AQA A-Level Biology Paper 1, June 2024: Question 2

✅ Correct Answer Framework

• General: Involves the movement of polar/charged molecules.
• Facilitated Diffusion: Movement down a concentration gradient via carrier or channel proteins.
• Active Transport: Movement against a concentration gradient via carrier proteins using ATP (energy).

💡 Key Knowledge

Make sure you can contrast passive and active processes. Facilitated diffusion is passive (relies on kinetic energy of molecules, no ATP required), whereas active transport requires metabolic energy in the form of ATP to change protein conformation and pump substances against their gradient.

🧠 Exam Technique

This is a comparison/description question split into two distinct sub-descriptors. Ensure you explicitly mention concentration gradients for both, but make sure ATP and carrier proteins only are exclusively linked to active transport.

❌ Common Errors

• Stating that active transport uses channel proteins (channel proteins are only for facilitated diffusion).
• Failing to state that facilitated diffusion moves substances down a concentration gradient.
• Forgetting to mention ATP for active transport.

✅ Correct Answer

A (highly) folded cell-surface membrane (or invaginations / projections / extensions of the membrane).

💡 Key Knowledge

Microvilli dramatically increase the surface area of epithelial cells lining the ileum, maximizing the rate of diffusion and co-transport of digestion products. Do not confuse microvilli (cell membrane folds) with villi (folds of the whole ileum wall containing many cells).

🧠 Exam Technique

This is a straightforward AO1 recall question worth just 1 mark. Keep it brief and precise. Mentioning "folds" and "cell-surface membrane" is essential.

❌ Common Errors

• Calling them "hairs" or "microscopic hairs" (the mark scheme explicitly rejects "hairs").
• Describing them as folds of the tissue or wall instead of specifically the cell-surface membrane.

✅ Correct Answer Framework

1. Vitamin A combines/mixes/joins with bile salts.
2. This helps make it more soluble in water.
3. Micelles break down close to the cells (or maintain a higher concentration gradient at the cell-surface membrane / transport vitamin A to the lining).
4. Vitamin A diffuses directly across the cell-surface membrane (lipid-soluble nature allows direct passage through the phospholipid bilayer).

💡 Key Knowledge

Because vitamin A is fat-soluble (lipophilic), it cannot travel freely through the aqueous environment of the lumen on its own. Micelles wrap around lipid-soluble vitamins and monoglycerides/fatty acids, carrying them to the epithelial cell surface where they release them.

🧠 Exam Technique

Read the stem carefully: it specifies vitamin A is a fat-soluble substance and involves micelles. Do not describe general co-transport (like sodium-glucose co-transport). Emphasize the role of bile salts, solubility, and simple diffusion across the membrane.

❌ Common Errors

• Stating that the whole micelle gets absorbed into the cell (micelles remain outside; only the lipid/vitamin diffuses across).
• Saying vitamin A uses facilitated diffusion or active transport (lipids and fat-soluble vitamins cross via simple diffusion through the phospholipid bilayer).