Units
All 7 units of Biology 20. Each one has a concepts page to learn from, practice questions marked as you go, and quizzes built at random.
Unit 1
Unit 1: Energy and matter exchange in the biosphere
Energy arrives once and leaves as heat; matter goes round and round. Trophic levels, the carbon and nitrogen cycles, and what decides which biome grows where.
Unit 2
Unit 2: Ecosystems and population change
How a community rebuilds itself after a disturbance, what limits a population's growth, and the three conditions that make natural selection inevitable.
Unit 3
Unit 3: Photosynthesis
How light becomes sugar, in two connected stages: the reactions in the thylakoid membrane that capture energy, and the cycle in the stroma that builds carbon into a molecule.
Unit 4
Unit 4: Cellular respiration
Getting the energy back out again: glycolysis, the Krebs cycle, the electron transport chain, and what a cell can still do when the oxygen runs out.
Unit 5
Unit 5: Digestion and nutrition
A tube from one end to the other, with a different pH and a different set of enzymes in each section, and a surface area the size of a room for absorbing the result.
Unit 6
Unit 6: Circulation and gas exchange
The pump, the pipes and the passenger: how blood is pushed around two circuits, what carries the oxygen, and where the actual exchange happens.
Studying for the exam?
Mix every unit together
Build a random quiz drawn from all 84 questions across every unit, at the level you choose. It is the closest thing here to a real exam.