A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Monomers, Polymers, and Chemical Reactions in Biological Molecules — mark scheme explained
The short answer
The variety of life, both past and present, is extensive, but the biochemical basis of life is similar for all living things. This section focuses on understanding monomers, polymers, and the chemical reactions that form and break these molecules. Monomers and Polymers Monomers: Monomers are the smaller units from which larger molecules are made.
The question
Describe the process of forming a dipeptide from two amino acids through a condensation reaction.
[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 identification questions, marks are typically awarded for correctly naming the monomers or polymers. For descriptive questions, marks are given for clear and accurate explanations of the processes involved. For application questions, marks are awarded for demonstrating a deep understanding of how these concepts apply to biological systems.
What the command words demand
- Identify
- Recognize and name specific monomers or polymers from a given list or description.
- Describe
- Provide a detailed explanation of the process, including key steps and components.
- Explain
- Give reasons for why certain processes occur and how they are relevant to biological systems.
- Apply
- Use your knowledge to solve problems or explain real-world scenarios involving monomers, polymers, and chemical reactions.
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.0 marksAmino acid 1 and Amino acid 2
- Describe the formation of a chemical bond.2 marksThe carboxyl group (-COOH) of one amino acid reacts with the amino group (-NH 2 ) of another amino acid to form a peptide bond (-CO-NH-).
- Explain the elimination of water.1 markA molecule of water (H 2 O) is released as a byproduct.
- Write the balanced equation.1 markAmino acid 1 + Amino acid 2 → Dipeptide + H 2 O
Final answer: Amino acid 1 + Amino acid 2 → Dipeptide + H 2 O
Work through every step correctly and you earn all 4 marks.
Another worked example
Explain how a dipeptide is broken down into its constituent amino acids through a hydrolysis reaction.
- Identify the reactants.1 markDipeptide and water (H 2 O)
- Describe the breaking of the chemical bond.1 markThe peptide bond (-CO-NH-) in the dipeptide is broken by the addition of a water molecule.
- Explain the formation of amino acids.1 markThe carboxyl group (-COOH) and the amino group (-NH 2 ) are restored, forming two separate amino acids.
- Write the balanced equation.1 markDipeptide + H 2 O → Amino acid 1 + Amino acid 2
Final answer: Dipeptide + H 2 O → Amino acid 1 + Amino acid 2
Work through every step correctly and you earn all 4 marks.
Common mistakes
Confusing monomers and polymers.
Why it happens: Students may mix up the definitions of monomers and polymers, leading to incorrect answers in questions that require identifying these molecules.
Fix: Always remember that monomers are the smaller units from which larger molecules (polymers) are made. Polymers are formed by joining many monomers together.
Forgetting to mention water in condensation and hydrolysis reactions.
Why it happens: Students often overlook the role of water in these reactions, leading to incomplete or incorrect descriptions.
Fix: Always include the role of water in your explanations. In a condensation reaction, water is eliminated as a byproduct. In a hydrolysis reaction, water is used to break a chemical bond.
Misidentifying examples of monomers and polymers.
Why it happens: Students may struggle to recall specific examples of monomers and polymers, leading to incorrect answers in identification questions.
Fix: Practice recalling examples of monomers (monosaccharides, amino acids, nucleotides) and polymers (polysaccharides, proteins, nucleic acids).
Failing to provide a balanced equation for chemical reactions.
Why it happens: Students may not write the complete balanced equation for condensation and hydrolysis reactions, leading to incomplete answers.
Fix: Always write the balanced equation for these reactions, including all reactants and products. For example, Amino acid 1 + Amino acid 2 → Dipeptide + H 2 O.
Not explaining the role of chemical bonds in condensation and hydrolysis reactions.
Why it happens: Students may not fully understand how chemical bonds are formed or broken in these reactions, leading to vague or incorrect explanations.
Fix: Practice describing the formation and breaking of chemical bonds. In a condensation reaction, a peptide bond is formed. In a hydrolysis reaction, a peptide bond is broken.
Failing to apply knowledge to biological processes.
Why it happens: Students may struggle to connect the concepts of monomers, polymers, and chemical reactions to real-world biological processes like digestion and protein synthesis.
Fix: Practice explaining how these reactions are involved in biological processes. For example, hydrolysis is crucial for breaking down food molecules during digestion.
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 |
|---|---|---|
| Describe Condensation Reaction | Describe how two amino acids join to form a dipeptide via condensation. | 4 |
| Hydrolysis Reaction | Explain how water breaks a peptide bond to split a dipeptide into two amino acids. | 4 |
| Identify Monomers | Name the individual monomer units that join together to form a given polysaccharide. | 2 |
| Describe A Process | Explain how water is produced when monomers join together during a condensation reaction. | 3 |
| Explain Hydrolysis Digestion | Describe how water breaks bonds in large molecules to release smaller absorbable ones during digestion. | 4 |
| Total across these question types | 17 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.