Other pathogens: stem rust, influenza and malariaEdexcel A-Level Biology B: Revision notes
Section 1
Three pathogens, three kinds of organism
Pathogens can be fungi, viruses or protoctists. This topic compares one example of each:
- Puccinia graminis, a fungus that causes stem rust in wheat
- Influenza virus, which infects the human respiratory tract
- Plasmodium species, protoctist parasites that cause malaria
For each you need to know how it is transmitted, how it infects and its pathogenic effect. Detailed life cycles are not required.
Section 2
Stem rust: Puccinia graminis in wheat
Transmission: spores are released from pustules and carried by the wind to other wheat plants, sometimes over long distances.
Mode of infection: a spore lands on a wheat plant and germinates. Fungal hyphae grow into the leaf or stem tissue, often entering through the stomata, and absorb nutrients from the host cells.
Pathogenic effect: red-brown pustules burst through the surface of stems and leaves. The damaged tissue photosynthesises less and loses water, and the fungus takes sugars from the plant. The grain is shrivelled and the crop yield falls.
Link the effect to photosynthesis and nutrient loss, as this explains the lower yield.
Section 3
Influenza virus
Transmission: influenza is spread in airborne droplets when an infected person coughs or sneezes, and also by touching contaminated surfaces.
Mode of infection: the virus enters epithelial cells lining the airways. It is not a cell, so it enters a host cell and uses the host cell's machinery to make new virus particles, which are released to infect other cells.
Pathogenic effect: infected cells are destroyed, so the airway lining is damaged and mucus and bacteria are not cleared properly. Patients have fever, aching muscles and a dry cough, and may develop a secondary bacterial infection of the chest.
Do not say that antibiotics kill viruses or that viruses reproduce by themselves. They replicate only inside host cells.
Section 4
Malaria: Plasmodium
Transmission: malaria is spread by a vector, the infected female Anopheles mosquito. When it bites, it injects Plasmodium in its saliva into the blood.
Mode of infection: the parasites are carried to the liver, enter liver cells and multiply. They are then released and infect red blood cells, where they multiply again.
Pathogenic effect: infected red blood cells burst, releasing parasites and toxins, which causes recurring fever. The loss of red blood cells causes anaemia, and in severe cases blocked blood vessels damage organs such as the brain.
Section 5
Comparing the three pathogens
- Transmission: stem rust by wind-blown spores, influenza by airborne droplets, malaria by a mosquito vector. Only malaria needs a vector.
- Target cells: wheat leaf and stem tissue, human airway epithelial cells, and human liver and red blood cells.
- Effect: crop damage and lower yield, damaged airways and fever, and fever with anaemia.
All three use the host's cells or nutrients and destroy or damage host tissue, but each acts in a different way, so a good answer names the route, the cells targeted and the symptoms.
Must Know
- Puccinia graminis is a fungus spread by wind-blown spores; hyphae invade wheat and cause pustules, less photosynthesis and lower yield
- Influenza virus is spread by airborne droplets; it infects and destroys airway epithelial cells
- Plasmodium is spread by the female Anopheles mosquito (vector); it infects the liver and red blood cells
- Malaria causes recurring fever and anaemia
- You do not need detailed life cycles
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Other pathogens: stem rust, influenza and malaria
- A wheat farmer finds red-brown pustules on the stems and leaves of her crop. Hyphae of the fungus Puccinia graminis are seen growing through the leaf tissue under a microscope. Spores from the pustules are carried across neighbouring fields, and the infected plants produce small, shrivelled grains and a much lower yield.Explain why wheat plants infected with Puccinia graminis produce a lower yield of grain.2 marks
- Over three days, many pupils in a school develop fever, aching muscles and a dry cough. Swabs show influenza virus, an enveloped RNA virus, in the cells lining the nose and throat of the pupils. One pupil later develops a bacterial chest infection.Explain why influenza makes a patient more likely to develop a bacterial chest infection.2 marks
- A traveller returns from a region where malaria is endemic. Ten days later she develops a fever that returns every two to three days, and she feels weak and pale. A blood test shows a low haemoglobin concentration, and Plasmodium parasites are seen inside her red blood cells.Describe how the traveller became infected with Plasmodium.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).