Photosynthesis Flashcards
What is photosynthesis?
A two stage reaction made up of the LDR & LIR
What does LDR stand for?
Light dependent reaction
What does LIR stand for?
Light independent rraction
Where does the light dependent reaction take place?
Thylakoid Membranes/Grana
Where does the light independent reaction take place?
Stroma
What are the 4 stages of the LDR?
1 - Photoionisation of chlorophyll
2 - Production of ATP + NADPH
3 - Chemiosmosis
4 - Photolysis
Photoionisation of chlorophyll (LDR)
Chlorophyll absorbs light energy. Light energy excites the electrons in the chlorophyll leading to their release from the chlorophyll molecule. The release of the electrons leads to the chlorophyll molecule becoming (+ve)ly charged - it has been photoionised.
The light independent reaction is also known as the…
Calvin Cycle
Production of NADPH (LDR)
Produced from the energy released from the photoionisation of chlorophyll during the LDR. It is needed in the LIR (Calvin cycle) to provide the H+ ions needed to reduce GP (glycerate-3-phosphate) to TP (triose phoshpate).
Production of ATP (LDR)
The electron transport chain consists of a chain of proteins in the thylakoid membrane through which excited electrons flow. As the excited e- move down the chain they lose energy, this energy is used to transport protons into the thylakoid. Forming a proton (electrochemical) gradient across the thylakoid membrane. The protons move down their conc gradient, into the stroma via ATP synthase. The energy from this movement combines ADP and Pi to form ATP.
Chemiosmosis (LDR)
Chemiosmotic Theory
The process of ATP synthesis via the ETC. The production of ATP involves electron transfer associated with the transfer of electrons down the electron transfer chain and passage of protons across chloroplast membranes and is catalysed by ATP synthase embedded in these membranes.
Photolysis (LDR)
Oxygen is produced through the photolysis of water during LDR. This is where light energy is used to split water into protons (H+ ions), electrons and oxygen.
Where is the enzyme rubisco found?
In a chloroplast - in the stroma as this is where the LIR takes place.
What is a photosystem?
A group of photosynthestic pigment molecules (such as chlorophylls a + b) attached to proteins in the thylakoid membranes of chloroplasts.
How are photosystems used by plants?
Photosystems are used by plants to capture light energy and transfer electrons to the chlorophyll molecule in the centre of the phtosystem.
The Light Independent Reaction
- carbon dioxide reacts with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP). This reaction is catalysed by the enzyme rubisco
- ATP and reduced NADP from the light-dependent reaction are used to reduce GP to triose phosphate
- some of the triose phosphate is used to regenerate RuBP in the Calvin cycle
- some of the triose phosphate is converted to useful organic substances.
Role of ATP in the LIR?
Used to convert GP to TP. The hydrolysis of ATP is also needed to regenrate RuBP (ribulose bisphosphate) from TP.
What happens to carbon dioxide during the LIR?
CO2 is combined with RuBP in a reaction catalysed by rubisco, leading to the formation of glycerate-3-phosphate (GP)
GP is a 3C compound. CO2 is a 1C compound.
The LIR does not directly use light - however it cannot take place at night - why?
LIR needs both ATP and NADPH to function. These are both produced by the LDR which does require light energy to be absorbed by chlorophyll (photoionisation). The LDR stops when it gets too dark. When the supply of ATP and NADPH runs out - the LIR stops too.
LIR is a cycle (calvin) Each cycle uses one molecule of CO2 - how many cycles are needed to produce one molecule of glucose?
6 - 2 molecules of TP is made. Because TP is a 3 carbon molecule that means that there are 6 molecules of carbon. Five of these are used to regenerate RuBP and one is molecule of carbon is used to make a hexose sugar e.g. glucose.
Why is it important that plants receive the correct wavelength of light?
The correct wavelengths are required in order to excite the electrons of different pigment molecules.
The higher the light intensity - the more energy is provided at these wavelengths for the LDR.
Most likely limiting factor on a warm, sunny, windless day?
Carbon dioxide concentration
What is a limiting factor?
The factor preventing the rate of photosynthesis from going any faster.
Suggest one way in which the carbon dioxide concentration in a glasshouse could be increased?
A small amount of propane could be burnt in a CO2 generator.
What affect will increasing the CO2 conc from 0.04% to 0.4% have on the rate of photosynthesis?
Increases the rate of photosynthesis because it makes collisions between molecules of CO2 and rubisco more likely.
Why won’t the rate of photosynthesis be affected when increasing the CO2 conc above 0.4%?
Once the concentration of CO2 reaches 0.4%, all of the rubisco enzymes’ active sites will be occupied. Increasing the carbon dioxide concentration further (beyond 0.4%) will therefore have no further effect on the rate of photosynthesis.
Why do plants usually contain a mixture of different photosynthetic pigments?
Each pigment will absorb a different wavelength of light so plants contain a mixture to maximise the range of light wavelengths they can absorb.
What is the role of dehydrogenase enzymes in the LDR of photosynthesis?
These catalyse the reduction of NADP during the LDR.
Describe the role of the solvent in chromatography.
It is the mobile phase, it dissolves the mixture being separated and carries it through the stationary phase. This allows the mixture to separate out.
How can you tell how many pigments an extract contains?
By counting the number of dots present above the point of origin.
What is chromatography used for?
To separate components of a mixture so that they can be identified.
Name two types of chromatography?
Paper chromatography
Thin layer chromatography
What is the mobile phase in paper chromatography and TLC?
Liquid solvent
What is the stationary phase in paper chromatography?
Chromatography paper
What is the stationary phase in TLC?
Thin layer of solid - silica gel on a plastic/glass plate
Basic principle of chromatography
1 - mobile phase moves through/over stationary phase
2 - components of mixture spend different amounts of time in each phase
3 - time spent in each phase is what separates out the components of the mixture e.g. components spending more time in the mobile phase travel faster/further.
Rf value
The distance travelled by a substance through the stationary phase in relation to the solvent.
How to calculate the Rf value
distance travelled by the spot / distance travelled by the solvent
- Investigating the pigments isolated from leaves of different plants using chromatography e.g. (using TLC to show the pigments present in shade tolerant and shade-intolerant plants)
Grind up leaves from shade tolerant plant w/ anhydrous sodium sulphate
ADD a few drops of propanone.
- Investigating the pigments isolated from leaves of different plants using chromatography
Transfer this to test tube
ADD some petroleum ether
Gently shake
2 distinct layers form - top layer is the pigments mixed with petroleum ether
- Investigating the pigments isolated from leaves of different plants using chromatography
Transfer some of top layer to 2nd test tube w/ some anhydrous sodium sulphate
- Investigating the pigments isolated from leaves of different plants using chromatography
Draw horizontal pencil line near bottom of TLC plate
Add several drops of liquid from STEP 3 (to create one concentrated drop) on the line.
Ensure each drop has dried before adding the next. This is your POINT OF ORIGIN
- Investigating the pigments isolated from leaves of different plants using chromatography
Make sure plate is dry - place into a small glass container w/ prepared solvent
Make sure solvent is just below point of origin.
Put lid on container and leave plate to develop.
- Investigating the pigments isolated from leaves of different plants using chromatography
Once solvent has nearly reached top - take it out and mark the solvent front w/ pencil
Leave plate to dry in a well-ventilated place.
- Investigating the pigments isolated from leaves of different plants using chromatography
There will be several coloured spots between point of origin and solvent front. These are the separated pigments. Calculate Rf value
- Investigating the pigments isolated from leaves of different plants using chromatography
Repeat process for the shade intolerant plant + compare pigments present in their leaves.
Why is it best to do steps 2 and 5 in a fume cupboard?
The chemicals used are volatile (will evaporate easily) and the vapours are hazardous.
Give an example of a shade tolerant plant:
hostas
Give an example of a shade intolerant plant:
chrysanthemums
What is the developed solvent made out of in the first experiment (step 5)
Mixture of propanone, cyclohexane and petroleum ether.
How could you used your results from the first experiment to identify each pigment?
Calculate each of their Rf values. Each pigment has a specific Rf value in a specific solvent, so look up the Rf value you have calculated in a database in order to identify the pigments.
- Investigating the activity of dehydrogenase in chloroplasts
Cut some leaves (spinach) into pieces - removing any tough stalks.
- Investigating the activity of dehydrogenase in chloroplasts
Pestle + mortar –> grind leaf pieces w/ chilled isolation solution + filter liquid into a beaker through funnel lined w/ muslin cloth
- Investigating the activity of dehydrogenase in chloroplasts
Transfer liquid to centrifuge tubes + centrifuge them at high speed for 10 mins. Makes chloroplasts gather at bottom of each tube = pellet
- Investigating the activity of dehydrogenase in chloroplasts
Remove supernatant - leaving pellet at bottom
- Investigating the activity of dehydrogenase in chloroplasts
Re-suspend pellets in fresh, chilled isolation solution. It is your chloroplast extract. Store on ice for duration of experiment.
- Investigating the activity of dehydrogenase in chloroplasts
Set up colorimeter w/ red filter + zero (standardise) it using a cuvette containing chloroplast extract + distilled water.
- Investigating the activity of dehydrogenase in chloroplasts
Set up test tube rack at a set distance from lamp. Switch lamp on.
- Investigating the activity of dehydrogenase in chloroplasts
Put a test tube in the rack - add a set volume of chloroplast extract to tube and a set volume of DCPIP. Mix contents of tube.
- Investigating the activity of dehydrogenase in chloroplasts
Immediately take a sample of mixture from tube and add it to a cuvette. Place in colorimeter + record absorbance. Do this every 2 mins for next 10 mins.
- Investigating the activity of dehydrogenase in chloroplasts
Repeat steps 7-9 for each distance under investigation.
- Investigating the activity of dehydrogenase in chloroplasts
Check whether absorbance changes at each distances in 2 negative control tubes. NO CHANGE should be seen in these controls.
What should the first negative control tube contain?
DCPIP and chilled isolation solution - wrapped in tin foil (so no light reaches contents of tube)
What is the isolation solution made up of? Step 2
A solution of sucrose, potassium chloride and phosphate buffer at pH 7.
How does the dehydrogenase experiment work?
In photosystem 1 (LDR) NADP acts as an e- acceptor, is reduced.
Reaction is catalysed by a dehydrogenase enzyme.
A redox indicator dye (DCPIP) also acts as an e- acceptor and can be reduced by dehydrogenase in chloroplasts.
As dye is reduced, see a colour change. DCPIP blue –> colourless.
How can a redox indicator dye be used to measure the rate of dehydrogenase activity in chloroplasts?
Measure the rate at which DCPIP is losing its colour. This is the rate at which DCPIP is being reduced.
Describe how you would isolate chloroplasts from a sample of leaf tissue.
Use pestle and mortar to grind up leaf tissue and mix with isolation solution. Filter through muslin cloth into beaker. Transfer liquid to centrifuge tubes and centrifuge at high speeds for 10 mins - this separates chloroplasts into pellet at bottom of tube.
What safety precaution would you take if you are carrying out TLC with a volatile, hazardous solvent?
Carry out experiment in a fume cupboard.
Why is a negative control used in the second experiment?
To show that it was the chloroplast extract making the redox indicator dye change colour and not the isolation solution.
Being closer to the light sources would achieve what change on a graph?
A steeper drop in absorbance. Dehydrogenase activity faster at a higher light intensity, so redox indicator dye losing colour faster - decreased absorbance.
What is reduction?
Electron acceptor. Gain in electrons.
What is oxidation?
Electron donor. Loss of electrons.
ATP is synthesised during photosynthesis through a photophosphorylation reaction. What 3 things are needed for this reaction to happen?
ADP + Pi + Light (energy)
What is meant by the term ‘photolysis’?
When light energy is used to split a molecule.
