Photosynthesis (A-level only) — AQA A-Level Biology
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Photosynthesis (A-level only) explained
Photosynthesis (A-level only) covers 7 specification points, including Chlorophyll absorbs light, leading to… and The light-independent reaction uses reduced….
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The light-dependent reaction in such detail as to show that: chlorophyll absorbs light, leading to photoionisation of chlorophyll some of the energy from electrons released during photoionisation is conserved in the production of ATP and reduced NADP 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 (chemiosomotic theory) photolysis of water produces protons, electrons and oxygen.
The light-dependent reaction takes place on the thylakoid membranes of the chloroplast. Chlorophyll absorbs light, and the energy raises an electron to a higher energy level so that it leaves the molecule, which is photoionisation. The electrons pass along a chain of electron carriers embedded in the membrane, losing energy at each transfer, and that energy actively moves protons from the stroma into the thylakoid space. A proton gradient builds across the membrane, and protons pass back into the stroma through ATP synthase; their movement drives the synthesis of ATP from ADP and inorganic phosphate, which is chemiosmosis, called photophosphorylation here. At the end of the chain, electrons and protons combine with NADP to form reduced NADP. The electrons lost from chlorophyll are replaced by photolysis of water, which splits into protons, electrons and oxygen.
The light-independent reaction uses reduced NADP from the light- dependent reaction to form a simple sugar.
The light-independent reaction occurs in the stroma of the chloroplast. It depends on the products of the light-dependent reaction, so it stops soon after light is removed even though light is not used directly in it. Reduced NADP is the reducing agent: it donates hydrogen to glycerate 3-phosphate (GP), converting it to triose phosphate (TP), and is oxidised back to NADP, which returns to the thylakoid membranes to be reduced again. ATP from the light-dependent reaction supplies energy for this reduction. Triose phosphate is the simple sugar produced; two triose phosphate molecules can be joined to give a hexose sugar such as glucose. In limiting-factor questions, less reduced NADP means less GP is reduced, so less triose phosphate and less sugar are formed.
The hydrolysis of ATP, also from the light-dependent reaction, provides the additional energy for this reaction.
Reduced NADP supplies the hydrogen for the reduction of glycerate 3-phosphate, but reduction also needs energy, and that comes from ATP made in the light-dependent reaction. ATP is hydrolysed to ADP and inorganic phosphate by ATP hydrolase, releasing energy in a small, immediately usable amount, and the inorganic phosphate released can phosphorylate other compounds and make them more reactive. Two of the three ATP molecules used in each turn of the Calvin cycle drive the conversion of GP to triose phosphate; the third is used in regenerating ribulose bisphosphate from triose phosphate, a step with an energy cost of its own. The ADP and phosphate produced return to the thylakoid membranes, where photophosphorylation converts them back to ATP, so the two halves of photosynthesis continuously recycle the same pool of nucleotide and coenzyme.
The light-independent reaction in such detail as to show that: 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.
In the stroma, carbon dioxide combines with the five-carbon ribulose bisphosphate (RuBP) in a reaction catalysed by rubisco. The six-carbon product is unstable and immediately splits into two molecules of the three-carbon glycerate 3-phosphate (GP), so one RuBP yields two GP. GP is then reduced to triose phosphate, using reduced NADP as the source of hydrogen and energy from the hydrolysis of ATP, both supplied by the light-dependent reaction. Some of the triose phosphate is used to regenerate RuBP so that the cycle can continue; this regeneration also requires ATP. The remaining triose phosphate leaves the cycle and is converted into useful organic substances, first hexose sugars such as glucose, then starch, cellulose, amino acids, lipids and nucleotides.
Students should be able to: identify environmental factors that limit the rate of photosynthesis evaluate data relating to common agricultural practices used to overcome the effect of these limiting factors.
The rate of photosynthesis at any moment is set by whichever factor is in shortest supply: light intensity, carbon dioxide concentration or temperature. On a rate graph the sloping part shows the named factor limiting the rate, and the plateau shows that something else has become limiting. Each factor acts by a different mechanism. Low light intensity slows photoionisation, so less ATP and reduced NADP are made and less GP is reduced. Low carbon dioxide leaves RuBP unreacted, so less GP is formed. Low temperature slows enzymes such as rubisco, while very high temperature denatures them and closes the stomata, cutting the carbon dioxide supply. Growers overcome these in glasshouses using artificial lighting, paraffin burners or cylinders to raise carbon dioxide, and heaters and ventilation to hold temperature near the optimum.
Required practical 7: Use of chromatography to investigate the pigments isolated from leaves of different plants, eg, leaves from shade-tolerant and shade-intolerant plants or leaves of different colours.
Pigments are extracted by grinding leaf tissue (e.g., from shade-tolerant and shade-intolerant plants) with sand and a solvent. A pencil origin line is drawn on chromatography paper. The extract is spotted repeatedly onto the origin. The paper is suspended in a closed vessel with the solvent below the origin line, preventing the spot from washing off. Pigments separate based on their solubility in the solvent and attraction to the paper; more soluble pigments travel further. The run stops before the solvent reaches the top. Rf values (distance moved by pigment divided by distance moved by solvent front) are calculated to identify and compare pigments between different plant types, revealing adaptations to varying light intensities.
Required practical 8: Investigation into the effect of a named factor on the rate of dehydrogenase activity in extracts of chloroplasts.
Chloroplasts are extracted by grinding leaf tissue in ice-cold, buffered, isotonic solution, then filtering and centrifuging. Cold reduces enzyme activity and tissue damage, the buffer holds pH constant so enzymes are not denatured, and the isotonic solution prevents osmotic damage. The suspension is mixed with DCPIP, a blue dye acting as an artificial electron acceptor in place of NADP. In the light, electrons from photoionisation and photolysis reduce DCPIP, which turns from blue to colourless; the faster the colour loss, the greater the dehydrogenase activity and the faster the light-dependent reaction. Rate is the reciprocal of time to decolourise, or is calculated from colorimeter readings at set intervals. A named factor, such as light intensity or temperature, is varied while others are controlled.
Your focus
- Describe the path of an electron from chlorophyll to reduced NADP, naming photoionisation and the electron transfer chain.
- Explain how a proton gradient across the thylakoid membrane leads to ATP synthesis by ATP synthase.
- State the three products of photolysis and say what happens to each of them, including how the electrons replace those lost from chlorophyll.
Show all 23 objectives
- Use the rate of oxygen production as a measure of the rate of the light-dependent reaction.
- Explain the role of reduced NADP in converting glycerate 3-phosphate to triose phosphate.
- Predict the effect of a reduced supply of reduced NADP on the amounts of GP, triose phosphate and sugar.
- State where in the chloroplast the light-independent reaction occurs and why it depends on the light-dependent reaction.
- Explain how the hydrolysis of ATP supplies the energy for the reduction of glycerate 3-phosphate.
- Identify the two steps of the Calvin cycle that require ATP and say what each one achieves.
- Predict the effect on GP, triose phosphate and RuBP of a fall in the supply of ATP from the light-dependent reaction.
- Describe one turn of the Calvin cycle, naming RuBP, GP, triose phosphate and rubisco in the correct order.
- Explain the separate roles of reduced NADP and of ATP in the reduction of GP.
- Account for the fate of triose phosphate, distinguishing the fraction that regenerates RuBP from the fraction that leaves the cycle.
- Predict how the concentrations of RuBP and GP change when the carbon dioxide supply is cut off.
- Identify the limiting factor in each region of a photosynthesis rate graph and justify your choice.
- Explain how low light, low carbon dioxide and low temperature each reduce the rate, in terms of named molecules.
- Evaluate an agricultural practice such as carbon dioxide enrichment, weighing yield against cost and the design of the trial.
- Describe how to separate leaf pigments using paper or thin-layer chromatography, detailing the correct setup.
- Calculate Rf values to identify and compare pigments isolated from leaves of different plants.
- Explain differences in pigment composition between shade-tolerant and shade-intolerant plants as adaptations to light availability.
- Explain why the chloroplast extraction medium must be ice-cold, buffered and isotonic.
- Describe how the reduction of DCPIP is used to measure the rate of the light-dependent reaction.
- Design a valid investigation into one named factor affecting dehydrogenase activity, naming the control and the controlled variables.
Photosynthesis (A-level only) exam tips
Marking Points
- Defining photoionisation as chlorophyll absorbing light and losing an electron, and stating that the electron is raised to a higher energy level before leaving the molecule.
- Describing electrons passing down the electron transfer chain in the thylakoid membrane, releasing energy that moves protons across the membrane.
- Explaining that protons pass back through ATP synthase so that ATP is made from ADP and inorganic phosphate, by chemiosmosis.
- Stating that electrons and protons reduce NADP to form reduced NADP, and that two electrons are needed overall to reduce one NADP.
- Stating that photolysis of water produces protons, electrons and oxygen, and that the electrons replace those lost from chlorophyll.
- States that reduced NADP reduces glycerate 3-phosphate to triose phosphate.
- Names triose phosphate, or the hexose formed from two triose phosphates, as the simple sugar produced.
- Explains that oxidised NADP returns to the light-dependent reaction to be reduced again.
- In a limiting-factor context, links less reduced NADP to less reduction of GP and therefore less triose phosphate and less sugar formed.
- Locates the light-independent reaction in the stroma of the chloroplast.
- Distinguishes the role of reduced NADP (hydrogen/reducing power) from that of ATP (energy) in the reduction of GP.
- one mark for ATP being hydrolysed to ADP and inorganic phosphate, releasing energy
- one mark for that energy being used, together with reduced NADP, to convert glycerate 3-phosphate to triose phosphate
- one mark for ATP also being used to regenerate ribulose bisphosphate from triose phosphate
- one mark for ADP and phosphate returning to the light-dependent reaction to be used in photophosphorylation
- one mark, in a limiting factor question, for less ATP causing less GP to be reduced and less RuBP to be regenerated
- States that carbon dioxide combines with ribulose bisphosphate, described as carbon fixation.
- States that the reaction is catalysed by rubisco and produces two molecules of glycerate 3-phosphate.
- States that GP is reduced to triose phosphate.
- States that reduced NADP is used in the reduction of GP.
- States that ATP is used in the reduction of GP (or in the regeneration of RuBP).
- States that some triose phosphate is used to regenerate ribulose bisphosphate.
- States that some triose phosphate is converted to a named useful organic substance (e.g. glucose, starch, cellulose, amino acids, lipids, nucleotides).
- one mark for naming light intensity, carbon dioxide concentration or temperature as the factor limiting the rate over a stated part of the graph
- one mark for the mechanism, such as less ATP and reduced NADP produced, or less RuBP reacting with carbon dioxide
- one mark for identifying the plateau as the point at which another factor has become limiting
- one mark, in an evaluation, for setting the increased yield against the cost of the practice
- one mark for a valid criticism of the trial, such as other variables not being controlled or only one growing season being tested
- Draw the origin line in pencil and apply the extract to that line, ensuring the solvent level is below the origin so the spot does not dissolve into the solvent.
- Stop the run and mark the solvent front before the solvent reaches the top of the paper.
- Explain separation: pigments have different solubilities in the mobile phase (solvent) and different affinities for the stationary phase (paper).
- Calculate the Rf value by dividing the distance moved by the centre of the pigment spot by the distance moved by the solvent front.
- Compare chromatograms from different plants (e.g., shade-tolerant vs shade-intolerant) to identify differences in pigment composition related to light adaptation.
- Explaining why the extraction medium is ice-cold, buffered and isotonic, giving a distinct reason for each condition.
- Explaining that DCPIP acts as an artificial electron acceptor in place of NADP, and that its reduction causes the colour change from blue to colourless.
- Describing how rate is measured, either as the reciprocal of the time to decolourise or from colorimeter readings taken at set intervals.
- Naming the factor varied and describing how it is changed, for example light intensity by changing lamp distance.
- Describing a valid control, such as a tube kept in the dark, together with named controlled variables.
Examiner Tips
- 💡Use the oxygen released as a measure of the rate of the light-dependent reaction: the faster oxygen appears, the faster the reaction.
- 💡Name all three products in a description, ATP, reduced NADP and oxygen, and say where each one goes.
- 💡Write reduced NADP in full, or NADPH; the mark scheme rejects reduced NAD in this context.
- 💡In any question about light falling, follow the chain: less reduced NADP, less GP reduced, less triose phosphate, less sugar.
- 💡State where the oxidised NADP goes, because that recycling step is a marking point many students omit.
- 💡When asked about the simple sugar, name triose phosphate and explain how two triose phosphate molecules form a hexose sugar such as glucose.
- 💡Pair the two requirements in one sentence: reduced NADP provides the hydrogen, hydrolysed ATP provides the energy.
- 💡Name both uses of ATP in the Calvin cycle whenever the question is worth two marks or more.
- 💡Track ADP and phosphate back to the light-dependent reaction to show that you understand the cycle is closed.
- 💡For a 'describe the light-independent reaction' question, write one short sentence for each stage: carbon fixation, reduction, regeneration, and product formation.
- 💡Name rubisco explicitly, because the enzyme is credited alongside the production of two GP.
- 💡An annotated cycle diagram can earn marks, provided every molecule on it is named in full.
- 💡Describe a rate graph in two halves: what limits the rate on the slope, and what must be limiting at the plateau.
- 💡Convert every limiting factor into named molecules, such as less ATP, less reduced NADP and less GP, to secure the explanation marks.
- 💡In evaluate questions give at least one point for and one against, because examiners often cap the mark for one-sided answers.
- 💡When comparing shade-tolerant and shade-intolerant plants, note that shade-tolerant plants often have a wider variety or higher concentration of accessory pigments to maximise light absorption.
- 💡Measure the Rf value from the origin line to the centre of each pigment spot, ensuring you use the same units (e.g., mm) for both the pigment and solvent front distances.
- 💡Justify each part of the extraction medium separately, because ice-cold, buffered and isotonic are three distinct reasons.
- 💡Define your measure of rate before describing the method; the reciprocal of the time to decolourise is the usual one.
- 💡Name the factor you vary and say how you vary it, since the practical requires a named factor.
Common Mistakes
- Writing NADP where reduced NADP is meant, or using NAD and reduced NAD, which examiners reject in a photosynthesis answer; correct this by naming reduced NADP (or NADPH) as the product.
- Saying light energy is converted to chemical energy without naming photoionisation, the electron transfer chain or ATP synthase; correct this by naming each stage explicitly.
- Placing the light-dependent reaction in the stroma instead of on the thylakoid membranes; correct this by locating it on the thylakoid membranes, with the stroma as the site of the light-independent reaction.
- Saying the oxygen released comes from carbon dioxide rather than from the photolysis of water; correct this by stating that oxygen is released when water is split.
- Describing protons as being pumped out of the thylakoid space instead of into it; correct this by stating that protons are moved from the stroma into the thylakoid space, creating the gradient.
- Omitting photolysis of water, or failing to say that it replaces the electrons lost from chlorophyll; correct this by stating that photolysis supplies the replacement electrons.
- Calling it the dark reaction and claiming it happens only at night. Correction: it is the light-independent reaction and can occur whenever its products are available, but it stops soon after light is removed because reduced NADP and ATP run out.
- Writing NADP where reduced NADP is required, so the reducing agent is never identified. Correction: reduced NADP is the reducing agent; NADP is the oxidised form that returns to the thylakoids.
- Using reduced NAD in place of reduced NADP. Correction: photosynthesis uses reduced NADP, not reduced NAD.
- Saying glucose is made directly from carbon dioxide, without passing through GP and triose phosphate. Correction: carbon dioxide combines with RuBP to form two molecules of GP, which is reduced to triose phosphate, and two triose phosphates can form a hexose sugar.
- Claiming that all triose phosphate becomes glucose. Correction: most triose phosphate is used to regenerate RuBP, and only some leaves the cycle to form organic substances.
- saying ATP 'gives energy' without naming its hydrolysis to ADP and inorganic phosphate
- crediting ATP with the reduction itself, when the hydrogen comes from reduced NADP
- forgetting that ATP is also needed to regenerate RuBP, not only to reduce GP
- describing ATP as a long-term energy store rather than an immediate energy source
- writing that ATP is broken down by respiration inside the chloroplast rather than hydrolysed by ATP hydrolase
- Naming GP wrongly, for example as glucose 3-phosphate. Correction: the correct name is glycerate 3-phosphate (GP).
- Using reduced NAD in place of reduced NADP for the reduction of GP. Correction: photosynthesis uses reduced NADP.
- Saying one RuBP gives one GP, missing that the six-carbon intermediate splits into two three-carbon molecules. Correction: one RuBP yields two GP.
- Forgetting that most triose phosphate regenerates RuBP, and claiming all of it becomes glucose. Correction: some triose phosphate regenerates RuBP and some is converted to organic substances.
- Confusing the roles of ATP and reduced NADP. Correction: reduced NADP supplies hydrogen/reducing power, while ATP supplies energy.
- saying a factor is limiting on the plateau of the graph, where by definition it no longer is
- naming the limiting factor without giving its biochemical consequence inside the chloroplast
- claiming temperature has no effect because the light-independent reaction does not need light, when its enzymes are temperature sensitive
- treating water as a limiting factor in the exam sense, when AQA expects light, carbon dioxide and temperature
- evaluating an agricultural practice on yield alone, with no mention of cost or of how the trial was run
- Drawing the origin line in ink. Correction: Always use a pencil, as ink contains pigments that will dissolve in the solvent and contaminate the chromatogram.
- Letting the solvent run off the top of the paper. Correction: Stop the run before the top and immediately mark the solvent front, otherwise Rf values cannot be calculated.
- Failing to compare different plant types. Correction: Ensure the investigation explicitly compares pigments from different leaves (e.g., shade-tolerant vs shade-intolerant) to meet the practical's objective.
- Explaining separation purely by 'pigment size'. Correction: Explain separation in terms of differing solubilities in the solvent and affinities for the paper.
- Saying DCPIP is oxidised when it is being reduced, or that it changes from colourless to blue; correct this by stating that DCPIP is reduced and turns from blue to colourless.
- Using distilled water as the extraction medium, so chloroplasts take in water and burst; correct this by using an isotonic solution to prevent osmotic damage.
- Leaving the extract at room temperature for long periods, so enzymes and organelles are damaged before the experiment begins; correct this by keeping the extract ice-cold throughout.
- Using a colorimeter without stating the filter or the quantity recorded, so the rate is never defined; correct this by naming the filter and the reading taken at each time interval.
- Treating decolourisation as evidence of photosynthesis as a whole, rather than of the light-dependent reaction; correct this by stating that DCPIP reduction measures the light-dependent reaction only.
- Failing to name the factor being varied, so the investigation does not match the required practical; correct this by naming the factor and describing how it is varied.