The light reactions of photosynthesis
Objective of the lecture:
1. To describe the structure of function of chloroplasts.
2. To define the light reactions of photosynthesis.
Text book pages:
198-212.
Photosynthesis Chapter 10 of text book
Plants use sunlight, carbon dioxide, and water to produce carbohydrate
with oxygen as a byproduct.
The overall chemical reaction summarizes the process as:
CO2 + 2 H2O + light energy  (CH2O)n + H2O + O2
where (CH2O)n stands for carbohydrate.
Usually, glucose (C6H12O6) is considered as the carbohydrate made so:
6 CO2 + 12 H2O + light energy  C6H12O6 + 6 H2O + 6 O2
... this may keep the chemists happy
... but a better summary is of how the process occurs is:
Light
energy
Sunlight
Light-dependent
reactions
H2O
O2
Thylakoid Reactions
Light reactions
Calvin cycle
Chemical
energy
ATP, NADPH
Chemical
energy
CO2
(CH2O)n
Stroma Reactions
Dark reactions
Plant structure, particularly cell structure
(1) makes the reactions possible,
(2) enables integration of light and dark reactions.
Leaves contain millions of chloroplasts.
Cell
Chloroplasts
Fig. 10.2
Chloroplasts are highly structured, membrane-rich organelles.
Outer membrane
membrane
Outer
Inner membrane
Inner
membrane
Thylakoids
Thylakoids
Granum
Granum
Stroma
Stroma
Recall that membranes are
composed of a lipid bilayer in
which are embeded proteins
that enable exchange of
materials across the
membrane.
Fig. 6.13
Phospholipids are
in constant lateral
motion, but rarely
flip to the other
side of the bilayer
Phospholipid
bilayer
Membrane proteins
Figure 6-18b
There are two processes in photosynthesis that capture light and produce
energy rich compounds that are used in carbon fixation. These are termed
Photosystem I, and
Photosystem II.
These processes are linked in what is termed the Z scheme of photosynthesis.
The Z refers to changes in redox potential of electrons.
Note that PSII comes before PSI in this scheme
Wavelength of maximum
absorption in the far red
Wavelength of maximum
absorption in the red
Light reactions occur in
the thylakoids (PSII) and
stroma lamella (PSI).
Dark reactions in
occur in the stroma
Thylakoid membranes appear stacked like coins but
in fact are highly folded and have a well defined
interior and exterior with respect to the stroma
Fig. 10.8
Chlorophyll is the most abundant pigment in the chloroplast.
All eukaryotic photosynthetic organisms contain both chlorophyll a
and chlorophyll b
Carotenoids transfer
energy from photons to
chlorophyll. They also
can quench free radicals
by accepting or stabilizing
unpaired electrons and so
protect chlorophyll
molecules
-carotene
Chlorophylls a and b
When a photon strikes its energy
can be transferred to an electron
in the “head” region. The
electron is excited, raised to a
higher electron shell, with greater
potential energy
Tail
Ring structure in “head”
(absorbs light)
Wavelengths (nm)
Gamma
UltraX-rays
rays
violet
Infrared
Microwaves
Radio
waves
The
electromagnetic
spectrum
Shorter
wavelength
Visible light
Longer
wavelength
nm
Higher
energy
Lower
energy
e–
Blue photons excite electrons to
an even higher energy state
Figure 10-9
e–
Red photons excite electrons
to a high-energy state
Photons
Energy state of electrons in chlorophyll
Fig. 10.6a Different pigments absorb different wavelengths of light.
Chlorophyll b
Chlorophylls absorb blue and red
light and transmit green light
Chlorophyll a
Carotenoids
Carotenoids absorb blue
and green light and
transmit yellow, orange,
or red light
Fig. 10.6b
Pigments that absorb blue and red photons are the
most effective at triggering photosynthesis.
The oxygen-seeking bacteria
congregate in the wavelengths
of light where the alga is
producing the most oxygen
Oxygenseeking
bacteria
O2
O2
Filamentous alga
Basic concept of energy transfer during photosynthesis
Three Fates for Excited Electrons in Photosynthesis
FLUORESCENCE
or
Electron drops back down to
lower energy level; heat and
fluorescence are emitted.
RESONANCE
or
Energy in electron is transferred to
nearby pigment.
Higher
REDUCTION/OXIDATION
Electron is transferred to
a new compound.
Electron
acceptor
Reaction
center
Photon
Photon
Fluorescence
e–
Heat
e–
Lower
Chlorophyll molecule
e–
Chlorophyll molecules in antenna complex
Reaction center
Photochemistry
The energy of the excited state causes chemical reactions to
occur. The photochemical reactions of photosynthesis are
among the fastest known chemical reactions. This extreme
speed is necessary for photochemistry to compete with the
other possible reactions of the excited state.
Funneling of excitation from antenna system toward reaction center
The excited-state energy of pigments
increases with distance from the
reaction center. Pigments closer to the
reaction center are lower in energy
than those farther from it. This energy
gradient ensures that excitation
transfer toward the reaction center is
energetically favorable and that
transfer back out to the peripheral
portions of the antenna is energetically
unvavorable.
2-D view of structure of the LHCII antenna complex from higher plants
Stroma
Thylakoid Lumen
In photosystem II, excited
electrons feed an electron
transport chain.
Pheophytin has the structure of
chlorophyll but without the Mg in
the porphyrin-like ring and acts as
an electron acceptor.
Higher
Pheophytin
e–
PQ
Cytochrome
complex
Photon
2. Electrons that reach pheophytin are transferred to
plastoquinone (PQ), which is lipid soluble, passed to
an electron transport chain (quinones and
cytochromes)
Chlorophyll
Lower
2H2O
O2+ 4H+ + 4e-
1. When an electron in the reaction center chlorophyll
is excited energetically the electron binds to pheophytin
and the reaction center chlorophyll is oxidized
Photosystem II Feeds an ETC that Pumps Protons
3. Passage of electrons along the chain
involves a series of reduction-oxidation
reactions that results in protons being pumped
from stroma to thylakoid lumen
Plastoquinone carries protons to
the inside of thylakoids, creating
a proton-motive force.
Stroma
Stroma Photon
Antenna
complex
H+
Photosystem II
The ph of the lumen reaches 5
while that of the stroma is
around 8 - the concentration of
H+ is 1000 times higher in the
lumen than the stroma.
Cytochrome
complex
e–
PQ
Pheophytin
e–
e–
Reaction
center
PQ
H2O
Thylakoid Lumen
(low pH)
O2+
+
H
H+
H+
H+
H+
H+
+
H
+
H
H+
H+
H+
H+
An essential component of the
reaction is the physical transfer
of the electron from the excited
chlorophyll. The transfer takes
~200 picoseconds (1 picosecond
= 10-12 s).
The oxidized reaction center of the chlorophyll that donated an electron is re-reduced by a
secondary donor and the ultimate donor is water and oxygen is produced.
Figure 10-14
Photosystem I
Higher
2e–
NADP+ + H+
Iron and sulphur
compounds
Ferredoxin
NADP reductase
2 Photons
Chlorophyll
Lower
NADPH
NADPH is an electron carrier that
can donate electrons to other
compounds and so reduce them.
The Z scheme linking Photosystem II and Photosystem I
Fig. 10.15
4e–
2 NADP+ + 2 H+
Higher
Pheophytin
Ferredoxin
4e–
PQ
4 Photons
Cytochrome
complex
4 Photons
2 NADPH
PC
ATP
produced via
proton-motive force
P700
Photosystem I
P680
Photosystem II
4e–
Lower
2 H2O
4 H+ + O 2
When electrons reach the end of the Photosystem II electron
chain they are passed to a protein plastocyanin that can diffuse
through the lumen of the thylakoid and donate electrons to
Photosystem I. Shuttle rate of 1000 electrons per second
between photosystems.
ATP synthase – only in the stroma lamella and edge of grana stacks
Chemiosmosis
Ion concentration differences
and electric potential
differences across
membranes are a source of
energy that can be utilized
Stroma
Hydrophilic
As a result of the light
reactions the stroma has
become more alkaline (fewer
H+ ions) and the lumen more
acid (more H+ ions)
The internal stalk and much
of the enzyme complex
located in the membrane
rotates during catalysis.
Hydrophobic
Thylakoid Lumen
T
The enzyme is actually a
tiny molecular motor
Transfer of electrons and protons in the thylakoid membrane is carried out vectorially
Stroma
Thylakoid Lumen
Protons diffuse to the site of ATP synthase
Dashed lines represent electron transfer
Solid lines represent proton movement
Organization and structure of the four major protein complexes
Stroma
LHCI, PSI, and ATP
synthase are all in the
stroma lamella or on the
edge of a grana
LHC light harvesting complex
Organization and structure of the four major protein complexes
Stroma
Thylakoid Lumen
Things you need to know ...
1. The structure of chloroplasts and how the light reactions are
distributed and supply ATP and NADPH to the dark reactions
2. The Z scheme of photosynthesis, its photochemical and electropotential characteristics and its spatial arrangement
through the chloroplast membrane system, acidification of
the thylakoid lumen and formation of ATP.
3. The energy transfer system during photosynthesis including the
role of different pigments, the antenna and reaction center