AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2020: Question 1

💡 Key knowledge

• Osmosis is the net movement of water through a partially permeable membrane from a region of higher water concentration to lower water concentration.
• In potato cells, water can move in or out depending on the concentration of the surrounding sugar solution.
• The independent variable is what the student changes on purpose.

🧠 Exam technique

• For graph questions, use the data carefully and plot each point accurately.
• If a line of best fit is needed, draw a smooth curve if the pattern is curved.
• When calculating percentage change, always use the starting mass in the denominator.

✅ Correct answer

Concentration of the sugar solution

1 mark for identifying the variable changed by the student.

💡 Key knowledge

The independent variable is the factor the student deliberately changes between tests. Here, each piece of potato is placed in a different concentration of sugar solution.

❌ Common errors

• Change in mass is the dependent variable, not the independent variable.
• Length of time is kept the same, so it is not the independent variable.
• Starting mass is measured, not changed on purpose.

✅ Correct answer

It gained water by osmosis because the concentration of water outside the potato was greater than inside the cells.

You could also say: the sugar solution outside was more dilute / had a higher water concentration than inside the potato.

💡 Key knowledge

• In 0.0 mol/dm³ sugar solution, the outside solution is pure water, so it has a high water concentration.
• Water moves into the potato cells through the partially permeable cell membranes.
• As water enters, the potato’s mass increases.

🧠 Exam technique

• For 2 marks, usually need what happened plus why.
• Use the word osmosis for one mark and a correct explanation of water concentration for the other.
• Examiner accepted reverse wording too, as long as the direction of water movement was correct.

❌ Common errors

• Saying the potato absorbed sugar — the mark scheme is about water.
• Saying “water moves from low to high concentration” without making clear it is water concentration.
• Talking about diffusion of sugar instead of osmosis of water.

✅ Correct answer

Plot all the points correctly from Table 1:

• (0.0, 2.20)
• (0.1, 1.15)
• (0.2, 0.25)
• (0.3, −0.40)
• (0.4, −0.75)
• (0.5, −0.95)

Then draw a curve of best fit through the points.

💡 Key knowledge

• The graph shows a clear downward trend: as sugar concentration increases, change in mass decreases.
• The line should be a smooth curve, not a zig-zag joining dot to dot.
• Marks were awarded for accuracy of plotting and for the line of best fit.

🧠 Exam technique

• Use the same scale as the graph paper and plot carefully to within about ± ½ a square.
• Do not extrapolate beyond the data unless asked.
• A good curve should pass close to all points and show the overall pattern.

❌ Common errors

• Connecting the points with straight line segments instead of drawing a curve.
• Swapping the axes: concentration is the x-axis, change in mass is the y-axis.
• Missing a point or plotting it in the wrong place.

✅ Correct answer

Concentration = 0.23–0.24 mol/dm³

1 mark for a correct reading from the graph. The exact value depends on the line drawn.

💡 Key knowledge

The mass does not change when the concentration inside the potato cells is equal to the concentration outside.

So you find the point where the line of best fit crosses change in mass = 0.

🧠 Exam technique

• Read from the x-axis where the curve crosses the y = 0 line.
• Use the graph carefully and quote the answer in mol/dm³.
• The mark scheme allowed 0.23 or 0.24 mol/dm³ if no line was drawn, showing that a sensible estimate from the table was acceptable.

❌ Common errors

• Giving the y-value instead of the x-value.
• Forgetting units.
• Choosing the wrong crossing point, or using the plotted points instead of the zero-crossing.

📐 Calculations — step by step

Use the values for 0.2 mol/dm³:

• Change in mass = 0.25 g
• Mass at start = 7.96 g

Step 1: Write the formula

percentage change in mass = (change in mass ÷ mass at start) × 100

Step 2: Substitute the values

(0.25 ÷ 7.96) × 100

Step 3: Calculate

= 3.14070352...

Step 4: Round to 3 significant figures

3.14%

✅ Correct answer

3.14%

Marks were available for the correct substitution, correct calculation, and correct rounding to 3 significant figures.

🧠 Exam technique

• Always use the starting mass, not the final mass.
• Keep the units clear: grams cancel in the calculation, so the final answer is a percentage.
• Round the final answer to the number of significant figures asked for.

❌ Common errors

• Using change in mass ÷ final mass instead of starting mass.
• Forgetting to multiply by 100.
• Giving a negative percentage when the mass increased.
• Rounding too early and losing accuracy.

💡 What top answers did well

• Used precise biological language: osmosis, water concentration, partially permeable membrane.
• Plotted the graph accurately and drew a smooth curve.
• Showed calculation working clearly and gave the answer to 3 significant figures.

❌ Where students lost marks

• Confusing the independent variable with the dependent variable.
• Explaining mass change using sugar moving instead of water moving.
• Reading the graph incorrectly or missing the zero-crossing.
• Using the wrong value in the percentage change formula.