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Today we are studying this section
of the specification:
Photosynthesis
Two-stage process:
• Light-dependent reaction
– Occurs only in the light; involves thylakoids.
• Light-independent reaction (Calvin cycle)
– Occurs in dark and light; involves stroma.
Evidence for two stages
• Temperature has a clear effect on the rate of
photosynthesis
– This suggests that although light levels initially limit
the rate, there is a second stage controlled by
temperature-sensitive enzymes.
• A plant given alternating periods of dark and
light forms more carbohydrate than one in
continuous light
– Period of darkness means that substances recycled
in the light independent stage are not limiting for the
light dependent stage.
Location of the reaction
The light-dependent reaction takes place in the thylakoid
membranes of the chloroplasts.
Situated in these membranes are light-harvesting systems
called photosystems. There are two types, photosystem I
(PSI) and photosystem II (PSII). Both have chlorophyll at
their centres.
photosystem
light
chlorophyll
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REMINDER: NADP and NADPH
Nicotinamide adenine dinucleotide phosphate (NADP) is
a coenzyme involved in the photosynthesis reactions.
The compound is a dinucleotide,
containing an adenine base and a
nicotinamide base. The nucleotides
are joined through their phosphate
groups. In addition there is an extra
phosphate on the ribose of the
adenine-containing nucleotide.
nicotinamide
adenine
ribose
NADP can accept electrons to be reduced to reduced NADP,
often called NADPH. NADPH can be oxidized back to NADP,
releasing electrons.
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Light-dependent reaction
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Excitation of electrons
Photon of light hits chlorophyll molecule
Energy transferred to the electrons in the molecule
Electrons raised to higher energy levels
If they are raised to a high enough level they will dissociate
Electrons picked up by an electron acceptor
ATP formed as the electron is passed along an electron
transport chain
Electron transfer chain
electron
acceptor
ATP
ADP + Pi
final
electron
acceptor
Non-cyclic photophosphorylation
electron
acceptor
e-
2H
ATP
electron
acceptor
eADP + Pi
2H+
e-
Photosystem I
e-
Photosystem II
light
light
H 2O
½O2
NADP
NADP
e-
2H
2H+
Reduced NADP
To the light
independent
reaction
Light-dependent reaction
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Cyclic photophosphorylation
electron
acceptor
eATP
ADP + Pi
e-
Chlorophyll
light
Light-dependent reaction
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Photolysis of water
H2O → 2
+
H
+2
e
+ ½ O2
NADP
Photosystem II
Reduced NADP
Light-dependent reaction: summary
Cyclic photophosphorylation
Non-cyclic
photophosphorylation
photolysis
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Products of the light-dependent reaction
Products of the light-dependent reaction that pass into the
light-independent reaction:

reduced NADP

ATP
Products of the light-dependent reaction that leave the plant:

oxygen
Products of the light-dependent reaction that are re-used
in another part of the light-dependent reaction:

H+ ions

electrons
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Light-dependent stage
• Occurs in the thylakoids
• Involves splitting of water by light –
photolysis
• ADP is converted to ATP by light –
photophosphorylation
– Cyclic photophosphorylation uses only PSI
– Non-cyclic photophosphorylation uses both PSI
and PSII
• Reduced NADP and ATP needed for light
independent reactions
• O2 released as a waste product.
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Today we are studying this section
of the specification:
Melvin Calvin
• Member of the Radiation
Laboratory at Berkeley,
University of California
• Studied using
radioactively labelled
carbon dioxide
• 14CO2 fed to Chlorella
algae and its path
tracked
Location of the reaction
The light-independent reaction takes place in the stroma of
the chloroplasts.
The light-independent reaction can also be called the “dark
reactions”, or the Calvin Cycle.
The reaction consists of a cycle of enzyme-controlled
processes making complex molecules such as carbohydrates.
stroma
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Three phases
1. Carboxylation
Carbon dioxide fixation with ribulose bisphosphate
(RuBP)
2. Reduction
Reduction of glycerate 3-phosphate (GP) to
glyceraldehyde 3-phosphate (GALP)
3. Regeneration
Re-formation of the CO2
acceptor molecules
Phase 1: Carboxylation
Rubisco
CO2
GP
6C
molecule
RuBP
GP
RUBISCO
Phase 2: Reduction
2ATP
2ADP + 2Pi
GP
GALP
GP
GALP
2NADPH + H+
2NADP+
Phase 3: Regeneration
ATP
ADP + Pi
GALP
RuBP
GALP
The light-independent reaction
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Products of the Calvin cycle
Products of the light-independent reaction that pass back
into the light-dependent reaction:

NADP

ADP

inorganic phosphate
Products of the light-independent reaction that are used in
other processes:

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triose phosphate – used to build complex
carbohydrates, amino acids and lipids.
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Making complex molecules
Triose phosphate is a three-carbon sugar that can be used
to make a variety of complex biological molecules.
Pairs of TP molecules combine to form
hexose sugars, such as glucose, some
of which may isomerize to form fructose.
These monosaccharides can combine to form disaccharides
such as sucrose, and polysaccharides such as cellulose and
starch.
TP can be converted to glycerol and this may be combined
with fatty acids to make lipids.
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Summary of photosynthesis
light
lightdependent
reactions
lightindependent
reactions
carbohydrates, other
complex molecules
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Synthesis of Organic Substances
GALP
GALP
hexose
GALP
GP
glycerol
fatty
acids
lipids
GALP
amino
acids
The Maths
• 2 molecules of GALP are made each turn
• 5 out of 6 molecules of GALP are used to
regenerate RuBP
• 2 molecules of GALP are needed to make
a hexose sugar
• How many turns are needed to make 1
molecule of hexose sugar?
• How many molecules of ATP and reduced
NADP are needed?