IB Biology HL C3.2 Defence against disease questions

Exam-style multiple choice and written questions with full markschemes. Check multiple choice answers straight away, or type, photograph or draw a written answer and get it marked.

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Question 1

Multiple choiceEasy1 mark

A hospital restricts antibiotics to cases in which they are needed. Some pathogenic bacterial strains are already resistant to several antibiotics. How can this careful use slow the increase of multiresistant strains?

Show answer and explanation

Answer: D. Antibiotic use selects for resistant bacteria because susceptible bacteria are more likely to be killed. With less unnecessary antibiotic use, the selective advantage of multiresistant bacteria is reduced, slowing their increase in frequency.

A: Treats antibiotics as directly causing resistance mutations rather than selecting bacteria that already have resistance.
B: Confuses reduced selection with blocked inheritance; resistant bacteria can still pass resistance to offspring.
C: Assumes resistance disappears without antibiotics; reduced use does not itself remove resistance.
Remember: antibiotics select among bacterial variants; they do not direct bacteria to become resistant.

Question 2

Paper 2Medium4 marks

Researchers added the same concentration of an antibiotic to bacterial cultures and to cultures of human lymphocytes, some of which were infected with HIV. Figure 1 shows measurements after 24 hours, expressed relative to the corresponding culture without antibiotic.

Same antibiotic concentration addedBacterial cultureBacterial cell number:22% of untreated cultureUninfected humanlymphocyte cultureViable lymphocyte number:98% of untreated cultureHIV-infected humanlymphocyte cultureHIV particles released:103% of untreated cultureFor each culture, the corresponding untreated value is 100%.
(a)

Compare the effect of the antibiotic on bacterial cells and uninfected human lymphocytes, using Figure 1.

[1]
(b)

Using Figure 1 and your knowledge of antibiotics, explain why the antibiotic does not control HIV.

[1]
(c)

Explain how HIV infection can result in opportunistic infections.

[2]

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