AQA A-Level Biology: Properties of Water in Biological Systems — mark scheme explained

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

What each mark is really for — in plain English — and the wording trap that loses it.

• S1

  Step 1: Identify the reactants and products.

• S2

  Two amino acids are the reactants, and a dipeptide (a molecule with a peptide bond) and water are the products.

• S3

  Step 2: Describe the condensation reaction.

• S4

  In 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).

• S5

  Step 3: Write the balanced equation.

• S6

  Amino Acid 1 + Amino Acid 2 → Dipeptide + H 2 O

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   Step 1: Identify the reactants and products.

2. S2

   Two amino acids are the reactants, and a dipeptide (a molecule with a peptide bond) and water are the products.

3. S3

   Step 2: Describe the condensation reaction.

4. S4

   In 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).

5. S5

   Step 3: Write the balanced equation.

6. S6

   Amino Acid 1 + Amino Acid 2 → Dipeptide + H 2 O

7. Final answer: In a condensation reaction, two amino acids form a peptide bond and release water (H 2 O).

Common mistakes

• Confusing condensation and hydrolysis reactions. — 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. — 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. — 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. — 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. — 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. — 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

• Full worked solution (all marking points)3 marks

Related questions

• AQA A-Level Biology: Monomers, Polymers, and Chemical Reactions in Biological Molecules — mark scheme explained →
• AQA A-Level Biology: Triglycerides and Phospholipids: Structure and Properties — mark scheme explained →