AQA AS Level Biology Paper 2, November 2020: Question 9

10 marks · Medium difficulty · Extended Answer

Describe the processes involved in the absorption and transport of digested lipid molecules from the ileum into lymph vessels, and describe how the structure of a protein depends on the amino acids it contains.

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Question

Question text

09.1 Describe the processes involved in the absorption and transport of digested lipid

molecules from the ileum into lymph vessels.

[5 marks]

09.2 Describe how the structure of a protein depends on the amino acids it contains.

[5 marks]

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

09.1 1. Micelles contain bile salts and fatty 5 1. Ignore other correct

acids/monoglycerides; components of

micelles

2. Make fatty acids/monoglycerides (more) soluble

(in water) 2. Accept lipid/fat for

fatty acid/

monoglyceride

Bring/release/carry fatty acids/monoglycerides to 3. Reject if absorbed

cell/lining (of the iluem) by facilitated

OR diffusion

3. Ignore if micelles

Maintain high(er) concentration of fatty themselves are

acids/monoglycerides to cell/lining (of the ileum); being absorbed

3. Fatty acids/monoglycerides absorbed by 4. Accept

diffusion; chylomicrons form

5. Accept exocytosis

4. Triglycerides (re)formed (in cells); for ‘vesicles move’

5. Vesicles move to cell membrane;

09.2 1. Structure is deternmined by (relative) position of 5 max 3. Accept alpha

amino acid/R group/interactions; helix/β-pleated

sheet for

2. Primary structure is sequence/order of amino

‘secondary

acids;

structure’

3. Secondary structure formed by hydrogen

bonding (between amino acids); 1., 4. and 6. Accept

4. Tertiary structure formed by interactions for ‘intereactions’,

(between R groups); hydrogen bonds/

disulfide

5. Creates active site in enzymes bridges/ionic

OR bonds/hydrophobic-

hydrophilic

Creates complementary/specific shapes in interactions

antibodies/carrier proteins/receptor (molecules);

6. Quaternary structure contains >1 polypeptide 6. Accept prosthetic

chain (group)

Quaternary structure formed by

interactions/bonds between polypeptides;

TOTAL 10

How to answer it

Lipid Absorption & Protein Structure

📌 What this question tests

This question assesses your recall and application of key biological processes and molecular structures from Unit 1 and Unit 2: the specialized mechanism of lipid digestion product absorption via micelles into lymph vessels (lacteals), and how the sequence and properties of amino acids dictate the four levels of protein structure and functional diversity.

Question 09.1

Describe the processes involved in the absorption and transport of digested lipid molecules from the ileum into lymph vessels. [5 marks]

✅ Marking Points (Mark Scheme)

Point 1: Micelles contain bile salts and fatty acids / monoglycerides.
Point 2: Make fatty acids / monoglycerides more soluble in water (or bring them to cell lining / maintain high concentration gradient).
Point 3: Fatty acids and monoglycerides are absorbed by simple diffusion across the cell membrane.
Point 4: Triglycerides are re-formed inside the endoplasmic reticulum / cells.
Point 5: Vesicles (chylomicrons) move to the cell membrane and leave via exocytosis into lymph vessels.

💡 Key Knowledge

Micelles: Act as transport vehicles because fatty acids and monoglycerides are non-polar and insoluble.
Diffusion: Because they are lipid-soluble molecules, they cross the phospholipid bilayer directly via simple diffusion (not facilitated diffusion or active transport).
Re-synthesis: Inside the epithelial cells, triglycerides are reassembled from monoglycerides and fatty acids.

🧠 Exam Technique

Keep the sequence chronological: Lumen → Epithelial cell → Lymph vessel (lacteal).
Use precise terminology: state monoglycerides and fatty acids , not just "fats".

❌ Common Errors

The Micelle Trap: Stating that micelles themselves are absorbed into the cell. Only the digestion products diffuse out of the micelle and into the cell.
Wrong Transport Mechanism: Claiming that lipids use facilitated diffusion or active transport.

Max Mark: 5

Question 09.2

Describe how the structure of a protein depends on the amino acids it contains. [5 marks]

✅ Marking Points (Mark Scheme)

Point 1: Structure is determined by the (relative) position of amino acids / R group interactions.
Point 2: Primary structure is the specific sequence / order of amino acids.
Point 3: Secondary structure is formed by hydrogen bonding between amino acids (forming alpha-helices or beta-pleated sheets).
Point 4: Tertiary structure is formed by interactions (hydrogen bonds, ionic bonds, disulfide bridges) between R groups.
Point 5 or 6: Functional consequence (creates active sites in enzymes, complementary shapes in antibodies/receptors) OR Quaternary structure involves more than one polypeptide chain.

💡 Key Knowledge

The R group (variable side chain) is what differs between the 20 naturally occurring amino acids and dictates bonding capabilities.
Bonds stabilize folds: hydrogen bonds are numerous but weak; disulfide bridges are covalent and very strong.

🧠 Exam Technique

Structure your answer logically from primary up to quaternary structure to pick up sequential marking points effortlessly.
Explicitly link the R group properties to tertiary structure interactions to secure top-level marks.

❌ Common Errors

Confusing peptide bonds (which link amino acids in the primary structure backbone) with hydrogen bonds or ionic bonds in secondary/tertiary structures.
Vagueness: Writing "bonds hold it together" without specifying *where* or *between which parts* (e.g., between R groups).

Max Mark: 5 (5 max)

Topics

Biology · 3.1 Biological molecules · 3.3 Organisms exchange substances with their environment

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.