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Today we are studying this section of the specification:
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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.
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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.
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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.
chlorophyll
light
photosystem
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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.
NADP can accept electrons to be reduced to reduced NADP, often called NADPH. NADPH can be oxidized back to NADP, releasing electrons.
nicotinamide
adenine
ribose
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Light-dependent reaction
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Excitation of electronsPhoton 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
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Electron transfer chain
final electron acceptor
ATP
ADP + Pi
electron acceptor
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Non-cyclic photophosphorylation
Photosystem II
Photosystem I
electron acceptor
electron acceptor
e-
e-
e-
ATP
ADP + Pi
light light
H2Oe-
2H+
2H
½O2
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NADP
e-
2H+
2H
NADP
Reduced NADP
To the light independent
reaction
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Light-dependent reaction
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Cyclic photophosphorylation
Chlorophyll
e-
e-
electron acceptor
light
ATP
ADP + Pi
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Light-dependent reaction
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Photolysis of water
H2O → 2 H+ + 2 e- + ½ O2
Photosystem II
NADP
Reduced NADP
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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:
Products of the light-dependent reaction that leave the plant:
Products of the light-dependent reaction that are re-used in another part of the light-dependent reaction:
reduced NADP
ATP
oxygen
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:
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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
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Location of the reaction
The light-independent reaction can also be called the “dark reactions”, or the Calvin Cycle.
The light-independent reaction takes place in the stroma of the chloroplasts.
The reaction consists of a cycle of enzyme-controlled processes making complex molecules such as carbohydrates.
stroma
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Three phases
1. CarboxylationCarbon dioxide fixation with ribulose bisphosphate (RuBP)
2. ReductionReduction of glycerate 3-phosphate (GP) to glyceraldehyde 3-phosphate (GALP)
3. RegenerationRe-formation of the CO2
acceptor molecules
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CO2
RuBP
6C molecule
GP
GP
Phase 1: Carboxylation
Rubisco
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RUBISCO
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Phase 2: Reduction
GP
GP
GALP
GALP
2ATP 2ADP + 2Pi
2NADPH + H+ 2NADP+
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Phase 3: Regeneration
GALP
GALP
RuBP
ATP ADP + Pi
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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:
Products of the light-independent reaction that are used in other processes:
NADP
ADP
triose phosphate – used to build complex carbohydrates, amino acids and lipids.
inorganic phosphate
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Making complex molecules
Pairs of TP molecules combine to form hexose sugars, such as glucose, some of which may isomerize to form fructose.
Triose phosphate is a three-carbon sugar that can be used to make a variety of complex biological molecules.
TP can be converted to glycerol and this may be combined with fatty acids to make lipids.
These monosaccharides can combine to form disaccharides such as sucrose, and polysaccharides such as cellulose and starch.
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Summary of photosynthesis
light- dependent reactions
light- independent
reactions
light
carbohydrates, other complex molecules
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Synthesis of Organic SubstancesGALP GALP GALP GP
hexose
glycerolfatty acids
lipids aminoacids
GALP
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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?