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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the reactants.
- S2
Amino acid 1 and Amino acid 2
- S3
Step 2: Describe the formation of a chemical bond.
- S4
The 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-).
- S5
Step 3: Explain the elimination of water.
- S6
A molecule of water (H 2 O) is released as a byproduct.
- S7
Step 4: Write the balanced equation.
- S8
Amino acid 1 + Amino acid 2 → Dipeptide + H 2 O
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the reactants.
- S2
Amino acid 1 and Amino acid 2
- S3
Step 2: Describe the formation of a chemical bond.
- S4
The 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-).
- S5
Step 3: Explain the elimination of water.
- S6
A molecule of water (H 2 O) is released as a byproduct.
- S7
Step 4: Write the balanced equation.
- S8
Amino acid 1 + Amino acid 2 → Dipeptide + H 2 O
Final answer: Amino acid 1 + Amino acid 2 → Dipeptide + H 2 O
Common mistakes
- Confusing monomers and polymers. — 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. — 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. — Practice recalling examples of monomers (monosaccharides, amino acids, nucleotides) and polymers (polysaccharides, proteins, nucleic acids).
- Failing to provide a balanced equation for chemical reactions. — 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. — 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. — 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
- Full worked solution (all marking points)4 marks