Interaction and Interdependence
IB Biology HL — Theme C: Interaction and Interdependence
This theme is really one idea repeated at eight different scales: something crosses a boundary and something else responds. An electron crosses a thylakoid membrane, a neurotransmitter crosses a synapse, energy crosses a trophic level, a hormone crosses a bloodstream. Every exam question here is testing whether you can name what crossed, where it came from, and what changed as a result — so read each subtopic with that lens rather than as isolated facts to memorise.
Overview: The Shape of This Topic
Why these eight subtopics sit together
Photosynthesis and cell respiration are the two engines that convert energy between light, chemical bond, and heat form. Defence and neural signalling are both about a cell recognising a signal (antigen or neurotransmitter) and mounting a specific, rapid response. Transfer of energy/matter and populations/communities scale the same logic up to ecosystems. Enzymes and metabolism are the molecular machinery that makes every one of those processes controllable and reversible. Integration of body systems is the HL synthesis: how several of these signalling systems run simultaneously without contradicting each other.
- Examiners love to ask 'what happens if you block step X' — Q7 (block Ca²⁺ channels) and Q2 (move chloroplasts to the dark) are exactly this move: predict the downstream consequence of removing one link in a chain.
- Data-based questions dominate this theme (isotope ratios in Q4, dose–response curves in Q8, dye reduction rates in Q1) — you are rarely just recalling a diagram, you're reading a graph or table through the lens of the mechanism.
- HL adds quantitative treatment almost everywhere: Km/Vmax, RQ, logistic growth (dN/dt), Simpson's index — SL students describe, HL students calculate and interpret gradients.
The shape of the chapter
Command terms that decide your mark in this theme
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Describe | State what happens, in sequence, with no reasoning required. | AO1/AO2 | Losing marks here usually means missing a STEP (e.g. describing an action potential but skipping the refractory period), not missing reasoning. |
| Explain | State what happens AND give the mechanism/reason why. | AO2/AO3 | Q3-style questions fail here: saying 'RuBP decreases' without saying WHY (fixation outpaces regeneration) scores half marks. |
| Compare | Give similarities AND differences, explicitly, ideally in a linked sentence. | AO3 | Listing features of A then B separately, with no direct comparative statement, loses the compare mark even if content is correct. |
| Distinguish | State the difference only — similarities not required. | AO2 | Wasting time describing what's shared between (e.g.) active and passive immunity costs time, not marks, but shows you've misread the term. |
| Determine / Calculate | Produce a numerical answer with working shown. | AO2 | Km/Vmax, RQ, logistic growth and Simpson's index questions: unit errors or an unlabelled final answer commonly cost the final mark even when the method is right. |
| Deduce | Reach a conclusion from given data/evidence, not from prior knowledge alone. | AO3 | Q1 and Q4 are deduce-style: your answer must reference the data given (dye colour change, δ¹³C values), not just general theory. |
| Evaluate | Weigh strengths and limitations to reach a judgement. | AO3 | Common in vaccine/monoclonal antibody questions — a one-sided answer (only benefits, no limitation) cannot access top marks. |
Key point
Overview