AQA GCSE Biology Biology Paper 1 (Foundation), June 2023: Question 5

✅ Correct Mark Scheme Answers

• 05.1: Any one from:
  • Growth
  • Development
  • Repair (or replacement of worn-out/damaged cells)
• 05.2: Tick Amino acids
• 05.3: Tick Biuret reagent
• 05.4: Tick Purple (lilac)

💡 Key Knowledge

• Mitosis produces identical diploid cells for growth and tissue repair, not for reproduction (which is meiosis).
• Proteins are long chains of monomers called amino acids folded into specific 3D shapes.
• Biuret test: Starts pale blue. Turns purple/lilac if peptide bonds are present.
• Benedict's: Tests for reducing sugars (turns green/yellow/brick red when heated).
• Ethanol: Tests for lipids (forms a milky white emulsion).

🧠 Exam Technique

For 05.1, do not just write "energy" or "survival". Examiners look for precise terms like growth or repair.

❌ Common Errors

• Mixing up Biuret (protein = purple) with Benedict's (sugar = red).
• Confusing amino acids with fatty acids (lipids) or glucose (starch).
• Writing "purple/blue" for Biuret — if blue is mentioned alongside purple without clarification, examiners can penalise as it looks like no reaction took place.

✅ Correct Matching & Reading

05.5 Line Matching:

• Has a special shape ➔ Only fits one molecule [1 mark]
• Is a catalyst ➔ Speeds up reactions [1 mark]

05.6 Reading Figure 3:

• pH = 9 (Accept any value in the range 8.5 to 9.5 ) [1 mark]

💡 Key Knowledge

• Biological Catalyst: A substance that speeds up a chemical reaction without being consumed or altered by the reaction.
• Specificity: Enzymes have a complementary active site that only binds to one specific substrate.
• Optimum pH: The pH value where the rate of reaction is at its highest (the peak of the curve). For Enzyme Y, the peak sits symmetrically at pH 9.

✅ Model Answers

05.7 [2 marks]:

• The fastest rate of breakdown / optimum is at pH 2 (allow low pH / acidic conditions). [1 mark]
• The stomach produces/contains hydrochloric acid (or has an acidic pH). [1 mark]

05.8 [2 marks]:

Complete the sentences using words from the box:

The part of the enzyme that changes shape is the active site. [1 mark]

The change in shape means the enzyme cannot bind to the substrate. [1 mark]

❌ Common Errors & Misconceptions

• Saying the enzyme "dies": Enzymes are chemical molecules, not living organisms! Always write denatures or active site changes shape.
• Vague stomach explanations: Stating only "the stomach digests food" without mentioning hydrochloric acid or low pH/acidic environment misses Mark 2.
• Antigen vs Substrate: Antigens are surface markers on pathogens. The molecule an enzyme acts on is always the substrate.

🧠 Exam Technique: 2-Part Explanations

When asked to "explain why an enzyme works best in [organ]", always link graph data to biological organ conditions:

1. Data fact: State the enzyme's optimum condition (e.g. "Enzyme X peaks at pH 2").
2. Organ link: State that the stomach environment matches this (e.g. "because the stomach contains hydrochloric acid").

💡 The Mechanism of Denaturation

At extreme pH values away from the optimum:

• Bonds holding the enzyme's 3D structure break.
• The active site changes shape.
• The substrate no longer fits (no enzyme-substrate complexes can form).

✅ Model Answer (Level 3, Full 6 Marks)

1. Set up: Place a drop of iodine solution into each well of a spotting tile using a pipette.
2. Measure volumes: Use a 10 cm³ measuring cylinder to measure fixed volumes (e.g. 2 cm³) of amylase solution, starch solution, and pH 5 buffer solution into a test tube.
3. Timing & Sampling: Mix the solutions and start the timer immediately.
4. Testing: Every 30 seconds, use a pipette to take a drop from the mixture and add it to a well on the spotting tile.
5. End-point: Observe the colour change. Initially, it turns blue-black (starch present). Continue until the iodine remains orange/brown (all starch has been broken down). Record the time taken.
6. Change independent variable: Repeat steps 1–5 using the same volumes and concentrations, but with different buffer solutions (e.g. pH 7 and pH 9).
7. Control variables: Keep the temperature constant (using a water bath at 35 °C), and use identical concentrations and volumes of starch and amylase solutions.

🧠 Examiner Checklist for Top Marks

To reach 5-6 marks, ensure your written plan covers:

• Independent Variable: Stating at least three different pH buffers (e.g., pH 5, 7, 9).
• Dependent Variable: Measuring the time taken for iodine to stop turning blue-black (remaining orange-brown).
• Controlled Variables: Explicitly mention controlling temperature, amylase volume/concentration, and starch volume/concentration.
• Equipment Named: Explicitly refer to the spotting tile, pipette, measuring cylinder, and timer.

❌ Major Pitfalls That Cap Marks at Level 1 or 2

• Forgetting control variables: Under the mark scheme, you cannot get Level 3 without explicitly naming control variables (temperature/volumes).
• Mixing up the reagents: Stating that you put Benedict's or Biuret in the spotting tile instead of iodine solution.
• Not sampling at regular intervals: Vaguely saying "wait until it changes colour" without explaining that a sample must be removed every 30 seconds.

💡 Why Does Iodine Stay Brown/Orange?

Iodine is an indicator for starch:

• Starch present: Turns blue-black.
• Starch absent: Stays orange/brown.

When amylase breaks down all the starch into maltose, there is no starch left to react with the iodine, confirming the reaction is complete.