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Pathogens and non-specific defences questions
Bacteria, viruses, fungi and protoctista with named diseases; how pathogens damage tissue through toxins and cell lysis; physical and chemical barriers including skin, mucus and cilia, stomach acid and lysozyme; inflammation and fever; and phagocytosis written as a sequence that ends with antigen presentation.
6 original questions · 21 marks · the pathogens and non-specific defences notes · Pathogens, disease and immunity
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Describe the stages of phagocytosis, in order, from the phagocyte approaching a bacterium to the display of its antigens.
Mark scheme
- B1 the phagocyte moves towards the bacterium along a concentration gradient of chemicals released by the pathogen or by damaged tissue, which is chemotaxis
- B1 receptors on the phagocyte's cell-surface membrane attach to antigens on the bacterium's surface
- B1 the membrane folds around the bacterium and the edges fuse, enclosing it in a vesicle called a phagosome
- B1 lysosomes move to the phagosome and fuse with it, emptying hydrolytic enzymes inside, and those enzymes hydrolyse the bacterium
- B1 the phagocyte moves some of the bacterium's antigens to its own cell-surface membrane and displays them, becoming an antigen-presenting cell
Compare physical barriers with chemical barriers as defences against infection, using a named example of each.
Mark scheme
- B1 both act against any pathogen without recognising which one it is, and both act at the same speed on every exposure
- B1 a physical barrier blocks or removes pathogens, whereas a chemical barrier destroys them
- B1 a named physical barrier with how it works, for example skin, which is dry, dead and keratinised, or mucus which traps particles while cilia sweep it to the throat
- B1 a named chemical barrier with how it works, for example stomach acid near pH 2, which denatures the enzymes and other proteins of swallowed bacteria, or lysozyme, which hydrolyses bacterial cell walls
A drug prevents lysosomes from fusing with phagosomes inside a phagocyte. Suggest how this would affect a patient's response to a bacterial infection.
Mark scheme
- B1 bacteria are still engulfed, because chemotaxis, attachment and the forming of the phagosome are unaffected
- B1 hydrolytic enzymes are not emptied into the phagosome, so the bacteria are not hydrolysed and may survive inside the phagocyte
- B1 fewer antigens become available to be moved to the phagocyte's cell-surface membrane, so antigen presentation is reduced
- B1 the specific response is therefore slow to begin, since clonal selection cannot start until antigen is presented, and the infection is likely to be more severe and to last longer
Explain why an antibiotic that cures tuberculosis has no effect whatever on influenza.
Explain how inflammation at the site of an infection helps the body to clear the pathogen.
Give one named human disease caused by a bacterium and one named human disease caused by a protoctistan.
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