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Admissions tests / ESAT / Biology / Cells, membranes and transport

Demanding. 15 questions, 15 marks, about 27 minutes.

ESAT Biology: Cells, membranes and transport, set 3

Sub-cellular components of eukaryotic and prokaryotic cells, diffusion, osmosis in terms of water potential, active transport, and the mitotic cell cycle.

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  • Answer all questions. No calculator.
  • Each question has exactly one correct answer.
  1. 11 mark

    A herbaceous (non-woody) plant such as a sunflower seedling relies partly on cell turgidity, rather than woody tissue, to hold its stem upright. Which sub-cellular structure is filled with cell sap and, by pressing outward against the cell wall when full of water, is most directly responsible for producing this turgor pressure?

    1. A Mitochondrion
    2. B Nucleus
    3. C Cell wall
    4. D Vacuole
  2. 21 mark

    Unlike the single chromosomal DNA molecule that carries most of a bacterium's genes, small circular pieces of DNA are also present in its cytoplasm, called plasmids. Which statement correctly distinguishes plasmid DNA from the bacterium's chromosomal DNA?

    1. A Plasmid DNA is found only in eukaryotic cells, whereas chromosomal DNA is found in prokaryotic cells.
    2. B Plasmid DNA is enclosed within a nucleus, whereas chromosomal DNA is free in the cytoplasm.
    3. C Plasmid DNA exists as small, separate circles in the cytoplasm, in addition to the main chromosomal DNA.
    4. D Plasmid DNA and chromosomal DNA are two names for the same single molecule of DNA in a bacterium.
  3. 31 mark

    The stomach's muscular wall is built from smooth muscle cells grouped into smooth muscle tissue, which combines with epithelial and glandular tissue to form the stomach itself. The stomach in turn works with the oesophagus and intestines as part of the digestive system. Which level of organisation does 'the stomach' represent in this description?

    1. A Organ
    2. B Cell
    3. C Tissue
    4. D Organ system
  4. 41 mark

    A tissue engineer grows two cube-shaped clumps of cells in a nutrient dish: one with sides of 2 mm, the other with sides of 4 mm. Relying only on diffusion to supply nutrients to the cells at the centre of each clump, which statement correctly compares them?

    1. A The 4 mm cube has a surface area to volume ratio of 3:1, twice that of the 2 mm cube, so it will be better supplied by diffusion.
    2. B The 2 mm cube has a surface area to volume ratio of 3:1, twice that of the 4 mm cube, so it will be better supplied by diffusion.
    3. C Both cubes have the same surface area to volume ratio of 3:1, since increasing every side length by the same factor increases surface area and volume by the same factor.
    4. D The 2 mm cube has a surface area to volume ratio of 1:3 and the 4 mm cube has a ratio of 1:1.5, so the 4 mm cube will be better supplied by diffusion.
  5. 51 mark

    A potato cylinder of mass 4.80 g is left in a concentrated sucrose solution for 24 hours. Afterwards it has a mass of 4.32 g. Calculate the percentage change in the potato's mass, and state what this shows about the water potential of the sucrose solution relative to the potato cells.

    1. A -11.1%; the solution has a lower water potential than the potato cells, so water left the cells by osmosis.
    2. B -10%; the solution has a higher water potential than the potato cells, so water left the cells by osmosis.
    3. C +10%; the solution has a lower water potential than the potato cells, so water left the cells by osmosis.
    4. D -10%; the solution has a lower water potential than the potato cells, so water left the cells by osmosis.
  6. 61 mark

    Soil water typically contains a lower concentration of nitrate ions than the cytoplasm of a root hair cell, yet the plant continues to absorb nitrate ions from the soil into the root. Which statement correctly explains this movement?

    1. A This is active transport, moving nitrate ions against their concentration gradient using energy from respiration.
    2. B This is diffusion, since ions always move into a cell regardless of the concentration gradient.
    3. C This is osmosis, since ions move down the water potential gradient created by the difference in nitrate concentration.
    4. D This is active transport, moving nitrate ions down their concentration gradient without using energy.
  7. 71 mark

    A body cell about to enter the cell cycle contains a certain amount of DNA before replication begins; call this amount x. During interphase this DNA is replicated. The cell then undergoes mitosis, producing two daughter cells. What amount of DNA does each daughter cell contain immediately after mitosis, compared with the original cell before replication?

    1. A 2x, since the DNA content doubled by replication remains in each daughter cell after mitosis.
    2. B 4x, since the DNA is doubled once during interphase and doubled again during mitosis before being shared out.
    3. C x, since mitosis shares the doubled DNA equally between the two daughter cells, restoring the original amount.
    4. D 0.5x, since mitosis halves the DNA content of the cell, in the same way that meiosis does.
  8. 81 mark

    A single-celled organism reproduces asexually by mitotic division, and each division doubles the number of cells present. Starting from a single cell, how many cells are present after 4 successive divisions?

    1. A 8
    2. B 16
    3. C 4
    4. D 32
  9. 91 mark

    A mutation occurs in a cell that damages a gene normally responsible for controlling when the cell divides. As a result the cell and its descendants divide repeatedly and without the usual controls, eventually forming a tumour. Which statement best explains why this counts as an example of cancer, as understood at this level?

    1. A Because the mutation has caused a change in the cell that leads to uncontrolled cell division.
    2. B Because the mutation has stopped the cell from replicating its DNA, so it can no longer divide by mitosis.
    3. C Because the mutation has converted the cell from undergoing mitosis to undergoing meiosis.
    4. D Because the mutation has caused the cell to lose its nucleus, preventing normal cell division.
  10. 101 mark

    A dog's body cells each contain 78 chromosomes. In the testes, primary germ cells undergo meiosis to produce sperm cells. If 6 primary germ cells each complete meiosis, how many sperm cells are produced in total, and how many chromosomes does each contain?

    1. A 12 sperm cells, each containing 39 chromosomes
    2. B 24 sperm cells, each containing 78 chromosomes
    3. C 6 sperm cells, each containing 39 chromosomes
    4. D 24 sperm cells, each containing 39 chromosomes
  11. 111 mark

    Full siblings (other than identical twins) share both parents but are not identical to each other or to either parent. Which explanation for this correctly involves the role of meiosis?

    1. A Meiosis produces gametes that are genetically identical to the parent, so mixing two such gametes at fertilisation is what produces variation between siblings.
    2. B Meiosis is not involved in producing siblings; sexual reproduction relies entirely on mitosis to generate variation between offspring.
    3. C Meiosis produces genetically different haploid gametes, so each fertilisation combines a different pair of gametes, giving each sibling a different combination of genetic material.
    4. D Meiosis doubles the chromosome number in each gamete, so each sibling inherits twice the normal amount of genetic material from each parent.
  12. 121 mark

    A population of bacteria reproduces asexually by simple division, while a population of rabbits in the same habitat reproduces sexually. Assuming no mutations occur in the bacteria, which statement correctly compares the genetic variation within each population that reproduction alone produces?

    1. A The bacteria show more variation than the rabbits, because asexual reproduction happens much faster.
    2. B The rabbits show more variation than the bacteria, because sexual reproduction combines genetic material from two parents while asexual reproduction with no mutation produces genetically identical offspring.
    3. C The bacteria show the same variation as the rabbits, because both populations are reproducing to produce offspring.
    4. D The bacteria show more variation than the rabbits, because bacteria reproduce with only one parent, and having only one parent increases variation.
  13. 131 mark

    A couple plan to have two children. Assuming each pregnancy is independent and each child is equally likely to be male or female, what is the probability that both children are female?

    1. A 1/2
    2. B 1/3
    3. C 1/8
    4. D 1/4
  14. 141 mark

    In species where sex is determined by an XX/XY system, offspring sex depends on which sex chromosome the sperm cell carries, since all egg cells carry an X chromosome. A mother has already had three daughters. What is the probability that her fourth child will be a son?

    1. A 1/2, since each pregnancy is an independent event and earlier children's sex has no effect on this one.
    2. B 1/16, since the family has now had four children, and the probability of a specific overall pattern of four children is 1/16.
    3. C Less than 1/2, since after three daughters a son is 'due' to balance out the sex ratio in the family.
    4. D 1/8, since three daughters have already occurred, leaving three remaining possible sex combinations for the fourth child.
  15. 151 mark

    A plant root hair cell and a red blood cell are each placed into pure water. The plant cell takes in water by osmosis and becomes turgid, but does not burst; the red blood cell also takes in water by osmosis, but continues to swell until it bursts. Which structure explains this difference between the two cells?

    1. A The plant cell has a nucleus, which controls how much water can enter the cell, whereas the red blood cell has no nucleus.
    2. B The plant cell has mitochondria, which pump excess water back out of the cell using energy, whereas the red blood cell has none.
    3. C The plant cell has a cell wall, which resists the internal pressure and prevents the cell bursting, whereas the red blood cell has no cell wall.
    4. D The plant cell has a partially permeable membrane, whereas the red blood cell's membrane is fully permeable to water.

Worked solutions

Every question below carries the reasoning, not just the answer. The official material for this test publishes a correct option letter and nothing else.

  1. Question 1Answer: D

    1. The vacuole is the plant-only structure that stores cell sap, largely a solution of water and dissolved substances.
    2. When full of water, the vacuole presses the cytoplasm and cell membrane firmly against the cell wall, creating turgor pressure.
    3. The mitochondrion (respiration) and the nucleus (genetic control) play no direct part in producing this pressure.
    4. The cell wall resists the pressure and keeps the cell from bursting, but it does not generate the pressure itself; that is produced by the vacuole's water content.
    5. Therefore the answer is D.
    • Why not A: This assigns the support role to the organelle that releases energy from respiration, but generating turgor pressure through water storage is not a function of the mitochondrion.
    • Why not B: This assigns the support role to the nucleus, confusing its function of storing and controlling access to genetic material with the separate, physical role of maintaining pressure through water storage.
    • Why not C: This is a partial truth: the cell wall does resist the outward pressure and gives the cell its shape, but it is the vacuole's water content that generates the pressure in the first place; the question asks for the structure responsible for producing that pressure, not resisting it.

  2. Question 2Answer: C

    1. The sub-cellular components of a prokaryotic cell include cell membrane, cytoplasm, cell wall, chromosomal DNA (with no true nucleus), and plasmid DNA: two distinct DNA components.
    2. A bacterium has no nucleus at all, so neither type of DNA is enclosed within one.
    3. Plasmids are a feature of prokaryotic cells, existing as small circles of DNA in the cytoplasm, separate from and in addition to the single chromosomal DNA molecule.
    4. Therefore the answer is C.
    • Why not A: This wrongly places plasmids in eukaryotic cells; plasmids are a feature of prokaryotic (bacterial) cells, alongside chromosomal DNA, not a replacement for it in a different cell type.
    • Why not B: This wrongly gives the bacterium a nucleus; a defining feature of a prokaryotic cell is that it has no true nucleus, so neither its chromosomal DNA nor its plasmid DNA is enclosed by one.
    • Why not D: This treats plasmid DNA and chromosomal DNA as the same thing, when the specification lists them as two separate components of a bacterial cell: one main chromosomal DNA molecule, and separate, smaller plasmid DNA.

  3. Question 3Answer: A

    1. The levels of organisation run from cells, to tissues, to organs, to organ systems.
    2. The stomach is described as built from more than one tissue type working together, which is exactly what makes it an organ rather than a single tissue.
    3. The stomach is also described as one part of the larger digestive system, alongside the oesophagus and intestines, so it cannot itself be the organ system.
    4. Therefore the answer is A.
    • Why not B: This treats the stomach as a single cell, when the passage already describes it as built from many cells grouped into tissues; a cell is the simplest level, and the stomach is far more complex than that.
    • Why not C: This treats the stomach as a single tissue, but the passage describes it as formed from smooth muscle tissue combined with epithelial and glandular tissue; combining more than one tissue type produces an organ, not a tissue.
    • Why not D: This treats the stomach itself as the organ system, but the passage places the stomach alongside the oesophagus and intestines as part of a larger digestive system; the stomach is one organ within that system, not the system itself.

  4. Question 4Answer: B

    1. Surface area of a cube = 6 x side^2; volume = side^3.
    2. For the 2 mm cube: surface area = 6 x 4 = 24 mm^2, volume = 8 mm^3, so the ratio is 24:8 = 3:1.
    3. For the 4 mm cube: surface area = 6 x 16 = 96 mm^2, volume = 64 mm^3, so the ratio is 96:64 = 1.5:1.
    4. The 2 mm cube's ratio (3:1) is twice the 4 mm cube's ratio (1.5:1), so the smaller cube has more surface area available for diffusion per unit of volume needing supply.
    5. A higher surface area to volume ratio means diffusion can supply the tissue more effectively, so the 2 mm cube is better supplied.
    6. Therefore the answer is B.
    • Why not A: This correctly finds that one cube's ratio is twice the other's, but assigns the larger ratio to the larger cube; surface area to volume ratio falls as an object gets bigger, so it is the smaller, 2 mm cube that has the higher ratio, not the 4 mm cube.
    • Why not C: This assumes the surface area to volume ratio stays constant when every dimension is scaled up by the same factor, but surface area scales with the square of the side length while volume scales with the cube of it, so doubling the side length here halves the ratio rather than leaving it unchanged.
    • Why not D: This inverts the ratio, calculating volume to surface area rather than surface area to volume, which reverses which cube looks 'better supplied'; a genuinely larger surface area to volume ratio means more exchange surface for a given volume, which favours the smaller cube, not the larger one.

  5. Question 5Answer: D

    1. Percentage change = (final mass - original mass) / original mass x 100.
    2. Change in mass = 4.32 - 4.80 = -0.48 g.
    3. Percentage change = -0.48 / 4.80 x 100 = -10%.
    4. The potato cylinder has lost mass, so net water movement was out of the potato cells and into the surrounding solution.
    5. Water moves by osmosis from a region of higher water potential to a region of lower water potential, so the potato cells had the higher water potential and the solution had the lower water potential.
    6. Therefore the answer is D.
    • Why not A: This divides the 0.48 g change by the final mass of 4.32 g rather than the original mass of 4.80 g; percentage change must always be calculated relative to the starting value, which gives 10%, not 11.1%.
    • Why not B: This correctly calculates the 10% fall in mass but reverses the water potential comparison; water moves from a higher to a lower water potential, and since water left the potato, the potato's cells must have had the higher water potential, making the solution's water potential the lower one, not the higher one.
    • Why not C: This correctly identifies the water potential relationship but reports the percentage change as a gain rather than a loss, ignoring that the potato's mass fell from 4.80 g to 4.32 g; a decrease in mass must be expressed as a negative percentage change.

  6. Question 6Answer: A

    1. The nitrate ions move from a region of lower concentration in the soil water into the root hair cell, which already has a higher concentration of nitrate ions: that is, against the concentration gradient.
    2. Diffusion and osmosis both move substances down a gradient (concentration or water potential), so neither can explain movement against a gradient.
    3. Active transport moves substances across a membrane against a concentration gradient, and it needs energy, supplied by respiration in the cell.
    4. Therefore the answer is A.
    • Why not B: This wrongly names the process diffusion, and wrongly claims particles move regardless of gradient; diffusion moves particles down a concentration gradient, from high to low concentration, but nitrate ions are here moving from a lower concentration (the soil) to a higher one (the root hair cell), against the gradient.
    • Why not C: This wrongly names the process osmosis, which specifically describes the movement of water across a partially permeable membrane, not the movement of dissolved mineral ions such as nitrate.
    • Why not D: This correctly names active transport but wrongly describes the ions as moving down their concentration gradient and needing no energy; active transport is defined by moving substances against a concentration gradient, which requires an energy input from respiration, not the absence of one.

  7. Question 7Answer: C

    1. Before replication, a body cell has a DNA content of x.
    2. During interphase, this DNA is replicated once, doubling the content to 2x, ready for division.
    3. Mitosis then shares this 2x of DNA equally between the two daughter cells it produces.
    4. Each daughter cell therefore ends up with 2x / 2 = x, the same DNA content as the original cell before replication.
    5. Therefore the answer is C.
    • Why not A: This correctly notes that interphase doubles the DNA content to 2x, but stops there rather than following the DNA through mitosis; mitosis is precisely the division that shares this doubled amount equally between two daughter cells, so each daughter ends up with x, not the full 2x.
    • Why not B: This wrongly assumes DNA replication happens a second time during mitosis itself; DNA replication happens once, during interphase, and mitosis is purely a division process that shares out the DNA already produced, not a second doubling.
    • Why not D: This wrongly applies the halving that is characteristic of meiosis, which produces haploid gametes with half the normal chromosome number; mitosis instead restores each daughter cell to the original, pre-replication DNA content, x, not half of that amount.

  8. Question 8Answer: B

    1. Starting from 1 cell, each mitotic division doubles the number of cells present.
    2. After 1 division: 2 cells. After 2 divisions: 4 cells. After 3 divisions: 8 cells. After 4 divisions: 16 cells.
    3. This is 2^4 = 16.
    4. Therefore the answer is B.
    • Why not A: This gives 2^3 = 8, one division short of the four asked for; after the fourth division the cell number doubles once more, from 8 to 16.
    • Why not C: This treats the number of divisions as the answer itself (4), rather than the doubling effect each division has on the cell count; after 4 doublings from a single cell, the count is 2^4, not simply 4.
    • Why not D: This gives 2^5 = 32, one division too many; after only 4 divisions, not 5, the cell count reaches 16.

  9. Question 9Answer: A

    1. Cancer results from changes in cells, including mutations, that lead to uncontrolled cell division.
    2. The scenario describes exactly this: a mutation damages a control gene, and the affected cell and its descendants divide repeatedly without the usual controls.
    3. This is not a loss of the ability to divide, a switch to meiosis, or a loss of the nucleus; it is specifically a loss of control over mitotic division.
    4. Therefore the answer is A.
    • Why not B: This reverses the described effect; the mutation has not stopped division, it has removed the controls on it, so the cell divides more, not less, than normal.
    • Why not C: This wrongly swaps the type of division involved; the uncontrolled division described here is still mitotic division, producing genetically identical daughter cells, not meiosis, which produces gametes.
    • Why not D: This invents a detail not given in the scenario; the passage describes a mutation affecting a gene that controls when the cell divides, not the loss of the nucleus itself.

  10. Question 10Answer: D

    1. Meiosis involves two divisions, producing four daughter cells from each parent (germ) cell, each with a single copy of each chromosome: that is, haploid.
    2. Half of 78 is 39, so each sperm cell contains 39 chromosomes.
    3. With 6 primary germ cells each producing 4 sperm cells, the total is 6 x 4 = 24 sperm cells.
    4. Therefore the answer is D.
    • Why not A: This correctly halves the chromosome number to 39, but only doubles the number of germ cells (6 x 2) rather than producing the four sperm cells that meiosis's two divisions give from each germ cell (6 x 4 = 24), so it undercounts the total sperm cells.
    • Why not B: This correctly counts 24 sperm cells in total, but forgets that meiosis halves the chromosome number; each sperm cell is haploid, with 39 chromosomes, not the full 78 found in the dog's body cells.
    • Why not C: This treats each germ cell as producing only one sperm cell, giving a total equal to the number of germ cells (6) rather than the four sperm cells that each germ cell's two meiotic divisions actually produce (6 x 4 = 24).

  11. Question 11Answer: C

    1. Meiosis produces genetically different haploid gametes from a parent, rather than identical copies.
    2. Because each gamete produced is different, each act of fertilisation combines a different pair of gametes, giving each resulting offspring (sibling) a different combination of genetic material.
    3. This is why siblings, despite sharing the same two parents, are not genetically identical to each other or to either parent.
    4. Therefore the answer is C.
    • Why not A: This wrongly claims meiosis produces genetically identical gametes; meiosis's defining outcome, unlike mitosis, is genetically different gametes, and this difference between gametes is exactly what allows different siblings to inherit different combinations.
    • Why not B: This wrongly removes meiosis from the picture; meiosis is specifically the division that produces the haploid gametes needed for sexual reproduction, and it is the source of the genetic variation between siblings.
    • Why not D: This wrongly claims meiosis doubles the chromosome number; meiosis halves the chromosome number, producing haploid gametes, so that fusion at fertilisation restores, rather than doubles beyond, the normal diploid number.

  12. Question 12Answer: B

    1. Asexual reproduction involves one parent, and with no mutation the offspring are genetically identical to that parent.
    2. Sexual reproduction involves two parents, and offspring are genetically different from each other and from both parents, leading to increased variation.
    3. The bacteria here reproduce asexually with no mutation, so they show no variation from reproduction alone; the sexually reproducing rabbits do show variation.
    4. Therefore the answer is B.
    • Why not A: This wrongly links speed of reproduction to genetic variation; how quickly a population reproduces has no bearing on whether reproduction itself, asexual or sexual, produces variation between offspring.
    • Why not C: This wrongly assumes reproduction always produces the same amount of variation regardless of type; asexual reproduction with no mutation produces genetically identical offspring, while sexual reproduction, involving two parents, produces offspring that differ from both parents and each other.
    • Why not D: This wrongly claims having only one parent increases variation; asexual reproduction from a single parent is exactly why the offspring are genetically identical (barring mutation), not more varied, while combining genetic material from two parents in sexual reproduction is what increases variation.

  13. Question 13Answer: D

    1. Each child is independently equally likely to be male or female, with probability 1/2 each.
    2. The probability of both children being female is the product of the two independent probabilities: 1/2 x 1/2 = 1/4.
    3. Therefore the answer is D.
    • Why not A: This gives the probability that a single child is female, but does not combine the two independent events; the probability of both children being female requires multiplying the two separate 1/2 probabilities together.
    • Why not B: This treats the outcome as one of three equally likely possibilities (e.g. two girls, two boys, or one of each), rather than correctly multiplying the independent probabilities of each child's sex; there are four equally likely combinations of two children's sexes, not three.
    • Why not C: This multiplies three 1/2 probabilities together (1/2 x 1/2 x 1/2), as if there were three children or three independent events, rather than the two children the question describes.

  14. Question 14Answer: A

    1. Sex is determined by which sex chromosome the sperm cell carries; every egg carries an X chromosome, and each sperm cell is independently equally likely to carry an X or a Y chromosome.
    2. Because each pregnancy is an independent event, the sex of the three earlier children has no influence on the sex of the fourth.
    3. The probability that the fourth child is a son, regardless of the previous three children's sexes, remains 1/2.
    4. Therefore the answer is A.
    • Why not B: This calculates the probability of a specific overall pattern of four children (three daughters then a son, or any particular ordered sequence of four), which is 1/2^4 = 1/16, rather than the probability of the sex of one single, independent, remaining birth, which is what the question asks.
    • Why not C: This is the gambler's fallacy: assuming that because three daughters have occurred, a son becomes more likely to 'balance things out'. Each pregnancy is an independent event, and the sex of earlier children has no effect on the sex of a later one; the probability of a son remains 1/2, not more.
    • Why not D: This treats the previous three births as somehow reducing the remaining possibilities, but each new pregnancy is a fresh, independent 1/2 chance of a son; there are not 'three remaining combinations' to divide among, since each birth's sex is decided independently of every other.

  15. Question 15Answer: C

    1. Both cells take in water by osmosis because both have a partially permeable cell membrane, so this cannot be what makes them behave differently.
    2. The plant cell additionally has a rigid cell wall surrounding its cell membrane, a structure present in plant cells (and, separately, in bacteria) but not in animal cells.
    3. As water enters the plant cell, the cell wall resists the resulting outward pressure, so the cell becomes turgid but does not burst.
    4. The red blood cell has no cell wall, so nothing resists the pressure of the entering water, and it continues to swell until it bursts.
    5. Therefore the answer is C.
    • Why not A: This wrongly gives the nucleus a role in regulating water entry; the nucleus controls the cell's genetic material and activities, but neither plant nor animal cells regulate osmotic water entry through the nucleus.
    • Why not B: This wrongly gives the mitochondrion an active, energy-using role in removing water; osmosis is a passive process, and mitochondria are not described as pumping water out of a cell.
    • Why not D: This wrongly claims the two cells' membranes differ in this way; both plant and animal cell membranes are partially permeable, which is exactly why both cells take in water by osmosis in the first place. The difference in outcome comes from a structure the plant cell has and the animal cell lacks, not a difference in the membrane itself.

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