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8.3 Photosynthesis
Understanding:- Light-dependent reactions take place in the
intermembrane space of the thylakoids- Reduced NADP and ATP are produced in the
light-dependent reactions- Light-independent reactions take place in
the stroma- Absorption of light by photosystems
generates excited electrons- Photolysis of water generates electrons for
use in the light-dependent reactions- Transfer of excited electrons occurs
between carriers in thylakoid membranes- Excited electrons from Photosystem II are
used to generate a proton gradient- In the light-independent reactions a
carboxylation of ribulose bisphosphate- Glycerate 3-phoshpate is reduced to triose
phosphate using reduced NADP and ATP- Triose phosphate is used to regenerate
RuBP and produce carbohydrates
Applications:- Calvin’s experiment to elucidate the
carboxylation of RuBP
Skills:- Annotation of a diagram to indicate
the adaptations of a chloroplast to its function
Nature of science:- Developments in scientific research follow improvemnts in apparatus: sources of 14C and autoradiography enabled Calvin to elucidate the pathways of carbon fixation
Understanding:- Ribulose bisphosphate is reformed
using ATP- The structure of the chloroplast is
adapted to its function in photosynthesis
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Chloroplast structure
Chloroplasts are adapted to their function
Draw a chloroplast and label the following onto it:- Thylakoid- Granum- Inner membrane- Outer membrane- Stroma
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Chloroplast structure
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Thylakoids – flattened membrane sacs with components for the absorption of light (first step of photosynthesis)
Grana – Stacked up thylakoids = more photosynthesis. Contains chlorophyll.
Stroma – fluid outside of the grana. Contains many enzymes and chemicals for photosynthesis.
Chloroplast structure
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Chloroplast structure
Chloroplast structure Function Allowed
Extensive membrane surface area of the thylakoids
Small space (lumen) within the thylakoidsStroma region similar to the cytosol of the cell
Double membrane on the outside
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Chloroplast structure
Chloroplast structure Function Allowed
Extensive membrane surface area of the thylakoids
Greater absorption of light by photosystems
Small space (lumen) within the thylakoids
Faster accumulation of protons to create a concentration gradient
Stroma region similar to the cytosol of the cell
Area for the enzymes necessary for the Calvin cycle to work
Double membrane on the outside
Isolates the working parts and enzymes of the chloroplast from the surrounding cytosol
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Photosynthesis
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Photosynthesis vs respiration
Essentially the opposite
Respiration = catabolism (breaks down moleculesinto smaller units to release energy)
Photosynthesis = anabolism (construct large molecules from smaller units using energy)
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Photosynthesis
Light dependent reactions
Light independent reactions
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Light dependent reactions
- Thylakoid membrane- Uses light directly - Use pigments to absorb the light (chlorophyll/carotenoids…)- Pigments organised into photosystems in thylakoids
Photosystems include:1. Chlorophyll a molecules2. Accessory pigments3. Reaction centre
A reaction centre in these photosystems contains:4. Pair of chlorophyll a molecules5. A Primary electron acceptor
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Light dependent reactions
Modern day plants have 2 photosystems
Each absorbs light efficiently at different wavelengths
Photosystem I: 700nm (P700)Photosystem II: 680nm (P680)
Work together to to transfer electrons to create energy
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Photosystem II
1. A photon of light is absorbed by a pigment in Photosystem II and is transferred to other pigment molecules until it reaches chlorophyll a. (Photoactivation)
2. Water split by enzymes to produce e-, H+ and oxygen – driven by light energy (photolysis)
3. Electrons supplied one by one to chlorophyll a
4. The photon energy excites a chlorophyll a electron to a higher energy state
5. Electron captured by primary acceptor of the reaction centre
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Electron transport chain
6. Excited electrons pass from primary acceptor down electron transport chain, losing energy at each step- Involves Plastoquinone (electron carrier) and plastocyanin
(electron acceptor)- Involves cytochrome complex where the e- transport chain occurs)
7. Protons pumped into thylakoid space against concentration gradient
This stored energy drives chemiosmosis – phosphorylation of ADP to ATP using ATP synthase
(ATP is the first product)
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Photosystem I
8. Another photon of light is absorbed by a pigment in Photosystem I.
9. Energy is transferred until it reaches chlorophyll a
10. Electron is energised and transferred to primary electron acceptor
11. De-energised electron from Photosystem II fills the void left by the newly energised electron
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Electron transport chain
12. High energy electron passed down a second electron transport chain that involves ferredoxin (electron carrier)
13. Enzyme NADP reductase catalyses the transfer of electron form ferredoxin to energy carrier NADP+
14. 2 electrons are needed to fully reduce NADP+ to NADPH
(NADPH is the second product)
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Light dependent reactions
NADPH and ATP are the final products
Supply energy for the light independent reactions to occur
Also where oxygen is released (water splitting in the first few steps)
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ATP
ATP production in photosynthesis and respiration are similar
Both use chemiosmosis to phosphorylate ADP to ATP (ATP synthase enzyme)
When it is produced using light = photophosphorylation
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ChemiosmosisRespiration chemiosmosis Photosynthesis chemiosmosis
Electron transport chain in cristae membranes
Energy released when electrons are exchanged from one carrier to another
Energy used to pump H+ into intermembrane space
H+ ions come from matrix
H+ ions diffuse back into matrix through channels of ATP synthase
ATP synthase catalyses the phosphorylation of ADP to ATP
Electron transport chain in thylakoid membranes
Energy released when electrons are exchanged from one carrier to another
Energy used to pump H+ into thylakoid space
H+ ions come from the stroma
H+ ions diffuse back into stroma through channels of ATP synthase
ATP synthase catalyses the photophosphorylation of ADP to ATP
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- In the stroma (contains enzymes)
- Uses ATP and NADPH from light dependent reaction
- Involves the Calvin cycle (begins and ends with the same substance)
Light Independent Reactions
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1. 5-carbon compound Ribulose bisphosphate (RuBP) binds to an incoming carbon dioxide molecule
2. Catalysed by enzyme RuBP carboxylase (rubisco) – results in an unstable 6-carbon compound
3. Breaks down into two 3-carbon compounds called glycerate 3-phosphate (GP)
Carbon Fixation
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1. GP uses ATP and NADPH to form triose phosphate (TP) – REDUCTION
2. TP can go in 2 directions
Reduction
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1. Some leave cycle to become sugar phosphates that become more complex carbohydrates (STARCH)
2. Most continue in the cycle to reproduce the originating compound of RuBP (uses ATP) REGENERATION
As much RuBP must be produced as is consumed.
Example:3 RuBP used = 6 TP molecules produced5 used to regenerate RuBP – 1 used to produce carbohydrates
Regeneration
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To form one 6 carbon sugar molecule:
12 TP molecules6 RuBP molecules12 NADPH18 ATP
Annotate onto your diagrams
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Calvin and his team worked out carbon fixation in plants.
Research the lollipop experiment- What is it?- What is the process?- What did he find out?
Calvin’s experiment
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Involves light dependent and light independent reactions
Light dependent reactions produce ATP and NADPH for the light independent reactions
Light is needed for the light independent reactions to occur – but not directly.
Photosynthesis summary
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Photosynthesis summaryLight dependent Light independent
Occurs in thylakoids
Uses light energy to form ATP and NADPH
Splits water in photolysis to proved replacement e- and H+ and to release oxygen to atmosphere
Includes 2 electron transport chains and photosystems I and II
Occurs in the stroma
Uses ATP and NADPH to form triose phosphate
Returns ADP, inorganic phosphate and NADP to the light dependent reactions
Involves the Calvin cycle