Module 0210101:
Molecular Biology and Biochemistry of the Cell
Lecture 16
Transduction of Light Energy in
Chloroplasts
Dale Sanders
9 March 2009
Objectives
By the end of the lecture you should understand…
1. How we can express the free energy stored in light;
2. What happens when chlorophyll absorbs a photon, and
how the free energy so gained can be lost;
3. How light harvesting chlorophylls funnel energy to
reaction centres;
4. That electrons are fed into the chloroplast redox chain via
a light-induced decrease in redox potential of the reaction
centre chlorophylls;
5. That the chloroplast redox chain comprises two
photosystems, and other redox components, arranged in
series to oxidize water and reduce NADP+
Reading
As in my previous lectures, all the topics today are well
covered by the standard big biochemistry textbooks.
One such text is
Voet & Voet (2004) Biochemistry (3rd Ed.) Chapter
24 and especially pp. 871-879
Also useful for a more in-depth treatment is
Nicholls, DG & Ferguson, SJ (2002) Bioenergetics 3
Chapter 6.
Buchanan BB et al. (2000) Biochemistry and
Molecular Biology of Plants. Chapter 12,
pp. 568-590.
Energetics of Photosynthesis
The basic equation (Van Niel, 1930’s)
CO2 + 2H2A light (CH2O) + H2O + 2A
“A” can be S, but is usually O
The relevant reactions (in higher plants) all located in
chloroplast. Equation 1 can be split into 4 separate
categories:
(i)
Absorption of light:
this lecture (16)
(ii) e- transport:
next lecture (17)
(iii) photophosphorylation:
lecture (13)
(iv) CO2 fixation:
lecture 18
Classes (i) – (iii) all occur at
THYLAKOID membrane
What is Light?
Electromagnetic radiation
10-14
Cosmic
rays
10-12
10-10
 rays
Wavelength (m)
10-8
UltraX-rays violet
10-6
10-4
Infra-red
10-2
Radio
waves
Visible
380-750 nm
The wavelength (λ) of light specifies the colour:
 / nm
400
500
Blue
600
Green
700
Red
1
102
Wavelength properties of light are exemplified by
interference filters.
But, at the molecular level, light has particulate
characteristics.
(Newton’s Corpuscular Theory; Einstein’s Quantum Theory)
Photon: a particle carrying a quantum of light
What is the Energy of a Photon?
Energy of a photon is inversely proportional to its
wavelength:
E = hc
(2)
λ
h = Planck’s constant = 6.63 x 10-34 J.s
c = Speed of light = 3.00 x 108 m.s-1
e.g. for light of λ = 600 nm
E = 3.32 x 10-19 J
Comparison of Light Energy with other
forms of Free Energy
Like any other molecular particle, we can speak of an
Avagadro’s Number (N) or “mole” of photons
1 “mole” of photons = 6.02 x 1023 photons
= 1 Einstein
i.e. Energy/mol photons, from Eq 2
E = Nhc
λ
e.g. light of λ = 600 nm,
E = 3.32 x 10-19 x 6.02 x 1023
 200 kJ/Einstein
(3)
kJ. Einstein-1
E
Just as units of chemical potential energy (J/mol) can be
converted to units of redox potential energy (V) by ÷ F,
light energy can be expressed in electrical units:
So, for light of λ = 600 nm,
E = 200 x 103 = 2.07 V
96500
Inverse
relationship of
300
energy and
wavelength
200
100
0
400
500
600
700
 / nm
Pigments, Absorption Spectra and Excited
States
The primary photosynthetic pigment in higher plants:
chlorophyll a (chl a)
A very hydrophobic molecule
localized exclusively
within the membrane.
Chlorophyll absorbs blue and red light
Absorption spectrum of chl a in acetone:
Interpretation

Absorption of “blue” photon pushes e- to 2nd excited
state
Only sufficient energy in “red” photon to get to1st state
[e- concerned is delocalised over tetrapyrrole ring by
alternating single and double bands.]
Transitions 2nd 1st excited state: energy lost as heat
1st ex state
ground state: energy can
be lost through emission of photon
of longer wavelength = fluorescence.
“Light Harvesting” is Accomplished
through a 3rd form of Energy Loss:
Resonance Energy Transfer
•Neighbouring chl responds to electric field of excited
chl (*chl): e- of *chl excites a lower energy e- in
neighbouring chl, thereby losing its own energy.
•Time: 10-12 s (1 ps).
•There is a SMALL energy loss in this process (i.e.
not quite 100% efficient):
Thus, in chl aggregates, excitation tends to pass from
species absorbing at shorter wavelengths to those
absorbing at longer wavelengths of light.
The Biological Importance of
Resonance Energy Transfer
In practice chl in thylakoid membranes is associated with
hydrophobic proteins. Micro-environments offered by the
proteins serve to broaden the absorption spectrum.
Energy is “funnelled” to chls absorbing lower energy
photons
This process is known as light-harvesting
Light-Harvesting (Antenna) Chlorophylls
A broad spectrum of wavelengths of visible light is
capable of funnelling energy to chl absorbing at long
wavelengths
Antenna and Reaction Centre Chlorophylls
The bulk of chlorophyll is in the light-harvesting complexes
e.g. 1 protein (LHC IIB):
binds: 8 molecules chlorophyll a
7 molecules chlorophyll b
Each reaction centre is associated with  300 antenna
chlorophyll molecules
Each reaction centre has its own special chlorophyll molecules
for participation in redox reactions
Chloroplasts contain 2 reaction centres:
Photosystem I (PSI) & Photosystem II (PSII)
Properties:
1. Absorption properties of the 2 photosystems are different
PS I: absorbs at 700 nm: Pigment is P700
PSII: absorbs at 680 nm: Pigment is P680
2. The 2 reaction centres are spatially-separated:
PS I: Stromal lamellae
PS II: Granal lamellae
Differential Location of the Two Photosystems
The 4th way in which Energy Stored in
Excited Chlorophyll (*Chl) can be lost
(but only in a reaction centre):
A redox reaction: the excited e- is lost from the
*chl to some other molecule
The redox reaction is facilitated by a change in E'o
of chl in the excited state:
Redox couple
P680/P680+
*P680/*P680+
E'o (mV)
+1000 highly oxidizing
- 650
ΔE'o = 1650
[Energy of 680 nm photon:
P700/P700+
*P700/*P700+
1820]
91% efficiency
+420
-1100 highly reducing
ΔE'o = 1550
[Energy of 700 nm photon:
1770]
88% efficiency
In both cases, there is a dramatic increase in
propensity to donate e- (negative shift in E'o)
*P700
-1000
e–
*P680
E'o (mV) 0
+1000
Acceptor
Acceptor
Light
Light
P680
e–
P700
PS II and PS I Act in Series to Catalyse
Electron Flow Between H2O and NADP+
NADP+ + H+
H2O
PS II
2e–
2e–
Redox
PS I
components
Redox
components
½O2
+2H+
NADPH
Light +
Resonance Energy
Transfer
Light +
Resonance Energy
Transfer
Evidence for series arrangement
of PS II and PS I:
Long λ light (700 nm: excites PS I):
OXIDATION of redox components between PS II and PS I
Shorter λ light ( ≤ 680 nm: excites mainly PS II):
REDUCTION of redox components between PS II and PS I
The Initial Redox Reactions in PS II and PS I
are Transmembrane Events
Summary
1.
2.
3.
Energy content of light can be expressed in kJ/Einstein, and is
inversely proportional to wavelength.
Energy absorbed by chl in leaf can be “funnelled” to longer
wavelength absorbing forms – light harvesting: Leads to
reaction centre excitation.
Higher plants have 2 photosytems:
(a) spatially distinct
(b) absorb at different wavelengths
4.
Excitation of reaction centre
change in E'o
5.
This change in E'o powers the thermodynamically unfavourable
reduction of NADP+ by H2O
6.
Initial redox event at RC chl involves charge separation across
thylakoid membrane (estromal side)