AQA GCSE Biology 4.2 Organisation: a complete overview of digestion, circulation, plants and disease

What module 4.2 actually demands

Organisation is about how living things are built up from cells and how their systems work together to stay alive. The examiners reward clear links between structure and function: why an enzyme's active site matters, why the left ventricle is muscular, why alveoli are thin, and why xylem is hollow. Two required practicals (food tests and the enzyme investigation) recur often.

This guide walks through the four areas of the module and ties together the matching dot-point pages, each with its own practice questions.

Levels of organisation and digestion

Cells form tissues, tissues form organs, and organs form organ systems. The digestive system is the worked example: it uses enzymes to break large insoluble food molecules into small soluble ones.

Enzymes are biological catalysts with a specific active site (the lock and key model). They have an optimum temperature and pH; beyond these the enzyme denatures. Carbohydrases break carbohydrates into sugars, proteases break proteins into amino acids, and lipases break lipids into fatty acids and glycerol. Bile neutralises stomach acid and emulsifies fats.

The circulatory system

The double circulatory system sends blood through the heart twice per circuit. The four-chambered heart pumps deoxygenated blood to the lungs (right side) and oxygenated blood to the body (left side), with the natural pacemaker in the right atrium setting the rhythm. Arteries carry blood away under high pressure, veins return it with valves, and capillaries allow exchange. In the lungs, alveoli provide a large, thin, moist surface for fast gas exchange.

Blood is a tissue: plasma carries dissolved substances; red blood cells carry oxygen with haemoglobin; white blood cells fight pathogens; and platelets clot the blood.

Plant tissues and transport

A leaf is an organ of several tissues, adapted for photosynthesis and gas exchange. Xylem carries water and mineral ions up from the roots (the transpiration stream), while phloem carries dissolved sugars to where they are needed (translocation). Water is lost through stomata, controlled by guard cells, and the rate of transpiration rises with higher temperature, more wind, more light and lower humidity.

Health and disease

Health is physical and mental wellbeing. Communicable diseases spread; non-communicable diseases (cardiovascular disease, type 2 diabetes, most cancers) do not and are linked to risk factors such as diet, smoking and alcohol. Cancer is uncontrolled cell division forming a tumour; malignant tumours can spread to form secondary tumours.

How module 4.2 is examined

• Structure and function. Explaining adaptations of the heart, blood vessels, alveoli, red blood cells and plant tissues.
• Enzyme questions. Graphs of rate against temperature or pH, and the lock and key model.
• Practical questions. Food tests and the enzyme investigation, including variables and conclusions.
• Data and evaluation. Risk-factor data and correlation versus causation for non-communicable disease.

Check your knowledge

A mix of recall and application questions covering module 4.2. Attempt them under timed conditions, then check against the solutions.

1. Put these in order from smallest to largest: organ, cell, organ system, tissue. (1 mark)
2. Explain what happens to an enzyme above its optimum temperature. (2 marks)
3. Name the products formed when a protease digests protein. (1 mark)
4. State the role of bile. (2 marks)
5. Explain why the left ventricle wall is thicker than the right. (2 marks)
6. State two adaptations of a red blood cell. (2 marks)
7. Give two factors that increase the rate of transpiration. (2 marks)
8. Define a non-communicable disease and give one example. (2 marks)