Unit Overview
Theme C: Interaction and Interdependence
IB Biology HL· 6 min read 📊 n/a
1. Unit at a Glance
This unit follows a hierarchical learning arc, starting with interactions at the smallest biological scales (molecules and cells) and expanding outward to whole organisms, populations, and entire global ecosystems. Core topics cover foundational concepts required for all IB Biology students, while additional higher level (AHL) topics extend these concepts for HL candidates.
A unifying theme across all sub-topics is that biological function depends entirely on interactions between components. Disruption of these interactions leads to dysfunction at every scale, from individual disease to ecosystem collapse.
This unit includes the following core and AHL sub-topics:
Core: Molecules
Covers molecular interactions between proteins, enzymes, and other biological molecules.
★★⏱ 15 min
Core: Cells
Explores interactions between cell components, membranes, and adjacent cells.
★★⏱ 18 min
Core: Organisms
Covers interactions between organ systems and organisms within a species.
★★★⏱ 16 min
Core: Ecosystems
Foundational concepts of energy flow, nutrient cycling, and species interactions.
★★⏱ 17 min
AHL: Cell signaling
Advanced study of how cells communicate via signal transduction pathways.
★★★⏱ 20 min
AHL: Neurobiology
Explores neuron communication and the organization of nervous systems.
★★★★⏱ 22 min
AHL: Plant hormone interactions
How plant hormones coordinate growth and responses to environmental stimuli.
★★★⏱ 18 min
AHL: Ecological niches
Detailed analysis of niche concept, resource partitioning, and competitive exclusion.
★★★⏱ 15 min
AHL: Population dynamics
Models of population growth, limiting factors, and population regulation.
★★★⏱ 17 min
AHL: Conservation biology
Concepts of biodiversity loss, conservation strategies, and habitat restoration.
★★★⏱ 19 min
AHL: Climate change impacts
How climate change disrupts biological interactions across all scales.
★★★⏱ 18 min
AHL: Ecosystem management
Strategies for sustainable management and restoration of damaged ecosystems.
★★★⏱ 20 min
2. Common Pitfalls
Wrong move:
Confusing interactions across different biological scales.
Why:
This leads to incorrect answers on questions that explicitly ask for connections between organizational levels.
Correct move:
Always anchor your answer to the scale specified in the question, and explicitly draw connections between levels if required.
Wrong move:
Assuming all interspecific interactions are permanently positive or negative.
Why:
Many interactions change their net effect depending on environmental context and species life stage.
Correct move:
Evaluate the net outcome of the interaction for each species under the specific conditions described.
3. Quick Reference Cheatsheet
Key Concept / Formula | Unit-Level Summary |
|---|---|
Enzyme-substrate interaction | Binding is specific, depends on molecular complementarity and the induced fit model |
Cell signaling pathway steps | Reception → Transduction → Cellular Response |
Action potential | Rapid depolarization followed by repolarization via voltage-gated ion channels |
Interspecific interaction types | Competition (-/-), predation (+/-), mutualism (+/+), commensalism (+/0) |
Exponential population growth | |
Logistic population growth | |
Competitive exclusion principle | Two species cannot occupy the identical niche in the same habitat indefinitely |
Plant tropism | Directional growth response to external stimuli, mediated by hormone signaling |
Trophic cascade | Disruption of top trophic levels causes cascading changes across the entire food web |
Biodiversity conservation | Protected areas and habitat corridors reduce extinction risk from habitat fragmentation |
What's Next
Begin your study of this unit with the first core sub-topic covering interactions between biological molecules. Work through all core topics first, then complete the AHL topics if you are studying for the IB Biology HL examination. Once you finish all sub-topics in this unit, you will progress to the next unit covering Theme D: Continuity and Change.