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

This guide teaches bacteria, viruses, tuberculosis, HIV and the body’s first lines of defence from scratch. It is aligned to the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2, and covers statements 6.5 to 6.9 of Topic 6. Topics 1 to 4 are International AS (IAS) and Topics 5 to 8 are International A2 (IA2), so this material sits in IA2. There are no tiers. Statements 6.10 onward (B and T cell roles, immunity types, antibiotics) are separate. Check your progress against the course hub and the checklist. When you finish, use the revision notes and the practice questions, then take a free diagnostic.

What this guide covers

Statement	What you must be able to do
6.5 (i)	Compare bacteria and viruses (nucleic acid, capsid, envelope) with reference to Ebola virus, TMV, HIV and lambda phage
6.5 (ii)	Explain the terms lytic and latency
6.6	Explain how Mycobacterium tuberculosis and HIV infect human cells and cause symptoms that may lead to death
6.7 (i)	Know the major routes pathogens take into the body
6.7 (ii)	Explain how skin, stomach acid, and gut and skin flora protect the body
6.8	Explain inflammation, lysozyme, interferon and phagocytosis
6.9	Explain the roles of antigens and antibodies, including plasma cells, macrophages and antigen-presenting cells

6.5 (i) Bacteria and viruses compared

A bacterium is a living cell. It is prokaryotic, so its DNA is not enclosed in a nucleus. It has a cell wall made of peptidoglycan, a cell surface membrane, 70S ribosomes, cytoplasm and usually a single circular chromosome, often with small plasmids. It carries out its own metabolism and divides by binary fission.

A virus is not a cell. It has no cytoplasm, no ribosomes and no metabolism. It is a piece of nucleic acid inside a protein coat called a capsid, and it can only replicate by using the machinery of a host cell. Some viruses also have an outer envelope, taken from the host membrane as the virus leaves.

The four viruses you must know differ in three ways: the type of nucleic acid, the shape of the capsid and the presence of an envelope.

Virus	Nucleic acid	Capsid	Envelope	Host
Ebola virus	Single-stranded RNA	Helical, in a long filament	Yes	Humans and other mammals
Tobacco mosaic virus (TMV)	Single-stranded RNA	Rigid helical rod of identical protein subunits	No	Tobacco and related plants
HIV	Two copies of single-stranded RNA, plus reverse transcriptase	Cone-shaped capsid	Yes, with attachment proteins	Human helper T cells and macrophages
Lambda phage	Double-stranded DNA	Icosahedral head with a tail and tail fibres	No	The bacterium Escherichia coli

Lambda phage is a bacteriophage, a virus that infects bacteria. Its tail fibres attach to the bacterial cell surface and it injects its DNA, leaving the empty capsid outside.

Worked example: size comparison

An electron micrograph shows one HIV particle as 38 mm across, at a magnification of ×300 000. A bacterium on a second micrograph has a real length of 2.0 µm.

1. Real size = image size ÷ magnification = 38 mm ÷ 300 000.
2. 38 ÷ 300 000 = 1.27 × 10⁻⁴ mm. Multiply by 10⁶ to convert mm to nm: 127 nm.
3. Convert the bacterium: 2.0 µm = 2000 nm.
4. Ratio = 2000 ÷ 127 = about 16 times longer than the virus.

Check units first. Most wrong answers come from converting mm to µm or nm incorrectly.

6.5 (ii) Lytic and latent infection

In a lytic infection the virus takes over the host quickly. It makes many copies of its genome and capsid proteins, assembles new particles and bursts the cell open (lysis), releasing the new viruses. Lambda phage can do this in E. coli.

Latency means the viral genetic material is present in the host cell but is not producing new viruses. Lambda phage can integrate its DNA into the bacterial chromosome. The integrated DNA is copied each time the bacterium divides, so every daughter cell carries it. Stress can switch the phage back to the lytic route.

HIV also shows latency. Its RNA is copied into DNA by reverse transcriptase and the DNA is inserted into the host genome as a provirus. In a resting T cell the provirus may stay silent for years, hidden from the immune system. When the cell is activated, it begins to make viral RNA and proteins.

6.6 How TB and HIV cause disease

Mycobacterium tuberculosis

M. tuberculosis is a bacterium. It spreads in droplets from the lungs of an infected person, released by coughing or sneezing. The bacteria are inhaled into the alveoli.

1. Macrophages in the alveoli engulf the bacteria by phagocytosis.
2. The bacteria resist digestion. Their waxy cell wall and their ability to stop the phagosome fusing with lysosomes let them survive and multiply inside the macrophage.
3. The immune system walls off the infection. White blood cells gather round infected macrophages and form small lumps in the lung called tubercles. The bacteria may stay alive but inactive inside, giving a latent infection with no symptoms.
4. If the immune system weakens, the bacteria multiply and the tubercles break down, forming cavities. Lung tissue is destroyed. Symptoms include a persistent cough, coughing up blood, weight loss, fever and night sweats. The bacteria can spread through the blood to other organs. Death follows from lung failure or widespread infection.

TB is a particular danger for people with HIV, because their weakened immune system cannot keep the tubercles intact.

HIV

HIV infects helper T cells (cells with the CD4 receptor) and macrophages.

1. HIV attachment proteins in the envelope bind to the CD4 receptor on the host cell.
2. The viral envelope fuses with the cell surface membrane and the capsid enters the cytoplasm.
3. Reverse transcriptase makes a DNA copy of the viral RNA. The DNA enters the nucleus and is inserted into the host genome.
4. The host cell transcribes and translates the viral genes. New capsids form around viral RNA and the particles bud from the membrane, taking an envelope with them.
5. Infected helper T cells are destroyed. Over years the count falls, and the immune system cannot respond to other pathogens.

AIDS is diagnosed when the person has opportunistic infections or cancers that rarely occur in healthy people. HIV itself does not kill. Death results from diseases such as TB and pneumonia that the weakened immune system cannot control.

Worked example: interpreting CD4 data

A patient has a CD4 count of 920 cells per mm³ at diagnosis and 410 cells per mm³ six years later.

1. Fall = 920 − 410 = 510 cells per mm³.
2. Percentage fall = 510 ÷ 920 × 100 = 55.4%.
3. Mean rate of fall = 510 ÷ 6 = 85 cells per mm³ per year.

When you comment on a graph, say the fall is a mean rate, since the real decline may not be steady.

6.7 Routes into the body and barriers

(i) Routes of entry

Route	Example
Respiratory tract (inhalation)	M. tuberculosis in droplets
Digestive tract (ingestion)	Bacteria in contaminated food and water
Breaks in the skin	Cuts, bites, injections, needles
Mucous membranes of the genital tract, eyes and mouth	HIV in sexual fluids
Mother to fetus	Across the placenta, or in blood at birth

(ii) Barriers

• Skin. The outer layer is dead cells filled with keratin and is tough and waterproof. Sebum keeps it slightly acidic and contains antimicrobial substances.
• Stomach acid. The stomach secretes hydrochloric acid. Its low pH (about 2) kills most ingested microorganisms and denatures their enzymes.
• Skin and gut flora. Harmless resident bacteria compete with pathogens for space and nutrients and release substances that inhibit them. If antibiotics kill gut flora, a pathogen can colonise the gap.

Mucus and cilia in the airways are a further barrier, trapping inhaled microorganisms and moving them away from the alveoli.

6.8 Non-specific responses

These work the same way against any pathogen, from the first moments of infection.

Inflammation. Damaged cells and mast cells release histamine. Arterioles nearby widen, so blood flow increases, and capillary walls become more permeable. Fluid and phagocytes pass into the tissue. The area becomes red, hot, swollen and painful. The extra fluid and cells help to clear the pathogen.

Lysozyme. This enzyme, found in tears, saliva and mucus, hydrolyses the peptidoglycan in bacterial cell walls. The wall weakens, water enters by osmosis and the bacterium bursts.

Interferon. A cell infected by a virus releases interferon. It binds to receptors on neighbouring cells and prompts them to make proteins that block viral replication, mainly by stopping viral protein synthesis. It protects neighbours but does not rescue the infected cell.

Phagocytosis. Neutrophils and macrophages recognise pathogens, engulf them in a phagosome and fuse lysosomes with it. Lysosomal enzymes digest the pathogen and useful fragments are taken up.

Worked example: interpreting a neutrophil count

A patient with an infected wound has 4 800 neutrophils per mm³ of blood before infection and 11 400 at 24 hours.

1. Increase = 11 400 − 4 800 = 6 600.
2. Percentage increase = 6 600 ÷ 4 800 × 100 = 137.5%.
3. Explanation: more neutrophils are released and attracted to the wound, so more pathogens are engulfed and digested.

6.9 Antigens, antibodies and the immune response

Any foreign molecule that provokes an immune response is an antigen, and most are proteins or glycoproteins. Pathogens carry antigens on their surfaces. The body also has its own antigens, but immune cells normally tolerate them.

An antibody is a protein made by plasma cells that binds one specific antigen. It is made of four polypeptide chains, two long heavy chains and two shorter light chains, held by disulfide bonds. The variable regions at the two tips differ between antibodies and form the antigen-binding sites, shaped to fit one antigen. The constant region is the same for a given class.

The response in outline:

1. A macrophage engulfs a pathogen and displays pieces of its antigens on the cell surface membrane, so acting as an antigen-presenting cell.
2. A helper T cell with a complementary receptor binds to the presented antigen and is activated.
3. A B cell with surface antibody matching the same antigen binds it. The activated helper T cell stimulates this B cell to divide by mitosis (clonal expansion).
4. Many of the clones become plasma cells, which have extensive rough endoplasmic reticulum and secrete antibodies into blood and tissue fluid.

Antibodies help to remove pathogens by agglutination (clumping), neutralising toxins, blocking attachment to host cells, and labelling pathogens so that phagocytes engulf them more readily.

Worked example: clonal expansion

A selected B cell divides every 6 hours for 72 hours.

1. Number of divisions = 72 ÷ 6 = 12.
2. Cells after 12 divisions = 2¹² = 4 096.

This shows why the response takes days to reach full strength.

Common errors

• Calling a virus a cell, or saying viruses respire or reproduce on their own.
• Saying HIV kills the person directly. It destroys helper T cells, and death comes from other infections.
• Giving reverse transcriptase the wrong direction: it makes DNA from RNA.
• Saying lysozyme digests viruses or kills by acid. It targets bacterial peptidoglycan.
• Saying interferon kills viruses. It makes neighbouring cells resistant.
• Confusing antigens with antibodies, or plasma cells with memory cells.

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.