Bacteria and virusesEdexcel A-Level Biology A: Subtopic test

1. 1
   A microbiologist is comparing two pathogens: Mycobacterium tuberculosis, a bacterium about 3 µm long, and the Human Immunodeficiency Virus (HIV), a virus about 120 nm across.

   (a)

   Which feature is found in HIV but not in Mycobacterium tuberculosis?

   [1 mark]

   • A70S ribosomes
   • BA peptidoglycan cell wall
   • CCircular DNA free in the cytoplasm
   • DA protein capsid enclosing RNA

   (b)

   Antibiotics such as penicillin are not effective against HIV. Which statement explains this?

   [1 mark]

   • AHIV has a cell wall made of cellulose
   • BHIV has no cell wall or metabolic processes of its own for the antibiotic to target
   • CHIV is a eukaryote and antibiotics only work on prokaryotes
   • DHIV is too large for antibiotics to enter

   (c)

   State two structural differences between Mycobacterium tuberculosis and HIV.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A patient has had a persistent cough, fever, night sweats and weight loss for several weeks. Tests show that she is infected with Mycobacterium tuberculosis. The bacteria entered her lungs and were engulfed by macrophages in the alveoli, but many of the bacteria were not destroyed.

   (a)

   How is tuberculosis mainly transmitted from person to person?

   [1 mark]

   • ABy inhaling droplets containing the bacteria from an infected person's cough
   • BBy sharing needles during injections
   • CBy the bite of an infected mosquito
   • DBy contact with infected blood during sexual intercourse

   (b)

   Which feature of Mycobacterium tuberculosis helps it to survive inside macrophages?

   [1 mark]

   • AIt has no cell wall, so macrophage enzymes cannot attach
   • BIt is too small to be recognised as a foreign cell
   • CIts waxy cell wall resists digestion by the macrophage's enzymes
   • DIt produces antibodies that destroy the macrophage

   (c)

   Suggest how untreated tuberculosis could eventually lead to the death of the patient.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   HIV has a lipid envelope containing attachment proteins, a protein capsid, two single-stranded RNA molecules and the enzyme reverse transcriptase. It infects helper T cells, which have CD4 receptors on their surface.

   (a)

   Describe how HIV enters a helper T cell and how its genetic material is used to produce viral DNA.

   [3 marks]

   (b)

   Explain how HIV infection can result in the death of an infected person, even though HIV itself does not directly cause most deaths.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A hospital team is treating a man who is infected with both HIV and Mycobacterium tuberculosis. His helper T cell count is far below normal, he has a persistent cough and he has lost a great deal of weight.

   (a)

   Compare the structure of a bacterium such as Mycobacterium tuberculosis with a virus such as HIV.

   [6 marks]

   (b)

   Explain why this man is much more likely than a healthy person to develop active tuberculosis and to die from it.

   [6 marks]

   Total for question 4: 12 marks

End of questions