AQA AS Level Biology Paper 2, June 2019: Question 5
10 marks · Medium difficulty · Short Answer
Investigate water movement in celery stalks involving transpiration rate measurements, transport mechanisms in xylem vessels, safe scalpel use, and statistical processing of distance data.
Practise this questionQuestion
Question text
05 A student used the apparatus shown in Figure 5 and a digital balance to determine
the rate of water movement in a celery stalk in grams per hour per group of xylem
vessels.
Figure 5
05.1 The student measured the time taken for water movement.
Give two other measurements he made to calculate the rate of water movement.
[2 marks]
05.2 Give the reason for adding a layer of oil to the water in the beaker.
[1 mark]
05.3 A different student used coloured water to investigate the movement of water in leaf
stalks of celery.
During the procedure she:
• cut equal lengths of stalk from each plant
• put the cut end of each stalk into coloured water
• left these stalks to take up the coloured water for 20 minutes
• used a sharp scalpel to cut slices from the stalks at 1 mm intervals until she reached
a slice with no coloured water.
Figure 6 shows a slice of leaf stalk with coloured water inside groups of xylem
vessels.
Figure 6
Explain why coloured water moved up the stalks.
[3 marks]
05.4 The student used a sharp scalpel to cut the celery. Describe how she should ensure
she handled the scalpel safely during this procedure.
[2 marks]
The student measured the distance the coloured water had travelled in eight celery
stalks.
Her results are shown in Table 1.
Table 1
Distance / mm
70 35 40 35 30 80 42 44
05.5 The student had to choose whether to summarise her measurements by calculating
the mean, the median or the mode.
Circle the most appropriate measure for this set of measurements.
Give a reason for your choice and find the value using the measurements from all
eight stalks.
[2 marks]
Mean* Median* Mode*
*circle one word.
Reason:
Calculation:
Answer =
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
1. Initial and final mass (of beaker and all 2 1. Accept change in
contents); mass/weight
2. Number of (groups of) xylem vessels;
1. Ignore volume
1. Reject change in
05.1 mass of
celery/beaker/water
alone
2. Accept amount for
‘number’
Prevent evaporation/water loss 1
OR
05.2
(So) evaporation/water loss/transpiration only from
celery;
1. Water evaporates/is transpired (from leaves/ 3 2. Accept negative
stalk/celery/plant); pressure for tension
2. Water potential gradient/lower water potential
creates tension/pulls up water
05.3
OR
Osmosis creates tension/pulls up water;
3. Hydrogen bonds/cohesion/adhesion maintains
column;
1. Cut away from body; 2 1. Accept description
2. Against hard/non-slip/flat surface; of cutting technique to
05.4 avoid cutting fingers
2. Accept named hard
surface eg tile/board
Median (no mark) 2 1. Accept anomalies /
1. (Presence of) outliers/80/70 extremes for ‘outliers’
OR
Small sample size/8 (measurements);
2. 41;
05.5
Accept for 1 mark,
Mean of 47
OR
Mode of 35
TOTAL 10
How to answer it
Water Movement in Plants & Transpiration
What this question tests
This question assesses your understanding of transpiration, the cohesion-tension theory of water transport in xylem, practical methodology (potometers and handling safety), and biological data analysis (choosing appropriate statistical summary measures like the median in the presence of anomalies).
Experimental Measurements for Rate of Water Movement
✅ Correct Answer
- 1. Initial and final mass (of beaker and all contents).
- 2. Number of (groups of) xylem vessels.
💡 Key Knowledge
To calculate a rate per hour per group of xylem vessels, you need to know the total mass lost over time (change in mass) and normalise it against the biological structure transporting the water (the xylem groups).
❌ Common Errors
Students often lose marks by writing vague terms like "change in mass" without specifying initial and final mass, or by referring to "volume" instead of mass. You cannot accept changes in the mass of the celery alone, as the mass lost is measured via the container setup.
The Role of Oil in Transpiration Experiments
✅ Correct Answer
Prevent evaporation / water loss (from the water surface / beaker), ensuring that any water loss measured is due to transpiration from the celery only.
🧠 Exam Technique
Always link control variables in potometer experiments directly to isolating the independent variable. The oil forms an impermeable barrier stopping direct evaporation into the air.
Explaining Water Movement up Stalks (Cohesion-Tension Theory)
✅ Correct Answer
- Water evaporates / is transpired from the leaves / celery.
- Creates a water potential gradient / lower water potential, creating tension (or negative pressure) that pulls water up.
- Hydrogen bonding / cohesion / adhesion maintains a continuous, unbroken water column.
💡 Key Knowledge
This is a classic AS Biology explanation question. You must hit all three logical stages in sequence: Evaporation at the leaves → Generation of tension via a water potential gradient → Maintenance of the column via cohesion/hydrogen bonds.
❌ Common Errors
Avoid saying "suction", as examiners penalize unscientific terminology. Use "tension" or "negative pressure". Ensure you explicitly mention hydrogen bonds or cohesion between water molecules.
Lab Safety: Using a Scalpel
✅ Correct Answer
- Cut away from the body.
- Cut against a hard, non-slip, flat surface (such as a tile or cutting board).
🧠 Exam Technique
Standard safety questions require actionable, preventative instructions. General statements like "be careful" or "wear goggles" do not answer how to safely use a scalpel to slice plant tissue.
Data Analysis and Statistical Choice
✅ Correct Answer
Circled: Median
Reason: Presence of outliers (80, 70) / extreme values / small sample size (8 measurements).
Calculation Answer: 41
📐 Step-by-Step Calculation
- Step 1: List the raw data values from Table 1 in ascending order:
30, 35, 35, 40, 42, 44, 70, 80 - Step 2: Identify the middle position for an even sample size ($n = 8$). The middle two values are the 4th and 5th terms ( 40 and 42 ).
- Step 3: Calculate the mean of these two middle values: (40 + 42) / 2 = 41 .
❌ Common Calculation Traps
Remember to rank data in order before finding the median! Failing to sort the numbers first leads to incorrect middle-value selection.
Topics
Biology · Practical skills · Experimental design · Data analysis · 3.3 Organisms exchange substances with their environment
Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.