AQA AS Level Biology Paper 1, June 2023: Question 8

✅ Correct Answer

Any one of the following:

• To break down links between cells / cell walls
• To separate cells / cell walls
• To break down / hydrolyse cellulose / cell wall
• Allowing the stain to pass / diffuse into cells
• Allowing cells to be more easily squashed
• To stop mitosis

❌ Common Errors

Students frequently lose marks by writing vague statements like "to kill the cells" without linking it to stopping mitosis, or incorrectly stating that acid is used to "denature enzymes" (which is too broad and rejected by examiners).

✅ Correct Answer

Any two of the following (2 marks max):

• Eye protection (Accept wear goggles)
• Gloves
• Add water to spills immediately
• Do not pour away down sink (or follow disposal instructions)

🧠 Exam Technique

Keep safety precautions specific and actionable. General statements like "be careful with hot acid" or "wear a lab coat" are insufficient. Always default to standard lab safety gear: eye protection and gloves.

✅ Correct Answer

Null hypothesis: There is no difference between the mitotic index at 200 µm and 300 µm.
Statistical test: t-test

💡 Key Knowledge

A null hypothesis must always state that there is no significant difference or no correlation depending on the test. For comparing two means (like mitotic indices at two distinct distances), a t-test is required.

📐 Step-by-Step Calculation

1. Read the graph (Figure 7): At 200 µm from the root tip, the mean mitotic index is 0.35 (or 35%).
2. Convert units for area: Area = 0.2 mm²
   Convert to µm²: 1 mm = 1000 µm, so (1 mm)² = 1,000,000 µm² (10⁶).
   Total area = 0.2 × 1,000,000 = 200,000 µm²
3. Calculate total number of cells in the field of view:
   Total area ÷ Mean area of one cell = 200,000 ÷ 3495 = 57.225 cells
4. Calculate cells undergoing mitosis using the mitotic index:
   Total cells × Mitotic Index = 57.225 × 0.35 = 20.028 cells

Final Answer Range: 19.95 to 20.03 cells (Accept 20)

❌ Calculation Traps

Failing to square the conversion factor when switching from mm² to µm² is the most common error. Students often multiply by 1000 instead of 1,000,000 (10⁶), resulting in mark penalties.

✅ Correct Answer

Any two controlled variables / methodological considerations (Max 2 marks):

• Roots / plants of the same age
• Same growing conditions (temperature, light, water)
• Same distance from root tip
• Same time in acid / same temperature of acid / same concentration of acid
• Same pressure of squash technique
• Several fields of view counted for each species
• Same time of day (to account for diurnal mitotic rhythms)

🧠 Exam Technique

When an experimental comparison is made between different species or conditions, always think about standard control variables that affect growth rate, cell cycle length, and microscopic observation quality.