AQA GCSE Biology Biology Paper 1 (Foundation), June 2025: Question 2

✅ Correct Answer

Temperature of pondweed [1 mark]

💡 Key Knowledge

Independent variable: The variable that you change deliberately.
Here, tubes are placed into water baths set at six different temperatures.

❌ Common Errors

Confusing control variables with the independent variable. The volume of water and length of pondweed should be kept constant (controlled).

✅ Correct Answer

• "Photosynthesis by the pondweed releases bubbles of oxygen." [1 mark]
• "The green substance needed for photosynthesis is called chlorophyll." [1 mark]

💡 Key Knowledge

Photosynthesis word equation:

carbon dioxide + water → glucose + oxygen

Light is absorbed by the green pigment chlorophyll, located inside chloroplasts.

🧠 Exam Technique

Ensure words are entered in the exact order requested by the prompt. Answers must come directly from the provided box.

✅ Correct Answer

• (Time) decreases [1 mark]
• then (time) increases [1 mark]

Also allowed: rate/speed increases, then rate/speed decreases.

🧠 Exam Technique: Two-part Trends

Notice that between 10 °C and 40 °C the time drops from 84 s to 25 s, but from 40 °C to 60 °C the time goes back up to 77 s. When a trend changes direction, you must describe both parts to secure both marks.

❌ Common Errors

• Only describing the first trend: "as temperature increases, time decreases" (loses the 2nd mark).
• Quoting figures without stating the pattern (mark scheme explicitly states: "ignore use of data").

📐 Step-by-Step Calculation

Step 1: Locate value from Table 2
At 50 °C, time taken to release 10 bubbles = 33 seconds.

Step 2: Substitute into the formula
Rate = 10 / 33 [1 mark]

Step 3: Calculate unrounded value
10 ÷ 33 = 0.3030... [1 mark]

Step 4: Round to 1 decimal place
The second digit after the decimal is 0 (less than 5), so round down:
Rate = 0.3 bubbles per second [1 mark]

❌ Calculation Traps

• Wrong temperature: Picking the time for 40 °C (25 s) or 60 °C (77 s). Always double check the row.
• Significant figures vs decimal places: Writing 0.30 (2 decimal places) is explicitly not accepted by the mark scheme.

✅ Correct Answer

40 °C [1 mark]

💡 Key Knowledge: Rate vs Time

Rate is inversely proportional to time:

Fastest rate = Shortest time taken

At 40 °C, the time taken to produce 10 bubbles is only 25 seconds, which is the lowest value in the table, meaning photosynthesis was occurring the fastest.

❌ Common Errors

Students often mistake the largest time (84 s at 10 °C) for the "fastest" rate, confusing a longer duration with higher speed.

✅ Correct Answer

More than 77 seconds [1 mark]

💡 Key Knowledge: Enzyme Denaturation

Photosynthesis is controlled by enzymes. Beyond the optimum temperature (around 40 °C):

• High temperatures break chemical bonds holding the enzyme's structure.
• The active site changes shape (denatures), meaning substrate molecules can no longer bind.
• By 80 °C, enzymes are severely denatured, slowing or stopping photosynthesis completely, meaning bubbles take much longer (or infinite time) to appear.

🧠 Exam Technique

Look at the pattern: from 40 °C (25 s) → 50 °C (33 s) → 60 °C (77 s). The time is climbing steeply. At 80 °C, it must be even longer than 77 s.

✅ Correct Answers

• Concentration of carbon dioxide [1 mark]
• Light intensity [1 mark]

💡 Key Knowledge: Limiting Factors

The main factors affecting the rate of photosynthesis are:

• Temperature (investigated here)
• Carbon dioxide concentration (a reactant)
• Light intensity (energy source)
• Chlorophyll amount (absorbs light)

❌ Common Errors

Selecting "Size of boiling tube" or "Type of stopwatch". While apparatus setup matters, these do not chemically or biologically alter the rate of photosynthesis.