AQA GCSE Biology Biology Paper 2 (Foundation), June 2024: Question 7

✅ Correct Answer

Either of the following:

• To compare with the other tubes (to see how roots grow without the chemical).
• To show the effect of chemical Q on root growth.

🧠 Exam Technique

When asked why a "zero concentration" or untreated setup is included, examiners want the word compare or an explicit statement about identifying the effect of the independent variable.

❌ Common Errors

• Writing "as a control" (simply repeating the question gets 0 marks).
• Calling it a "control variable" (a control tube is a baseline comparison, not a variable being kept constant).
• Vague claims like "to make it a fair test".

💡 Key Knowledge

A control experiment provides a baseline measurement. It proves that any observed root growth in tubes 2–6 was caused specifically by chemical Q, rather than just water or room temperature.

📐 Step-by-Step Calculation

1. Identify values from Figure 8:
   Concentration in Tube 6 = 100 arbitrary units
   Concentration in Tube 2 = 0.01 arbitrary units
2. Set up division ratio:
   100 ÷ 0.01 (1 mark)
3. Calculate the final value:
   10 000 or 10⁴ times (1 mark)

❌ Common Calculation Traps

• Subtracting instead of dividing: 100 - 0.01 = 99.99 (gets 0 marks; "how many times more" means a ratio/division).
• Dividing upside down: 0.01 ÷ 100 = 0.0001 .
• Miscounting zeros when dividing by a decimal: 100 ÷ 0.01 is equivalent to 100 × 100 = 10 000 , not 1 000.

✅ Correct Answer

Tick the second box:

☑ 0.1 arbitrary units

🧠 Exam Technique

Scan Figure 8 for the stem showing the greatest root development. Tube 3 visibly possesses the longest, most extensive cluster of roots, which matches a concentration of 0.1 arbitrary units.

✅ Correct Answer

• Reduced / no root growth at high(er) concentrations.
• OR specifically stating: no roots grew in Tube 6 / at concentration 100.

❌ Common Errors

• Stating unqualified trends like "the higher the concentration, the fewer roots grow" (this is false across the whole range because roots increase up to 0.1).
• Saying "the plant died" without referring to the observed evidence (root growth).

✅ Key Points Needed (Indicative Content)

• Genetically Identical Offspring: Cuttings are clones produced by asexual reproduction involving mitosis.
• Phenotype Guarantee: All new plants will inherit the same alleles/genes as Plant A, guaranteeing large, brightly-coloured flowers.
• No Variation: Seeds are produced by sexual reproduction (involving meiosis and fertilisation/pollination), which mixes alleles and causes variation—flowers might end up small or dull.
• Speed: Growing cuttings is quicker because you don't need to wait for seed formation, pollination, or germination.

🧠 Level 2 Marking Criteria (3–4 marks)

To reach Level 2, you must give reasons and clearly link them to biological causes:

• Link cuttings to clones / identical alleles → ensuring desired flower appearance.
• Link seeds to sexual reproduction / mixing of alleles → causing unpredictable variation.

💡 Biology in Context

Cuttings (Asexual Reproduction):

• 1 parent plant
• Cell division: Mitosis only
• Offspring: Clones (no genetic variation)

Seeds (Sexual Reproduction):

• Gamete fusion (pollen + ovule)
• Cell division: Meiosis to make gametes
• Offspring show genetic variation

❌ Common Errors

• Confusing mitosis and meiosis (cuttings grow into plants by mitosis).
• Saying "cuttings produce identical traits" without explaining why (they have the exact same genes/alleles).
• Forgetting to explain why seeds are disadvantageous here (cross-pollination causes variation).