost are replaced by
splitting water
2H2O  4H+ + 4e- + O2
So 1 molecule of oxygen released
requires 4 photons of light
© 2010 Paul Billiet ODWS
The photosystems
Two types of pigment systems have been
found
 PHOTOSYSTEM I Mainly chlorophyll a
 PHOTOSYSTEM II Chlorophyll b, some
chlorophyll a plus other pigments
© 2010 Paul Billiet ODWS
The photosystems
These photosystems bring about three reactions:
 Photolysis of water to provide electrons (e-)
and protons (H+)
 Photophosphorylation to produce ATP from
coupled redox reactions in an electron transport
chain
 Reduction of NADP to NADPH + H+ (NADP is
therefore the final electron acceptor)
© 2010 Paul Billiet ODWS
More +ve
eCyclic
photophosphorylation
Ferredoxin
e-
NADPH
reductase
REDOX POTENTIAL
Plastoquinone
NADP
Cytochrome b6 – f
complex
ADP
ATP
More -ve
PHOTOSYSTEM II
H 2O
Non-cyclic
photophosphorylation
Plastocyanin
PHOTOSYSTEM I
O2 + 4H+
REACTION PATHWAY
© 2010 Paul Billiet ODWS
NADPH
+ H+