(1) Chloroplasts contain the light-dependent reactions of photosynthesis, which capture energy from sunlight and use it to produce ATP and NADPH. (2) These reactions occur in the thylakoid membranes through two photosystems that absorb light and transfer electrons. This powers an electron transport chain that pumps protons across the membrane. (3) The resulting proton gradient drives ATP synthesis when protons diffuse back through ATP synthase. Oxygen is also released as a byproduct of splitting water.

1. 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.

2. 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.
Photosynthesis Chapter 10 of text book
... but a better summary is of how the process occurs is:
Light
energy
Sunlight H2O O2
Light-dependent
reactions
ATP, NADPH
Chemical
energy
CO2
Calvin cycle
(CH2O)n
Chemical
energy
... this may keep the chemists happy
Thylakoid Reactions
Light reactions
Stroma Reactions
Dark reactions
Usually, glucose (C6H12O6) is considered as the carbohydrate made so:
6 CO2 + 12 H2O + light energy  C6H12O6 + 6 H2O + 6 O2

3. Plant structure, particularly cell structure
(1) makes the reactions possible,
(2) enables integration of light and dark reactions.
Leaves contain millions of chloroplasts.
Chloroplasts
Cell
Fig. 10.2

4. Phospholipid
bilayer
Figure 6-18b
Membrane proteins
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
Chloroplasts are highly structured, membrane-rich organelles.
Outer membrane
Inner membrane
Thylakoids
Granum
Stroma
Outer membrane
Inner membrane
Thylakoids
Granum
Stroma

5. 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.
Wavelength of maximum
absorption in the red
Wavelength of maximum
absorption in the far red
The Z refers to changes in redox potential of electrons.
Note that PSII comes before PSI in this scheme

6. 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

7. Chlorophylls a and b
Ring structure in “head”
(absorbs light)
-carotene
Tail
Fig. 10.8
Chlorophyll is the most abundant pigment in the chloroplast.
All eukaryotic photosynthetic organisms contain both chlorophyll a
and chlorophyll 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
Carotenoids transfer
energy from photons to
chlorophyll. They also
can quench free radicals
by accepting or stabilizing
unpaired electrons and so
protect chlorophyll
molecules

8. The
electromagnetic
spectrum
Wavelengths (nm)
Gamma
rays X-rays
Ultra-
violet Infrared
Micro-
waves
Radio
waves
Shorter
wavelength
Visible light
Longer
wavelength
nm
Higher
energy
Lower
energy

9. Figure 10-9
Photons
Energy state of electrons in chlorophyll
e–
e–
Blue photons excite electrons to
an even higher energy state
Red photons excite electrons
to a high-energy state

10. Different pigments absorb different wavelengths of light.
Chlorophyll b
Chlorophyll a
Carotenoids
Carotenoids absorb blue
and green light and
transmit yellow, orange,
or red light
Chlorophylls absorb blue and red
light and transmit green light
Fig. 10.6a

11. Oxygen-
seeking
bacteria
Pigments that absorb blue and red photons are the
most effective at triggering photosynthesis.
Filamentous alga
O2
O2
The oxygen-seeking bacteria
congregate in the wavelengths
of light where the alga is
producing the most oxygen
Fig. 10.6b

12. Basic concept of energy transfer during photosynthesis

13. Three Fates for Excited Electrons in Photosynthesis
Reaction
center
Fluorescence
Heat
Photon
Photon
e–
e–
Electron
acceptor
Chlorophyll molecules in antenna complex Reaction center
Chlorophyll molecule
Lower
Higher
e–
FLUORESCENCE
Electron drops back down to
lower energy level; heat and
fluorescence are emitted.
REDUCTION/OXIDATION
or
Electron is transferred to
a new compound.
RESONANCE
or
Energy in electron is transferred to
nearby pigment.
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.

14. 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.

15. 2-D view of structure of the LHCII antenna complex from higher plants
Stroma
Thylakoid Lumen

16. Chlorophyll
e–
Lower
Photon
Pheophytin
Cytochrome
complex
Higher
PQ
1. When an electron in the reaction center chlorophyll
is excited energetically the electron binds to pheophytin
and the reaction center chlorophyll is oxidized
2. Electrons that reach pheophytin are transferred to
plastoquinone (PQ), which is lipid soluble, passed to
an electron transport chain (quinones and
cytochromes)
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.
2H2O O2+ 4H+ + 4e-

17. Photosystem II Feeds an ETC that Pumps Protons
Cytochrome
complex
PQ
PQ
e–
e–
e–
Pheophytin
Antenna
complex
Reaction
center
Photosystem II
Stroma Photon H+
H+
(low pH) H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
H+
Stroma
Thylakoid Lumen
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.
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 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.
+
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.
H2O
O2

18. Figure 10-14
2e–
2 Photons
H+
NADP+
NADPH
Lower
Higher
Chlorophyll
Ferredoxin
+
Photosystem I
Iron and sulphur
compounds
NADP reductase
NADPH is an electron carrier that
can donate electrons to other
compounds and so reduce them.

19. 4e–
4 Photons
2 H+
2 NADP+
2 NADPH
Lower
Higher
Photosystem I
Ferredoxin
+
4e–
4 Photons
4e–
Photosystem II
4 H+
PQ
PC
P700
ATP
produced via
proton-motive force
Cytochrome
complex
Pheophytin
P680
+ O2
2 H2O
Fig. 10.15
The Z scheme linking Photosystem II and Photosystem I
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.

20. T
Chemiosmosis
Ion concentration differences
and electric potential
differences across
membranes are a source of
energy that can be utilized
As a result of the light
reactions the stroma has
become more alkaline (fewer
H+ ions) and the lumen more
acid (more H+ ions)
Hydrophilic
Hydrophobic
The internal stalk and much
of the enzyme complex
located in the membrane
rotates during catalysis.
The enzyme is actually a
tiny molecular motor
Stroma
Thylakoid Lumen
ATP synthase – only in the stroma lamella and edge of grana stacks

21. 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

22. Organization and structure of the four major protein complexes
Stroma
LHC light harvesting complex
LHCI, PSI, and ATP
synthase are all in the
stroma lamella or on the
edge of a grana

23. Organization and structure of the four major protein complexes
Stroma
Thylakoid Lumen

24. 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 electro-
potential 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