AQA AS Level Biology Paper 2, November 2020: Question 8

9 marks · Medium difficulty · Practical Techniques & Data Analysis

Calculate the percentage error of mean phloem pressure from table data, explain how pressure is generated via the mass flow hypothesis, describe the relationship between phloem pressure and xylem water movement from graphs, and explain why phloem pressure is reduced during the hottest part of the day.

Practise this question

Question

A four-part biology question based on phloem pressure and water movement in plants. Question 08.1 provides a table (Table 3) of 9 phloem pressure measurements and asks to calculate the percentage error of the mean. Question 08.2 asks to explain how pressure is generated inside the phloem tube using the mass flow hypothesis. Question 08.3 provides two line graphs (Figure 6) showing phloem pressure and rate of water movement in xylem over a 24-hour period, asking to describe their relationship. Question 08.4 asks to explain why phloem pressure is reduced during the hottest part of the day using Figure 6 and understanding of transpiration and mass flow.
Question text

08.1 A scientist measured the pressure in a phloem tube in a willow plant stem.

He repeated his measurements to obtain nine readings.

His results are shown in Table 3.

Table 3

Phloem pressure / arbitrary units

7.4 8.0 7.0 8.6 8.2 9.3 7.4 9.1 8.8

The percentage error of the mean phloem pressure in this phloem tube is calculated

using this equation.

uncertainty in measurement

Percentage error = ×100

mean

The uncertainty in measurement is half the range of the measured values.

Calculate the percentage error of the mean phloem pressure in this phloem tube.

Show your working.

[2 marks]

Percentage error23 %

08.2 The mass flow hypothesis is used to explain the movement of substances through

phloem.

Use your understanding of the mass flow hypothesis to explain how pressure is

generated inside this phloem tube.

[3 marks]

08.3 The scientist also measured changes in the phloem pressure and changes in the rate

of water movement in the xylem of a willow plant at intervals during a day.

His results are shown in Figure 6.

Figure 6

Describe the relationship between phloem pressure and the rate of water movement

in xylem in this plant.

[1 mark]

08.4 Phloem pressure is reduced during the hottest part of the day. Use information in

Figure 6 along with your understanding of transpiration and mass flow to explain why.

[3 marks]

Mark scheme

Show the mark scheme The mark scheme provides answers for questions 08.1 to 08.4. For 08.1, the correct percentage error is 14% or 14.02% (2 marks), with intermediate marks for correct mean (8.2) or uncertainty (1.15). For 08.2, points include active transport of sucrose into phloem, lowering water potential, and water moving in by osmosis from xylem (3 marks). For 08.3, answers note that phloem pressure falls as water movement in xylem increases or that they are inversely proportional (1 mark). For 08.4, points cover high transpiration/evaporation, water lost through stomata leading to high tension in xylem, and less water moving from xylem to phloem due to insufficient water potential (3 marks). Total marks: 9.

Question Marking Guidance Mark Comments

08.1 Correct answer for 2 marks = 14/14.02/14.024;; 2

Accept for 1 mark,

mean = 8.2

OR

uncertainty = 1.15

08.2 1. Sucrose actively transported (into phloem); 3

2. Lowering/reducing water potential

OR

More negative water potential;

3. Water moves (into phloem) by osmosis (from

xylem);

08.3 Phloem pressure falls as (rate of) water movement 1 Accept converse

(in xylem) increases

OR

Inversely proportional;

08.4 1. High (rate of) transpiration/evaporation; 3

2. Water lost through stomata

OR

(High) tension in xylem;

3. (Causes) less water movement from xylem to

phloem

OR

Insufficient water potential in phloem to draw

water from xylem;

TOTAL 9

How to answer it

Transport in Plants: Phloem Pressure & Mass Flow

What this question tests

This question assesses your understanding of transport in plants, specifically the mechanism of the mass flow hypothesis, data interpretation from graphs and tables, calculating percentage errors using ranges and means, and linking transpiration rates in the xylem to pressure changes in the phloem.

Question 08.1

Calculating Percentage Error from Experimental Data

📐 Step-by-Step Calculation

  1. Identify values from Table 3: 7.4, 8.0, 7.0, 8.6, 8.2, 9.3, 7.4, 9.1, 8.8 (9 readings).
  2. Calculate the mean: Sum of values (73.8) ÷ 9 = 8.2 .
  3. Find the uncertainty (half the range): Maximum value (9.3) − Minimum value (7.0) = 2.3. Then, 2.3 ÷ 2 = 1.15 .
  4. Apply formula: Percentage error = (1.15 ÷ 8.2) × 100 = 14.024% .

✅ Correct Answer & Marking

Answer: 14 / 14.02 / 14.024 %

Mark breakdown (2 marks):
• 2 marks for correct final answer.
• 1 mark awarded for correctly identifying either the mean ( 8.2 ) or the uncertainty ( 1.15 ) if the final calculation is incorrect.

❌ Common Student Errors

  • Forgetting to halve the range: Many students use the full range (2.3) directly in the numerator instead of calculating half the range (1.15).
  • Arithmetic slips: Miscounting or misadding the raw data values leading to an incorrect mean. Always re-add your values twice!
Question 08.2

The Mass Flow Hypothesis: Generating Phloem Pressure

💡 Key Knowledge Required

  • Assimilation loading occurs at source cells (leaves). Sucrose is actively transported into the sieve tube elements.
  • This lowers the water potential (makes it more negative) inside the phloem sieve tube.
  • Water follows passively down its water potential gradient from the adjacent xylem into the phloem by osmosis, increasing hydrostatic pressure.

🧠 Exam Technique & Marking Points

This is a classic 3-mark sequence question. Examiners look for cause-and-effect chains in order:

  • Point 1: Sucrose actively transported (into phloem).
  • Point 2: Lowers water potential / makes water potential more negative.
  • Point 3: Water moves into phloem by osmosis from the xylem.
Question 08.3

Interpreting Graph Trends

✅ Correct Answer & Examiner Guidance

Answer: Phloem pressure falls as the rate of water movement in the xylem increases (or stating they are inversely proportional).

Mark breakdown (1 mark): Awarded for a clear, comparative statement linking the two variables shown in Figure 6. Make sure to reference both phloem pressure and xylem water movement explicitly.
Question 08.4

Linking Transpiration, Xylem Tension, and Phloem Pressure

💡 Key Knowledge Required

  • During the hottest part of the day, light intensity and temperature peak, causing a high rate of transpiration/evaporation of water through open stomata.
  • This creates high tension (negative pressure) pulling water up the xylem column.
  • Because water is pulled strongly up the xylem, less water moves sideways into the phloem by osmosis (or there is insufficient water potential gradient between xylem and phloem), reducing hydrostatic pressure in the phloem.

🧠 Exam Technique & Examiner Insights

Top-level responses seamlessly bridge two distinct topics: water transport in xylem (transpiration pull) and translocation in phloem (mass flow). Ensure you explicitly connect environmental conditions (hot part of the day) to stomatal water loss and subsequent water potential gradients.

Mark breakdown (3 marks):
1. High rate of transpiration / water lost through stomata.
2. High tension in xylem.
3. Causes less water movement from xylem to phloem (or insufficient water potential to draw water from xylem).

Topics

Biology · Practical skills · 3.3 Organisms exchange substances with their environment · Data analysis

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.