Cambridge IGCSE · 0610

Biology

Cells and movement of substances — Topics 1-3

Name: ____________________Date: October 10, 2026

1.
A cell image is 48 mm long. The actual cell is 0.12 mm long. Calculate the magnification.
[3 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Magnification compares image size with actual size. Both measurements are already in millimetres, so no unit conversion is needed.
2. Use M = image size / actual size = 48/0.12 = 400. The image is 400 times the actual length.
3. Report ×400, not 400 mm: the matching length units cancel. Check that 0.12 mm × 400 = 48 mm.
Marking points
- Uses magnification = image size ÷ actual size.
- Substitutes 48 ÷ 0.12 using matching units.
- Obtains ×400.
Examiner tip: Convert units first if the image and actual size use different units.
2.
Explain why a plant cell becomes turgid when placed in pure water.
[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: Pure water has a higher water potential than the cell sap. 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: Water enters through the partially permeable membrane by osmosis. 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: The vacuole expands and pushes the cell contents against the wall. 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: The cell wall resists further expansion, producing turgor. 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: Include both the water-potential gradient and the partially permeable membrane.
Marking points
- Pure water has a higher water potential than the cell sap.
- Water enters through the partially permeable membrane by osmosis.
- The vacuole expands and pushes the cell contents against the wall.
- The cell wall resists further expansion, producing turgor.
Examiner tip: Include both the water-potential gradient and the partially permeable membrane.
3.
State three structures found in a plant cell but not in an animal 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 the cell wall. 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 chloroplasts. 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 large permanent vacuole. 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: These three structures relate directly to functions animal cells do not need: support (wall), photosynthesis (chloroplasts) and turgor/storage (vacuole).
Marking points
- States the cell wall.
- States chloroplasts.
- States a large permanent vacuole.
Examiner tip: These three structures relate directly to functions animal cells do not need: support (wall), photosynthesis (chloroplasts) and turgor/storage (vacuole).
4.
Define diffusion, and explain why a substance diffuses faster when the concentration gradient is steeper.
[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: Defines diffusion as the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. 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 steeper concentration gradient means a larger difference in concentration between the two regions. 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 larger difference results in a greater net (overall) rate of particle movement from the high to the low concentration region. 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: Diffusion requires no energy input from the cell — it occurs passively due to the random motion of particles down a concentration gradient.
Marking points
- Defines diffusion as the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.
- States that a steeper concentration gradient means a larger difference in concentration between the two regions.
- States that this larger difference results in a greater net (overall) rate of particle movement from the high to the low concentration region.
Examiner tip: Diffusion requires no energy input from the cell — it occurs passively due to the random motion of particles down a concentration gradient.
5.
Explain why a red blood cell placed in distilled water eventually bursts (lyses), while a plant cell placed in distilled water does not.
[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 both cases, water enters the cell by osmosis, since distilled water has a higher water potential than the cell contents. 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 red blood cell has no cell wall, so as it takes in water and swells, there is nothing to prevent the membrane from stretching until it bursts. 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 plant cell has a rigid cell wall, which resists the expansion of the cell and prevents the membrane from stretching to bursting point, so the cell becomes turgid 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: The presence or absence of a cell wall is the single key structural difference that determines whether excess water intake causes bursting or simply turgor.
Marking points
- States that in both cases, water enters the cell by osmosis, since distilled water has a higher water potential than the cell contents.
- States that the red blood cell has no cell wall, so as it takes in water and swells, there is nothing to prevent the membrane from stretching until it bursts.
- States that the plant cell has a rigid cell wall, which resists the expansion of the cell and prevents the membrane from stretching to bursting point, so the cell becomes turgid instead.
Examiner tip: The presence or absence of a cell wall is the single key structural difference that determines whether excess water intake causes bursting or simply turgor.
6.
Explain why active transport, unlike diffusion, requires energy from 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 active transport moves substances against their concentration gradient, from a region of lower concentration to a region of higher concentration. 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 moving particles against their natural direction of diffusion does not happen spontaneously and requires energy input. 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 energy is supplied by ATP produced during cell respiration, and is used by carrier proteins in the membrane to actively move the substance. 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: Diffusion is passive (down the gradient, no energy needed); active transport is the opposite (against the gradient, energy required) — the two processes are natural contrasts.
Marking points
- States that active transport moves substances against their concentration gradient, from a region of lower concentration to a region of higher concentration.
- States that moving particles against their natural direction of diffusion does not happen spontaneously and requires energy input.
- States that this energy is supplied by ATP produced during cell respiration, and is used by carrier proteins in the membrane to actively move the substance.
Examiner tip: Diffusion is passive (down the gradient, no energy needed); active transport is the opposite (against the gradient, energy required) — the two processes are natural contrasts.
7.
Root hair cells are adapted for the efficient uptake of water and mineral ions. Describe two features of a root hair cell that adapt it for this function.
[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 root hair cell has a long, thin extension (hair) that greatly increases its surface area in contact with the soil. 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 increased surface area increases the rate of water and ion uptake by diffusion and active transport. 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 cell contains many mitochondria, providing the ATP energy needed for the active transport of mineral ions into the cell against their concentration gradient. 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: Mineral ions are usually more concentrated inside root hair cells than in the surrounding soil, so their uptake generally requires active transport, not just diffusion.
Marking points
- States that the root hair cell has a long, thin extension (hair) that greatly increases its surface area in contact with the soil.
- States that this increased surface area increases the rate of water and ion uptake by diffusion and active transport.
- States that the cell contains many mitochondria, providing the ATP energy needed for the active transport of mineral ions into the cell against their concentration gradient.
Examiner tip: Mineral ions are usually more concentrated inside root hair cells than in the surrounding soil, so their uptake generally requires active transport, not just diffusion.
8.
Outline the function of the nucleus, cell membrane and cytoplasm in a generalised animal 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 the nucleus contains the genetic material (DNA) and controls the cell's activities. 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 cell membrane controls the movement of substances into and out of the cell. 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 cytoplasm is the site of many chemical (metabolic) reactions within the cell. 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: Link each organelle to its function directly — simply naming organelles without stating what they do will not gain full marks.
Marking points
- States that the nucleus contains the genetic material (DNA) and controls the cell's activities.
- States that the cell membrane controls the movement of substances into and out of the cell.
- States that the cytoplasm is the site of many chemical (metabolic) reactions within the cell.
Examiner tip: Link each organelle to its function directly — simply naming organelles without stating what they do will not gain full marks.
9.
A student is given an unlabelled slide and asked to determine whether the cells shown are plant or animal cells. State two observable features that would allow this distinction.
[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 the presence of a clearly defined, regular (often rectangular) cell wall, visible as a distinct outer boundary, as evidence of a plant cell. 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 the presence of visible chloroplasts (small green structures) as further evidence of a plant cell. 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: A visible cell wall and chloroplasts are the two most reliable, easily observed features distinguishing a plant cell from an animal cell under a light microscope.
Marking points
- States the presence of a clearly defined, regular (often rectangular) cell wall, visible as a distinct outer boundary, as evidence of a plant cell.
- States the presence of visible chloroplasts (small green structures) as further evidence of a plant cell.
Examiner tip: A visible cell wall and chloroplasts are the two most reliable, easily observed features distinguishing a plant cell from an animal cell under a light microscope.
10.
Explain why, in a osmosis experiment using visking (dialysis) tubing, glucose can pass through the tubing but starch cannot.
[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 visking tubing is partially permeable, containing pores of a limited, fixed size. 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 molecules are small enough to pass through these pores. 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 starch molecules are much larger (as they are polymers of many glucose units) and are too large to pass through the pores. 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 partially permeable membrane always selects by molecular size — visking tubing is a common model for the selective properties of real cell membranes.
Marking points
- States that visking tubing is partially permeable, containing pores of a limited, fixed size.
- States that glucose molecules are small enough to pass through these pores.
- States that starch molecules are much larger (as they are polymers of many glucose units) and are too large to pass through the pores.
Examiner tip: A partially permeable membrane always selects by molecular size — visking tubing is a common model for the selective properties of real cell membranes.
11.
Explain, using an example, the difference between a tissue, an organ and an organ system.
[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 a tissue is a group of similar cells working together to perform a shared function, e.g. muscle tissue. 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 an organ is made of several different tissues working together to perform a specific function, e.g. the stomach, which contains muscle tissue, glandular tissue and epithelial tissue. 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 an organ system is a group of organs working together to perform a broad function, e.g. the digestive system. 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 hierarchy (cell → tissue → organ → organ system → organism) always increases in complexity, with each level built from units of the level below it.
Marking points
- States that a tissue is a group of similar cells working together to perform a shared function, e.g. muscle tissue.
- States that an organ is made of several different tissues working together to perform a specific function, e.g. the stomach, which contains muscle tissue, glandular tissue and epithelial tissue.
- States that an organ system is a group of organs working together to perform a broad function, e.g. the digestive system.
Examiner tip: This hierarchy (cell → tissue → organ → organ system → organism) always increases in complexity, with each level built from units of the level below it.
12.
Describe three features of a sperm cell that adapt it for its function of fertilising an egg 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 it has a long tail (flagellum) that allows it to swim towards the egg cell. 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 it contains many mitochondria in its middle section, providing the energy (ATP) needed for the tail to move. 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 it has an acrosome at its head, containing enzymes that digest through the egg cell's outer layers to allow fertilisation. 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 structural feature of the sperm cell links to a specific step in fertilisation: the tail for movement, the mitochondria for the energy that movement needs, and the acrosome for penetrating the egg.
Marking points
- States that it has a long tail (flagellum) that allows it to swim towards the egg cell.
- States that it contains many mitochondria in its middle section, providing the energy (ATP) needed for the tail to move.
- States that it has an acrosome at its head, containing enzymes that digest through the egg cell's outer layers to allow fertilisation.
Examiner tip: Each structural feature of the sperm cell links to a specific step in fertilisation: the tail for movement, the mitochondria for the energy that movement needs, and the acrosome for penetrating the egg.
13.
Describe three features of a nerve cell (neurone) that adapt it for its function of rapidly transmitting electrical impulses.
[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 it has a long axon, allowing electrical impulses to travel quickly over long distances within the body. 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 many neurones have a myelin sheath surrounding the axon, which insulates it and speeds up the transmission of impulses. 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 it has many branched dendrites at its ends, allowing it to connect with, and receive impulses from, many other neurones. 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 myelin sheath acts like electrical insulation on a wire — it is why myelinated neurones transmit impulses considerably faster than unmyelinated ones.
Marking points
- States that it has a long axon, allowing electrical impulses to travel quickly over long distances within the body.
- States that many neurones have a myelin sheath surrounding the axon, which insulates it and speeds up the transmission of impulses.
- States that it has many branched dendrites at its ends, allowing it to connect with, and receive impulses from, many other neurones.
Examiner tip: The myelin sheath acts like electrical insulation on a wire — it is why myelinated neurones transmit impulses considerably faster than unmyelinated ones.
14.
Describe three features of a palisade mesophyll cell 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 it contains many chloroplasts, to absorb as much light as possible and maximise 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 it has a regular, column-like (elongated) shape and is packed closely together near the upper surface of the leaf, where light intensity is greatest. 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 it has a thin cell wall, reducing the distance carbon dioxide must diffuse to reach the chloroplasts. 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 arrangement of palisade cells (tall, closely packed, near the upper surface) maximises light capture, since light intensity decreases with depth into the leaf.
Marking points
- States that it contains many chloroplasts, to absorb as much light as possible and maximise the rate of photosynthesis.
- States that it has a regular, column-like (elongated) shape and is packed closely together near the upper surface of the leaf, where light intensity is greatest.
- States that it has a thin cell wall, reducing the distance carbon dioxide must diffuse to reach the chloroplasts.
Examiner tip: The arrangement of palisade cells (tall, closely packed, near the upper surface) maximises light capture, since light intensity decreases with depth into the leaf.
15.
Describe three features of a red blood cell that adapt it for its function of transporting oxygen.
[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 it has a biconcave disc shape, which increases its surface area to volume ratio for efficient gas exchange (diffusion of 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 it has no nucleus, leaving more internal space for haemoglobin. 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 it contains haemoglobin, a protein that binds reversibly to oxygen, allowing the cell to carry oxygen around the body. 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: Losing its nucleus is a highly unusual adaptation among human cells, made specifically to maximise the red blood cell's carrying capacity for haemoglobin (and therefore oxygen).
Marking points
- States that it has a biconcave disc shape, which increases its surface area to volume ratio for efficient gas exchange (diffusion of oxygen).
- States that it has no nucleus, leaving more internal space for haemoglobin.
- States that it contains haemoglobin, a protein that binds reversibly to oxygen, allowing the cell to carry oxygen around the body.
Examiner tip: Losing its nucleus is a highly unusual adaptation among human cells, made specifically to maximise the red blood cell's carrying capacity for haemoglobin (and therefore oxygen).
16.
An image of a cell has a magnification of ×250. If the image is 20 mm long, calculate the actual length of the cell, in micrometres (μm).
[3 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
2. Work through this mathematical step: Uses actual size = image size ÷ magnification. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
3. Work through this mathematical step: Obtains actual size = 20 ÷ 250 = 0.08 mm. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
4. Work through this mathematical step: Converts to micrometres: 0.08 × 1000 = 80 μm. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Rearranging magnification = image size ÷ actual size gives actual size = image size ÷ magnification — always double-check which quantity is unknown before rearranging.
Marking points
- Uses actual size = image size ÷ magnification.
- Obtains actual size = 20 ÷ 250 = 0.08 mm.
- Converts to micrometres: 0.08 × 1000 = 80 μm.
Examiner tip: Rearranging magnification = image size ÷ actual size gives actual size = image size ÷ magnification — always double-check which quantity is unknown before rearranging.
17.
A cell has an actual length of 0.025 mm. Calculate the length of its image, in mm, at a magnification of ×2000.
[2 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
2. Work through this mathematical step: Uses image size = actual size × magnification. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
3. Work through this mathematical step: Obtains image size = 0.025 × 2000 = 50 mm. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The magnification triangle (magnification, image size, actual size) works exactly like other rate/ratio triangles — cover the unknown quantity to see which operation to use.
Marking points
- Uses image size = actual size × magnification.
- Obtains image size = 0.025 × 2000 = 50 mm.
Examiner tip: The magnification triangle (magnification, image size, actual size) works exactly like other rate/ratio triangles — cover the unknown quantity to see which operation to use.
18.
A student places equal-sized cylinders of potato tissue into different concentrations of sugar solution and measures their mass before and after 24 hours. State what would happen to the mass of a potato cylinder placed in (a) pure water, and (b) a concentrated sugar solution, explaining each in terms of osmosis.
[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 in pure water, the potato cylinder's mass would increase. 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 this is because pure water has a higher water potential than the cells of the potato, so water moves into the cells by osmosis. 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 a concentrated sugar solution, the potato cylinder's mass would decrease. 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: Explains that this is because the sugar solution has a lower water potential than the cells of the potato, so water moves out of the cells by osmosis. 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 experiment can also identify the potato's own water potential: the specific sugar concentration at which the cylinder's mass does not change at all is the point where the external solution's water potential exactly matches the potato cells'.
Marking points
- States that in pure water, the potato cylinder's mass would increase.
- Explains that this is because pure water has a higher water potential than the cells of the potato, so water moves into the cells by osmosis.
- States that in a concentrated sugar solution, the potato cylinder's mass would decrease.
- Explains that this is because the sugar solution has a lower water potential than the cells of the potato, so water moves out of the cells by osmosis.
Examiner tip: This experiment can also identify the potato's own water potential: the specific sugar concentration at which the cylinder's mass does not change at all is the point where the external solution's water potential exactly matches the potato cells'.
19.
Explain why surface area to volume ratio decreases as an organism (or cell) increases in size, and state why this is important for larger organisms.
[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 as an object's linear size increases, its volume increases faster than its surface area (volume increases with the cube of size, while surface area increases with the square of size). 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 means surface area to volume ratio decreases as size increases. 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 because diffusion alone becomes too slow to supply and remove substances across a relatively small surface area for a large volume, larger organisms need specialised exchange surfaces and transport systems to move substances efficiently. 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 the fundamental reason large multicellular organisms need circulatory and respiratory systems, while single-celled organisms can rely entirely on diffusion across their outer membrane.
Marking points
- States that as an object's linear size increases, its volume increases faster than its surface area (volume increases with the cube of size, while surface area increases with the square of size).
- States that this means surface area to volume ratio decreases as size increases.
- States that because diffusion alone becomes too slow to supply and remove substances across a relatively small surface area for a large volume, larger organisms need specialised exchange surfaces and transport systems to move substances efficiently.
Examiner tip: This is the fundamental reason large multicellular organisms need circulatory and respiratory systems, while single-celled organisms can rely entirely on diffusion across their outer membrane.
20.
State three factors, other than the concentration gradient, that affect the rate of diffusion.
[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 temperature affects the rate of diffusion: a higher temperature increases the rate, since particles have more kinetic energy and move faster. 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 surface area affects the rate of diffusion: a larger surface area increases the rate, since more particles can cross at once. 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 distance affects the rate of diffusion: a shorter distance (thinner membrane or exchange surface) increases the rate, since particles travel a shorter distance. 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: Efficient exchange surfaces in living organisms (such as alveoli or root hairs) are adapted to maximise surface area and minimise diffusion distance simultaneously — both factors working together.
Marking points
- States that temperature affects the rate of diffusion: a higher temperature increases the rate, since particles have more kinetic energy and move faster.
- States that surface area affects the rate of diffusion: a larger surface area increases the rate, since more particles can cross at once.
- States that distance affects the rate of diffusion: a shorter distance (thinner membrane or exchange surface) increases the rate, since particles travel a shorter distance.
Examiner tip: Efficient exchange surfaces in living organisms (such as alveoli or root hairs) are adapted to maximise surface area and minimise diffusion distance simultaneously — both factors working together.
21.
Explain how facilitated diffusion allows large or charged molecules, such as glucose, to cross a cell membrane, and state one way it is similar to simple diffusion.
[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 facilitated diffusion uses specific carrier (or channel) proteins embedded in the cell membrane to transport molecules that cannot easily pass directly through the phospholipid bilayer. 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 molecule binds to the carrier protein, which changes shape to move the molecule across the membrane. 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, like simple diffusion, facilitated diffusion is passive: it moves substances down their concentration gradient and requires no energy from respiration. 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: Facilitated diffusion is still passive (no energy required) and still moves substances down their concentration gradient — the only difference from simple diffusion is the need for a specific carrier protein.
Marking points
- States that facilitated diffusion uses specific carrier (or channel) proteins embedded in the cell membrane to transport molecules that cannot easily pass directly through the phospholipid bilayer.
- States that the molecule binds to the carrier protein, which changes shape to move the molecule across the membrane.
- States that, like simple diffusion, facilitated diffusion is passive: it moves substances down their concentration gradient and requires no energy from respiration.
Examiner tip: Facilitated diffusion is still passive (no energy required) and still moves substances down their concentration gradient — the only difference from simple diffusion is the need for a specific carrier protein.
22.
State what a plant cell wall is made of, and describe two functions it performs.
[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 cell wall is made of cellulose. 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 it provides structural support/strength to the cell, helping the plant remain upright. 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 it is fully permeable and prevents the cell from bursting, by resisting expansion, when the cell takes in water by osmosis and becomes turgid. 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 cell wall is fully permeable (unlike the partially permeable cell membrane just inside it) — it never restricts what can pass through it, only providing mechanical strength.
Marking points
- States that the cell wall is made of cellulose.
- States that it provides structural support/strength to the cell, helping the plant remain upright.
- States that it is fully permeable and prevents the cell from bursting, by resisting expansion, when the cell takes in water by osmosis and becomes turgid.
Examiner tip: The cell wall is fully permeable (unlike the partially permeable cell membrane just inside it) — it never restricts what can pass through it, only providing mechanical strength.
23.
State a formal definition of osmosis, in terms of water potential and a partially permeable membrane.
[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 osmosis is the net movement of water molecules. 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 movement occurs from a region of higher water potential to a region of lower 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 this movement occurs through a partially permeable membrane. 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: Osmosis is simply a special case of diffusion that applies specifically to water molecules moving across a partially permeable membrane — all three parts of this definition are needed for full marks.
Marking points
- States that osmosis is the net movement of water molecules.
- States that this movement occurs from a region of higher water potential to a region of lower water potential.
- States that this movement occurs through a partially permeable membrane.
Examiner tip: Osmosis is simply a special case of diffusion that applies specifically to water molecules moving across a partially permeable membrane — all three parts of this definition are needed for full marks.
24.
Describe three features of a xylem vessel that adapt it for the transport of water.
[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 xylem vessels are made of dead cells, with no cell contents (cytoplasm or nucleus), forming a continuous hollow tube with no obstruction to water flow. 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 cell walls are thickened and strengthened with lignin, providing strength to withstand the pressure of water movement and to support the plant. 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 end walls between adjacent cells have broken down, forming one continuous, uninterrupted tube. 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: Xylem vessels being made of dead cells is actually essential to their function: living contents would only obstruct the continuous, open tube needed for efficient water transport.
Marking points
- States that xylem vessels are made of dead cells, with no cell contents (cytoplasm or nucleus), forming a continuous hollow tube with no obstruction to water flow.
- States that the cell walls are thickened and strengthened with lignin, providing strength to withstand the pressure of water movement and to support the plant.
- States that the end walls between adjacent cells have broken down, forming one continuous, uninterrupted tube.
Examiner tip: Xylem vessels being made of dead cells is actually essential to their function: living contents would only obstruct the continuous, open tube needed for efficient water transport.
25.
Describe three features of phloem tissue that adapt it for the transport of dissolved sugars (translocation).
[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 sieve tube cells are living cells arranged end to end, with perforated sieve plates between them allowing sugar solution to flow through. 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 sieve tube cells have very few organelles, and lack a nucleus, leaving space for the flow of sugar solution. 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 each sieve tube cell is closely associated with a companion cell, which contains a nucleus and many mitochondria, providing the energy needed to actively load sugars into the sieve tube. 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: Sieve tube cells rely entirely on their neighbouring companion cells for metabolic support, since they themselves have lost their nucleus and most organelles to maximise space for sugar transport.
Marking points
- States that sieve tube cells are living cells arranged end to end, with perforated sieve plates between them allowing sugar solution to flow through.
- States that sieve tube cells have very few organelles, and lack a nucleus, leaving space for the flow of sugar solution.
- States that each sieve tube cell is closely associated with a companion cell, which contains a nucleus and many mitochondria, providing the energy needed to actively load sugars into the sieve tube.
Examiner tip: Sieve tube cells rely entirely on their neighbouring companion cells for metabolic support, since they themselves have lost their nucleus and most organelles to maximise space for sugar transport.