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	$d N / d t = r_{ma x} N$
Logistic population growth	$d N / d t = r_{ma x} N \frac{( K - N )}{K}$
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.