IGCSE Biology / The variety of living organisms
Viruses and the main pathogen groups revision notes
Edexcel International GCSE Biology (4BI1), chapter 2: The variety of living organisms, specification point 1.4. Free lesson.
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Viruses have a simple structure and can reproduce only inside a living host cell. Pathogens belong to four groups, and you should know examples of each and why antibiotics do not work on viruses.
What a virus is made of
A virus is much smaller and simpler than a bacterium. It is not a cell. Its outer layer is a protein coat, with genetic material inside. That material is DNA or RNA, not both. A virus has no nucleus, so its genetic material is not enclosed in one. It has no cell membrane, cytoplasm, organelles, ribosomes or mitochondria.
Why viruses need a host cell
Without ribosomes, a virus cannot make proteins. Without mitochondria, it cannot respire. It cannot carry out these processes or reproduce by itself, so it uses the machinery inside a living host cell to reproduce. This makes viruses parasites. Because a virus is not a cell and cannot function independently, it belongs to none of the five kingdoms.
The four groups of pathogen
A pathogen is a microorganism that causes disease. Pathogens come from four groups. Learn the named examples and match each one to the disease it causes. Virus: influenza virus causes flu; tobacco mosaic virus discolours tobacco plant leaves; HIV causes AIDS. Bacterium: Pneumococcus causes pneumonia. Bacteria are cells without a nucleus, and antibiotics can work on them. Fungus: athlete’s foot fungus causes athlete’s foot by growing on the skin. Protoctist: Plasmodium causes malaria, which is spread by mosquitoes.
Why antibiotics do not work on viruses
Antibiotics affect bacteria, not viruses. A virus has no cell wall, ribosomes of its own or metabolism for an antibiotic to disrupt. So a virus is unaffected by antibiotics, rather than resistant to them. Resistance means a population has evolved to survive a drug that used to kill it. Antibiotics never killed viruses, so viral resistance to antibiotics is not the correct explanation.