Biology unit guide
Form and function
Gas exchange, transport, muscle, adaptation, classification and cladistics.
Unit 2 is the clearest example of the principle the whole course is built on: structure explains function. Almost every question here can be answered by naming a feature and saying what it makes possible, and almost every lost mark comes from doing only the first half. Alveoli are not examined because they are thin - they are examined because being thin shortens the diffusion path.
Topics in this unit
What gets examined
- Explaining how alveoli, gills and leaves are adapted for gas exchange, in terms of surface area, thickness and maintaining a gradient.
- Relating the structure of arteries, veins and capillaries to the pressure each carries.
- The cardiac cycle, and how valves and pressure changes control the direction of flow.
- The sliding filament model of muscle contraction, including the role of calcium and ATP.
- Distinguishing adaptation from acclimatisation, and both from evolution.
- Reading a cladogram: what a node represents, and what branch order does and does not tell you.
How to revise it
Answer every structure question with a because
Thin walls because the diffusion path is short. Many capillaries because surface area is large. The feature alone is half an answer, and mark schemes award the other half.
Learn the three gas-exchange requirements once
Large surface area, short diffusion distance, a maintained concentration gradient. Alveoli, gills and leaves are the same three points with different anatomy, which turns three topics into one.
Follow blood through the heart in order
Vena cava, right atrium, right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium, left ventricle, aorta. Questions name a point in that sequence, so the sequence is the map.
Read cladograms by node, not by position
A node is a common ancestor. Two species next to each other on the page are not necessarily most closely related, and questions test precisely that misreading.
Where marks get lost
- Describing an adaptation without saying what it enables.
- Saying arteries carry oxygenated blood - the pulmonary artery does not.
- Confusing the sliding filament model with the filaments themselves shortening.
- Treating acclimatisation as evolution: one happens within a lifetime, the other across generations.
- Reading a cladogram left to right as an order of appearance.
Practise this unit
Reading about a unit only goes so far. Work through real IB-style questions on it, with mark schemes.