Unit 3 · Theme C: Interaction and Interdependence
The Complete IB Biology Guide · IB BiologyUnit 3 · Theme C: Interaction and Interdependence
- The Complete IB Biology Guide
- IB Biology
- 4 sections
International Baccalaureate does not publish a per-unit weighting for this exam. Exam structure (official): SL: Paper 1 (90 min, 36%) · Paper 2 (90 min, 44%) · Internal Assessment (20%); HL: Paper 1 (120 min, 36%) · Paper 2 (150 min, 44%) · Internal Assessment (20%). Be ready to carry the same unit skills and representations into a complete answer on every paper. Theme C: Interaction and Interdependence links molecular detail to patterns at cellular, organismal, and ecological scales. The Theme C: Interaction and Interdependence table identifies three high-value routes for revision. In Theme C: Interaction and Interdependence, start with the biological object, then name the comparison or mechanism, and only then select a calculation or diagram.
- How IB Biology assesses this International Baccalaureate publishes no per-unit percentage for IB Biology; Paper 1 accounts for 36% and Paper 2 for 44% of the total at both SL and HL. · SL: Paper 1 (90 min, 36%) · Paper 2 (90 min, 44%) · Internal Assessment (20%) | HL: Paper 1 (120 min, 36%) · Paper 2 (150 min, 44%) · Internal Assessment (20%) · show the model with metabolic pathway or signaling diagram, energy-transfer or population table, rate, diversity, or growth graph
- Key skills Enzymes and Metabolism, Cell Respiration, Photosynthesis, Neural Signalling, Integration of Body Systems, Defense Against Disease, Populations, Communities, and Transfers of Energy and Matter, Chemical Signalling (HL)
- How to study for Unit 3 This page turns metabolic pathway or signaling diagram, energy-transfer or population table, rate, diversity, or growth graph into one route: Write the biological balance or information pathway before substituting measured values..
- The organizing decision Track matter, energy, and signals through interacting systems without confusing gross values, net values, and transfer efficiencies.
What IB Biology Unit 3 covers
Use this map to connect each assessed skill to the relationship or representation that makes it visible.
Enzymes and Metabolism
enzyme rate; activation energy; metabolic pathway02Cell Respiration
respiratory quotient; ATP yield; aerobic respiration03Photosynthesis
net photosynthetic rate; gross photosynthesis; carbon fixation04Neural Signalling
neural conduction speed; action potential; synapse05Integration of Body Systems
homeostasis; feedback loop; system integration06Defense Against Disease
prevalence; antigen; immune response07Populations, Communities, and Transfers of Energy and Matter
logistic growth; energy-transfer efficiency; Simpson diversity index08Chemical Signalling (HL)
hormone serial dilution; receptor; signal transductionInternational Baccalaureate does not publish a per-unit weighting for this exam.
Official unit name and weighting: IB subject guide / subject brief.
Theme C: Interaction and Interdependence
Connect the published share to the unit model
Theme C: Interaction and Interdependence links molecular detail to patterns at cellular, organismal, and ecological scales. The Theme C: Interaction and Interdependence table identifies three high-value routes for revision. In Theme C: Interaction and Interdependence, start with the biological object, then name the comparison or mechanism, and only then select a calculation or diagram.
The Theme C: Interaction and Interdependence examples use independently created measurements. Those Theme C: Interaction and Interdependence settings are practice material, not past-paper questions. The purpose in Theme C: Interaction and Interdependence is to make units, assumptions, and limits visible while keeping the biological interpretation attached to every number.
For qualitative prompts in Theme C: Interaction and Interdependence, predict the biological relationship before reading any options. For Theme C: Interaction and Interdependence, reject a choice that reverses cause and effect, changes the stated scale, or claims more certainty than the evidence supports. For quantitative work in Theme C: Interaction and Interdependence, list values with units and estimate the likely magnitude before calculating.
A complete Theme C: Interaction and Interdependence response distinguishes observation from explanation. Within Theme C: Interaction and Interdependence, a pattern states what changes; a mechanism names how a biological process produces that change; an evaluation identifies the design feature that limits confidence. These Theme C: Interaction and Interdependence roles should not be blended into a single vague sentence.
The decision that organizes this unit
Define the system and choose the route before calculating
Track matter, energy, and signals through interacting systems without confusing gross values, net values, and transfer efficiencies.
Write the biological balance or information pathway before substituting measured values.
Mechanism route and repair branches
Relationships to preserve
- Gross photosynthesis equals net photosynthesis plus respiration under the stated measurement assumption.
- Efficiency compares a transferred quantity with the available quantity using named numerator and denominator.
- A signaling or population rate requires compatible time and concentration units before division.
Representations to read
- metabolic pathway or signaling diagram
- energy-transfer or population table
- rate, diversity, or growth graph
Branches to reject
- subtracting respiration twice when recovering a gross rate
- reversing numerator and denominator in a quotient
- extending a measured relationship beyond the tested conditions
| Key concept | Why it is hard | What scores |
|---|---|---|
| Metabolism | Net and gross rates differ | Write the balance first |
| Communities | Efficiency and diversity have definitions | Name numerator and denominator |
| Signalling | Time and concentration units shift | Convert before division |
How IB Biology assesses Theme C: Interaction and Interdependence
What a complete response must make visible
Match the task to evidence that a reader can audit, then check the most likely reasoning failure before finalizing the response.
| Task | Evidence to show | Hurdle |
|---|---|---|
| Enzymes and Metabolism | enzyme rate; activation energy; metabolic pathway | subtracting respiration twice when recovering a gross rate |
| Cell Respiration | respiratory quotient; ATP yield; aerobic respiration | reversing numerator and denominator in a quotient |
| Photosynthesis | net photosynthetic rate; gross photosynthesis; carbon fixation | extending a measured relationship beyond the tested conditions |
| Neural Signalling | neural conduction speed; action potential; synapse | subtracting respiration twice when recovering a gross rate |
| Integration of Body Systems | homeostasis; feedback loop; system integration | reversing numerator and denominator in a quotient |
| Defense Against Disease | prevalence; antigen; immune response | extending a measured relationship beyond the tested conditions |
| Populations, Communities, and Transfers of Energy and Matter | logistic growth; energy-transfer efficiency; Simpson diversity index | subtracting respiration twice when recovering a gross rate |
| Chemical Signalling (HL) | hormone serial dilution; receptor; signal transduction | reversing numerator and denominator in a quotient |
Net photosynthetic rate
Read each question as it appears on the exam paper, choose your answer, then tap it to see the feedback.
Practice 1: Recovering gross photosynthetic rate
An aquatic plant releases 7.6 mg of oxygen per hour in light and consumes 2.1 mg of oxygen per hour in darkness.
Estimate the gross photosynthetic rate and state the assumption required when using the dark measurement.
Show your working.
State the rate in mg oxygen per hour.
A. Gross rate = 7.6 + 2.1 = 9.7 mg oxygen per hour, assuming respiration in darkness represents respiration in light.
✅ Correct! 😊 This restores the oxygen used in respiration to the measured net release and states the assumption that limits the estimate.
B. Gross rate = 7.6 − 2.1 = 5.5 mg oxygen per hour because respiration must be removed from net release.
❌ Not quite 😭 Net release already reflects oxygen consumed by respiration. Subtracting respiration again removes it twice and underestimates gross photosynthesis.
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C. Gross rate = 7.6 mg oxygen per hour because the light measurement records photosynthesis alone.
❌ Not quite 😭 The light measurement is net oxygen release because photosynthesis and respiration can occur during the measurement. Treating it as gross rate ignores oxygen consumption.
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D. Gross rate = 7.6 ÷ 2.1 = 3.6 mg oxygen per hour, with no assumption about respiration required.
❌ Not quite 😭 Dividing the two rates does not follow the biological balance and the resulting quantity is not a gross rate. The use of dark respiration also requires an explicit assumption.
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Full explanation
- Step 1Write the balance: net photosynthesis = gross photosynthesis − respiration.
- Step 2Rearrange it: gross photosynthesis = net photosynthesis + respiration.
- Step 3Substitute the measured rates: 7.6 + 2.1 = 9.7 mg oxygen per hour.
- Step 4State that the estimate assumes the dark respiration rate represents respiration in light.
Practice 2: Interpreting energy-transfer efficiency
A producer population stores 24 000 kJ m⁻² year⁻¹, and the primary-consumer population feeding on it stores 3 000 kJ m⁻² year⁻¹.
Which statement correctly calculates and interprets the energy-transfer efficiency?
Select the best answer.
A. Efficiency = 3 000 ÷ 24 000 × 100 = 12.5%; the remainder follows several pathways rather than all being a single loss.
✅ Correct! 😊 This places consumer energy over available producer energy, converts the ratio to a percentage and avoids treating every untransferred quantity as biologically identical.
B. Efficiency = 24 000 ÷ 3 000 × 100 = 800%; the larger producer store belongs in the numerator.
❌ Not quite 😭 This reverses transferred and available energy, producing an impossible efficiency above 100%. The next trophic-level store belongs in the numerator.
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C. Efficiency = 24 000 − 3 000 = 21 000 kJ m⁻² year⁻¹ because efficiency is the energy that does not transfer.
❌ Not quite 😭 Subtraction gives an energy difference, not an efficiency. Efficiency is a dimensionless ratio converted to a percentage.
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D. Efficiency = 3 000 ÷ 24 000 = 0.125%; all remaining energy is lost as heat.
❌ Not quite 😭 The ratio is 0.125, but multiplying by 100 gives 12.5%, not 0.125%. The remainder may also be respired, uneaten or leave as waste, so the heat claim is too narrow.
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Full explanation
- Step 1Name the quantities: 3 000 is transferred to consumers and 24 000 is available in producers.
- Step 2Calculate 3 000 ÷ 24 000 = 0.125 using matching energy, area and time bases.
- Step 3Multiply by 100 to obtain an energy-transfer efficiency of 12.5%.
- Step 4Limit the interpretation: the remainder can be respired, remain uneaten or leave as waste and heat.
Key terms for Unit 3: Theme C: Interaction and Interdependence
Models, uses, and boundaries
- Gross photosynthetic rate
- Add the measured dark respiration rate to the net light rate to estimate gross photosynthesis. Use when light and dark oxygen measurements are supplied and the respiration assumption is stated.
- Respiratory quotient
- Divide carbon dioxide produced by oxygen consumed to find respiratory quotient. Use when matching gas volumes are given for respiration.
- Temperature coefficient
- Divide the rate after a ten-degree temperature rise by the rate before the rise. Use when comparing enzyme rates across an interval of exactly ten degrees Celsius.
- Energy-transfer efficiency
- Divide energy stored at the next trophic level by energy available at the previous level, then multiply by one hundred. Use when energy stores are reported with matching area and time units.
IB Biology Unit 3 FAQ
How much of IB Biology does Unit 3 carry?
International Baccalaureate does not publish a per-unit weighting for this exam.
What is the first move on a Theme C: Interaction and Interdependence problem?
Write the biological balance or information pathway before substituting measured values.
Which relationships should I preserve?
Gross photosynthesis equals net photosynthesis plus respiration under the stated measurement assumption. Efficiency compares a transferred quantity with the available quantity using named numerator and denominator. A signaling or population rate requires compatible time and concentration units before division.
Which representations should I practice?
Practice moving among metabolic pathway or signaling diagram, energy-transfer or population table, rate, diversity, or growth graph.
What error should I check before submitting an answer?
Check for subtracting respiration twice when recovering a gross rate; reversing numerator and denominator in a quotient; extending a measured relationship beyond the tested conditions.
Evidence workshop
Continue from the free model into complete practice
The full unit guide continues with the chapter’s worked examples, figures, scoring tables, and answer checks.
- Net photosynthetic rate
- Respiratory quotient
- Temperature coefficient for enzyme activity
- Energy-transfer efficiency
Full unit practice. Open the complete guide for the full evidence workshop and synthesis.
Related IB Biology unit guides
IB Biology Exam Guide & Review
The whole exam and its official unit sequence.01Theme A: Unity and Diversity
International Baccalaureate does not publish a per-unit weighting for this exam.02Theme B: Form and Function
International Baccalaureate does not publish a per-unit weighting for this exam.04Theme D: Continuity and Change
International Baccalaureate does not publish a per-unit weighting for this exam.How to study IB Biology Unit 3
Start with the organizing decision
Before solving, restate the decision in operational terms: Track matter, energy, and signals through interacting systems without confusing gross values, net values, and transfer efficiencies.. Your first written move should be to Write the biological balance or information pathway before substituting measured values..
Practice the same idea in several representations
Rotate through metabolic pathway or signaling diagram, energy-transfer or population table, rate, diversity, or growth graph. Use each representation to practice Enzymes and Metabolism, Cell Respiration, Photosynthesis, Neural Signalling, Integration of Body Systems, Defense Against Disease, Populations, Communities, and Transfers of Energy and Matter, Chemical Signalling (HL), and explain what stays invariant when the surface form changes.
Turn each error into a repair check
After every attempt, audit the response for subtracting respiration twice when recovering a gross rate; reversing numerator and denominator in a quotient; extending a measured relationship beyond the tested conditions. Then redo only the first step that made the reasoning diverge, keeping units, direction, and model conditions visible.
Confirm current course details in the official IB subject guide / subject brief for the 2027 administration.