Organisation and Maintenance of the Organism
This domain examines how cells are organised into tissues, organs and systems, and how those systems maintain a stable internal environment. Questions frequently link structure to function in digestion, gas exchange, transport and homeostasis.
Organisation and transport
Cells with similar functions form tissues; tissues form organs; organs form organ systems. Digestive enzymes are specific: amylase breaks starch to maltose, protease breaks proteins to amino acids, and lipase breaks fats to fatty acids and glycerol. Bile emulsifies fats and neutralises stomach acid. Villi increase surface area, have thin walls and a rich blood supply. Alveoli have the same exchange features. Arteries carry blood away from the heart, veins return it, and capillaries allow exchange.
Homeostasis
Negative feedback reverses a change. Receptors detect the stimulus, a coordination centre processes it, and effectors respond. Examples include insulin lowering blood glucose and vasodilation cooling the body.
Worked example
Explain why a villus speeds up absorption. Reasoning: its finger-like shape gives a large surface area; the epithelium is one cell thick, shortening the diffusion path; the blood capillary removes absorbed molecules, maintaining a steep concentration gradient. All three points are needed for full marks.
Traps
- The pulmonary artery carries deoxygenated blood away from the heart, so 'artery = oxygenated' is false.
- Bile does not digest fat; it emulsifies it.
- Villi absorb; they do not 'make' enzymes.