This set covers diffusion, osmosis, active transport, percentage change in mass and cautious conclusions. The questions run from easier to harder. All data are invented for practice.
Write a full answer on paper before opening each worked answer. Notice which type of error you make, because the section at the end sends each type to the right lesson.
Questions and worked answers
Q1. Complete the definition: “Diffusion is the ___ movement of particles from a region of ___ concentration to a region of ___ concentration, down a concentration ___, as a result of their ___ movement.”
Show answer
Net; higher; lower; gradient; random. The word net matters because particles move in all directions, but more move down the gradient than up it.
Q2. Oxygen is at 0.09 units outside a cell and 0.02 units inside. State the direction of net movement and give a reason.
Show answer
Net movement is into the cell. The concentration is higher outside (0.09) than inside (0.02), so oxygen moves down its concentration gradient by diffusion.
Q3. Calculate the surface area to volume ratio of a cube with sides 3 cm. Compare it with a 1 cm cube, which has a ratio of 6 : 1.
Show answer
Surface area = 6 × 3 × 3 = 54 cm². Volume = 3 × 3 × 3 = 27 cm³. Ratio = 54 : 27 = 2 : 1. The 3 cm cube has a smaller ratio than the 1 cm cube (6 : 1), so diffusion to its centre is slower for its size.
Q4. A bag made of a partially permeable membrane holds 25% sugar solution and is placed in 5% sugar solution. The model states that only water can cross. Predict what happens to the mass of the bag and explain.
Show answer
The solution inside (25%) is more concentrated, so it has the lower water potential. Water moves by osmosis from the 5% solution (higher water potential) into the bag. The bag gains water, so its mass increases. The sugar cannot cross, so it does not move out.
Q5. A potato cylinder is placed in distilled water and becomes firmer. Explain why.
Show answer
Distilled water has a higher water potential than the cell contents. Water enters the cells by osmosis through the partially permeable membrane. The cells become swollen and pressed against their walls, so the cylinder feels firmer and its mass increases.
Q6. A cylinder’s mass changes from 2.50 g to 2.20 g. Calculate the percentage change in mass.
Show answer
Change = 2.20 − 2.50 = −0.30 g. Percentage change = −0.30 ÷ 2.50 × 100 = −12%. The negative sign shows a loss of mass.
Q7. A second cylinder’s mass changes from 1.80 g to 2.07 g. Calculate the percentage change and say what it tells you about the solution.
Show answer
Change = 2.07 − 1.80 = +0.27 g. Percentage change = 0.27 ÷ 1.80 × 100 = +15%. The cylinder gained water by osmosis, so the solution had a higher water potential than the cell contents, meaning it was more dilute.
Q8. For each situation, say whether it is diffusion or active transport and give one reason.
(a) Carbon dioxide moves from a cell with a high concentration into blood with a low concentration. (b) A root hair cell takes in an ion from soil water at 0.02 units into a cell at 0.15 units, and uptake slows when oxygen is reduced.
Show answer
(a) Diffusion. The movement is down the concentration gradient and needs no energy from the cell.
(b) Active transport. The ion moves against the gradient (from 0.02 to 0.15), and the dependence on oxygen shows that it needs energy from respiration.
Q9. The rate of ion uptake by root cells is 12 units with oxygen and 3 units when oxygen is removed. Calculate the percentage fall and explain what it suggests.
Show answer
Fall = 12 − 3 = 9 units. Percentage fall = 9 ÷ 12 × 100 = 75%. Less oxygen means less aerobic respiration, so less energy is available. A large fall suggests that uptake depends on energy, which is consistent with active transport.
Q10. On a graph, the percentage change in mass is +9% at 0.1 mol/dm³ and −3% at 0.3 mol/dm³. Estimate the concentration at which there is no change in mass.
Show answer
The total difference is 9 − (−3) = 12 percentage points. Zero is 9 ÷ 12 = 0.75 of the way along the 0.2 mol/dm³ gap, so 0.1 + 0.75 × 0.2 = 0.25 mol/dm³. This is an estimate, because the true line may curve.
Q11. A student says: “At equilibrium, diffusion stops and the particles stop moving.” Correct this and explain why it matters in an answer.
Show answer
At equilibrium the net movement stops, but particles continue to move randomly, with equal numbers crossing in each direction. Saying “net” shows you understand that diffusion is the result of random movement, and it avoids losing the mark for a vague statement.
Q12. In an experiment, 10 onion cells in a concentrated sucrose solution all lost water. A student writes: “This proves that all plants die in strong solutions.” Write a better conclusion and name one thing that would be needed to support the student’s claim.
Show answer
Better conclusion: “In this experiment all 10 onion cells lost water by osmosis in the concentrated sucrose solution, which suggests that plant cells lose water when the outside solution has a lower water potential.” The claim about dying plants needs evidence from whole plants over a longer time, for example measuring survival or growth, and a larger and more varied sample.
If you got these wrong
Match the type of error to the lesson:
| Type of error | Questions | Go to |
|---|---|---|
| Missing key words in a diffusion explanation, or the equilibrium idea | Q1, Q2, Q3, Q11 | Explain diffusion using a concentration difference |
| Water direction backwards, or following the solute | Q4, Q5 | Predict water movement in a stated osmosis model |
| Confusing active transport with diffusion | Q8, Q9 | Distinguish active transport from diffusion |
| Percentage change or estimate mistakes | Q6, Q7, Q10 | Interpret a mass-change dataset |
| Claims that go further than the evidence | Q12 | Separate a cell response from a whole-organism claim |
Keep a record of repeated slips in the mistake log and retest queue, and return to the movement across membranes overview to check the study order.
If a particular type of question keeps defeating you, a teacher in online one-to-one Biology tuition can work through your own working and set a short retest.