AQA AS Level Biology Paper 1, November 2020: Question 2

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

1. Accept annotated

02.1 1. Bilayer 2

diagram for

OR ‘bilayer’

Water is present inside and outside a cell;

Accept

cytoplasm/tissue fluid

2. Hydrophobic (fatty acid) tails point away/are for water

repelled from water

Accept for two marks,

OR annotated diagram of

bilayer with water

Hydrophilic (phosphate) heads point to/are in/are

labelled on each side

attracted to water;

Ignore

hydrophilic/phosphate

heads protect

hydrophobic/fatty acid

tails

02.2 1. Condensation (reaction) 2 Accept labelled

diagram

OR

Loss of water;

2. Between of glycerol and fatty acid;

02.3 1. High (specific) heat capacity; 2

2. Buffers changes in temperature; 2. Accept ideas such

as a lot of energy

needed/gained to

change

temperature

TOTAL 6

How to answer it

Cell Membranes and Biological Molecules Study Guide

What this question tests

This question assesses your core knowledge of biological molecules and cellular ultrastructure (AS Topic 1 & 3). Specifically, it tests your understanding of phospholipid bilayer arrangements, the chemical formation of ester bonds via condensation reactions, and the thermal properties of water that stabilize cellular environments.

Question 02.1

Explain the arrangement of phospholipids in a cell-surface membrane. [2 marks]

✅ Correct Answer

Arranged as a bilayer (or water is present both inside and outside the cell).
Hydrophobic (fatty acid) tails point away from water / are repelled from water OR Hydrophilic (phosphate) heads point towards / are in / are attracted to water.

💡 Key Knowledge

Phospholipids are amphipathic molecules possessing both polar (hydrophilic) heads and non-polar (hydrophobic) tails.
The aqueous environment of the cytoplasm (inside) and tissue fluid/blood (outside) dictates this structural orientation.

🧠 Exam Technique

You can earn full marks here by drawing a fully-labelled annotated diagram showing a bilayer with water indicated on both sides. Examiners accept terms like 'cytoplasm' or 'tissue fluid' in place of 'water'.

❌ Common Errors

A frequent misconception is stating that "hydrophilic heads protect the hydrophobic tails". The examiner explicitly ignores this statement; marks are awarded strictly based on the interaction/orientation relative to water.

Mark allocation: 1 mark for mentioning the bilayer/water context + 1 mark for the correct orientation of hydrophobic tails or hydrophilic heads relative to water.

Question 02.2

Describe how an ester bond is formed in a phospholipid molecule. [2 marks]

✅ Correct Answer

Condensation reaction (or loss of water).
Formed between glycerol and fatty acid.

💡 Key Knowledge

A phospholipid consists of one glycerol molecule, two fatty acid chains, and a phosphate group.
Ester bonds form via a condensation reaction where a molecule of water is removed between the hydroxyl group of glycerol and the carboxyl group of a fatty acid.

🧠 Exam Technique

Be precise with chemical terminology. Always state "condensation" rather than just "water is lost". You must explicitly name the two reacting components: glycerol and fatty acid.

❌ Common Errors

Students often confuse ester bonds with peptide or glycosidic bonds, or incorrectly state that water is added (which is hydrolysis, not condensation).

Mark allocation: 1 mark for identifying condensation/loss of water + 1 mark for stating it occurs between glycerol and fatty acid.

Question 02.3

State and explain the property of water that helps to prevent temperature increase in a cell. [2 marks]

✅ Correct Answer

Property: High (specific) heat capacity.
Explanation: Buffers changes in temperature (or a lot of energy is required/gained to change temperature).

💡 Key Knowledge

Water molecules form an extensive network of hydrogen bonds with one another.
A large amount of thermal energy is required to break these hydrogen bonds, allowing water to absorb significant heat without a major rise in temperature.

🧠 Exam Technique

Make sure you clearly separate the Property name from the Explanation as requested by the answer lines on the paper. Do not confuse specific heat capacity with latent heat of vaporization.

❌ Common Errors

A very common error is writing "high melting point" or "high boiling point" instead of high specific heat capacity. Vague answers like "it keeps temperature stable" without explaining why (energy needed to break hydrogen bonds) fail to score the second mark.

Mark allocation: 1 mark for stating "high specific heat capacity" + 1 mark for explaining that it buffers temperature changes / requires a lot of energy to alter temperature.

Topics

Biology · 3.1 Biological molecules