4.1-Capturing Solar Energy:
Light Dependent Reactions
SBI4U1
Photosynthesis
Overall Chemical Equation:
6CO2 (g) + 6H2O (l) + energy  C6H12O6 (s) + 6 O2 (g)
Simply stated…
Carbon dioxide + water with addition of energy
from the sun yields glucose + oxygen.
Photosynthesis…
-Photosynthesis transforms radiant energy of
sunlight into chemical energy
-Photosynthesizing agents/organisms use
approx. 2% of the sun’s energy.
• Photosynthesis allows plants to:
– Make glucose
• Converted to cellulose in cell walls
• Starch for energy storage
Two sets of reactions that make up
photosynthesis…
“Photo” = light
“Synthesis” = rxns that synthesize carbohydrate
Rx. 1- Light Dependent Reaction:
Light energy is trapped and used to generate ATP
and NADPH( similar to NADH in Cell. Resp)
Rx. 2- Light Independent Reaction
Energy from ATP and reducing power of NADPH are
used to make glucose
Light Dependent Rxn: rxn
that uses solar energy to
generate ATP and NADPH
(similar to NADH)
Light Independent Rxn: rxn
that uses the energy of
ATP and reducing power of
NADPH to make a high
energy organic molecule
*Note: cyanobacteria, algae and plants all carry
out photosynthesis, but we are just going to
focus on plants for simplicity
Structure of the Chloroplasts
• Photosynthesis factory
• 3-8µm in length and 2-3µm in diameter
• Outer and inner membranes enclose a space
filled w/ protein rich semi-liquid material
called stroma
– Thylakoids are within the stroma
• Flattened discs
– Stacked thylakoids are called grana, unstacked
thylakoids are called lamellae
Levels of Organization in a
Plant Leaf
Carbon dioxide and
water that are used to
synthesize glucose
through
photosynthesis are
taken up
by the leaf and then
enter into plant cells
and chloroplasts.
Water enters the leaf
through veins, and
carbon dioxide enters
via openings called
stomata.
Grana
• Inside the thylakoid sac is the thylakoid lumen
– Water filled
• Chlorophyll and ETC proteins are embedded
in thylakoid membrane
– Chlorophyll is a green coloured pigment that
absorbs light
– Common Forms of chlorophyll:
chlorophyll a and chlorophyll b
Why Chlorophyll Appears Green
A) Leaves appear green because chlorophyll molecules in leaf cells reflect green and yellow
wavelengths of light and absorb other wavelengths (red and blue).
B) This absorbance spectrum for three photosynthetic pigments shows that each pigment
absorbs a different combination of colours of light.
Absorption of
Light
• Light is absorbed in packets
of energy called photons
• Wavelengths (colour) of light
are related to energy
– Shorter wavelength  more
energy
– Longer wavelength  less
energy
– Electrons can absorb a photon
only if it carries exactly enough
energy to allow the electron to
move up to another allowed
energy level.
Pigment: compound that absorbs visible light
– E.g. Chlorophyll a and chlorophyll b
Chlorophyll a absorbs at 400-450 nm and 650-700 nm
Chlorophyll b absorbs at 450-500 nm and 600-650 nm
– Both types of chlorophyll reflect green
Photosystems
• Protein based complexes composed of clusters of
pigments that absorb light energy in the thylakoid
membrane
• When a pigment molecule absorbs a photon,
the molecule passes the energy to the
chlorphyll a molecules.
• Light reactions of photosynthesis occur in the
thylakoid membrane
• Divided into 3 parts
1. Photoexcitation: e- gets excited
2. Electron transport: e- transferred to e- carriers,
and protons pump into lumen
3. Chemiosmosis: formation of ATP
Photosystems are made up of two parts:
1. Antenna complex
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Made up of chlorophyll molecules and other
pigments
Absorbs a photon and transfers energy from pigment
to pigment until it reaches chlorophyll a in reaction
centre
2. Reaction Centre:
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Transmembrane protein complex
Contains chlorophyll a
e- absorb energy and passes e- to an electron acceptor
The Reaction Centre
• The antenna complex is
also sometimes referred
to as the lightharvesting complex
because it gathers
(harvests) energy from
light so that the energy
can be directed
to the P680 molecule in
the reaction centre.
There are 2 Photosystems:
1. Photosystem One - PS I:
– Primary pigment is chlorophyll a.
– Absorption peak at 700nm
– Called P700
2. Photosystem Two - PS II:
– Primary pigment is chlorophyll b
– Absorption peak at 680nm
– Called P680
Arrangement of Photosystem I
and Photosystem II
In the light-dependent reactions, photosystem II passes electrons to
photosystem I via an electron transport system, which contains the b6-f
complex. This complex acts as a proton pump to produce a proton gradient
across the thylakoid membrane. The electrons lost from the reaction centre
of photosystem II are replenished by the oxidation of water. Photosystem I
uses the electrons to reduce NADP+ to NADPH.
PSI and PSII work together to produce ATP and NADPH
•PSII passes electrons to PSI via an electron transport
system (w/ b6-f complex  proton pump)
•e- lost from PSII are replenished by oxidation of H2O
•PSI uses e- to reduce NADP+ to NADPH
Animation: http://www.biology4all.com/resources_library/source/61a.swf
• ATP synthesis in light-dependent rxns is the
same as in aerobic respiration
• It is called photophosphorylation
– Using photons to drive phosphorylation of ADP to
produce ATP via chemiosmosis
• PSI and PSII, an electron transport system, and
ATP synthase enzyme are in the thylakoid
membrane
• They works together to make ATP
Animation:
http://www.stolaf.edu/people/giannini/flashanimat/metabolism/photosynthesis.swf
Making ATP by Chemiosmosis
Photosystem I, photosystem II, an electron transport system,
and the ATP synthase enzyme are embedded in the thylakoid
membrane of chloroplasts. ATP synthesis by chemiosmosis in
chloroplasts occurs in a way that is very similar to the way it
occurs in mitochondria.
Cyclic Photophosphorylation
Cyclic Phosphorylation
In cyclic photophosphorylation, an electron in
P700 is excited by a photon and begins taking
the same path that it took in noncyclic
photophosphorylation. However, the electron
is not used to reduce NADP+ but instead is
passed back to the b6-f complex, where the
energy is used to generate the proton
gradient.
• Chloroplast produce more ATP through cyclic
photophosphorylation
– Only PSI
– Photon excited an e- from P700 in PSI
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it follows the same path as in noncyclic
e- is not used to reduce NADP+
It is passed back to b6-f complex
Creatinfga proton gradient to generate ATP, but not
NADPH
– Believed to be used by early bacteria
Learning Expectations...
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Reactants and products of photosynthesis
Structure and function of chloroplasts
Chlorophyll a vs. chlorophyll b
Photosystems (PSI vs. PSII)
ATP production and chemiosmosis in plants
Cyclic vs. noncyclic photophosphorylation
** there are a lot of details here, re-watching the animations may be beneficial**