Organisation of Living Things
What you must understand
Life is organised in levels: organelle, cell, tissue, organ, organ system, organism. In multicellular organisms, specialised cells are grouped into tissues with common structure and function (epithelial, connective, muscle, nervous in animals; epidermal, vascular, ground in plants). Organs combine tissues, and systems coordinate organs.
You must understand transport systems. In mammals, the circulatory system moves gases, nutrients, wastes and heat; the heart has four chambers, with the left ventricle thickest because it pumps blood around the whole body. Blood vessels differ in structure and function: arteries carry blood away from the heart at high pressure, veins return blood at low pressure with valves, and capillaries have thin walls for exchange. In plants, xylem transports water and minerals upward (dead cells, transpiration pull), while phloem transports sugars bidirectionally (living cells, translocation).
Also understand gas exchange surfaces and how structure supports efficient exchange: large surface area, thin barriers, moist surfaces, concentration gradients maintained by ventilation and blood flow. Know the relationship between structure and function in leaves (stomata, mesophyll, vascular bundles) and lungs (alveoli).
Traps in questions
- Xylem is unidirectional; phloem is bidirectional - mixing these loses marks.
- Transpiration is loss of water vapour, not "sweating" by plants.
- Do not confuse arteries/veins with oxygenated/deoxygenated blood; the pulmonary artery carries deoxygenated blood.
- When asked why a structure is efficient, name the feature and link it to the function.
Rule of thumb
For any exchange surface, apply the "large, thin, moist, gradient" checklist - large surface area, thin diffusion distance, moist surface, and a maintained concentration gradient.
Worked example
Alveoli are efficient because they provide a huge total surface area, walls one cell thick, a moist lining, and a steep gradient maintained by ventilation and blood flow.