AQA A-Level Biology Paper 1, June 2025: Question 1

✅ Model Answer Points

• Bile salts break large lipid droplets into smaller droplets, which increases the surface area [Mark 1].
• This allows for a faster rate of hydrolysis by lipase (or increased lipase enzyme activity) [Mark 2].

🧠 Exam Technique & Precision

• Always state the enzyme name: Say "faster hydrolysis by lipase" rather than generic words like "faster digestion" or "faster breakdown".
• Emulsification is physical, not chemical: Bile salts do NOT digest lipids into fatty acids; they simply produce smaller droplets to aid lipase.

❌ Common Errors in Emulsification

• Writing that bile salts "break bonds" or "chemically digest" lipids.
• Stating only that lipids are broken down, omitting the term lipase or hydrolysis.
• Saying "increases surface area to volume ratio" instead of simply increasing the surface area of lipid droplets (mark schemes explicitly ignore surface area to volume ratio here).

✅ Model Answer Points

• Micelles carry fatty acids and monoglycerides to the cell-surface membrane of epithelial cells (or make them more soluble in aqueous ileum fluid) [Mark 3].
• At the epithelial membrane, micelles break down, and fatty acids/monoglycerides diffuse directly into the cell through the phospholipid bilayer [Mark 4].

💡 Key Knowledge: What are Micelles?

Micelles are microscopic complexes (4–7 nm) formed when bile salts associate with the non-polar products of lipase digestion (monoglycerides and fatty acids). They keep these insoluble products dissolved in the aqueous chyme and transport them to the microvilli brush border.

❌ Critical Misconceptions on Micelles

• Wrong: "Micelles enter the cell by diffusion" or "Micelles cross the membrane." The micelle does not cross the membrane! It delivers fatty acids/monoglycerides, which then leave the micelle and diffuse across the membrane independently.
• Wrong: Referring to micelles carrying "lipids" or "triglycerides". Triglycerides have already been hydrolysed; you must name fatty acids and/or monoglycerides.
• Wrong: Saying "facilitated diffusion". Fatty acids are lipid-soluble and enter via simple diffusion.

✅ Model Answer Points

• Description: Phospholipids are arranged as a bilayer [Mark 1].
• Explanation:
  • Hydrophilic (polar) heads are attracted to / point towards water (outside and inside the cell).
  OR
  • Hydrophobic (non-polar) tails are repelled by / point away from water (towards the interior of the membrane) [Mark 2].

💡 Chemical Structure Recall

• Phosphate head: Electrically charged / polar → interacts with polar H₂O molecules (hydrophilic).
• Fatty acid tails: Non-polar hydrocarbon chains → insoluble in aqueous surroundings (hydrophobic).

❌ Common Errors

• Swapping the terms: saying heads are hydrophobic and tails are hydrophilic.
• Stating that the bilayer forms because tails "attract each other" rather than correctly explaining that tails are repelled by water (or heads are attracted to water).
• Forgetting to state the word bilayer (saying "double layer" is acceptable, but "bilayer" is standard A-Level biological terminology).

🧠 "Describe and Explain" Strategy

• Notice the command words: Describe AND Explain.
• Describe = What does it look like? → A phospholipid bilayer.
• Explain = Why does it take this form? → Heads are hydrophilic and face aqueous solutions, while tails are hydrophobic and face inwards.