WJEC A-Level Biology Unit 2 Biodiversity and Physiology of Body Systems: a deep dive on classification, gas exchange, transport, nutrition and disease

What Unit 2 actually demands

Biodiversity and Physiology of Body Systems takes the cell-level biology of Unit 1 and scales it up to whole organisms. The unit runs from how we classify and measure the diversity of life, through how organisms exchange gases, transport substances and obtain food, to how infectious disease spreads and how the body defends itself. Examiners test recall of structures and processes alongside the application of a few powerful principles to many different organisms.

This guide walks through all five clusters of the unit, then sets out the exam patterns WJEC repeats. Each cluster has a matching dot-point page with practice questions; this overview ties them together.

Classification and biodiversity

Organisms are classified in a hierarchy from domain to species, with each given a binomial Latin name. Know the three domains (Archaea, Bacteria, Eukarya) and the five kingdoms, and understand that classification is revised as DNA and protein sequencing reveal evolutionary relationships. Biodiversity is the variety of life; the index of diversity measures species diversity using both the number of species and the number of individuals in each, so an evenly distributed community scores higher than one dominated by a single species.

Adaptations for gas exchange

The unifying idea is the efficient exchange surface: large surface area, short diffusion distance and a maintained concentration gradient. Apply it to the alveoli (vast thin moist surface, ventilated and well supplied with blood), the fish gill (counter-current flow that keeps the gradient along the whole lamella), the insect tracheal system (air delivered directly to tissues through tracheoles), and the leaf (stomata and air spaces). The surface area to volume argument explains why large organisms need specialised systems at all.

Adaptations for transport

Mammals use a double circulation so blood is repressurised after the lungs. Learn the four-chambered heart, the cardiac cycle (atrial systole, ventricular systole, diastole) and the conduction system from the sinoatrial node through the atrioventricular node and Purkyne fibres. Oxygen transport centres on the S-shaped oxygen dissociation curve and the Bohr effect, which shifts the curve right at respiring tissues so more oxygen is unloaded. In plants, water rises in the xylem by transpiration pull (cohesion-tension theory) and assimilates move in the phloem by translocation.

Adaptations for nutrition

Distinguish autotrophs from heterotrophs, then follow food through the human gut. Carbohydrases, proteases and lipase (with bile salts) digest the three main food groups, and the products are absorbed at the ileum, whose villi and microvilli give a huge surface area, with glucose and amino acids taken up by co-transport. Finish with the dental and gut adaptations of herbivores and carnivores, and the role of gut microbes in digesting cellulose, which mammals cannot do themselves.

The effect of infectious disease

Pathogens (bacteria, viruses, fungi, protoctists) cause disease with a major global impact, as in tuberculosis and malaria. The body defends with non-specific barriers and phagocytosis, then a specific response: T cells coordinate the cellular response and B cells make antibodies and memory cells in the humoral response. Antibiotics kill bacteria, often by disrupting the cell wall, but resistance evolves by natural selection when a random mutation is selected by the antibiotic and spreads, often on plasmids.

How Unit 2 is examined

A typical WJEC profile for this unit:

• Comparison and recall. Comparing exchange surfaces, distinguishing xylem and phloem, and listing the immune cells.
• Graph and data questions. Reading and shifting the oxygen dissociation curve, interpreting biodiversity data, and analysing eutrophication or disease statistics.
• Process explanations. The cardiac cycle, counter-current flow, absorption at the ileum, and the spread of antibiotic resistance.
• Extended answers. Why a double circulation is efficient, how the immune response works, and how natural selection drives resistance are all predictable.

Check your knowledge

A mix of recall and application questions covering the whole of Unit 2. Attempt them under timed conditions, then check against the solutions.

1. State the order of the taxonomic hierarchy from domain to species. (2 marks)
2. Explain how the alveoli are adapted for efficient gas exchange. (3 marks)
3. Explain why counter-current flow makes fish gills efficient. (2 marks)
4. Describe the three phases of the cardiac cycle. (3 marks)
5. Explain what the Bohr effect is and why it helps respiring tissues. (3 marks)
6. Explain how the ileum is adapted for the absorption of digested food. (4 marks)
7. Explain how leaching of fertiliser can cause the death of fish by eutrophication. (4 marks)
8. Explain how antibiotic resistance arises and spreads in a bacterial population. (4 marks)