What are the four groups of pathogens, and how do they cause disease?

OCR examines bacteria (for example tuberculosis), viruses (for example influenza and HIV, plus tobacco mosaic virus in plants), fungi (for example athlete's foot and black sigatoka) and protoctists (for example Plasmodium causing malaria, and the protoctist causing potato blight). Pathogens cause disease either by directly damaging host tissues, for example breaking down cells or using their nutrients, or by producing toxins that harm the host. You need named examples in both animals and plants.

Why is the secondary immune response faster than the primary response?

In the primary response the correct B and T lymphocytes must first be selected (clonal selection) and then divide (clonal expansion), which is slow, so antibody levels rise slowly and the person may feel ill. Memory cells are left behind. On re-exposure (the secondary response) these memory cells recognise the antigen immediately and divide rapidly into plasma cells, so antibodies are produced faster, in greater quantity and for longer, usually destroying the pathogen before symptoms appear. This is the basis of long-term immunity and vaccination.

How do you calculate and interpret Simpson's index of diversity?

Simpson's index of diversity is D equals 1 minus the sum of (n divided by N) squared, where n is the number of individuals of each species and N is the total number of individuals (an equivalent form uses n(n-1) over N(N-1)). The value runs from 0 to 1: a value closer to 1 means higher diversity, with more species and a more even distribution, and a more stable ecosystem; a value closer to 0 means low diversity, often one dominant species, and a less stable community. Always interpret the number, do not just state it.

Why did the three-domain system replace the five-kingdom system?

Molecular evidence, especially comparing ribosomal RNA sequences along with differences in membrane lipids, cell wall chemistry and enzymes, showed that the prokaryotes are not one group: the Archaea differ fundamentally from the Bacteria and resemble eukaryotes in some features. The three-domain system (Bacteria, Archaea and Eukarya) therefore splits the old single prokaryote kingdom into two domains. This difference was invisible to the eye, which is why the older system, based on observable features, had grouped them together.

What is the difference between the three types of natural selection?

Directional selection favours one extreme of a phenotype, so the mean of the population shifts towards it over time, for example antibiotic resistance or dark peppered moths in polluted areas. Stabilising selection favours the intermediate phenotype and selects against both extremes, so the mean stays the same but the range narrows, for example human birth mass. Disruptive selection favours both extremes against the intermediate, splitting the distribution into two peaks, and can begin the formation of new species.

EnglandBiology

OCR A-Level Biology Module 4 Biodiversity, evolution and disease: disease, immunity, biodiversity and evolution

A deep-dive OCR A-Level Biology guide to Module 4 Biodiversity, evolution and disease. Covers communicable diseases and primary defences, the specific immune response and vaccination, measuring and sampling biodiversity with Simpson's index, classification and the three domains, and evolution by natural selection, with the exam patterns OCR repeats.

Generated by Claude Opus 4.820 min readOCR-H420-Module-4Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What Module 4 actually demands
Communicable diseases and primary defences
The immune response and vaccination
Measuring and sampling biodiversity
Classification and phylogeny
Evolution by natural selection
How Module 4 is examined
Check your knowledge

What Module 4 actually demands

Biodiversity, evolution and disease is the "biological diversity" core of OCR A-Level Biology A, assessed mainly in Paper 2. It links four big themes: how pathogens cause communicable disease and how organisms defend themselves, how the specific immune system and vaccination work, how biodiversity is measured and why it matters, and how classification and evolution by natural selection account for the diversity of life. The examiners reward precise terminology (clonal selection, opsonin, species evenness, directional selection) and the ability to apply it to data, calculations and unfamiliar examples.

This guide ties together the five dot-point pages for Module 4 and sets out the exam patterns OCR repeats.

Communicable diseases and primary defences

A communicable disease is caused by a pathogen (bacterium, virus, fungus or protoctist) and spreads by direct or indirect transmission. Primary non-specific defences act against all pathogens: in plants the waxy cuticle, cell walls, callose and antimicrobial chemicals; in animals the skin, mucous membranes, stomach acid, lysozyme and blood clotting. If a pathogen breaches these, phagocytes engulf it into a phagosome, fuse it with a lysosome and digest it with hydrolytic enzymes, and macrophages present the antigens, linking to the specific response.

The immune response and vaccination

The specific response has two arms: the cell-mediated response (T helper cells coordinate via interleukins, T killer cells destroy infected cells) and the humoral response (B cells undergo clonal selection and expansion into plasma cells and memory cells). Antibodies are Y-shaped globular proteins with a variable region complementary to one antigen. The secondary response is faster, larger and longer than the primary because of memory cells. Vaccination provides antigens to make memory cells safely, and herd immunity protects the unvaccinated. Immunity is active (own antibodies, long-lasting) or passive (ready-made, short-lived), natural or artificial. Antibiotic resistance evolves by mutation then selection.

Measuring and sampling biodiversity

Biodiversity has three levels: habitat, species (richness and evenness) and genetic. Non-motile organisms are sampled by random quadrats (density or percentage cover), gradients by transects, and motile animals by mark-release-recapture. Simpson's index of diversity, $D = 1 - \sum (n/N)^2$ , runs from 0 to 1, with values closer to 1 meaning higher, more stable diversity. Biodiversity is maintained for ecological, economic and aesthetic or ethical reasons.

Classification and phylogeny

Classification is hierarchical (domain, kingdom, phylum, class, order, family, genus, species), and each species has a binomial name. The five-kingdom system was refined into the three-domain system (Bacteria, Archaea, Eukarya) using molecular evidence. Phylogeny is the evolutionary relationship between organisms; molecular evidence (DNA, ribosomal RNA and amino acid sequences) is the most reliable, because more similar sequences indicate a more recent common ancestor and it avoids being misled by convergent evolution.

Evolution by natural selection

Evolution is a change in allele frequency over time. Natural selection acts on heritable variation (whose source is random mutation): with overproduction and a selection pressure, individuals with advantageous alleles survive and reproduce more, raising the allele frequency. Directional selection shifts the mean to one extreme (antibiotic resistance), stabilising selection favours the intermediate (human birth mass), and disruptive selection favours both extremes. Evidence comes from fossils, comparative anatomy (homologous structures) and molecular biology.

How Module 4 is examined

A typical OCR profile for Biodiversity, evolution and disease:

Multiple choice and short answer. Matching a pathogen to a disease, ordering the steps of phagocytosis, classifying a type of immunity, naming the three domains.
Maths. Calculating Simpson's index of diversity and a mark-release-recapture population estimate, and interpreting an antibody-concentration graph.
Applied and data questions. Explaining antibiotic resistance, interpreting a phylogenetic tree, and evaluating a sampling method.
Level-of-Response extended answers. The specific immune response, why the secondary response is faster, and a worked natural-selection example (peppered moths, resistance) are all predictable.

Check your knowledge

A mix of recall and application questions covering the whole of Module 4. Attempt them under timed conditions, then check against the solutions.

Describe how a phagocyte destroys a pathogen. (4 marks)
Explain why the secondary immune response is faster and larger than the primary response. (4 marks)
Distinguish between active and passive immunity. (3 marks)
A field contains three plant species with counts 20, 10 and 5 (total 35). Calculate Simpson's index of diversity. (3 marks)
Explain why molecular evidence gives a more reliable classification than observable features. (3 marks)
Name the three domains and explain why the prokaryotes are split into two of them. (3 marks)
Explain how antibiotic resistance arises in a bacterial population. (4 marks)
Distinguish between directional and stabilising selection, with an example of each. (4 marks)

Solutions

Q1: The phagocyte is attracted to the pathogen and recognises its antigens as foreign. It engulfs the pathogen by endocytosis, enclosing it in a vesicle (phagosome). A lysosome fuses with the phagosome, and its hydrolytic enzymes (lysozymes) digest and destroy the pathogen. A macrophage then presents the antigens.
Q2: In the primary response, the correct lymphocytes must be selected (clonal selection) and divide (clonal expansion), which is slow, so antibody production is delayed. Memory cells remain afterwards. On re-exposure, these memory cells recognise the antigen at once and divide rapidly into many plasma cells, so antibodies are produced faster, in greater quantity and for longer.
Q3: Active immunity is when the body makes its own antibodies and memory cells (after infection or vaccination); it is slow to develop but long-lasting. Passive immunity is when ready-made antibodies are received (from the placenta, breast milk or an injection); it is immediate but short-lived because no memory cells are made.
Q4: $D = 1 - \sum (n/N)^2$ with N = 35. $(20/35)^2 = 0.3265$ ; $(10/35)^2 = 0.0816$ ; $(5/35)^2 = 0.0204$ . Sum $= 0.4286$ . $D = 1 - 0.4286 = 0.57$ (2 significant figures).
Q5: The DNA base sequence and the amino acid sequences it codes for directly reflect an organism's genes; more closely related species share more similar sequences because fewer mutations have accumulated since they diverged. Observable features can mislead, because unrelated organisms can evolve similar features (convergent evolution), so molecular data is more objective and reliable.
Q6: Bacteria, Archaea and Eukarya. The prokaryotes were split because molecular evidence (ribosomal RNA, membrane lipids, cell wall chemistry and enzymes) showed Archaea differ fundamentally from Bacteria, a difference not visible in their similar appearance.
Q7: A random mutation gives a bacterium resistance to the antibiotic. When the antibiotic is present it acts as a selection pressure: non-resistant bacteria are killed, but resistant bacteria survive and reproduce, passing on the resistance allele (including by plasmids). Over generations the frequency of the resistance allele increases, so more of the population is resistant.
Q8: Directional selection favours one extreme of a phenotype, so the mean shifts towards it (for example antibiotic resistance, or dark peppered moths). Stabilising selection favours the intermediate phenotype and selects against both extremes, so the mean stays the same but the range narrows (for example human birth mass).