Pearson Edexcel International A Level Biology: Bacteria, Viruses, TB, HIV and the Body's Defences (YBI11) – Revision Notes

These revision notes condense statements 6.5 to 6.9 of Topic 6 of the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2. Topic 6 is International A2 (IA2); Topics 1 to 4 are International AS (IAS). There are no tiers. For full explanations and worked examples, read the study guide. Then test yourself with the practice questions, tick off the checklist, find the course hub and try a free diagnostic.

6.5 Bacteria and viruses

Feature	Bacterium	Virus
Cellular?	Yes, prokaryotic cell	No, acellular
Genetic material	Circular DNA, plus plasmids	DNA or RNA, never both
Protein coat	None (has a peptidoglycan cell wall)	Capsid
Ribosomes	70S	None
Metabolism	Own enzymes	None; uses host
Reproduction	Binary fission	Only inside host cells
Envelope	No	Some viruses

The four viruses

Virus	Nucleic acid	Capsid	Envelope
Ebola virus	ssRNA	Helical filament	Yes
TMV	ssRNA	Rigid helical rod	No
HIV	2 × ssRNA, plus reverse transcriptase	Cone	Yes
Lambda phage	dsDNA	Icosahedral head, tail, tail fibres	No

Memory hook: three of the four have RNA and one has DNA. Ebola and TMV both have helical capsids, but only Ebola has an envelope. HIV is the other enveloped virus. Lambda phage is the only DNA virus here and the only one that infects bacteria.

Lytic and latency

• Lytic: the virus replicates at once, assembles new particles and bursts the host cell.
• Latency: viral genetic material is in the host but makes no new particles. For lambda phage the DNA is integrated into the bacterial chromosome and copied at each division. For HIV the DNA copy (provirus) is in the host genome of a resting T cell.
• A latent virus can switch to the lytic cycle when conditions change.

Method: compare structures

1. Name the nucleic acid for each virus (DNA or RNA, single or double strand).
2. Describe the capsid shape.
3. State whether an envelope is present.
4. Add the host.

6.6 TB and HIV

TB in steps

1. Inhale droplets containing M. tuberculosis.
2. Alveolar macrophages engulf the bacteria.
3. Bacteria survive in macrophages by resisting digestion.
4. Tubercles form around infected cells; infection is walled off, often latent.
5. Weakened immunity reactivates the infection. Lung tissue is destroyed, cavities form.
6. Symptoms: cough with blood, weight loss, fever, night sweats. Death from lung damage or spread.

HIV in steps

1. Attachment proteins bind to CD4 on helper T cells and macrophages.
2. Envelope fuses with the cell surface membrane.
3. Reverse transcriptase makes DNA from RNA.
4. DNA is integrated as a provirus (latency possible).
5. Host cell makes viral proteins and RNA; new viruses bud off with an envelope.
6. Helper T cells fall in number, so immunity fails and opportunistic infections cause death.

Worked reminder: a CD4 count falling from 760 to 340 cells per mm³ is a fall of 420, which is 420 ÷ 760 × 100 = 55.3%.

6.7 Entry routes and barriers

Route	Example
Inhalation	TB droplets
Ingestion	Contaminated water
Skin breaks	Needles, cuts, bites
Mucous membranes	HIV in sexual fluids
Placenta or birth	Mother to child

Barrier	How it protects
Skin	Keratinised dead cells form a physical layer; sebum is antimicrobial
Stomach acid	pH about 2 kills or denatures ingested microorganisms
Skin and gut flora	Compete for space and food; produce inhibitory substances
Mucus and cilia	Trap microorganisms in airways and sweep them upward

6.8 Non-specific responses

Response	Key points	Linked to
Inflammation	Histamine from mast cells; vasodilation; more permeable capillaries; fluid and phagocytes enter tissue	Redness, heat, swelling, pain
Lysozyme	Hydrolyses peptidoglycan; bacterium bursts by osmosis	Tears, saliva, mucus
Interferon	Released by virus-infected cells; binds neighbouring cells; they make proteins that block viral replication	Viruses only
Phagocytosis	Engulf, phagosome, lysosome fusion, enzyme digestion	Neutrophils, macrophages

Phagocytosis in order

1. Pathogen recognised and binds to the phagocyte.
2. Membrane engulfs it into a phagosome.
3. Lysosome fuses and releases hydrolytic enzymes.
4. Pathogen is digested and the products are absorbed or expelled.

6.9 Antigens and antibodies

• Antigen: a molecule that triggers an immune response; foreign antigens are on pathogens.
• Antibody: a protein secreted by plasma cells; two heavy and two light polypeptide chains; variable regions form two identical binding sites; complementary in shape to one antigen.
• Macrophage: engulfs a pathogen and presents antigen fragments, becoming an antigen-presenting cell.
• Plasma cell: a clone of an activated B cell with extensive rough endoplasmic reticulum, secreting antibody.

Sequence

1. Macrophage engulfs a pathogen and presents its antigen.
2. A helper T cell with a matching receptor binds and is activated.
3. A B cell with a matching surface antibody binds the same antigen and is stimulated to divide by mitosis (clonal expansion).
4. Plasma cells form and secrete antibodies.

Antibody actions

• Agglutination (clumping pathogens together).
• Neutralising toxins.
• Blocking attachment sites on pathogens or viruses.
• Opsonisation, marking pathogens for phagocytes.

Worked reminder: if one B cell divides every 8 hours for 48 hours, there are 48 ÷ 8 = 6 divisions, and 2⁶ = 64 cells.

Growth reminder for TB

Suppose a data item says M. tuberculosis doubles about every 20 hours. Starting from 50 bacteria, after 100 hours there are 100 ÷ 20 = 5 doublings, so 50 × 2⁵ = 1 600 bacteria. Slow division is one reason TB develops over weeks or months and why treatment is long. Use the figure given in the question rather than one you remember.

Antibody structure checklist

• Four polypeptide chains: two heavy, two light.
• Chains are joined by disulfide bonds.
• A flexible hinge region lets the two arms move.
• Two identical antigen-binding sites in the variable regions.
• The constant region is the same within a class and is not the binding site.
• The shape of the binding site is complementary to one antigen only, which is why a different pathogen needs a different antibody.

Command words and how to answer

Command word	What to write
State	One short fact, no explanation
Describe	What happens, in order
Explain	Give the reason, using “because” or “so”
Compare	Similarities and differences, with both sides stated each time
Suggest	Apply what you know to an unfamiliar context

For a comparison, write the virus and the bacterium in the same sentence, for example “HIV has RNA whereas a bacterium has DNA”. A list of separate facts about each does not show a difference.

Must-know distinctions

• Antigen versus antibody: one triggers, the other binds.
• Lytic versus latent: bursting now versus waiting inside.
• Lysozyme versus interferon: bacterial walls versus antiviral proteins in neighbours.
• Macrophage versus plasma cell: presents antigen versus secretes antibody.
• Non-specific versus specific: responds the same to anything versus matched to one antigen.

Quick self-test

1. State what is meant by a capsid.
2. Name the enzyme that makes DNA from the RNA of HIV.
3. Which of the four viruses in the specification infects bacteria?
4. What does latency mean?
5. Name two symptoms of advanced tuberculosis.
6. Give two ways skin protects the body.
7. State why gut flora help to prevent infection.
8. What causes swelling during inflammation?
9. What does lysozyme digest?
10. How does interferon protect a neighbouring cell?
11. A single B cell divides every 4 hours for 24 hours. How many cells result?
12. A virus is 44 mm across on an image magnified ×200 000. State its real size in nm.

Answers

1. The protein coat that surrounds the viral nucleic acid.
2. Reverse transcriptase.
3. Lambda phage.
4. The viral genetic material is in the host cell but makes no new virus particles.
5. Any two of: persistent cough, coughing blood, weight loss, fever, night sweats.
6. Any two of: tough keratinised layer, waterproof, sebum with antimicrobial action, skin flora competition.
7. They compete with pathogens for food and space and release inhibitory substances.
8. Capillaries become more permeable, so fluid leaks into the tissue.
9. Peptidoglycan in bacterial cell walls.
10. It binds to receptors and triggers production of proteins that block viral replication.
11. 24 ÷ 4 = 6 divisions, so 2⁶ = 64 cells.
12. 44 ÷ 200 000 = 2.2 × 10⁻⁴ mm, so 220 nm.

Where marks are usually lost

• Writing “bacteria” when the answer is “virus” for statements on reverse transcriptase or capsids.
• Saying HIV “kills the immune system” rather than naming helper T cells.
• Omitting that TB bacteria survive inside macrophages.
• Giving “stomach acid kills bacteria” without a link to pH or enzyme denaturation.
• Describing inflammation symptoms without the mechanism (vasodilation and permeability).
• Saying interferon “kills the virus”.
• Writing “antibodies kill pathogens” instead of a precise action such as agglutination.
• Mixing up the unit when converting millimetres to nanometres.
• Calculating a percentage fall against the final value instead of the starting value.
• Describing a viral life cycle without saying where the host cell machinery is used.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology (YBI11), Specification Issue 2, published by Pearson Education Limited, Topic 6, statements 6.5 to 6.9.