A Level Biology: Infectious diseases — Practice Questions (Cambridge 9700)

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — Cambridge International holds copyright in its own papers. Use these alongside the official past papers available from your board.

Each question practises a skill tested in the June 2024 Paper 22. After each answer there is an examiner insight, a mark-scheme insight or a tip and, where one matches, the real question to try next.

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Questions

1. Answer these questions about pathogens.

(a) Give the full name of the virus that causes HIV/AIDS.

(b) Give the scientific names of the bacteria that cause cholera and tuberculosis (TB).

(c) Name the type of organism that causes malaria and give the name of one species. [4]

2. A person with HIV/AIDS has been taking anti-retroviral therapy (ART) but stops the treatment. A few months later the person develops a serious lung infection caused by a fungus that rarely causes disease in healthy people. Explain why stopping ART makes this infection more likely. [3]

3. Endolysins are enzymes coded for by viruses (bacteriophages) that infect bacteria. When purified endolysins are added to some types of bacteria from the outside, they hydrolyse bonds in the peptidoglycan of the cell wall. Compare the way endolysins and penicillin kill bacteria, giving similarities and differences. [4]

4. Explain why penicillin:

(a) has no effect on HIV

(b) has little effect on bacteria that are not growing or dividing. [3]

5. Suggest why TB spreads more quickly in overcrowded housing, and why people living with HIV are more likely to develop active TB. [3]

6. A patient stops taking a course of antibiotic as soon as they feel better. Explain how this could lead to an increase in antibiotic-resistant bacteria. [3]

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Answers

1. (a) Human immunodeficiency virus [1].

(b) Cholera: Vibrio cholerae [1]. TB: Mycobacterium tuberculosis (also accept Mycobacterium bovis) [1].

(c) A protoctist, e.g. Plasmodium falciparum (also accept P. vivax, P. ovale or P. malariae) [1].

Examiner insight (Cambridge 9700 June 2024 examiner report, Paper 22, Question 6(a)): Candidates must spell pathogen names correctly. For HIV, most errors were in “immunodeficiency” (such as “immune deficiency” or doubled letters). Some also wrote “human immunodeficiency disease”.

Source for the examiner insights on this page: Cambridge International AS & A Level Biology 9700 June 2024 Principal Examiner Report for Teachers, Paper 9700/22 section, paraphrased.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 22, Question 6(a).

2. ART stops HIV replicating, keeping the viral load low [1]. Without ART, HIV multiplies and infects and destroys T-helper lymphocytes [1]. With fewer T-helper cells, B-lymphocytes and other immune cells are not activated, so the immune response is weak [1]. The person is then open to opportunistic infections that a healthy immune system would control [1]. (Any three.)

Examiner insight (Cambridge 9700 June 2024 examiner report, Paper 22, Question 6(b)): Full-credit answers explained the benefit of ART to the immune system and then linked this to opportunistic infections being prevented or less likely; fewer mentioned lower viral numbers or maintained T-lymphocyte numbers. Some wrongly thought ART was a drug that treats the opportunistic infections themselves.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 22, Question 6(b).

3. Similarities (any one or two): both act on the peptidoglycan cell wall and weaken it [1]; both cause osmotic lysis, because the weakened wall cannot withstand the pressure as water enters [1]. Differences (any two or three): endolysin is an enzyme (a protein) that hydrolyses bonds in existing peptidoglycan, whereas penicillin is an antibiotic that inhibits the enzymes (transpeptidases) that form cross-links in new peptidoglycan [1]; penicillin only works on bacteria that are making new cell wall, whereas endolysin can break down any wall it reaches [1]; endolysin is a much larger molecule and can be denatured by heat [1]. (Max 4; at least one similarity and one difference are needed.)

Examiner insight (Cambridge 9700 June 2024 examiner report, Paper 22, Question 3(b)): Answers giving only differences could not earn full marks when similarities and differences were asked for. A common misconception was that penicillin makes “holes” in the wall; in fact it stops the action of the enzyme that forms the cross-links.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 22, Question 3(b).

4. (a) HIV is a virus with no cell wall or peptidoglycan, and it uses the host cell’s metabolism, so there is nothing for penicillin to act on [1].

(b) Penicillin inhibits the enzymes that build cross-links in new cell wall [1]. Bacteria that are not growing are not making new cell wall, so their wall is not weakened and they do not burst [1].

Mark-scheme insight (Cambridge 9700 June 2024 mark scheme, Paper 22, Question 3(b)): The mark scheme credited the point that penicillin is effective only while bacterial cells are growing and making new cell wall (in that question it was contrasted with lysozyme, which acts at any stage).

Source for the mark-scheme insights on this page: Cambridge International AS & A Level Biology 9700 June 2024 mark scheme for Paper 22 (9700/22), paraphrased. Cambridge’s 9700 past papers page publishes the Paper 21 mark scheme from this series, not the Paper 22 one.

Try the real question next: Cambridge International AS & A Level Biology 9700, June 2024, Paper 22, Question 3(b).

5. TB spreads through droplets in the air when an infected person coughs or sneezes [1]. In overcrowded housing people are close together for long periods, often with poor ventilation, so more people breathe in the bacteria [1]. HIV destroys T-helper lymphocytes, weakening the immune response, so TB bacteria that were held in check become active, or a new infection is not controlled [1].

Tip: Name the method of transmission first, then link each social or biological factor to it.

6. Stopping early leaves some bacteria alive. These are more likely to be ones with a mutation giving some resistance [1]. The antibiotic acts as a selection pressure: susceptible bacteria were killed, so resistant ones face less competition and reproduce [1]. They pass the resistance allele (gene) to their offspring (and may share plasmids with other bacteria), so the resistant population grows [1].

Tip: The antibiotic does not cause the mutation. It selects bacteria that already carry it.

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Where marks are usually lost

• Misspelling “immunodeficiency” or a pathogen’s scientific name.
• Saying ART treats opportunistic infections, instead of explaining that it protects T-helper cells by reducing viral load.
• Giving only differences when a question asks for similarities and differences.
• Saying penicillin makes holes in the cell wall instead of stopping cross-links from forming.
• Suggesting that antibiotics cause the mutations that give resistance.