WJEC A-Level Biology Unit 3 Energy, Homeostasis and the Environment: a deep dive on photosynthesis, respiration, microbiology, ecosystems, the kidney and the nervous system
A deep-dive WJEC A-Level Biology guide to Unit 3, Energy, Homeostasis and the Environment. Covers ATP and photosynthesis, respiration, microbiology, population size and ecosystems, human impact on the environment, homeostasis and the kidney, and the nervous system, with the exam patterns WJEC repeats.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What Unit 3 actually demands
Energy, Homeostasis and the Environment is the largest A2 unit and the heart of the A-level. It runs from how energy is captured and released at the molecular level, through how microorganisms grow and how ecosystems work, to how the body keeps its internal environment stable and signals through nerves. Examiners want detailed, accurate accounts of multi-step processes alongside data analysis and the application of homeostatic principles.
This guide walks through all seven 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.
The importance of ATP and photosynthesis
ATP is the cell's energy currency, releasing usable energy when hydrolysed to ADP and phosphate. Photosynthesis stores light energy as chemical energy in two stages: the light-dependent reactions in the thylakoids make ATP by photophosphorylation, reduce NADP, and split water by photolysis to release oxygen; the light-independent Calvin cycle in the stroma fixes carbon dioxide onto RuBP to make triose phosphate, regenerating RuBP. Rate is set by the limiting factors of light intensity, carbon dioxide concentration and temperature.
Respiration
Aerobic respiration releases energy in four stages: glycolysis (cytoplasm), the link reaction and Krebs cycle (matrix), and oxidative phosphorylation on the inner mitochondrial membrane, where most ATP is made. Oxygen is the final electron acceptor, forming water and keeping the electron transport chain running. Without oxygen, anaerobic respiration regenerates NAD by forming lactate in animals or ethanol and carbon dioxide in yeast, yielding far less ATP.
Microbiology
Bacteria are prokaryotes grown on sterile media using aseptic technique to avoid contamination. In a closed batch culture the population follows the growth curve of lag, log, stationary and death phases; continuous culture keeps cells in the log phase. Growth is measured by viable counts, turbidity or total counts, and the distinction between a viable count (living cells) and a total count (all cells) is regularly tested.
Population size and ecosystems
Energy enters ecosystems as light, is fixed by producers, and flows through trophic levels with only about ten percent passing on, because energy is lost in respiration as heat and in faeces and excretion. Nutrients are recycled through the carbon and nitrogen cycles, the latter involving nitrogen-fixing, nitrifying and denitrifying bacteria. Populations grow to a carrying capacity set by limiting factors, and succession changes communities over time to a climax community.
Human impact on the environment
Human activity reduces biodiversity through deforestation, intensive agriculture and pollution. Eutrophication is the standout process: nutrient enrichment causes an algal bloom, light is blocked, plants die, and decomposers use up the oxygen, killing fish. Greenhouse gas emissions drive climate change, and conservation aims to protect species and habitats sustainably.
Homeostasis and the kidney
Homeostasis keeps the internal environment stable by negative feedback. The kidney filters blood by ultrafiltration in the glomerulus, reclaims useful substances by selective reabsorption, and controls water potential by osmoregulation: when the blood is too concentrated, osmoreceptors trigger ADH release, which makes the distal tubule and collecting duct more permeable to water, so more water is reabsorbed and concentrated urine is made.
The nervous system
Neurones carry electrical impulses. The resting potential (about minus seventy millivolts) is maintained by the sodium-potassium pump. A stimulus reaching threshold triggers an action potential: sodium influx depolarises, then potassium efflux repolarises. The impulse jumps between nodes of Ranvier in myelinated neurones (saltatory conduction). At a synapse, calcium triggers neurotransmitter release, which binds receptors on the next neurone to continue the signal in one direction only.
How Unit 3 is examined
A typical WJEC profile for this unit:
- Process accounts. The stages of photosynthesis and respiration, the growth curve, and the action potential, with precise locations and products.
- Data and graph questions. Limiting-factor graphs, growth curves, energy-flow diagrams, and action-potential traces.
- Homeostasis explanations. Negative feedback applied to osmoregulation and the role of ADH.
- Extended answers. Energy transfer losses, eutrophication, the role of oxygen in respiration, and synaptic transmission are all predictable.
Check your knowledge
A mix of recall and application questions covering the whole of Unit 3. Attempt them under timed conditions, then check against the solutions.
- Describe what happens in the light-dependent reactions of photosynthesis. (4 marks)
- Explain the role of oxygen in aerobic respiration. (2 marks)
- Describe the four phases of a bacterial growth curve in a batch culture. (4 marks)
- Explain why only about ten percent of energy is transferred between trophic levels. (3 marks)
- Explain how leaching of fertiliser leads to eutrophication. (3 marks)
- Explain how the body responds when the water potential of the blood falls too low. (4 marks)
- Describe how an action potential is generated. (3 marks)
- Explain why nervous transmission across a synapse occurs in only one direction. (2 marks)