IB DP Biology A2.3 Viruses-FA 2025- IB Style Questions For HL Paper 2

Question

The diagrams show a virus (200 nm) and a bacterium (1 μm):

a. Calculate the magnification of the bacterium. [1]

b. State the method that bacteria use to divide. [1]

c. Outline the effectiveness of antibiotics against viruses and bacteria. [1]

d(i). Define saprotrophic organisms. [1]

d(ii). State one role of saprotrophic organisms in the ecosystem. [1]

▶️ Answer/Explanation

Solution

a. Magnification Calculation:

• Measured image size ≈ 4.5 cm (45,000 μm)

• Actual size = 1 μm

• Magnification = Image size ÷ Actual size = 45,000 μm ÷ 1 μm = 45,000×

(Accept 44,000-46,000 due to measurement tolerance)

b. Bacterial Division Method:

✅ Binary fission

• Asexual reproduction where one cell divides into two identical daughter cells

c. Antibiotic Effectiveness:

✅ Effective against bacteria but not viruses

• Antibiotics target bacterial cell structures (cell walls, ribosomes)

• Viruses lack these structures and reproduce inside host cells

d(i). Saprotrophic Organisms Definition:

✅ Organisms that decompose dead organic matter by secreting digestive enzymes and absorbing nutrients

• Examples: Fungi (like Mucor), some bacteria

d(ii). Ecological Role:

✅ Nutrient recycling (breaking down dead matter into inorganic compounds)

• Maintains ecosystem balance by returning nutrients to the soil

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