20 April 2015

#79 Question 2

The diagram shows the bacterium Mycobacterium tuberculosis, which causes tuberculosis (TB).

(a) M. tuberculosis is taken up by macrophages and multiplies inside them.
Suggest how this strategy could help to protect M. tuberculosis from the immune response by B cells. (3 marks)
(b) In an experiment to investigate how M tuberculosis avoids destruction by macrophages, bacteria were added to a culture of macrophages obtained from the alveoli of mice. At the same time, a quantity of small glass beads, equivalent in size to the bacteria, were added to the culture. The experiment was repeated using increasing quantities of bacteria and glass beads.

After 4 hours, the macrophages were sampled to find out how many had taken up either glass beads or bacteria. The results are shown in the graph. The x-axis shows the initial ratio of bacteria or glass beads to macrophages in the mixture.

Discuss what these results suggest about the ability of macrophages to take up M. tuberculosis.
(3 marks)

(c) When M tuberculosis is present inside a phagosome of a macrophage, it secretes glycolipids that accumulate in lysosomes and prevent the lysosomes fusing with the phagosome.

Explain how this prevents the macrophage from destroying the bacterium.                            (3 marks)
Total: 9 marks

Candidate A

(a) It stops the B cells seeing them, so they don't make antibodies ü against them.

* This is not a very clear answer. B cells do not 'see', so this is not a good term to use. The 'they' in the second half of the sentence could refer to either B cells or the bacteria. 1/3

(b) The macrophages took up more glass beads than bacteria .ü So they are not very good at taking up the bacteria .ü

* Just enough for two marks, although the second sentence is weak. 2/3

(c) tysosornes contain digestive enzymes, ü so if they don't fuse with the phagosome the bacteria won't get digested. ü

* Once again, the candidate has the right ideas, but does not give enough biological detail to get full marks. 2/3

 Candidate B

(a) B cells only become active when they meet the specific antigen ü to which they are able to respond. If the bacteria are inside a macrophage. then the B cell's receptors won't meet the antigen ü on the bacteria. This means that the B cells will not divide to produce plasma cells , üand will not secrete antibodies üagainst the bacteria.

* A good answer. 3/3

(b) The cells only started to take up any bacteria when the particle.macrophage ratio was 1 ü On the other hand, they took up glass beads even when the ratio was above 0.01. üWhen the ratio of particles to macrophages was 10, only about 30% of the macrophages had taken up bacteria, whereas over 75% of them had taken up glass beads. ü This shows the macrophages do take up the bacteria,
but not as well as they take up glass beads. ü

* A good answer, which does attempt to 'discuss' by providing statements relating to the relatively low ability of the macro phages to take up the bacteria, but also stating that they do take them up. In general, it is always a good idea to quote data where they are relevant in your answer. 3/3

(c) Normally, lysosomes fuse with phagosomes and release hydrolytic enzymes üinto them. These enzymes then hydrolyse (digest) whatever is in the phagosome. ü If this doesn't happen, then the bacteria can live inside the phagesome ü without being digested.

* All correct. 3/3

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