AQA GCSE Biology Biology Paper 2 (Higher), June 2022: Question 9
11 marks · High Demand difficulty · Short Answer
Investigate plant shoot responses to light and the distribution and breakdown of auxin under different conditions.
Practise this questionQuestion
Question text
09 Students investigated the response of plant shoots to one-sided light.
Figure 13 shows how the students set up three experiments.
Figure 13
09.1 Suggest two control variables the students should have used in their investigation.
[2 marks]
09.2 Describe how experiment B and experiment C acted as controls for the investigation.
*34* [2 marks]
Experiment B
Experiment C
09.3 Give two conclusions that the students could make from the ink marks on the shoot
in experiment A.
[2 marks]
09.4 Name the type of response shown by the seedling in experiment A.
[1 mark]
Auxin is a plant hormone. Auxin is made in the shoot tip.
Scientists investigated the role of auxin in the response of shoot tips to light.
This is the method used.
1. Grow four seedlings in the dark for a few days.
2. Cut the tip off the shoot of each seedling.
3. Place each shoot tip on a small block of agar jelly.
4. Place the shoot tips and agar in different conditions as shown in Figure 14.
5. After 24 hours, measure the mass of auxin in the agar blocks.
Figure 14
The numbers under each block show the mass of auxin that diffused into the blocks
from the shoot tips.
The mass of auxin is given in arbitrary units.
09.5 A scientist made a hypothesis:
‘Light causes auxin to move from the side of the shoot nearest to the light to the
side furthest from the light.’
Describe the evidence from Figure 14 which supports the hypothesis.
[3 marks]
09.6 Another scientist made a different hypothesis:
‘Light causes the breakdown of auxin.’
Give the evidence from Figure 14 that shows that auxin is not broken down by light.
[1 mark]
Mark scheme
Show the mark scheme
Question 9
AO /
Question Answers Extra information Mark
Spec. Ref.
09.1 any two from: 2 AO3
• (same volume of) water allow (same amount of) water 4.5.4.1
• (same) temperature RPA8
• (same) species / type of plant
• (same) age of shoot
allow (amount of) minerals / ions
/ salts
ignore (same) time
ignore (same) height of shoot
ignore carbon dioxide
(concentration)
do not accept light
AO /
Spec. Ref.
09.2 B shows response (in A) is due allow B gives a comparison 1 AO2
to light (as B has no light) between no light and (one- 4.5.4.1
sided) light RPA8
ignore B shows the effect of no
light
C shows response (in A) is due allow C gives a comparison 1
to one-sided light (as C has light between all-round light and one-
from all sides) sided light
ignore C shows the effect of
light from all sides
AO /
Spec. Ref.
ignore reference to auxin
09.3 ignore reference to left and right AO3
sides 4.5.4.1
RPA8
growth / elongation occurs (just) allow no growth or very little 1
behind / below / at the tip growth at the base 29
ignore shoot is taller
bending occurs (just) behind / 1
below / at the tip
or
more growth on the side away
from the light
AO /
Spec. Ref.
09.4 phototropism / phototropic allow positive phototropism 1 AO1
ignore tropism 4.5.4.1
RPA8
do not accept negative
phototropism
do not accept phototrophic /
phototrophism
AO /
Spec. Ref.
ignore references to D and E
09.5 AO3
ignore references to left and 4.5.4.1
right sides unless at least one is
identified as side nearer to or
further from the light
in F there is more auxin in the 1
side furthest from the light
in G there is the same mass of 1
auxin in each side
in F auxin can move through the 1
shoot but in G the glass
prevents this
AO /
Spec. Ref.
09.6 all four (blocks) have allow examples such as mass of 1 AO3
(approximately) the same mass auxin in D / E is (about) the 4.5.4.1
of auxin same as in F / G
or
those in light have same mass
of auxin as those in dark
Total Question 9 11
How to answer it
Investigating Phototropism & Auxin Distribution
What this question tests
This 11-mark question assesses understanding of AQA Biology Required Practical 8 (RPA 8) and the hormonal control of plant responses (Specification 4.5.4.1):
- Variables: Selecting and justifying suitable control variables in seedling growth experiments.
- Control Experiments: Explaining why negative and positive control treatments (complete darkness vs. all-round light) are essential to establish valid conclusions.
- Data Interpretation: Deducing the exact location of cell elongation and bending along a shoot using sequential ink markings.
- Terminology: Recalling the precise biological term for directional growth in response to light.
- Hypothesis Testing & Data Evaluation: Analysing quantitative agar-block diffusion data (with and without impermeable glass barriers) to distinguish between lateral auxin transport and auxin destruction.
Control Variables in Shoot Growth Investigations
Suggest two control variables the students should have used in their investigation.
✅ Accepted Answers (Any two)
- (Same volume / amount of) water
- (Same) temperature
- (Same) species / type of plant (or seed)
- (Same) age of shoot / seedling
- (Same amount of) minerals / ions / salts
❌ Common Traps & Rejected Answers
- "Light" or "light intensity": Light is the independent variable being tested, so it cannot be kept constant across the setups!
- "Time": Time (12 hours) was already specified in the question diagram.
- "Carbon dioxide concentration": Often ignored by mark schemes for simple seedling growth unless in an airtight chamber.
- "Height of shoot": Starting height varies biologically; age or species should be controlled instead.
Role of Controls in Scientific Investigations
Describe how experiment B and experiment C acted as controls for the investigation.
✅ Correct Answers
- Experiment B (kept in darkness): Shows that the bending response (in A) is due to light (since B has no light and grows straight).
- Experiment C (all-round light): Shows that the bending response (in A) is due to one-sided / unilateral light (since C receives equal light from all directions and grows straight).
🧠 Exam Technique: How to Answer "Role of a Control"
Always state what condition is isolated and how it compares to the test setup (A):
- Do not just describe what was done (e.g. "B was put in the dark" gets 0 marks).
- Explicitly link to the observation: "B proves the bending is caused by light, not just normal upward growth."
- For C, emphasize the word one-sided (unilateral): "C proves it is the direction of light, not just the presence of light, that causes the shoot to bend."
Interpreting Growth from Ink Marks
Give two conclusions that the students could make from the ink marks on the shoot in experiment A.
✅ Correct Answers (Any two)
- Growth / cell elongation occurs (just) behind / below / at the tip.
- Bending occurs (just) behind / below / at the tip.
- There is more growth / elongation on the side away from the light (the shaded side).
- There is no growth (or very little growth) at the base of the shoot.
❌ Common Errors to Avoid
- Mentioning auxin: The question asks what can be concluded from the ink marks. Auxin cannot be seen on the shoot.
- Vague answers: "The shoot gets taller" (ignored—does not use the ink marks).
- Confusing sides: Stating growth occurs on the side nearest the light (incorrect; the shaded side elongates more, forcing the shoot to bend toward the light).
Identifying the Tropism
Name the type of response shown by the seedling in experiment A.
✅ Correct Answer
Phototropism (or positive phototropism / phototropic response).
💡 Key Vocabulary
- Tropism: A directional growth response of a plant towards or away from a directional stimulus.
- Phototropism: Growth towards (positive) or away from (negative) light.
- Gravitropism / Geotropism: Growth in response to gravity.
❌ Spelling & Precision Traps
- Do not write just "tropism" (too vague – needs the "photo-" prefix).
- Do not write "negative phototropism" (shoots show positive phototropism).
- Spelling errors like "phototrophic" or "phototrophism" are rejected by examiners because "trophic" refers to feeding levels, not growth movements!
Evaluating Evidence for Auxin Lateral Movement
A scientist made a hypothesis: 'Light causes auxin to move from the side of the shoot nearest to the light to the side furthest from the light.' Describe the evidence from Figure 14 which supports the hypothesis.
• D (Dark, intact agar): 25.3 units total.
• E (Dark, split agar with glass divider in base only): Left = 12.5, Right = 12.4 (symmetrical 50:50 distribution).
• F (One-sided light from right, split agar, glass in base only): Shaded side (furthest from light) = 17.2; Lit side (nearest light) = 7.8.
• G (One-sided light from right, glass sheet completely dividing the shoot tip): Shaded side = 12.6; Lit side = 12.6.
✅ 3 Key Evidence Points Required
- In F: There is more auxin in the block on the side furthest from the light / shaded side (17.2 vs 7.8 units).
- In G: There is an equal mass of auxin on each side (12.6 units each).
- Comparison / Mechanism: In F, auxin can move laterally across the shoot tip, but in G, the glass barrier through the tip prevents this lateral movement.
🧠 Exam Technique: Citing Evidence
- Identify the relevant treatments: only F and G test one-sided light with and without a physical barrier in the tip.
- Do not just write "left and right" – always relate it to the stimulus: "side nearest the light" and "side furthest from light".
- Reference to D and E is ignored here because they were kept in the dark and cannot test what light causes auxin to do.
Evidence Against Auxin Breakdown
Another scientist made a different hypothesis: 'Light causes the breakdown of auxin.' Give the evidence from Figure 14 that shows that auxin is not broken down by light.
🔢 Step-by-Step Data Analysis
Calculate the total mass of auxin recovered in each setup:
- Tip D (Dark): 25.3 units
- Tip E (Dark): 12.5 + 12.4 = 24.9 units
- Tip F (Light): 17.2 + 7.8 = 25.0 units
- Tip G (Light): 12.6 + 12.6 = 25.2 units
All totals are virtually identical (~25 units).
✅ Accepted Answers (Any one)
- All four setups / blocks have (approximately) the same total mass of auxin (~25 units).
- The total mass of auxin in tips exposed to light (F and G) is the same as tips kept in the dark (D and E).
❌ Common Error
Looking only at the lit block of F (7.8) and claiming auxin decreased. You must look at the total across both halves (17.2 + 7.8 = 25.0). If light broke auxin down, the total sum in F and G would be significantly less than in D (25.3).
Topics
Biology · Required Practicals · B5: Homeostasis and Response · Required Practicals
Question and mark scheme from the AQA GCSE Biology examination, Biology Paper 2 (Higher), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.