AQA AS Level Biology Paper 2, November 2020: Question 5

✅ Correct Answer / Mark Scheme

• Mark 1: Starch (solution) as the independent variable in the first column.
• Mark 2: Clear column headings showing starch concentration ( mg dm⁻³ ) and time for hydrolysis ( minutes , min , or seconds ). Brackets are acceptable (e.g., Starch concentration / mg dm⁻³).

🧠 Exam Technique

• Always put the independent variable (what you change) in the first column on the left.
• Include both the variable name and correct units separated by a slash or in brackets.
• Do not include columns for replicates or means if the question asks for basic completed column headings unless specified, to avoid losing marks on strict formatting rules.

❌ Common Errors

• Omitting units ( mg dm⁻³ or time units) from the headings.
• Putting the dependent variable (time) in the first column instead of the independent variable (concentration).
• Drawing a graph instead of a table.

✅ Correct Answer

As starch concentration increases, the time taken to hydrolyse/digest the starch increases (or converse).

💡 Key Knowledge

Higher substrate concentrations require more time for a fixed concentration of enzyme to completely break down all available glycosidic bonds because there is a greater total mass of substrate to process.

✅ Correct Answer / Mark Scheme

• Mark 1: Inhibitor has a similar shape to the substrate / is complementary to the active site.
• Mark 2: Inhibitor fits / binds directly into the active site of the enzyme.
• Mark 3: This prevents or reduces the formation of enzyme-substrate complexes.

🧠 Exam Technique

Be precise with terminology. Examiners penalise phrases like "same shape"—always use "similar shape" or "complementary shape". Ensure you explicitly state that it blocks the active site and stops enzyme-substrate complexes from forming.

❌ Common Errors

• Stating the inhibitor alters the shape of the active site (which describes a non-competitive inhibitor).
• Saying the inhibitor has the "same shape" instead of "similar" or "complementary".

✅ Correct Answer / Mark Scheme

• Mark 1: Less hydrolysis of starch.
• Mark 2: Less breakdown into maltose (reducing sugar).
• Mark 3: Less absorption of glucose into the blood stream OR more carbohydrate/starch egested in faeces.

💡 Key Knowledge

Retrograded starch acts as a competitive inhibitor to amylase in the gut. Less digested starch means fewer monosaccharides (glucose) are available for active transport / cotransport across the ileum epithelial cells, lowering net calorie intake and reducing weight gain.

🧠 Exam Technique

Link the molecular inhibition directly back to the stem context (eating stale bread, reduced digestion, less absorption, and ultimate body mass impact).