Pearson Edexcel International A Level Biology: Bacteria, Viruses, TB, HIV and the Body's Defences (YBI11) – Practice Questions

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 – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover statements 6.5 to 6.9 of the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2. Topic 6 is International A2 (IA2) and there are no tiers. Learn the content in the study guide and revision notes first. The course hub and checklist show the wider course, and the diagnostics help you find weak topics.

Questions

1. Compare the structure of a bacterium with that of a virus. [4]

2. For each part, name the virus or viruses from this list: Ebola virus, TMV, HIV, lambda phage. (a) The virus that infects bacteria. [1] (b) The two viruses that have an envelope. [1] (c) The virus with double-stranded DNA. [1]

3. Explain the difference between a lytic infection and latency, using lambda phage as your example. [3]

4. Explain how Mycobacterium tuberculosis causes damage to the lungs after it is inhaled. [5]

5. A patient infected with HIV has a CD4 count of 640 cells per mm³ at diagnosis and 280 cells per mm³ four years later. (a) Calculate the percentage fall in the count. [2] (b) Calculate the mean rate of fall per year. [1] (c) Explain why this patient is more likely to develop tuberculosis. [3]

6. An electron micrograph shows a bacterium 52 mm long at a magnification of ×20 000. Another micrograph shows a TMV particle 90 mm long at ×300 000. (a) Calculate the real length of the bacterium in µm. [2] (b) Calculate the real length of the TMV particle in nm. [1] (c) Calculate how many times longer the bacterium is than the TMV particle. [1]

7. (a) Name two routes by which pathogens can enter the body, with an example of each. [2] (b) A patient takes a long course of broad-spectrum antibiotics and then develops a gut infection. Explain why. [2]

8. A splinter breaks the skin and the area becomes red and swollen. Explain these changes. [4]

9. (a) Explain how lysozyme in tears helps to protect the eye. [2] (b) Explain why interferon protects neighbouring cells but not the cell that releases it. [2]

10. Describe how a neutrophil removes a bacterium from the tissues. [4]

11. A single B cell is selected and divides by mitosis every 5 hours for 60 hours. (a) Calculate the number of cells produced. [2] (b) Explain two features that allow a plasma cell to secrete large quantities of antibody. [2]

12. Describe the structure of an antibody and explain how antibodies, plasma cells and antigen-presenting cells together lead to the removal of a pathogen. [8]

Answers

1. Bacterium is a cell with cytoplasm and ribosomes whereas a virus has no cytoplasm or ribosomes [1]. Bacterium has a cell wall (peptidoglycan) whereas a virus has a protein capsid [1]. Bacterial genetic material is DNA only; a virus has either DNA or RNA [1]. Bacterium has its own metabolism and divides by binary fission whereas a virus can replicate only inside a host cell [1]. [4] Examiner insight: A comparison answer gains credit when both organisms appear in the same statement; two separate lists of features often show no clear difference.

2. (a) Lambda phage [1] (b) Ebola virus and HIV [1] (c) Lambda phage [1] Examiner insight: “Both” or “two” in the question means you must give all required names; naming only one of the enveloped viruses cannot earn the mark.

3. In a lytic infection the phage DNA is copied at once and new phages are assembled [1], then the bacterium bursts, releasing the phages [1]. In latency the phage DNA is integrated into the bacterial chromosome and copied at each division without making new phages [1]. [3] Examiner insight: Define both terms and tie each to the phage; stating only “lytic means lysis” is too thin for three marks.

4. Bacteria in droplets reach the alveoli, where macrophages engulf them [1]. The bacteria resist digestion and multiply inside the macrophages [1]. White blood cells wall off the infected cells, forming tubercles [1]. If immunity weakens, bacteria multiply and tubercles break down, forming cavities and destroying lung tissue [1]. The result is a persistent cough, coughing blood, weight loss and eventually lung failure or spread to other organs [1]. [5] Examiner insight: Give the process in order and show survival inside macrophages; “it damages the lungs” repeats the question and gains nothing.

5. (a) Fall = 640 − 280 = 360 [1]. 360 ÷ 640 × 100 = 56.3% [1] (b) 360 ÷ 4 = 90 cells per mm³ per year [1] (c) HIV destroys helper T cells [1], so the immune response to other pathogens is weak [1]; the tubercles cannot be kept intact, so latent M. tuberculosis is reactivated or a new infection is not controlled [1]. [6] Examiner insight: Show the subtraction and division. Calculate the change against the starting count, not the final count, since the final count gives a different and misleading percentage.

6. (a) 52 ÷ 20 000 = 0.0026 mm [1], which is 2.6 µm [1] (b) 90 ÷ 300 000 = 0.0003 mm = 300 nm [1] (c) 2.6 µm = 2 600 nm; 2 600 ÷ 300 = 8.7 times [1] [4] Examiner insight: Unit conversions are where most marks are lost; write the unit on every line and convert both lengths to the same unit before dividing.

7. (a) Any two of: inhalation (TB droplets) [1]; ingestion (contaminated water); skin breaks (needles); mucous membranes (HIV); across the placenta [1]. (b) The antibiotics kill gut flora as well as pathogens [1], so there is less competition and a pathogen can colonise the gut [1]. [4] Examiner insight: An example is needed with each route; naming the route alone does not show understanding.

8. Damaged cells and mast cells release histamine [1]. Arterioles dilate, so more blood flows to the area and it becomes red and hot [1]. Capillary walls become more permeable [1], so fluid leaks into the tissue and causes swelling, and phagocytes enter [1]. [4] Examiner insight: Link each sign to a mechanism. Writing “inflammation causes swelling” restates the question.

9. (a) Lysozyme hydrolyses peptidoglycan in bacterial cell walls [1], so the wall weakens and the bacterium bursts by osmosis [1]. (b) The infected cell releases interferon, which binds to receptors on neighbouring cells [1]; they make proteins that block viral replication, but the infected cell is already making virus [1]. [4] Examiner insight: “Kills bacteria” is not enough for lysozyme; name the substrate and the consequence for the cell.

10. The neutrophil recognises and binds the bacterium [1]. It engulfs it in a phagosome [1]. A lysosome fuses with the phagosome [1] and enzymes digest the bacterium [1]. [4] Examiner insight: A description is a sequence. Put the steps in order and use the terms phagosome and lysosome.

11. (a) 60 ÷ 5 = 12 divisions [1]; 2¹² = 4 096 cells [1] (b) Extensive rough endoplasmic reticulum for protein synthesis [1]; many mitochondria for ATP, or Golgi apparatus to modify and package antibody [1]. [4] Examiner insight: For “explain two features”, each feature needs its function; a bare list of organelles gains only partial credit.

12. Antibody has four polypeptide chains, two heavy and two light [1], joined by disulfide bonds [1]. The variable regions form two binding sites complementary to one antigen [1]. A macrophage engulfs the pathogen and presents its antigen, becoming an antigen-presenting cell [1]. A helper T cell with a matching receptor binds and is activated [1]. A B cell with matching surface antibody binds the antigen and is stimulated to divide [1]. The clone forms plasma cells that secrete antibody [1]. Antibodies bind the pathogen causing agglutination or neutralisation, so phagocytes can engulf it [1]. [8] Examiner insight: For an extended answer, order the steps and name each cell; a vague “the immune system makes antibodies” cannot access the later marks.

Where marks are usually lost

• Calling a virus a cell, or giving it ribosomes or its own metabolism.
• Failing to compare in a single statement when the command word is compare.
• Describing latency as “dormant” without saying the genetic material is integrated.
• Forgetting that M. tuberculosis survives inside macrophages.
• Calculating percentage change from the wrong value or mixing mm, µm and nm.
• Writing “interferon kills viruses” instead of the effect on neighbouring cells.
• Saying antibodies “kill” pathogens rather than naming agglutination, neutralisation or opsonisation.
• Confusing a plasma cell with the macrophage that presents antigen.

Next steps

• Revise from the revision notes or the study guide.
• Tick off statements on the checklist and browse the course hub.
• Take one of the free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology (YBI11), Specification Issue 2, published by Pearson Education Limited, Topic 6, statements 6.5 to 6.9.