AQA A-Level Biology Paper 1, June 2022: Question 2

✅ Correct Answer Structure

• Point 1: Pre-existing genetic variation (alleles for resistance) already present in the bacterial population.
• Point 2: Antibiotics act as a selection pressure, killing non-resistant bacteria while resistant strains survive and reproduce.
• Point 3: High usage of antibiotics in hospitals creates an environment where resistant alleles increase in frequency.

💡 Key Knowledge

Antibiotics do not cause mutations to create resistance. Mutations are random; antibiotics merely select for pre-existing resistant variants by killing off competition.

🧠 Exam Technique

Ensure you link the use of antibiotics directly to differential survival and reproduction. Use precise terminology like selection pressure and allele frequency rather than vague statements about bacteria "getting used" to drugs.

❌ Common Errors

• Stating that "antibiotics cause bacteria to mutate/become resistant."
• Claiming that patients develop immunity to antibiotics.
• Failing to mention pre-existing resistance alleles.

✅ Correct Answer

Maltose (or phonetic spellings like maltose/moltose)

💡 Key Knowledge

Maltose is a disaccharide formed by the condensation of two alpha-glucose molecules joined by a glycosidic bond (specifically a 1,4-glycosidic bond).

❌ Common Errors

Confusing the disaccharide with the enzyme ending in -ase ( maltase ). The question asks to name a disaccharide, so the suffix must be -ose .

✅ Correct Answer Steps

• Step 1: Disinfect work surfaces and wash hands before starting to eliminate contaminating microorganisms.
• Step 2: Use a sterile pipette or syringe to accurately transfer 0.3 cm³ of bacterial culture.
• Step 3: Flame the neck of the culture bottle and lift the agar plate lid only slightly at an angle to prevent entry of airborne contaminants. Work close to a Bunsen burner flame to utilize upward convection currents.

💡 Key Knowledge

Aseptic technique protects both the operator from pathogenic bacteria like C. difficile and prevents the culture from becoming contaminated with environmental microbes.

🧠 Exam Technique

Action verbs matter! Always specify how equipment is kept sterile (e.g., using sterile pipettes, flaming bottle necks, lifting lids minimally).

❌ Common Errors

Writing vague statements like "sterilize the air" or "keep the Petri dish wide open." Air currents cannot be sterilized entirely; you must use convection currents and restricted exposure times instead.

✅ Evaluation Arguments (For & Against)

For the hypothesis:

• Antibiotic-resistant C. difficile grows significantly faster in the presence of trehalose.
• This selective advantage means resistant strains are likely to increase in frequency and outcompete non-resistant strains in people consuming high trehalose diets.

Against the hypothesis (Limitations):

• The investigation was done in a laboratory petri dish, not inside the human body.
• No statistical test was performed to prove the differences in growth rates are statistically significant rather than due to chance.
• No data provided regarding other dietary sugars, bacterial species interactions, or varying concentrations of trehalose.

🧠 Exam Technique

For a 3-mark "evaluate" question based on a graph, top-mark responses must balance evidence supporting the conclusion with valid methodological limitations or missing variables shown in the data.