Microbiology, Immunity and ForensicsEdexcel International A Level Biology: Topic test

1. 1
   A culture of the bacterium Bacillus subtilis is grown in nutrient broth at 37 °C. At the start of exponential growth the culture contains 1.0 × 10⁴ cells cm⁻³, and the cells divide once every 30 minutes. The technician follows the growth of the culture by measuring its turbidity with a colorimeter.

   (a)

   How many cells cm⁻³ will the culture contain after 3 hours of exponential growth?

   [1 mark]

   • A3.2 × 10⁵
   • B1.2 × 10⁵
   • C6.4 × 10⁵
   • D6.0 × 10⁴

   (b)

   Why is turbidity measured with a colorimeter not a reliable measure of the number of viable cells?

   [1 mark]

   • AIt also measures dead cells, because dead cells scatter light as well as living ones.
   • BIt only detects the cells that are dividing.
   • CThe turbidity falls as the number of cells increases.
   • DA colorimeter cannot detect bacterial cells.

   (c)

   Later, the number of viable cells in the culture stopped increasing. Explain why.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A speck of soil carrying bacteria enters a child's eye. Her eye waters, and her tears contain the enzyme lysozyme.

   (a)

   How does lysozyme help to protect the eye?

   [1 mark]

   • AIt binds to antigens on the bacteria and labels them for phagocytes
   • BIt engulfs the bacteria by phagocytosis
   • CIt stops viruses replicating in neighbouring cells
   • DIt hydrolyses the cell walls of bacteria, so the bacteria burst

   (b)

   Which statement about phagocytosis of a bacterium is correct?

   [1 mark]

   • AAntibodies in the cytoplasm of the phagocyte digest the bacterium
   • BThe bacterium is enclosed in a vesicle, which fuses with a lysosome so that the bacterium is digested by enzymes
   • CThe phagocyte secretes interferon, which digests the bacterium
   • DThe bacterium enters through a channel protein in the membrane of the phagocyte

   (c)

   Explain why lysozyme is described as a non-specific defence.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A boy had measles when he was four years old. At the age of ten he was exposed to the measles virus again, but he did not become ill.

   (a)

   Explain why the boy did not become ill when he was exposed to measles virus for a second time.

   [3 marks]

   (b)

   Describe how the boy's immune system responded when he was infected for the first time.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A patient is diagnosed with tuberculosis (TB), caused by the bacterium Mycobacterium tuberculosis. The bacteria multiply slowly, and some live inside macrophages in the lungs. The patient is prescribed a combination of four antibiotics to be taken for six months, although his symptoms disappear after about three weeks. In the community where he lives, some cases of TB are caused by strains of the bacterium that are resistant to the most commonly used antibiotic.

   (a)

   Explain why the patient is given a combination of antibiotics for six months, even though his symptoms disappear after three weeks.

   [6 marks]

   (b)

   Explain how M. tuberculosis causes the symptoms of TB and avoids destruction by the immune system. Explain why this is an example of an evolutionary race between pathogen and host.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Peatlands in cold upland areas are waterlogged, so the water in them is acidic and contains little oxygen. Dead plant material builds up as peat instead of being completely broken down. A farmer plans to drain a peatland so that she can grow crops.

   (a)

   Which organisms are mainly responsible for the decomposition of dead plant material in soil?

   [1 mark]

   • APrimary consumers, which eat living plant tissue
   • BBacteria and fungi, which secrete enzymes onto the dead material and absorb the products
   • CProducers, which use dead plants as a source of carbon dioxide
   • DViruses, which digest the cellulose of plant cell walls

   (b)

   Which statement best explains why dead plant material decomposes very slowly in waterlogged peat?

   [1 mark]

   • AWater makes the enzymes of decomposers denature
   • BThere are no microorganisms in waterlogged soil
   • CCarbon dioxide cannot be released in respiration
   • DThere is little oxygen, so aerobic microorganisms respire and grow slowly

   (c)

   Explain why draining the peatland would increase the amount of carbon dioxide in the atmosphere.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A body is found in a shallow grave in woodland. The soft tissues are only partly decomposed, and fly larvae and beetles are present in the soil around the body. A body left on the soil surface in a similar woodland decomposes much faster.

   (a)

   Different species of insect are found on a body at different times after death. Which statement best explains this?

   [1 mark]

   • AThe changing condition of the body at each stage of decomposition attracts different species
   • BAll the insects arrive at the same time but the larger species eat the smaller species
   • CInsects only colonise bodies that are still warm
   • DDecomposition stops as soon as insects arrive

   (b)

   Which is the main reason why a body becomes stiff (rigor mortis) a few hours after death?

   [1 mark]

   • ANerve impulses continue to stimulate the muscles for many hours
   • BLactic acid denatures myosin so that it can no longer bind to actin
   • CATP is no longer made, so myosin heads cannot detach from actin and the muscles stay contracted
   • DGlycogen builds up in the muscle fibres and makes them swell

   (c)

   Suggest why the body in the shallow grave decomposed more slowly than the body on the surface.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   DNA was extracted from a small blood stain at a crime scene and from cheek swabs taken from three suspects. Fragments containing regions of short repeated sequences were amplified by the polymerase chain reaction (PCR) and then separated by gel electrophoresis. The profile from the blood stain matched the profile of suspect 2 at all ten regions that were tested. The profile of suspect 1 matched at only seven.

   (a)

   Describe how PCR is used to amplify the DNA from the blood stain.

   [3 marks]

   (b)

   Explain how gel electrophoresis separates the amplified DNA fragments and how the results show that suspect 2 is more likely than suspect 1 to be the source of the blood.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   In an outbreak of a respiratory illness caused by a new enveloped RNA virus, many patients also develop bacterial pneumonia. Health officials plan to vaccinate the population with a vaccine containing a surface protein from the virus, and to give emergency injections of antibodies purified from the blood of recovered patients to hospital staff who have been exposed to the virus.

   (a)

   Compare how protection is produced by the vaccine with how protection is produced by the injection of antibodies from recovered patients.

   [6 marks]

   (b)

   Explain why antibiotics are of no use against the virus but may be used against the bacterial pneumonia, with reference to the structure and replication of the virus. Explain also why antibiotics should be prescribed only for the bacterial infection.

   [6 marks]

   Total for question 8: 12 marks

End of questions