Cambridge International A-Level Biology 9700: Frequent mistakes

Cambridge Principal Examiner Reports for Biology 9700 identify recurring errors in multiple choice and structured questions. Enzyme kinetics, genetics terminology and immune system functions are particularly challenging.

Biochemistry and cell structure

Reducing sugars

Weaker candidates incorrectly conclude that reducing sugars were present in the original mixture when conditions suggest otherwise. Confusion about Benedict’s test and hydrolysis.

Fix: Reducing sugars give brick-red with Benedict’s. Sucrose is non-reducing unless hydrolysed first. Check the experimental setup.

Protein structure and bonds

Many incorrectly suggest bond 2 is correct for primary structure when it is only part of it. Confusion between primary (sequence), secondary (α-helix, β-sheet), tertiary and quaternary structure.

Fix: Primary = amino acid sequence (peptide bonds). Secondary = local folding (H-bonds). Tertiary = 3D shape. Quaternary = multiple polypeptides.

Enzyme kinetics

Candidates struggle to understand that substrate concentration remaining above zero means enzyme-substrate complexes continue to form. Not all substrate is used at any moment.

Fix: At V_max, enzyme is saturated. Substrate still present; rate is limited by enzyme availability.

Genetics and protein synthesis

Mutations

Less than half correctly understand that mutations change the order of nucleotides in DNA, not amino acids in genes. Amino acid order changes as a consequence of nucleotide change.

Fix: Mutation = change in DNA nucleotide sequence. May alter mRNA → may alter amino acid sequence.

tRNA anticodon

More than half misread questions and don’t correctly identify the tRNA anticodon (complementary to mRNA codon). Confusion between codon and anticodon.

Fix: mRNA codon (e.g. AUG) pairs with tRNA anticodon (UAC). Anticodon on tRNA.

Immunology

B-lymphocytes vs. other cells

Over half of weaker candidates incorrectly identify a cell as a B-lymphocyte when it is another type. Need to recognise cell function and location.

Fix: B-lymphocytes: produce antibodies; mature in bone marrow. T-lymphocytes: cell-mediated; mature in thymus.

Immunity vs. resistant genes

Confusion between immunity (acquired/natural resistance to pathogen) and resistant genes (genetic resistance, e.g. in bacteria).

Fix: Immunity = host defence. Resistant genes = genetic traits (e.g. antibiotic resistance in bacteria).

T-lymphocyte role

Over half unaware that T-lymphocytes play roles outside the blood (e.g. in tissues, lymph nodes).

Fix: T-cells: blood, lymph, lymphoid tissues. Not confined to blood.

General patterns

• Misreading questions—check focus (e.g. anticodon not codon)
• Levels of response—structure extended answers; address all parts
• Precise terminology—use syllabus terms

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Based on Cambridge International A-level Biology 9700 Principal Examiner Reports (2021–2025).