A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Properties of Water in Biological Systems — mark scheme explained
The short answer
Water is a crucial component of all living cells, and its unique properties play vital roles in biological processes. This section delves into the specific properties of water that are essential for life, including its role as a metabolite, solvent, temperature buffer, cooling agent, and cohesive molecule.
The question
Explain how water is involved in the formation of a peptide bond between two amino acids.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For explanation questions, marks are typically awarded for providing a detailed description that includes relevant principles and relationships. For descriptive questions, marks are given for clear accounts using appropriate terminology. For comparison questions, marks are awarded for identifying both similarities and differences. For evaluation questions, marks are given for assessing the significance or effectiveness of a concept or process.
What the command words demand
- Explain
- Provide a detailed description of the concept, including relevant principles and relationships.
- Describe
- Give a clear account of the process or phenomenon, using appropriate terminology.
- Compare
- Identify similarities and differences between two or more concepts or processes.
- Evaluate
- Assess the significance or effectiveness of a concept or process.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, provide detailed answers, and check your work.
- Identify the reactants and products.1 markTwo amino acids are the reactants, and a dipeptide (a molecule with a peptide bond) and water are the products.
- Describe the condensation reaction.2 marksIn a condensation reaction, the carboxyl group (-COOH) of one amino acid reacts with the amino group (-NH 2 ) of another amino acid. This results in the formation of a peptide bond (-CO-NH-) and the elimination of water (H 2 O).
- Write the balanced equation.0 marksAmino Acid 1 + Amino Acid 2 → Dipeptide + H 2 O
Final answer: In a condensation reaction, two amino acids form a peptide bond and release water (H 2 O).
Work through every step correctly and you earn all 3 marks.
Another worked example
Describe how the high heat capacity of water helps maintain stable internal conditions in organisms.
- Define heat capacity.0 marksHeat capacity is the amount of heat required to raise the temperature of a substance by 1°C.
- Explain the significance of high heat capacity.3 marksWater has a high heat capacity, meaning it can absorb or release a significant amount of heat without a large change in temperature. This property helps buffer environmental changes and maintain stable internal conditions (homeostasis) in organisms.
- Provide an example.1 markFor instance, when the external environment becomes hot, water in the body absorbs heat, preventing rapid increases in body temperature. Conversely, when it is cold, water releases heat slowly, helping to maintain a constant body temperature.
Final answer: The high heat capacity of water helps buffer environmental changes and maintain stable internal conditions (homeostasis) in organisms by absorbing or releasing significant amounts of heat without large temperature changes.
Work through every step correctly and you earn all 4 marks.
Common mistakes
Confusing condensation and hydrolysis reactions.
Why it happens: Students may mix up the definitions and processes of these two types of reactions, leading to incorrect answers.
Fix: Remember that in condensation reactions, water is eliminated as a product, while in hydrolysis reactions, water is added as a reactant. Practice writing balanced equations for both types of reactions.
Failing to explain the polar nature of water and its role as a solvent.
Why it happens: Students may not fully understand why water is an excellent solvent, leading to vague or incorrect explanations.
Fix: Practice explaining that water's polar nature allows it to form hydrogen bonds with other molecules, facilitating their dissolution. Provide specific examples of ionic and polar compounds that dissolve in water.
Not linking the high heat capacity of water to temperature regulation.
Why it happens: Students may describe the property without explaining its biological significance, leading to incomplete answers.
Fix: Always link the high heat capacity of water to its role in buffering environmental changes and maintaining stable internal conditions (homeostasis) in organisms. Provide examples of how this property helps maintain body temperature.
Misunderstanding the latent heat of vaporisation and its cooling effect.
Why it happens: Students may not fully grasp the concept of latent heat or its significance for cooling mechanisms, leading to incorrect answers.
Fix: Practice explaining that the high latent heat of vaporisation means water absorbs a significant amount of heat during evaporation, providing an efficient cooling effect with minimal water loss. Provide examples such as sweating in humans and transpiration in plants.
Failing to describe how cohesion supports water transport in plants.
Why it happens: Students may not fully understand the role of cohesion in xylem vessels, leading to incomplete or incorrect explanations.
Fix: Practice explaining that cohesion between water molecules helps maintain continuous columns of water in xylem vessels, allowing for efficient transport from roots to leaves. Emphasize the importance of preventing air bubbles and ensuring water can be pulled up against gravity.
Not explaining the significance of surface tension in biological processes.
Why it happens: Students may not fully understand how surface tension affects biological systems, leading to vague or incorrect explanations.
Fix: Practice describing how surface tension allows small organisms to walk on water and influences the movement of water through soil and plant tissues. Provide specific examples to illustrate these points.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Condensation Reaction | Explain how water is eliminated when a peptide bond forms between two amino acids. | 3 |
| Explain Water Property | Explain how water's high heat capacity buffers temperature changes to maintain stable internal conditions. | 4 |
| Explain Cooling Effect | Explain how latent heat of vaporisation cools a surface while losing little water. | 4 |
| Explain Transpiration Mechanism | Explain how hydrogen bonding creates cohesion that maintains unbroken water columns in xylem. | 4 |
| Total across these question types | 15 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.