AQA AS Level Biology Paper 2, June 2024: Question 1

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

1. Buffers changes in temperature; 1. Accept descriptions

2 of buffering changes

2. (Provides a) cooling effect (via evaporation);

01.1 (2 x in temperature, eg

AO1) resists temperature

changes

1. Evaporation/transpiration (from the porous pot);

2. Tension created moves water (upwards); 2. Accept ‘negative

pressure’ for tension

3. Cohesion maintains the column of water

3 or ‘water pulled’ or

OR ‘suction’.

01.2 (1 x

Cohesion is due to hydrogen bonds between AO2, 2 Ignore adhesion but

water (molecules); x AO3) reject mark point 3 if

adhesion used to

describe attraction

between water

molecules.

0.8/0.85/0.848/0.8478 (using 3.14); 1

01.3 OR (1 x

0.8482 (using π on calculator) AO2)

How to answer it

Water Properties and the Cohesion-Tension Theory

📌 What This Question Tests

This question assesses fundamental physical properties of water and their biological relevance, your ability to explain experimental physical models of transpiration, and quantitative rate calculations.

Biological roles of water: High specific heat capacity (thermal stability) vs. latent heat of vaporisation (evaporative cooling).
Mass transport in plants: How physical apparatus (porous pot) models the cohesion-tension mechanism in xylem vessels.
Mathematical skills: Cylinder volume calculation ( πr²d ) and unit conversion (minutes to hours).

Question 01.1

Biological Importance of Water Properties

2 Marks · AO1 (Recall and Understanding)

✅ Mark Scheme Criteria

High heat capacity: Buffers changes in temperature (or resists temperature fluctuations). [1 mark]
Large latent heat of vaporisation: Provides a cooling effect (via evaporation / sweating / transpiration). [1 mark]

💡 Key Knowledge

Due to extensive hydrogen bonding between H₂O molecules:

High specific heat capacity: A large amount of heat energy is required to raise the temperature. This ensures stable aquatic environments and internal body temperatures so enzymes do not denature.
Latent heat of vaporisation: A lot of thermal energy is absorbed to break hydrogen bonds and vaporise water, allowing organisms to lose heat with minimal water loss (e.g. sweating, panting).

🧠 Exam Technique

Always state the importance to living organisms, not just the physical definition. Saying "it takes a lot of energy to heat up" describes the property, but does not explain why organisms care. Use active biological terms: buffers temperature and cools organism.

❌ Common Errors

Confusing heat capacity with latent heat.
Vague statements like "keeps animals warm" or "prevents water boiling".
Omitting the word "evaporation" when referencing cooling.

Award 1 mark per distinct matching property and biological importance. Maximum 2 marks.

Question 01.2

Porous Pot Model & Cohesion-Tension Theory

3 Marks · 1 × AO2, 2 × AO3 (Application & Evaluation)

✅ Mark Scheme Points

Mark Point 1: Evaporation / transpiration (of water) occurs from the porous pot. [1 mark]
Mark Point 2: Tension (negative pressure / suction) is created which pulls water upwards. [1 mark]
Mark Point 3: Cohesion maintains an unbroken continuous column of water OR cohesion is due to hydrogen bonds between water molecules. [1 mark]

💡 The Porous Pot Analogy

Experimental Model	Real Plant Equivalent
Porous pot surface	Mesophyll cell walls & stomata (site of evaporation)
Glass tubing	Xylem vessels (rigid dead tubes)
Beaker reservoir	Soil water absorbed by roots

🧠 Top-Level Examiner Advice

Break your answer down directly to match the name of the theory:

Evaporation: Where does water leave? (Porous pot).
Tension: Water loss creates negative pressure pulling the column up.
Cohesion: Hydrogen bonding keeps water molecules sticking together so the column does not break.

❌ Critical Misconceptions

Adhesion vs Cohesion: Do not say "cohesion between water and glass". Cohesion is strictly between water molecules. If you state adhesion is attraction between water molecules, you will lose Mark Point 3.
Attributing water movement to "capillary action" or "root pressure" alone; this model demonstrates cohesive pull from the top.

Total: 3 marks. 'Suction' or 'water pulled' is credited for tension.

Question 01.3

Calculating the Rate of Water Movement

1 Mark · AO2 (Mathematical Processing)

📐 Step-by-Step Calculation

Given values:

Distance moved ( d ) = 1.5 cm
Radius ( r ) = 0.6 cm
Time = 120 minutes
Formula for volume: V = πr²d (where π = 3.14 )
Target unit: cm³ hour⁻¹

Step 1: Calculate the volume of water displaced ( cm³ )

V = π × r² × d
V = 3.14 × (0.6)² × 1.5
V = 3.14 × 0.36 × 1.5 = 1.6956 cm³

Step 2: Convert time to hours

120 minutes = 120 / 60 = 2 hours

Step 3: Calculate the rate ( Volume ÷ Time )

Rate = 1.6956 cm³ ÷ 2 hours = 0.8478 cm³ hour⁻¹

✅ Acceptable Final Answers

Using π = 3.14: 0.85 or 0.848 or 0.8478 (also accepts 0.8)
Using calculator π: 0.8482

❌ Common Calculation Traps

Forgetting time conversion: Dividing by 120 gives 0.0141 cm³ min⁻¹ . Always check the required unit in the prompt ( cm³ hour⁻¹ ).
Squaring error: Forgetting to square only the radius ( 0.6² = 0.36 ).
Diameter trap: The question gives radius directly ( 0.6 cm ). Do not halve it!

1 mark awarded for correct final answer. No working mark available (single-mark question).

Topics

Biology · Practical skills · 3.1 Biological molecules · 3.3 Organisms exchange substances with their environment · Data analysis