AQA AS Level Biology Paper 2, June 2019: Question 1

✅ Correct Answer Structure

• Mark Point 1: Hydrolysis of peptide bonds.
• Mark Point 2: Endopeptidases act in the middle of proteins/polypeptides to produce shorter polypeptides (increasing ends).
• Mark Point 3: Exopeptidases act at the ends of proteins/polypeptides to produce dipeptides or amino acids.
• Mark Point 4: Dipeptidases act on dipeptides (between two amino acids) to produce single amino acids.

💡 Key Knowledge

Protein digestion relies on three distinct classes of proteases acting sequentially. You must learn the exact anatomical substrate and product specifications for each enzyme type to secure full marks.

🧠 Exam Technique

Be precise with terminology. Always state that enzymes cause hydrolysis rather than just "breaking down". Ensure you match the correct enzyme to its specific cleavage location (middle vs. ends vs. between two amino acids).

❌ Common Errors

Students frequently lose marks by stating incorrect pairings, such as claiming "endopeptidases produce dipeptidases". Another common error is discussing organ locations (like the stomach or pancreas) when the question only asks for the role of the enzymes.

✅ Correct Answer Structure

• Mark Point 1: No significant difference (in protein absorption).
• Mark Point 2: Because the plus/minus 2 standard deviations overlap.
• Mark Point 3: Therefore, the mean percentage absorbed is not affected by the percentage in the diet.
• Mark Point 4 / 5: Alternative conclusions include noting that protein content in this diet range is not a limiting factor, or that the range tested was too small.

💡 Key Knowledge

When error bars (representing standard deviations or standard errors) overlap between data sets, it indicates that the differences between means are likely due to chance, meaning there is no significant difference.

🧠 Exam Technique

When questions ask what you can "conclude", always look for statistical indicators like error bars. State your conclusion clearly first, then immediately justify it by referencing the overlap of the error bars or standard deviations.

❌ Common Errors

Avoid using casual words like "not accurate" or "roughly the same". Examiners strictly penalise answers that fail to use the statistical term "no significant difference" or omit the specific justification regarding overlapping error bars.

✅ Correct Answer Structure

• Mark Point 1: More or remaining/undigested protein is broken down.
• Mark Point 2: Resulting in more amino acids being absorbed.
• Mark Point 3: Because the food/protein passes again through the stomach or ileum.

💡 Key Knowledge

Caecotrophy in rabbits allows a "second pass" through the true digestive tract (stomach and ileum) for material that was fermented past the primary site of enzymatic absorption, maximizing nutrient extraction.

🧠 Exam Technique

Link the stimulus material (caecum, caecal droppings, stomach, ileum) directly to biological processes. Explain the sequence: eating droppings re-introduces protein to digestive enzymes for further hydrolysis and absorption.

❌ Common Errors

A frequent pitfall is assuming microorganisms do all the work—students forget that repeating passage through the mammalian stomach and ileum exposes the protein to endogenous mammalian enzymes (like pepsin and trypsin) for a second time.