Barriers and non-specific defencesEdexcel A-Level Biology A: Subtopic test

1. 1
   Pathogens can enter the body by several routes and the body has barriers against each one. In hospital, patient 1 has taken a broad-spectrum antibiotic for ten days and has developed severe diarrhoea caused by Clostridium difficile in the gut. Patient 2 takes a drug that raises the pH of the stomach contents from about pH 2 to about pH 5.

   (a)

   Why is patient 2 more likely to develop a gut infection?

   [1 mark]

   • AThe enzyme pepsin is activated at the higher pH
   • BMore ingested bacteria survive because their enzymes are no longer denatured by acid
   • CLess mucus is secreted by the stomach lining
   • DGut flora are killed by the higher pH

   (b)

   Which describes how the lining of the airways helps to protect against inhaled pathogens?

   [1 mark]

   • ASquamous cells release lysozyme into the blood
   • BThe cells secrete stomach acid onto the surface
   • CGoblet cells release antibodies directly onto the pathogens
   • DCilia sweep mucus containing trapped pathogens away from the lungs

   (c)

   Explain why taking a broad-spectrum antibiotic could lead to the infection in patient 1.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student cuts her finger on broken glass. Within a few minutes the area around the cut becomes red, warm, swollen and painful, although the cut was cleaned straight away.

   (a)

   Which substance released by damaged tissue and mast cells causes vasodilation and increases the permeability of capillaries?

   [1 mark]

   • AInterferon
   • BLysozyme
   • CHistamine
   • DKeratin

   (b)

   What causes the swelling around the cut?

   [1 mark]

   • APlasma leaks out of the more permeable capillaries into the tissue
   • BBacteria multiply and form a solid mass
   • CRed blood cells leave the capillaries and form tissue fluid
   • DVasoconstriction traps blood in the finger

   (c)

   Explain how inflammation helps the body to fight an infection in the cut.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Interferon is released by cells infected with a virus. In an experiment, two dishes of cultured cells were exposed to the same dose of influenza virus. One dish had been treated with interferon for six hours beforehand and one was untreated. After 48 hours the untreated dish had 120 plaques (areas of dead cells) and the interferon-treated dish had 18 plaques.

   (a)

   Describe how interferon protects cells from viral infection.

   [3 marks]

   (b)

   Calculate the percentage reduction in the number of plaques caused by interferon, and suggest why some plaques still formed in the treated dish.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A man has a deep leg wound contaminated with soil. Three days later the skin around the wound is red, swollen and warm, and pus containing white blood cells and bacteria is draining from it.

   (a)

   Explain how the non-specific response to the wound accounts for the redness, swelling and warmth, and for the pus.

   [6 marks]

   (b)

   Describe how a neutrophil destroys the bacteria in the wound and explain how lysozyme helps to defend against these bacteria.

   [6 marks]

   Total for question 4: 12 marks

End of questions