Unit 2 · Theme B: Form and Function
The Complete IB Biology Guide · IB BiologyUnit 2 · Theme B: Form and Function
- 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 B: Form and Function links molecular detail to patterns at cellular, organismal, and ecological scales. The Theme B: Form and Function table identifies three high-value routes for revision. In Theme B: Form and Function, 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 membrane or organelle diagram, rate table with compound units, transport or scaling graph
- Key skills Carbohydrates and Lipids, Proteins, Membranes and Membrane Transport, Organelles, Compartmentalization, and Cell Specialization, Gas Exchange, Transport, Adaptation to Environment and Ecological Niches, Muscle and Motility (HL)
- How to study for Unit 2 This page turns membrane or organelle diagram, rate table with compound units, transport or scaling graph into one route: Name the structure and measured function, then write the relevant rate or scaling relationship with its units..
- The organizing decision Relate biological structure to function while carrying the area, time, volume, and mass bases that make rates comparable.
What IB Biology Unit 2 covers
Use this map to connect each assessed skill to the relationship or representation that makes it visible.
Carbohydrates and Lipids
monosaccharide; triglyceride; phospholipid02Proteins
amino acid; peptide bond; protein conformation03Membranes and Membrane Transport
diffusion flux; selective permeability; concentration gradient04Organelles, Compartmentalization, and Cell Specialization
organelle; compartmentalization; cell specialization05Gas Exchange
ventilation rate; gas-exchange surface; partial pressure06Transport
cardiac output; stroke volume; transpiration rate07Adaptation to Environment and Ecological Niches
niche overlap; allometric scaling; adaptation08Muscle and Motility (HL)
specific force; cross-sectional area; sarcomereInternational Baccalaureate does not publish a per-unit weighting for this exam.
Official unit name and weighting: IB subject guide / subject brief.
Theme B: Form and Function
Connect the published share to the unit model
Theme B: Form and Function links molecular detail to patterns at cellular, organismal, and ecological scales. The Theme B: Form and Function table identifies three high-value routes for revision. In Theme B: Form and Function, start with the biological object, then name the comparison or mechanism, and only then select a calculation or diagram.
The Theme B: Form and Function examples use independently created measurements. Those Theme B: Form and Function settings are practice material, not past-paper questions. The purpose in Theme B: Form and Function is to make units, assumptions, and limits visible while keeping the biological interpretation attached to every number.
For qualitative prompts in Theme B: Form and Function, predict the biological relationship before reading any options. For Theme B: Form and Function, 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 B: Form and Function, list values with units and estimate the likely magnitude before calculating.
A complete Theme B: Form and Function response distinguishes observation from explanation. Within Theme B: Form and Function, 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 B: Form and Function 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
Relate biological structure to function while carrying the area, time, volume, and mass bases that make rates comparable.
Name the structure and measured function, then write the relevant rate or scaling relationship with its units.
Mechanism route and repair branches
Relationships to preserve
- Membrane transport depends on both the driving difference and the area and time over which movement is measured.
- Whole-organ transport measures combine component rates such as beats per minute and volume per beat.
- Scaling relationships must state whether a value is a total or is normalized by mass or area.
Representations to read
- membrane or organelle diagram
- rate table with compound units
- transport or scaling graph
Branches to reject
- reporting an amount as though it were a rate
- dropping area or time from a compound unit
- interpreting an interval mean as an instantaneous rate
| Key concept | Why it is hard | What scores |
|---|---|---|
| Membranes | Rates need area and time bases | Carry compound units |
| Transport | Whole-organ measures combine factors | Cancel beats or breaths |
| Adaptation | Scaling laws use explicit exponents | Interpret totals and per-mass values |
How IB Biology assesses Theme B: Form and Function
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 |
|---|---|---|
| Carbohydrates and Lipids | monosaccharide; triglyceride; phospholipid | reporting an amount as though it were a rate |
| Proteins | amino acid; peptide bond; protein conformation | dropping area or time from a compound unit |
| Membranes and Membrane Transport | diffusion flux; selective permeability; concentration gradient | interpreting an interval mean as an instantaneous rate |
| Organelles, Compartmentalization, and Cell Specialization | organelle; compartmentalization; cell specialization | reporting an amount as though it were a rate |
| Gas Exchange | ventilation rate; gas-exchange surface; partial pressure | dropping area or time from a compound unit |
| Transport | cardiac output; stroke volume; transpiration rate | interpreting an interval mean as an instantaneous rate |
| Adaptation to Environment and Ecological Niches | niche overlap; allometric scaling; adaptation | reporting an amount as though it were a rate |
| Muscle and Motility (HL) | specific force; cross-sectional area; sarcomere | dropping area or time from a compound unit |
Enzyme rate and substrate concentration
Read each question as it appears on the exam paper, choose your answer, then tap it to see the feedback.
Practice 1: Calculating mean enzyme rate
An enzyme-catalysed reaction produces 36 micromoles of product in 4.0 minutes.
Calculate the mean rate of product formation and state one limitation of this value.
Show your working.
State the rate in micromoles per minute.
A. 36 ÷ 4.0 = 9.0 micromoles per minute; this mean may hide changes in rate during the measured interval.
✅ Correct! 😊 This divides product formed by elapsed time, reports amount per time and recognizes that an interval mean does not show within-interval changes.
B. 36 micromoles is the mean rate because this is the total amount of product formed.
❌ Not quite 😭 This reports an amount as though it were a rate. Without division by elapsed time, the response lacks the time basis required for a mean rate.
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C. 4.0 ÷ 36 = 0.11 minutes per micromole; this is the mean rate of product formation.
❌ Not quite 😭 This inverts the required relationship and produces time per amount. The question asks for product formed per minute.
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D. 36 ÷ 4.0 = 9.0 micromoles; this proves the rate was constant throughout the four minutes.
❌ Not quite 😭 The numerical division is right, but the per-minute basis is missing and the conclusion is unsupported. An interval mean cannot prove an instantaneous rate stayed constant.
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Full explanation
- Step 1Write the relationship: mean rate = product formed ÷ elapsed time.
- Step 2Substitute both values with units: 36 micromoles ÷ 4.0 minutes.
- Step 3Calculate 9.0 micromoles per minute, retaining the amount and time bases.
- Step 4State that the mean summarises the whole interval and may conceal changes within it.
Practice 2: Comparing membrane diffusion flux
In 100 seconds, membrane P transports 4.0 micromoles across 2.0 cm², while membrane Q transports 6.0 micromoles across 6.0 cm².
Which statement correctly compares the diffusion flux across the two membranes?
Select the best answer.
A. Q has the greater flux because it transports 6.0 micromoles, the larger total amount.
❌ Not quite 😭 This compares totals without accounting for different membrane areas. A larger transported amount does not necessarily mean greater transfer for each square centimeter per second.
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B. P has a flux of 0.020 micromoles cm⁻² s⁻¹ and Q has 0.010 micromoles cm⁻² s⁻¹, so P has the greater flux.
✅ Correct! 😊 This divides each amount by the same elapsed time and then by its membrane area. The comparison is therefore made on the same area-and-time basis.
C. P and Q have equal flux because both were measured for 100 seconds.
❌ Not quite 😭 Equal measurement time does not remove the differences in amount transported and membrane area. Both factors must be included in the flux calculation.
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D. P has a flux of 2.0 micromoles cm⁻² and Q has 1.0 micromole cm⁻², so elapsed time is irrelevant.
❌ Not quite 😭 These values describe amount per area, not flux, because the time basis has been dropped. Diffusion flux requires both cm⁻² and s⁻¹.
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Full explanation
- Step 1Use diffusion flux = amount transported ÷ elapsed time ÷ membrane area.
- Step 2For P, calculate 4.0 ÷ 100 ÷ 2.0 = 0.020 micromoles cm⁻² s⁻¹.
- Step 3For Q, calculate 6.0 ÷ 100 ÷ 6.0 = 0.010 micromoles cm⁻² s⁻¹.
- Step 4Compare the normalised values and conclude that P has twice the diffusion flux of Q.
Key terms for Unit 2: Theme B: Form and Function
Models, uses, and boundaries
- Enzyme rate
- Divide the amount of product formed by elapsed time to find mean enzyme rate. Use when total product and a measurement interval are given.
- Diffusion flux
- Divide amount transported by membrane area and elapsed time to find diffusion flux. Use when transport must be compared per unit area per unit time.
- Percentage mass change
- Subtract initial mass from final mass, divide by initial mass, and multiply by one hundred. Use when osmosis data compare tissue mass before and after treatment.
- Cardiac output
- Multiply heart rate by stroke volume and convert milliliters to liters when needed. Use when blood volume per beat and beats per minute are given.
IB Biology Unit 2 FAQ
How much of IB Biology does Unit 2 carry?
International Baccalaureate does not publish a per-unit weighting for this exam.
What is the first move on a Theme B: Form and Function problem?
Name the structure and measured function, then write the relevant rate or scaling relationship with its units.
Which relationships should I preserve?
Membrane transport depends on both the driving difference and the area and time over which movement is measured. Whole-organ transport measures combine component rates such as beats per minute and volume per beat. Scaling relationships must state whether a value is a total or is normalized by mass or area.
Which representations should I practice?
Practice moving among membrane or organelle diagram, rate table with compound units, transport or scaling graph.
What error should I check before submitting an answer?
Check for reporting an amount as though it were a rate; dropping area or time from a compound unit; interpreting an interval mean as an instantaneous rate.
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.
- Enzyme rate and substrate concentration
- Diffusion flux across a membrane
- Osmosis and percentage mass change
- Cardiac output from heart rate
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.03Theme C: Interaction and Interdependence
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 2
Start with the organizing decision
Before solving, restate the decision in operational terms: Relate biological structure to function while carrying the area, time, volume, and mass bases that make rates comparable.. Your first written move should be to Name the structure and measured function, then write the relevant rate or scaling relationship with its units..
Practice the same idea in several representations
Rotate through membrane or organelle diagram, rate table with compound units, transport or scaling graph. Use each representation to practice Carbohydrates and Lipids, Proteins, Membranes and Membrane Transport, Organelles, Compartmentalization, and Cell Specialization, Gas Exchange, Transport, Adaptation to Environment and Ecological Niches, Muscle and Motility (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 reporting an amount as though it were a rate; dropping area or time from a compound unit; interpreting an interval mean as an instantaneous rate. 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.