Biology of Viruses, Bacteria and Fungi
What you must understand
Know that viruses are acellular obligate intracellular parasites: nucleic acid (DNA or RNA) inside a protein capsid, sometimes an envelope. They replicate only inside a host cell using the host's machinery. Learn the lytic vs lysogenic cycles. For bacteria (prokaryotes): no nucleus, circular DNA, ribosomes, cell wall (peptidoglycan), capsule, flagella, pili; reproduction by binary fission; roles in nutrient cycling and disease; antibiotic resistance through mutation and selection. For fungi: eukaryotic, heterotrophic, cell walls of chitin, body made of hyphae forming a mycelium; reproduction by spores; examples - yeasts (unicellular), moulds, mushrooms; ecological roles as decomposers and in symbiosis (lichens = fungus + alga/cyanobacterium, mycorrhizae).
Traps candidates fall into
- Calling a virus a "prokaryote" - it is neither prokaryotic nor eukaryotic; it is acellular.
- Saying antibiotics kill viruses - antibiotics target bacterial structures; viruses need antivirals/vaccines.
- Confusing bacterial cell wall (peptidoglycan) with fungal cell wall (chitin) and plant cell wall (cellulose).
- Thinking all bacteria are harmful - most are harmless or beneficial.
- Describing fungi as plants - fungi are heterotrophic and lack chlorophyll.
Worked example / rule of thumb
If asked "why doesn't an antibiotic cure influenza?", answer: influenza is caused by a virus, which lacks the bacterial targets (cell wall, bacterial ribosomes) that antibiotics act on. Rule of thumb for classification: nucleus absent + peptidoglycan wall = bacterium; chitin wall + hyphae/spores = fungus; nucleic acid + capsid, no cells = virus.