AQA GCSE Biology Biology Paper 2 (Foundation), June 2023: Question 5

✅ Correct Answer

Fungi

💡 Key Knowledge

The two primary groups of decomposers are bacteria and fungi. They break down dead organic matter by secreting digestive enzymes externally and absorbing the broken-down soluble nutrients.

❌ Common Errors

• Selecting Viruses: Viruses are non-cellular entities that replicate inside living host cells; they do not cause decomposition.
• Selecting Plants: Plants are primary producers (autotrophs), not decomposers.

✅ Correct Answer

The pH of the milk

🧠 Exam Technique

• Independent variable: What you intentionally change (here, temperature: 5 °C, 20 °C, 30 °C).
• Dependent variable: What you measure (here, the pH of the milk).
• Control variables: What you keep the same (volume of milk, type of milk).

❌ Common Errors

Confusing control variables (like volume of milk) with the dependent variable (the actual reading you record every day).

✅ Correct Answer

Put the beaker in a water bath

💡 Key Knowledge

A thermostatically controlled water bath (or incubator) supplies continuous heat to keep a steady temperature higher than room temperature over long periods.

❌ Common Errors

• "Wrap cloth around the beaker": Insulation slows down heat loss, but it cannot warm milk up or maintain 30 °C above ambient temperature for 7 days.
• "Put a lid on": This reduces evaporation, not temperature regulation.

✅ Correct Answer

Fatty acid

💡 Key Knowledge

Lipids (fats and oils) are broken down by lipase enzymes into:

Lipid → Glycerol + Fatty Acids

The accumulation of free fatty acids causes the pH to decrease (become more acidic).

❌ Common Errors

Selecting Amino acid (the product of protein digestion by proteases) or Hydrochloric acid (produced in the stomach, not by lipase digestion).

✅ Correct Answer

All the fat had been digested.

💡 Key Knowledge

Notice on Figure 9 that both the 20 °C and 30 °C curves flatten out at exactly pH 4.5. Once all substrate (fat/lipid) has been completely hydrolysed, no more fatty acids can form, so the pH stops falling.

❌ Common Errors

Thinking the reaction stopped because it was "too cold" or "too fast". The plateau level is identical at 20 °C and 30 °C because the starting quantity of fat was identical in both samples.

✅ Correct Answers (any one)

• (There is) more kinetic energy / particles move faster / more frequent collisions.
• Enzyme activity is higher / enzymes work faster.
• Bacteria / microorganisms divide / reproduce faster (or there are more bacteria).

🧠 Exam Technique

You can also give the converse for 5 °C (e.g., "at 5 °C particles have less kinetic energy"). Always use precise scientific vocabulary: mention kinetic energy or enzymes.

❌ Common Errors

• Writing just "it is warmer" — this is an observation, not an explanation!
• Writing "enzymes work better" — too vague; write faster or have higher activity.
• Writing "bacteria grow faster" — mark scheme specifies reproduction/division or number of bacteria.

📐 Step-by-Step Calculation

1. Read day 2 at 30 °C: Follow Day 2 vertically to the 30 °C curve:
   pH = 6.0 (tolerance ± 0.05).
2. Read day 3 at 30 °C: Follow Day 3 vertically to the 30 °C curve:
   pH = 4.7 (tolerance ± 0.05).
3. Calculate the fall per day:
   Rate = (pH at day 2) − (pH at day 3) / 1 day
   Rate = 6.0 − 4.7 = 1.3 pH units/day

🧠 Graph Reading Tip

Check the grid scale carefully before reading:

• 1 major grid division (10 small squares) = 0.5 pH units.
• Each small vertical square = 0.05 pH units.
• At day 3, the cross is 4 small squares above 4.5:
  4.5 + (4 × 0.05) = 4.70.

📐 Step-by-Step Calculation

1. Identify the two rates:
   Rate at 30 °C = 1.3 pH units/day (from 05.7)
   Rate at 5 °C = 0.1 pH units/day (given in question)
2. Set up the division:
   Ratio = 1.3 / 0.1
3. Calculate the final value:
   Rate is 13 times faster.

❌ Common Errors

• Adding a unit: The question asks "how many times faster". This is a pure ratio. The mark scheme explicitly states: "do not accept if a unit is given" (e.g. writing "13 pH/day" will lose Mark 2!).
• Subtracting instead of dividing (e.g. calculating 1.3 − 0.1 = 1.2).