Photosynthesis (A Level only) (AQA A Level Biology): Flashcards
Exam code: 7402
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Define photolysis.
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Define photolysis.
The splitting of water using light energy, which occurs in the thylakoid lumen and produces protons (H+), electrons and oxygen:
H2O → 2H+ + 2e- + ½O2
Define photoionisation of chlorophyll.
When light energy hits a photosystem, an electron in a chlorophyll molecule is excited to a higher energy level and is emitted from the chlorophyll molecule.
Where in the chloroplast does the light-dependent reaction take place?
On the thylakoid membranes.
The photosynthetic pigments (e.g. chlorophyll) are arranged in photosystems embedded in these membranes.
What are the three products of the light-dependent reaction?
ATP
Reduced NADP (NADPH)
Oxygen (from the photolysis of water)
Describe how ATP is produced during the light-dependent reaction (chemiosmosis).
Excited electrons pass along the electron transport chain, releasing energy as they do so.
This energy is used to pump protons (H+) across the thylakoid membrane, from the stroma into the thylakoid lumen.
Protons return to the stroma down their concentration gradient by facilitated diffusion through ATP synthase.
ATP synthase catalyses the addition of inorganic phosphate to ADP: ADP + Pi → ATP (photophosphorylation).
Define chemiosmosis in the light-dependent reaction.
The movement of protons (H+) down their concentration gradient across the thylakoid membrane, through ATP synthase, which drives the production of ATP.
What happens to the protons, electrons and oxygen produced by photolysis?
NADP combines with protons (H+) from the stroma and electrons from the electron transport chain:
2H^+^ + 2e^-^ + NADP → NADPH
How is reduced NADP (NADPH) formed in the light-dependent reaction?
NADP combines with protons (H+) from the stroma and electrons from the electron transport chain:
2H+ + 2e- + NADP → NADPH
True or False?
Protons are pumped across the thylakoid membrane by active transport.
False.
The protons are pumped across using energy released by electrons passing down the electron transport chain, not energy from ATP, so this is not active transport.
During photolysis, water is split into protons, and oxygen.
During photolysis, water is split into protons, electrons and oxygen.
Why is a large surface area of thylakoid membranes important?
It increases the number of light-dependent reactions that can occur, by holding many photosystems and electron carriers.
The production of ATP using light energy in the light-dependent reaction is called .
The production of ATP using light energy in the light-dependent reaction is called photophosphorylation.
Why do chloroplasts contain several different pigments?
Different pigments absorb different wavelengths of light, maximising the light energy that can be absorbed by the plant.
Which wavelengths do chlorophylls and carotenoids absorb?
Chlorophylls absorb the blue-violet and red regions of the spectrum.
Carotenoids absorb mainly the blue-violet region.
What is chromatography?
An experimental technique used to separate mixtures, e.g. a mixture of photosynthetic pigments.
How does thin-layer chromatography (TLC) differ from paper chromatography?
In TLC the mixture passes through a thin layer of adsorbent (e.g. silica gel), through which it travels faster and separates more distinctly.
Why is the origin line drawn in pencil?
Pen ink would separate into pigments during the experiment and obscure the results.
Why is acetone added when grinding the leaf sample?
Acetone is an organic solvent that dissolves the membranes (cell, chloroplast and thylakoid), releasing the pigments.
The paper is suspended in the solvent so that the solvent level is the pencil line, and is removed before the solvent reaches the .
The paper is suspended in the solvent so that the solvent level is below the pencil line, and is removed before the solvent reaches the top.
How is the R~f~ value of a pigment calculated?
R~f~ value = distance travelled by pigment ÷ distance travelled by the solvent
What does a higher R~f~ value indicate about a pigment?
The molecule has a higher affinity for the liquid mobile phase, e.g. because it is non-polar, highly soluble or small.
Rank carotenoids, chlorophyll *a and **chlorophyll *b by Rf value.
Carotenoids have the highest Rf value, usually close to 1.
Chlorophyll *a* has an intermediate Rf value.
Chlorophyll *b* has the lowest Rf value.
When calculating Rf values, where should distances be measured to?
Always measure to the centre of each spot.
Define the Calvin cycle.
The series of reactions making up the light-independent reaction, which uses ATP and reduced NADP to fix carbon dioxide into simple sugars while continuously regenerating RuBP.
Where in the chloroplast does the light-independent reaction (Calvin cycle) occur?
In the stroma of the chloroplast.
Which two products of the light-dependent reaction are used in the Calvin cycle?
ATP
Reduced NADP (NADPH)
Describe the fixation of carbon dioxide in the Calvin cycle.
Carbon dioxide combines with the 5C sugar ribulose bisphosphate (RuBP), in a reaction catalysed by the enzyme rubisco.
The resulting 6C compound is unstable and splits into two molecules of glycerate 3-phosphate (GP), a 3C compound.
How is glycerate 3-phosphate (GP) converted into triose phosphate?
GP is reduced to triose phosphate using ATP and reduced NADP from the light-dependent reaction.
ATP provides energy (released by its hydrolysis) and reduced NADP provides the reducing power.
How is ribulose bisphosphate (RuBP) regenerated in the Calvin cycle?
ATP is used to convert five of the six carbons present in the two triose phosphate molecules back into RuBP.
What is the role of the enzyme rubisco?
It catalyses the reaction joining carbon dioxide to ribulose bisphosphate (RuBP) during carbon fixation.
What useful organic substances can triose phosphate be converted into?
Starch, sucrose and cellulose (via hexose phosphates).
Lipids (triose phosphate → glycerol; GP → fatty acids).
Amino acids for protein synthesis.
True or False?
The light-independent reaction can continue indefinitely in the dark.
False.
It relies on ATP and reduced NADP from the light-dependent reaction; in darkness these inputs run out, so the Calvin cycle cannot continue indefinitely.
Carbon dioxide combines with RuBP to form an unstable 6C compound that splits into two molecules of .
Carbon dioxide combines with RuBP to form an unstable 6C compound that splits into two molecules of glycerate 3-phosphate (GP).
In the Calvin cycle, is used to regenerate RuBP from triose phosphate.
In the Calvin cycle, ATP is used to regenerate RuBP from triose phosphate.
Explain why the light-independent reaction is not truly independent of light.
It requires ATP and reduced NADP produced by the light-dependent reaction.
Without light these inputs are no longer made and quickly run out, so the reaction cannot continue.
Define a limiting factor of photosynthesis.
A factor that, when below its optimum level, reduces the rate of photosynthesis, even if all the other factors are at their optimum.
Name the three main limiting factors of photosynthesis.
Light intensity
Carbon dioxide concentration
Temperature
Explain how light intensity affects the rate of photosynthesis.
As light intensity increases, the rate increases, because more electrons undergo photoionisation and more ATP and reduced NADP are produced, so the Calvin cycle runs faster.
Eventually the rate plateaus, when another factor (e.g. temperature or carbon dioxide concentration) becomes limiting.
Explain how carbon dioxide concentration affects the rate of photosynthesis.
As carbon dioxide concentration increases, the rate increases, because more CO2 can combine with RuBP during carbon fixation, producing more GP and therefore more organic compounds.
The rate plateaus once another factor becomes limiting.
Explain how temperature affects the rate of photosynthesis.
Photosynthesis is an enzyme-controlled reaction (e.g. rubisco, ATP synthase), so increasing temperature increases the kinetic energy of molecules and raises the rate.
Above the optimum, enzymes begin to denature and the rate decreases.
True or False?
Temperature has a significant effect on the light-dependent reactions.
False.
The light-dependent reactions are driven by light energy rather than the kinetic energy of molecules, so temperature has no significant effect on them.
A plant is moved from bright light to low light. Explain the effect on the levels of GP, triose phosphate and RuBP.
Less ATP and reduced NADP are produced by the light-dependent reaction.
So GP cannot be reduced to triose phosphate, and GP builds up.
Triose phosphate and RuBP levels fall.
The carbon dioxide concentration around a plant is reduced. Explain the effect on the levels of GP, RuBP and triose phosphate.
Less carbon dioxide combines with RuBP.
So RuBP is not converted into GP, and RuBP builds up.
GP and triose phosphate levels fall.
You should always refer to the limiting factor of light as light , never just 'light'.
You should always refer to the limiting factor of light as light intensity, never just 'light'.
Above the optimum temperature, the enzymes controlling photosynthesis begin to and the rate decreases.
Above the optimum temperature, the enzymes controlling photosynthesis begin to denature and the rate decreases.
How can an understanding of limiting factors be used to increase crop yields?
In glasshouses, sensors and computers can monitor and adjust light intensity, carbon dioxide concentration and temperature to keep them near the optimum for photosynthesis.
Farmers must balance the higher yield against the cost of lighting and heating.
On a graph of rate of photosynthesis against light intensity, what does a plateau indicate?
That light intensity is no longer the limiting factor; another factor, such as temperature or carbon dioxide concentration, is now limiting the rate.
Which organisms can be used to investigate the rate of photosynthesis?
Aquatic plants such as Elodea pondweed, or algae / algal beads.
In the pondweed practical, how is the rate of photosynthesis measured?
By measuring the volume of oxygen gas collected in a gas syringe over a set period of time.
Why is sodium hydrogencarbonate added to the water around the pondweed?
It gives the pondweed a controlled supply of dissolved carbon dioxide.
Why is the pondweed stem cut at an angle?
To increase the surface area for bubble formation.
The apparatus is set up in a room so that light from external sources can be controlled.
The apparatus is set up in a darkened room so that light from external light sources can be controlled.
Give one advantage of using immobilised algal beads instead of pondweed.
Beads have a known surface area and volume, so standardisation between repeats is easier.
What is a redox indicator, and how does it change colour in the dehydrogenase practical?
A substance that changes colour when reduced; e.g. DCPIP changes from blue (oxidised) to colourless (reduced) as it accepts electrons.
In isolated chloroplasts, what does DCPIP replace during the light-dependent reaction?
DCPIP acts as the electron acceptor in place of NADP, taking up the electrons emitted from chlorophyll.
What does the rate of DCPIP colour change measure?
The rate of dehydrogenase activity, and therefore the rate of the light-dependent reaction.
Why are leaves ground in an isolation medium when extracting chloroplasts?
The isolation medium prevents cell and chloroplast damage due to osmosis or extreme pH.
True or False?
As light intensity decreases, the DCPIP takes longer to change from blue to colourless.
True.
At lower light intensity, chlorophyll releases fewer electrons, so DCPIP accepts fewer electrons and takes longer to become reduced (colourless).
What control tubes can be used when judging DCPIP colour change by eye?
A tube wrapped in foil as a reference for the original blue colour, and a tube without DCPIP as a reference for the final green colour.
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