Transport
What this domain tests
Transport examines how substances move within organisms: diffusion and osmosis, water uptake, transpiration, xylem and phloem, blood, the heart and blood vessels. You must link structure to function and interpret direction of flow.
Core rules
- Root hair cells increase surface area for water and mineral uptake. Water enters by osmosis; minerals may enter by active transport.
- Xylem carries water and mineral ions upward; it is made of dead, hollow cells strengthened with lignin. Phloem carries sucrose and amino acids up or down; cells are living and have sieve plates.
- Transpiration is loss of water vapour from stomata. It creates a pull that draws water up the xylem. Rate rises with temperature, wind, low humidity and light; stomata close in drought.
- Arteries carry blood away from the heart, have thick elastic walls and narrow lumens. Veins carry blood to the heart and have valves. Capillaries are one cell thick for exchange.
- The heart has four chambers; the left ventricle has the thickest wall because it pumps blood to the whole body. Double circulation separates pulmonary and systemic circuits.
Worked example
A plant is watered with salty water and wilts. Salty soil has fewer free water molecules, so its water potential is lower than that of the root hair cell sap. Water leaves the root hair cells by osmosis, cells lose turgor, and the plant wilts. The reasoning must compare water potentials, not simply say that salt is toxic.
Traps
Do not confuse xylem and phloem direction. Transpiration is not respiration. Arteries do not always carry oxygenated blood-the pulmonary artery carries deoxygenated blood. Do not say veins have thick walls; they are thin-walled with valves.