Bacteria, viruses and infection — Edexcel International A Level Biology
Comparing bacteria and viruses, the lytic and latent cycles, how TB bacteria and HIV infect cells, and the routes and barriers that decide infection.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Bacteria are single prokaryotic cells with a cell wall, cytoplasm, 70S ribosomes and circular DNA. Viruses are acellular: nucleic acid enclosed in a protein capsid, sometimes surrounded by an envelope from the host membrane. Viruses have no ribosomes or metabolism and replicate only inside host cells.
In the lytic cycle a virus takes over the host cell, makes many new virus particles and bursts the cell open to release them. In latency the viral genome stays in the host cell, often integrated into the host DNA, without producing virus, and can later be triggered into the lytic cycle.
Mycobacterium tuberculosis is inhaled, taken into macrophages in the lungs and survives there, forming tubercles that can break down and damage the lungs. HIV infects helper T cells using reverse transcriptase, destroying them, so the immune system fails and opportunistic infections can kill.
Pathogens enter the body through the gas exchange system, the digestive system, breaks in the skin and the genital and eye mucous membranes. Barriers that protect include the skin, stomach acid, and gut and skin flora, which are non-specific because they act against all pathogens in the same way.
Exam questions
7 exam-style questions (22 marks), each with its mark scheme.
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16 cards: flip them, mark what you knew, and practise the rest.
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The whole of microbiology, immunity and forensics on one page, so you can see where this subtopic fits.
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Quick check questions
What is the difference between a capsid and an envelope?
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The capsid is the protein coat around the viral nucleic acid. The envelope is an outer lipid layer, taken from the host cell membrane, present in some viruses.
Why do viruses only reproduce inside host cells?
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They have no ribosomes or metabolism, so they use the host's enzymes, ribosomes and ATP.
What is a provirus?
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A DNA copy of the HIV genome made by reverse transcriptase and integrated into the host cell's chromosome.
Where does Mycobacterium tuberculosis survive in the body?
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Inside macrophages in the lungs, which cannot digest it.
Name two ways that gut flora protect against infection.
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They compete with pathogens for nutrients and space, and they produce substances that inhibit pathogens.
Frequently asked questions
What is the difference between bacteria and viruses?
Bacteria are living prokaryotic cells with a cell wall, cytoplasm, ribosomes and circular DNA, and reproduce by binary fission. Viruses are acellular: nucleic acid in a protein capsid, sometimes with an envelope. They have no metabolism, reproduce only inside host cells, and antibiotics do not work against them.
What do lytic and latent mean for a virus?
In the lytic cycle a virus replicates immediately, makes many new particles and bursts the host cell. In latency the viral genome stays in the host cell without making new virus, often integrated into the host DNA as a prophage or provirus, until something triggers the lytic cycle.
How does HIV cause AIDS?
HIV binds to the CD4 receptor on helper T cells, enters, uses reverse transcriptase to make DNA from its RNA, integrates it and then makes new viruses that burst out. As helper T cells are destroyed, the immune system fails, and opportunistic infections such as TB cause death.
Why does Mycobacterium tuberculosis survive inside macrophages?
The bacterium has a thick waxy cell wall and stops lysosomes fusing with the phagosome, so the macrophage cannot digest it. The bacteria multiply inside the macrophage. The immune response walls them off in tubercles, but they can escape and damage the lungs if immunity is weakened.
How does the body stop pathogens entering?
Barriers include the skin, which is a keratinised, acidic physical barrier, stomach acid at about pH 1 to 2, and the gut and skin flora, which compete with pathogens for nutrients and space and make inhibitory substances. Mucus and cilia in the airways also trap and remove microorganisms.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026