Interaction and Interdependence
IB Biology SL — Interaction and Interdependence
This theme is really one argument told eight times: living systems only work because matter and energy move across boundaries — through an enzyme's active site, a cell membrane, a synapse, a food web. Examiners love hyperbolic and sigmoid graphs here (Michaelis-Menten, population growth, photosynthesis limiting factors) because reading a gradient correctly separates the students who understand the biology from the students who've memorised the shape.
Overview — the shape of this theme
Every subtopic asks the same underlying question: what controls the rate and direction of a transfer? At the molecular level it's enzyme kinetics; at the ecosystem level it's trophic energy loss; at the organism level it's a nerve impulse or a hormone. Command terms like determine, compare and explain recur constantly because the exam wants mechanism, not description.
- Metabolism, respiration and photosynthesis are enzyme-controlled pathways — the Km/Vmax logic from enzymes reappears everywhere.
- Defence and neural signalling are both about specific, threshold-based responses to a stimulus (antigen or depolarisation).
- Populations, communities and energy transfer scale the same interaction ideas up to whole ecosystems.
The shape of the chapter
Command terms that dominate this theme
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Determine | Find a numerical or definite value using given data or a formula | AO2 | Show working and units — a bare correct number without method often loses the method mark |
| Explain | Give reasons or a mechanism, using biological detail | AO2/AO3 | Description alone (what happens) without the 'because' loses marks |
| Compare | State similarities AND differences explicitly | AO3 | Two separate descriptions with no linking comparison scores poorly |
| Evaluate | Weigh strengths/weaknesses or judge a claim against evidence | AO3 | Must reach a reasoned judgement, not just list points for and against |
| Predict | Give a plausible outcome based on a pattern/theory, with justification | AO2 | Unjustified answer scores 0 even if the value happens to be right |
Key point
Overview