Cambridge IGCSE · 0610

Biology

Photosynthesis and respiration — Topics 6 and 12

Name: ____________________Date: October 10, 2026

1.
Write the word equation for photosynthesis, and state the two limiting factors that most commonly reduce its rate on a cool, cloudy day.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: Writes carbon dioxide + water → glucose + oxygen (with light energy required). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States low light intensity as a limiting factor. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States low temperature as a limiting factor. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A cloudy day points to light intensity; a cool day points to temperature — link each condition to its factor.
Marking points
- Writes carbon dioxide + water → glucose + oxygen (with light energy required).
- States low light intensity as a limiting factor.
- States low temperature as a limiting factor.
Examiner tip: A cloudy day points to light intensity; a cool day points to temperature — link each condition to its factor.
2.
Explain why plant leaves need stomata for both photosynthesis and respiration to occur efficiently.
[4 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: Stomata allow carbon dioxide to diffuse into the leaf for photosynthesis. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: Stomata allow oxygen produced by photosynthesis to diffuse out. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: Stomata allow oxygen to diffuse in for respiration and carbon dioxide to diffuse out. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Develop this part of the answer: Without stomata, gas exchange would be too slow to support either process efficiently. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Address gas movement for both processes separately rather than describing stomata only once.
Marking points
- Stomata allow carbon dioxide to diffuse into the leaf for photosynthesis.
- Stomata allow oxygen produced by photosynthesis to diffuse out.
- Stomata allow oxygen to diffuse in for respiration and carbon dioxide to diffuse out.
- Without stomata, gas exchange would be too slow to support either process efficiently.
Examiner tip: Address gas movement for both processes separately rather than describing stomata only once.
3.
Describe an investigation using a plant, sodium hydrogencarbonate solution, and light of varying distance to show that light intensity is a limiting factor of photosynthesis.
[4 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that a piece of pondweed is placed in sodium hydrogencarbonate solution (providing a carbon dioxide source) in a test tube. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that a lamp is placed at a series of measured distances from the plant, and the number of oxygen bubbles produced per minute is counted at each distance. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that as the lamp moves closer, light intensity increases and the rate of bubbling (photosynthesis) increases, up to a point. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Develop this part of the answer: States that all other variables (e.g. temperature, carbon dioxide concentration) must be kept constant so that light intensity is the only variable being tested. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Counting bubbles is used as an indirect measure of the rate of oxygen production, which is itself a direct measure of the rate of photosynthesis.
Marking points
- States that a piece of pondweed is placed in sodium hydrogencarbonate solution (providing a carbon dioxide source) in a test tube.
- States that a lamp is placed at a series of measured distances from the plant, and the number of oxygen bubbles produced per minute is counted at each distance.
- States that as the lamp moves closer, light intensity increases and the rate of bubbling (photosynthesis) increases, up to a point.
- States that all other variables (e.g. temperature, carbon dioxide concentration) must be kept constant so that light intensity is the only variable being tested.
Examiner tip: Counting bubbles is used as an indirect measure of the rate of oxygen production, which is itself a direct measure of the rate of photosynthesis.
4.
Write the word equation for aerobic respiration, and state one difference between aerobic and anaerobic respiration in terms of the products formed.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: Writes glucose + oxygen → carbon dioxide + water. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that aerobic respiration requires oxygen and fully breaks down glucose into carbon dioxide and water, releasing more energy. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that anaerobic respiration in animal cells produces lactic acid (no carbon dioxide), while in plant/yeast cells it produces ethanol and carbon dioxide, without needing oxygen. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Aerobic respiration releases far more energy per glucose molecule than anaerobic respiration, since the breakdown of glucose is much more complete.
Marking points
- Writes glucose + oxygen → carbon dioxide + water.
- States that aerobic respiration requires oxygen and fully breaks down glucose into carbon dioxide and water, releasing more energy.
- States that anaerobic respiration in animal cells produces lactic acid (no carbon dioxide), while in plant/yeast cells it produces ethanol and carbon dioxide, without needing oxygen.
Examiner tip: Aerobic respiration releases far more energy per glucose molecule than anaerobic respiration, since the breakdown of glucose is much more complete.
5.
Explain the role of chlorophyll in photosynthesis, and state where in the plant cell it is located.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that chlorophyll is a green pigment that absorbs light energy (particularly red and blue light). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that this absorbed light energy is used to drive the reactions of photosynthesis, converting carbon dioxide and water into glucose and oxygen. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that chlorophyll is located within chloroplasts in plant cells. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Chlorophyll appears green because it reflects green light while absorbing red and blue light most strongly, which it uses to power photosynthesis.
Marking points
- States that chlorophyll is a green pigment that absorbs light energy (particularly red and blue light).
- States that this absorbed light energy is used to drive the reactions of photosynthesis, converting carbon dioxide and water into glucose and oxygen.
- States that chlorophyll is located within chloroplasts in plant cells.
Examiner tip: Chlorophyll appears green because it reflects green light while absorbing red and blue light most strongly, which it uses to power photosynthesis.
6.
Describe the starch test used to determine whether a leaf has photosynthesised, and explain why the leaf must first be de-starched (destarched) before an investigation.
[4 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that the leaf is boiled in water then in ethanol to remove chlorophyll (so colour changes are visible), then rinsed and softened in warm water. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that iodine solution is added to the leaf, which turns blue-black in the presence of starch (and stays orange-brown if no starch is present). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that plants must be de-starched (kept in the dark for at least 24 hours before the investigation) to use up any existing starch stores. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Develop this part of the answer: States that de-starching ensures any starch detected afterward was produced during the investigation itself, giving a valid (fair) test of the conditions being investigated. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: De-starching is a control step: without it, you cannot be sure whether any starch detected was made during the experiment or was already present beforehand.
Marking points
- States that the leaf is boiled in water then in ethanol to remove chlorophyll (so colour changes are visible), then rinsed and softened in warm water.
- States that iodine solution is added to the leaf, which turns blue-black in the presence of starch (and stays orange-brown if no starch is present).
- States that plants must be de-starched (kept in the dark for at least 24 hours before the investigation) to use up any existing starch stores.
- States that de-starching ensures any starch detected afterward was produced during the investigation itself, giving a valid (fair) test of the conditions being investigated.
Examiner tip: De-starching is a control step: without it, you cannot be sure whether any starch detected was made during the experiment or was already present beforehand.
7.
State three raw materials or conditions required for photosynthesis to occur.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States carbon dioxide. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States water. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States light (energy) and/or chlorophyll. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Photosynthesis requires both raw materials (carbon dioxide and water) and conditions (light and chlorophyll) — a complete answer should distinguish these where relevant.
Marking points
- States carbon dioxide.
- States water.
- States light (energy) and/or chlorophyll.
Examiner tip: Photosynthesis requires both raw materials (carbon dioxide and water) and conditions (light and chlorophyll) — a complete answer should distinguish these where relevant.
8.
Explain why increasing carbon dioxide concentration increases the rate of photosynthesis only up to a certain point, beyond which the rate no longer increases.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that at low carbon dioxide concentration, carbon dioxide is the limiting factor, so increasing it increases the rate of photosynthesis. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that beyond a certain concentration, carbon dioxide is no longer the limiting factor. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that another factor (e.g. light intensity or temperature) has become limiting instead, so further increases in carbon dioxide have no additional effect on the rate. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: This is another application of the limiting factors principle — the same reasoning applies whether the factor under discussion is light, carbon dioxide, or temperature.
Marking points
- States that at low carbon dioxide concentration, carbon dioxide is the limiting factor, so increasing it increases the rate of photosynthesis.
- States that beyond a certain concentration, carbon dioxide is no longer the limiting factor.
- States that another factor (e.g. light intensity or temperature) has become limiting instead, so further increases in carbon dioxide have no additional effect on the rate.
Examiner tip: This is another application of the limiting factors principle — the same reasoning applies whether the factor under discussion is light, carbon dioxide, or temperature.
9.
Describe the exchange of gases between a leaf and the atmosphere during the day, when both photosynthesis and respiration are occurring simultaneously.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that both photosynthesis (using carbon dioxide, producing oxygen) and respiration (using oxygen, producing carbon dioxide) occur continuously in the leaf during the day. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that during bright daylight, the rate of photosynthesis usually exceeds the rate of respiration. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that as a result, there is a net uptake of carbon dioxide from the atmosphere and a net release of oxygen into it. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Both processes occur simultaneously at all times in a photosynthesising leaf; the observed net gas exchange simply reflects whichever process is currently occurring faster.
Marking points
- States that both photosynthesis (using carbon dioxide, producing oxygen) and respiration (using oxygen, producing carbon dioxide) occur continuously in the leaf during the day.
- States that during bright daylight, the rate of photosynthesis usually exceeds the rate of respiration.
- States that as a result, there is a net uptake of carbon dioxide from the atmosphere and a net release of oxygen into it.
Examiner tip: Both processes occur simultaneously at all times in a photosynthesising leaf; the observed net gas exchange simply reflects whichever process is currently occurring faster.
10.
Explain why, at night, a plant releases only carbon dioxide and no oxygen into the atmosphere, even though it produced oxygen during the day.
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that photosynthesis requires light and therefore cannot occur at night, so no oxygen is produced. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that respiration continues at night (it does not depend on light), consuming oxygen and releasing carbon dioxide as the only gas exchange occurring. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Respiration occurs continuously in all living plant cells, day and night; photosynthesis occurs only when light is available — this is why gas exchange reverses direction at night.
Marking points
- States that photosynthesis requires light and therefore cannot occur at night, so no oxygen is produced.
- States that respiration continues at night (it does not depend on light), consuming oxygen and releasing carbon dioxide as the only gas exchange occurring.
Examiner tip: Respiration occurs continuously in all living plant cells, day and night; photosynthesis occurs only when light is available — this is why gas exchange reverses direction at night.
11.
During vigorous exercise, muscle cells may not receive enough oxygen to meet their energy demands through aerobic respiration alone. Describe what happens in muscle cells under these conditions, and explain what is meant by 'oxygen debt'.
[4 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that muscle cells switch to (or supplement aerobic respiration with) anaerobic respiration, breaking down glucose without oxygen. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that this produces lactic acid as the only product, and releases much less energy per glucose molecule than aerobic respiration. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that lactic acid builds up in the muscles, contributing to muscle fatigue. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Develop this part of the answer: Defines oxygen debt as the extra oxygen the body needs to take in after exercise to break down (oxidise) the accumulated lactic acid. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Anaerobic respiration in muscle releases far less energy per glucose molecule than aerobic respiration, which is why it can only supplement, not replace, aerobic respiration for sustained activity.
Marking points
- States that muscle cells switch to (or supplement aerobic respiration with) anaerobic respiration, breaking down glucose without oxygen.
- States that this produces lactic acid as the only product, and releases much less energy per glucose molecule than aerobic respiration.
- States that lactic acid builds up in the muscles, contributing to muscle fatigue.
- Defines oxygen debt as the extra oxygen the body needs to take in after exercise to break down (oxidise) the accumulated lactic acid.
Examiner tip: Anaerobic respiration in muscle releases far less energy per glucose molecule than aerobic respiration, which is why it can only supplement, not replace, aerobic respiration for sustained activity.
12.
Yeast can respire anaerobically in a process called fermentation. Write the word equation for this process, and state one industrial use of yeast fermentation.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: Writes glucose → ethanol + carbon dioxide. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that this process, unlike anaerobic respiration in animal muscle, produces ethanol and carbon dioxide rather than lactic acid. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States a valid industrial use, e.g. brewing (producing alcoholic drinks) or baking, where the carbon dioxide produced makes bread dough rise. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The same anaerobic process (glucose broken down without oxygen) produces different final products depending on the organism: lactic acid in animal muscle, but ethanol and carbon dioxide in yeast and plants.
Marking points
- Writes glucose → ethanol + carbon dioxide.
- States that this process, unlike anaerobic respiration in animal muscle, produces ethanol and carbon dioxide rather than lactic acid.
- States a valid industrial use, e.g. brewing (producing alcoholic drinks) or baking, where the carbon dioxide produced makes bread dough rise.
Examiner tip: The same anaerobic process (glucose broken down without oxygen) produces different final products depending on the organism: lactic acid in animal muscle, but ethanol and carbon dioxide in yeast and plants.
13.
Describe an experiment using germinating seeds and limewater to show that respiration produces carbon dioxide, including a control needed to make the experiment valid.
[4 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that germinating seeds are placed in a sealed container/flask connected by a tube to limewater. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that as the seeds respire, carbon dioxide produced passes into the limewater, which turns milky/cloudy. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that a control flask, containing an equal quantity of dead (boiled) seeds, is set up under identical conditions, and its limewater should remain clear. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Develop this part of the answer: States that this control shows that the carbon dioxide is produced by the living process of respiration in the germinating seeds, not by some other, non-biological factor. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Using dead (boiled) seeds as a control isolates respiration as the variable being tested — since boiling destroys the seeds' enzymes and living cells, any remaining carbon dioxide production cannot be biological.
Marking points
- States that germinating seeds are placed in a sealed container/flask connected by a tube to limewater.
- States that as the seeds respire, carbon dioxide produced passes into the limewater, which turns milky/cloudy.
- States that a control flask, containing an equal quantity of dead (boiled) seeds, is set up under identical conditions, and its limewater should remain clear.
- States that this control shows that the carbon dioxide is produced by the living process of respiration in the germinating seeds, not by some other, non-biological factor.
Examiner tip: Using dead (boiled) seeds as a control isolates respiration as the variable being tested — since boiling destroys the seeds' enzymes and living cells, any remaining carbon dioxide production cannot be biological.
14.
Describe three features of a typical leaf that adapt it for efficient photosynthesis.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that the leaf is broad and thin, giving a large surface area for light absorption and a short diffusion distance for gases. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that the leaf contains air spaces within the spongy mesophyll layer, allowing carbon dioxide and oxygen to diffuse easily to and from the photosynthesising cells. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that the leaf has a network of veins, transporting water to the leaf in the xylem, and transporting the glucose produced away from the leaf in the phloem. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Each leaf adaptation serves photosynthesis in a different way: shape maximises light capture, internal air spaces maximise gas exchange, and veins supply raw materials while removing products.
Marking points
- States that the leaf is broad and thin, giving a large surface area for light absorption and a short diffusion distance for gases.
- States that the leaf contains air spaces within the spongy mesophyll layer, allowing carbon dioxide and oxygen to diffuse easily to and from the photosynthesising cells.
- States that the leaf has a network of veins, transporting water to the leaf in the xylem, and transporting the glucose produced away from the leaf in the phloem.
Examiner tip: Each leaf adaptation serves photosynthesis in a different way: shape maximises light capture, internal air spaces maximise gas exchange, and veins supply raw materials while removing products.
15.
Describe how guard cells control the opening and closing of a stoma.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that when a guard cell takes in water by osmosis and becomes turgid, its uneven (thicker inner) cell wall causes it to bend, opening the stomatal pore between the pair of guard cells. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that when a guard cell loses water and becomes flaccid, it straightens, closing the stomatal pore. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that stomata are typically open during the day, when light is available for photosynthesis and gas exchange is needed, and often closed at night, helping to reduce water loss. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Guard cells control stomatal opening through the same turgor-pressure mechanism seen throughout plant biology — the uneven thickness of their cell wall is what converts turgor into bending rather than simple swelling.
Marking points
- States that when a guard cell takes in water by osmosis and becomes turgid, its uneven (thicker inner) cell wall causes it to bend, opening the stomatal pore between the pair of guard cells.
- States that when a guard cell loses water and becomes flaccid, it straightens, closing the stomatal pore.
- States that stomata are typically open during the day, when light is available for photosynthesis and gas exchange is needed, and often closed at night, helping to reduce water loss.
Examiner tip: Guard cells control stomatal opening through the same turgor-pressure mechanism seen throughout plant biology — the uneven thickness of their cell wall is what converts turgor into bending rather than simple swelling.
16.
Explain why the rate of respiration increases as temperature rises from 10 °C to the optimum temperature, but decreases sharply above this optimum.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that increasing temperature increases the kinetic energy of molecules, increasing the frequency of successful collisions between the enzymes controlling respiration and their substrates. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that this increases the rate of the enzyme-controlled reactions of respiration, up to the optimum temperature. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that above the optimum temperature, the enzymes controlling respiration become denatured, since their active site changes shape, so the rate of reaction decreases sharply. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Respiration is itself a sequence of enzyme-controlled reactions, so its temperature response follows exactly the same rise-then-fall pattern as any other enzyme-controlled process.
Marking points
- States that increasing temperature increases the kinetic energy of molecules, increasing the frequency of successful collisions between the enzymes controlling respiration and their substrates.
- States that this increases the rate of the enzyme-controlled reactions of respiration, up to the optimum temperature.
- States that above the optimum temperature, the enzymes controlling respiration become denatured, since their active site changes shape, so the rate of reaction decreases sharply.
Examiner tip: Respiration is itself a sequence of enzyme-controlled reactions, so its temperature response follows exactly the same rise-then-fall pattern as any other enzyme-controlled process.
17.
Describe how discs cut from a leaf can be used to investigate the rate of photosynthesis at different light intensities.
[4 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that leaf discs are first submerged in a solution containing sodium hydrogencarbonate (a carbon dioxide source), with air removed from their air spaces, so that they initially sink. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that the discs are placed under a light source at a measured distance (intensity). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that as the discs photosynthesise, oxygen produced collects in their air spaces, making them buoyant, so they rise to the surface. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Develop this part of the answer: States that the time taken for a certain number (or proportion) of discs to rise to the surface is measured and used as an indirect measure of the rate of photosynthesis, where a shorter time indicates a faster rate. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: This method uses the physical buoyancy created by trapped oxygen gas as an indirect, easily observable measure of photosynthetic rate, avoiding the need to directly measure gas volumes.
Marking points
- States that leaf discs are first submerged in a solution containing sodium hydrogencarbonate (a carbon dioxide source), with air removed from their air spaces, so that they initially sink.
- States that the discs are placed under a light source at a measured distance (intensity).
- States that as the discs photosynthesise, oxygen produced collects in their air spaces, making them buoyant, so they rise to the surface.
- States that the time taken for a certain number (or proportion) of discs to rise to the surface is measured and used as an indirect measure of the rate of photosynthesis, where a shorter time indicates a faster rate.
Examiner tip: This method uses the physical buoyancy created by trapped oxygen gas as an indirect, easily observable measure of photosynthetic rate, avoiding the need to directly measure gas volumes.
18.
State what is meant by the 'compensation point' of a plant, in terms of the rates of photosynthesis and respiration.
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that the compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that at the compensation point, there is no net exchange of gases between the plant and its surroundings, since all gases produced by one process are used up by the other. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Below the compensation point (in dim light or darkness), a plant is a net consumer of oxygen and net producer of carbon dioxide, exactly as if it were not photosynthesising at all.
Marking points
- States that the compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration.
- States that at the compensation point, there is no net exchange of gases between the plant and its surroundings, since all gases produced by one process are used up by the other.
Examiner tip: Below the compensation point (in dim light or darkness), a plant is a net consumer of oxygen and net producer of carbon dioxide, exactly as if it were not photosynthesising at all.
19.
State three uses that a plant makes of the glucose produced during photosynthesis.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that glucose is used in respiration to release energy for the plant's life processes. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that glucose is converted into starch for storage, since starch is insoluble and does not affect the cell's water potential. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that glucose is used to make cellulose for building new cell walls, or combined with other nutrients such as nitrates to make amino acids and proteins. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Converting glucose to starch for storage is essential precisely because glucose, being soluble, would otherwise lower the cell's water potential and disrupt osmotic balance if stored in large quantities.
Marking points
- States that glucose is used in respiration to release energy for the plant's life processes.
- States that glucose is converted into starch for storage, since starch is insoluble and does not affect the cell's water potential.
- States that glucose is used to make cellulose for building new cell walls, or combined with other nutrients such as nitrates to make amino acids and proteins.
Examiner tip: Converting glucose to starch for storage is essential precisely because glucose, being soluble, would otherwise lower the cell's water potential and disrupt osmotic balance if stored in large quantities.
20.
A graph shows the rate of photosynthesis increasing steeply with light intensity at first, then levelling off (plateauing) at higher light intensities. Explain what is limiting the rate of photosynthesis in each of these two regions of the graph.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that in the steep initial region, light intensity is the limiting factor, since increasing light intensity directly increases the rate. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that in the plateau (flat) region, light intensity is no longer limiting, since further increases in light intensity produce no further increase in rate. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that in the plateau region, a different factor, such as carbon dioxide concentration or temperature, has become the limiting factor instead. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Whenever a rate-against-factor graph plateaus, this is always a signal that a different variable has become the new limiting factor — the plateau itself never means the process has simply 'stopped needing' the original factor.
Marking points
- States that in the steep initial region, light intensity is the limiting factor, since increasing light intensity directly increases the rate.
- States that in the plateau (flat) region, light intensity is no longer limiting, since further increases in light intensity produce no further increase in rate.
- States that in the plateau region, a different factor, such as carbon dioxide concentration or temperature, has become the limiting factor instead.
Examiner tip: Whenever a rate-against-factor graph plateaus, this is always a signal that a different variable has become the new limiting factor — the plateau itself never means the process has simply 'stopped needing' the original factor.
21.
Compare photosynthesis and respiration in terms of the type of energy transfer involved and where each occurs within a plant cell.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that photosynthesis is an energy-absorbing process, converting light energy into chemical energy stored in glucose, and occurs only in chloroplasts. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that respiration is an energy-releasing process, releasing the chemical energy stored in glucose for the cell to use, and occurs in the cytoplasm and mitochondria. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that respiration occurs continuously in all living plant cells, while photosynthesis only occurs in cells containing chlorophyll when light is available. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Photosynthesis and respiration are essentially opposite processes in terms of energy flow, yet both occur simultaneously in a green plant cell during daylight, in different organelles.
Marking points
- States that photosynthesis is an energy-absorbing process, converting light energy into chemical energy stored in glucose, and occurs only in chloroplasts.
- States that respiration is an energy-releasing process, releasing the chemical energy stored in glucose for the cell to use, and occurs in the cytoplasm and mitochondria.
- States that respiration occurs continuously in all living plant cells, while photosynthesis only occurs in cells containing chlorophyll when light is available.
Examiner tip: Photosynthesis and respiration are essentially opposite processes in terms of energy flow, yet both occur simultaneously in a green plant cell during daylight, in different organelles.
22.
Hydrogencarbonate indicator solution is red in atmospheric air, turns yellow in a higher concentration of carbon dioxide, and turns purple in a lower concentration of carbon dioxide. A test tube containing hydrogencarbonate indicator and a green plant is left in bright light for several hours. Predict and explain the colour change observed.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: Predicts that the indicator will turn purple. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: Explains that in bright light, the plant's rate of photosynthesis exceeds its rate of respiration, so there is a net uptake (removal) of carbon dioxide from the air in the tube. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that this decrease in carbon dioxide concentration causes the indicator to turn purple. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Hydrogencarbonate indicator is a practical tool for detecting the direction of net carbon dioxide exchange indirectly, without needing to measure gas volumes directly.
Marking points
- Predicts that the indicator will turn purple.
- Explains that in bright light, the plant's rate of photosynthesis exceeds its rate of respiration, so there is a net uptake (removal) of carbon dioxide from the air in the tube.
- States that this decrease in carbon dioxide concentration causes the indicator to turn purple.
Examiner tip: Hydrogencarbonate indicator is a practical tool for detecting the direction of net carbon dioxide exchange indirectly, without needing to measure gas volumes directly.
23.
Explain why a person's breathing rate remains higher than normal for some time after they stop vigorous exercise.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that during vigorous exercise, some anaerobic respiration occurred in the muscles, producing lactic acid and building up an oxygen debt. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that after exercise stops, extra oxygen is still needed to break down (oxidise) this accumulated lactic acid. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that the elevated breathing rate continues until this oxygen debt has been fully repaid, supplying the extra oxygen needed. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The continued heavy breathing after exercise is direct physical evidence of the oxygen debt built up during anaerobic respiration — it stops only once all the lactic acid has been broken down.
Marking points
- States that during vigorous exercise, some anaerobic respiration occurred in the muscles, producing lactic acid and building up an oxygen debt.
- States that after exercise stops, extra oxygen is still needed to break down (oxidise) this accumulated lactic acid.
- States that the elevated breathing rate continues until this oxygen debt has been fully repaid, supplying the extra oxygen needed.
Examiner tip: The continued heavy breathing after exercise is direct physical evidence of the oxygen debt built up during anaerobic respiration — it stops only once all the lactic acid has been broken down.
24.
Explain why respiration occurs continuously in every living cell, both day and night, while photosynthesis only occurs in green plant cells when light is available.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that respiration releases the energy that all living cells need for their continuous life processes, such as active transport, movement, protein synthesis and cell division, so it cannot stop even temporarily. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that respiration does not require light, so it can occur at any time, day or night. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that photosynthesis only occurs in cells containing chlorophyll (mainly green plant or algal cells) and requires light energy, so it stops whenever light is unavailable. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Respiration is a universal requirement of all living cells in all organisms, while photosynthesis is a specialised process restricted to cells with chlorophyll — this asymmetry is why one process can never stop but the other regularly does.
Marking points
- States that respiration releases the energy that all living cells need for their continuous life processes, such as active transport, movement, protein synthesis and cell division, so it cannot stop even temporarily.
- States that respiration does not require light, so it can occur at any time, day or night.
- States that photosynthesis only occurs in cells containing chlorophyll (mainly green plant or algal cells) and requires light energy, so it stops whenever light is unavailable.
Examiner tip: Respiration is a universal requirement of all living cells in all organisms, while photosynthesis is a specialised process restricted to cells with chlorophyll — this asymmetry is why one process can never stop but the other regularly does.
25.
Explain the role of ATP as the immediate energy source in cells, linking it to the process of respiration.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that the energy released from the breakdown of glucose during respiration is used to make ATP (adenosine triphosphate). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that ATP is the immediate, usable form of energy in cells, and is broken down to release energy exactly when and where it is needed for a cell process. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States a valid example of an energy-requiring process powered by ATP, e.g. active transport, muscle contraction, or protein synthesis. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: ATP acts like a rechargeable battery inside the cell: respiration 'recharges' it using energy from glucose, and cell processes then 'discharge' it wherever energy is needed.
Marking points
- States that the energy released from the breakdown of glucose during respiration is used to make ATP (adenosine triphosphate).
- States that ATP is the immediate, usable form of energy in cells, and is broken down to release energy exactly when and where it is needed for a cell process.
- States a valid example of an energy-requiring process powered by ATP, e.g. active transport, muscle contraction, or protein synthesis.
Examiner tip: ATP acts like a rechargeable battery inside the cell: respiration 'recharges' it using energy from glucose, and cell processes then 'discharge' it wherever energy is needed.