AQA AS Level Biology Paper 2, June 2024: Question 9

✅ Mark Scheme Breakdown (Any 5 from 6)

1. Hb loads / associates / binds oxygen in the lungs [1]
2. At high partial pressure of oxygen (high pO₂) [1]
3. Binding of an oxygen molecule to Hb makes binding of subsequent oxygen molecules easier (cooperative binding) [1]
4. Oxygen is transported as oxyhaemoglobin in red blood cells [1]
5. Hb unloads / dissociates oxygen in respiring cells / tissues [1]
6. At low partial pressure of oxygen (low pO₂) OR at high partial pressure of carbon dioxide (high pCO₂) [1]

💡 Key Knowledge to Secure

• Affinity & Conditions: High pO₂ at alveoli favours high affinity (association). Low pO₂ and high pCO₂ (acidic conditions in respiring muscle) lowers affinity (dissociation / Bohr effect).
• Cooperative Binding: Haemoglobin has a quaternary structure with 4 haem groups. The binding of the 1st O₂ alters the tertiary/quaternary conformation, exposing other haem groups and steepening the dissociation curve.
• Terminology: Use association / loading and dissociation / unloading. State pO₂ or "partial pressure of oxygen" rather than just "oxygen concentration".

🧠 Exam Technique & Structure

• Follow the prompt's order: Organise your response into three clear stages: Loading → Transport → Unloading.
• Pair action with condition: Never just say "it binds oxygen". Always state where (lungs/alveoli) and under what condition (high pO₂).
• Name the vehicle: State that transport occurs inside red blood cells (erythrocytes) in the form of oxyhaemoglobin.

❌ Common Traps & Lost Marks

• Vague terminology: Saying "oxygen concentration" or "oxygen levels" instead of partial pressure of oxygen ( pO₂ ).
• Missing cooperative binding: Describing that 4 molecules bind, but forgetting to explain that the binding of the first molecule makes subsequent binding easier.
• Omitting location: Forgetting to mention that oxygen is carried specifically in red blood cells.

✅ Mark Scheme Breakdown (Any 5 from 7)

1. mRNA attaches to ribosomes (OR rough endoplasmic reticulum – must be written in full) [1]
2. tRNA anticodons bind to complementary mRNA codons [1]
3. tRNA brings a specific amino acid [1]
4. Amino acids join by peptide bonds [1]
5. Amino acids join together with the use of ATP [1]
6. tRNA is released after amino acid joins the polypeptide [1]
7. The ribosome moves along the mRNA to form the polypeptide [1]

💡 Key Knowledge to Secure

• Codon vs Anticodon: A codon is a triplet of bases on mRNA; an anticodon is a complementary triplet of bases on tRNA.
• Role of ATP: Hydrolysis of ATP provides the energy required to activate amino acids / attach them to tRNA and catalyse peptide bond formation.
• Ribosomal action: Ribosome accommodates two tRNAs at a time and moves incrementally (translocates) one codon at a time towards the stop codon.

🧠 Exam Technique & Structure

• Chronological narrative: Write your answer as a chronological step-by-step story from initiation to chain elongation.
• Pair complementary terms: Always pair the term anticodon with tRNA, and codon with mRNA.
• Do not abbreviate blindly: If mentioning the rough ER, write rough endoplasmic reticulum in full, as standard mark schemes disallow "RER" on this question.

❌ Common Traps & Lost Marks

• Confusing Transcription & Translation: Describing DNA unzipping, RNA polymerase, or splicing will score 0 marks. Translation begins at the ribosome.
• Forgetting "Specific": Stating "tRNA carries an amino acid" misses the mark point; you must state it carries a specific amino acid.
• Neglecting ATP: Forgetting to mention the requirement of ATP for the condensation of amino acids into a polypeptide.