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A-Level Biology October/November 2024 Q4(b): Students investigated the composition of the cell wall of leaf cells of thale cress, Ar…
A-Level Biology · Paper 9700/23 · October/November 2024 · Question 4(b) · [4 marks]
Students investigated the composition of the cell wall of leaf cells of thale cress, Arabidopsis thaliana. The students began by isolating the cell wall components from the rest of the cell material. The students used enzymes extracted from a fungal pathogen of A. thaliana to hydrolyse the cell wall components to smaller molecules. The students prepared a reaction mixture containing the cell wall components and the enzymes. After 24 hours, they separated and identified the smaller molecules found in the reaction mixture. Four types of molecule were identified: short chains of β-glucose, β-glucose, peptides, amino acids. Explain the presence of these molecules in the reaction mixture after 24 hours of hydrolysis.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The molecules identified are the products of enzymatic hydrolysis of the polymers found in the A. thaliana cell wall.
- The cell wall contains the polysaccharide cellulose and structural proteins.
- The presence of β-glucose indicates the complete hydrolysis of cellulose, which is a polymer made of β-glucose monomers. The fungal enzymes must include cellulase, which breaks the glycosidic bonds in cellulose.
- The presence of amino acids indicates the complete hydrolysis of proteins, which are polymers made of amino acid monomers. The fungal enzymes must include proteases, which break the peptide bonds in the proteins.
- The presence of short chains of β-glucose and peptides shows that hydrolysis was incomplete or partial after 24 hours. Some cellulose and protein molecules were not fully broken down to their respective monomers, leaving these intermediate fragments.
How the marks are awarded
- B1 — Identifying that both cellulose and protein were the original components present in the cell wall, as stated in point 1.
- B1 — Explaining that cellulose is broken down into its monomer, β-glucose, as described in point 2.
- B1 — Explaining that protein is broken down into its monomers, amino acids, as detailed in point 3.
- B1 — Explaining that the presence of short chains of β-glucose and peptides is due to partial or incomplete hydrolysis, as stated in point 4.
Common mistakes
- Forgetting that plant cell walls contain a protein component, and therefore being unable to explain the presence of peptides and amino acids.
- Confusing cellulose with starch, or β-glucose with α-glucose.
- Failing to explain the presence of the intermediate products (short chains and peptides) by not mentioning the concept of partial or incomplete hydrolysis.
- Stating that the cell wall is 'made of' the products (e.g., 'the cell wall is made of amino acids'), rather than the original polymers.
Examiner tip: When analysing experimental results, always work backwards from the products identified to deduce the original substrate molecules and the specific enzymes or processes involved.
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