Unit 1 · Theme A: Unity and Diversity
The Complete IB Biology Guide · IB BiologyUnit 1 · Theme A: Unity and Diversity
- 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 A: Unity and Diversity links molecular detail to patterns at cellular, organismal, and ecological scales. The Theme A: Unity and Diversity table identifies three high-value routes for revision. In Theme A: Unity and Diversity, 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 molecular or cell diagram with a scale bar, sequence, frequency, or diversity table, cladogram or population graph
- Key skills Water, Nucleic Acids, Cell Structure, Diversity of Organisms, Evolution and Speciation, Conservation of Biodiversity, Origins of Cells and Viruses (HL), Classification and Cladistics (HL)
- How to study for Unit 1 This page turns molecular or cell diagram with a scale bar, sequence, frequency, or diversity table, cladogram or population graph into one route: Identify the biological object, then name the comparison or mechanism before selecting a calculation or diagram..
- The organizing decision Connect molecular detail to patterns at cellular, organismal, and ecological scales while keeping the biological interpretation attached to every measurement.
What IB Biology Unit 1 covers
Use this map to connect each assessed skill to the relationship or representation that makes it visible.
Water
hydrogen bonding; cohesion; solvent02Nucleic Acids
nucleotide; GC content; nucleic acid03Cell Structure
scale bar; surface-area-to-volume ratio; magnification04Diversity of Organisms
species richness; evenness; biodiversity05Evolution and Speciation
allele frequency; speciation; genetic distance06Conservation of Biodiversity
species richness; sampling area; conservation07Origins of Cells and Viruses (HL)
endosymbiosis; capsid; protocell08Classification and Cladistics (HL)
cladogram; parsimony score; common ancestorInternational Baccalaureate does not publish a per-unit weighting for this exam.
Official unit name and weighting: IB subject guide / subject brief.
Theme A: Unity and Diversity
Connect the published share to the unit model
Theme A: Unity and Diversity links molecular detail to patterns at cellular, organismal, and ecological scales. The Theme A: Unity and Diversity table identifies three high-value routes for revision. In Theme A: Unity and Diversity, start with the biological object, then name the comparison or mechanism, and only then select a calculation or diagram.
The Theme A: Unity and Diversity examples use independently created measurements. Those Theme A: Unity and Diversity settings are practice material, not past-paper questions. The purpose in Theme A: Unity and Diversity is to make units, assumptions, and limits visible while keeping the biological interpretation attached to every number.
For qualitative prompts in Theme A: Unity and Diversity, predict the biological relationship before reading any options. For Theme A: Unity and Diversity, 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 A: Unity and Diversity, list values with units and estimate the likely magnitude before calculating.
A complete Theme A: Unity and Diversity response distinguishes observation from explanation. Within Theme A: Unity and Diversity, 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 A: Unity and Diversity 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
Connect molecular detail to patterns at cellular, organismal, and ecological scales while keeping the biological interpretation attached to every measurement.
Identify the biological object, then name the comparison or mechanism before selecting a calculation or diagram.
Mechanism route and repair branches
Relationships to preserve
- A pattern states what changes, while a mechanism explains how a biological process produces that change.
- A frequency comparison needs a stated baseline before it can describe evolutionary change.
- A diversity measure must keep richness and evenness distinct when the result is interpreted.
Representations to read
- molecular or cell diagram with a scale bar
- sequence, frequency, or diversity table
- cladogram or population graph
Branches to reject
- hiding a scale conversion by dropping units
- treating frequency change as percentage-point change without naming the baseline
- claiming a mechanism when the evidence shows only a pattern
| Key concept | Why it is hard | What scores |
|---|---|---|
| Nucleic acids | Scale conversions hide unit traps | Show cancellation before rounding |
| Diversity | Indices blend richness and evenness | Define the index used |
| Evolution | Frequency is not percentage-point change | State the comparison baseline |
How IB Biology assesses Theme A: Unity and Diversity
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 |
|---|---|---|
| Water | hydrogen bonding; cohesion; solvent | hiding a scale conversion by dropping units |
| Nucleic Acids | nucleotide; GC content; nucleic acid | treating frequency change as percentage-point change without naming the baseline |
| Cell Structure | scale bar; surface-area-to-volume ratio; magnification | claiming a mechanism when the evidence shows only a pattern |
| Diversity of Organisms | species richness; evenness; biodiversity | hiding a scale conversion by dropping units |
| Evolution and Speciation | allele frequency; speciation; genetic distance | treating frequency change as percentage-point change without naming the baseline |
| Conservation of Biodiversity | species richness; sampling area; conservation | claiming a mechanism when the evidence shows only a pattern |
| Origins of Cells and Viruses (HL) | endosymbiosis; capsid; protocell | hiding a scale conversion by dropping units |
| Classification and Cladistics (HL) | cladogram; parsimony score; common ancestor | treating frequency change as percentage-point change without naming the baseline |
DNA dilution and concentration
Read each question as it appears on the exam paper, choose your answer, then tap it to see the feedback.
Practice 1: Recovering DNA concentration after dilution
A DNA extract is diluted by mixing 0.20 mL of extract with 1.80 mL of buffer; the diluted mixture has a DNA concentration of 18.0 ng/mL.
Calculate the DNA concentration of the original extract.
Show your working.
State the concentration in ng/mL.
A. Dilution factor = 2.00 ÷ 0.20 = 10.0; original concentration = 18.0 × 10.0 = 180 ng/mL.
✅ Correct! 😊 This uses total mixture volume over extract volume, then multiplies the diluted reading by the unitless factor. The final value is correctly linked to the original extract.
B. Dilution factor = 1.80 ÷ 0.20 = 9.0; original concentration = 18.0 × 9.0 = 162 ng/mL.
❌ Not quite 😭 This uses buffer volume instead of total mixture volume in the dilution factor. It therefore underestimates the original concentration.
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C. Dilution factor = 2.00 ÷ 0.20 = 10.0; original concentration = 18.0 ÷ 10.0 = 1.80 ng/mL.
❌ Not quite 😭 Although the dilution factor is correct, division moves in the wrong direction. Reversing a dilution must give an original concentration greater than the diluted reading.
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D. Original concentration = 18.0 × 2.00 = 36.0 ng because the final volume is 2.00 mL.
❌ Not quite 😭 This multiplies concentration by volume without comparing final and initial volumes, producing an amount rather than the requested concentration. The dilution factor is missing.
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Full explanation
- Step 1Calculate total mixture volume: 0.20 mL + 1.80 mL = 2.00 mL.
- Step 2Find the unitless dilution factor: 2.00 mL ÷ 0.20 mL = 10.0.
- Step 3Recover the original concentration: 18.0 ng/mL × 10.0 = 180 ng/mL.
- Step 4Confirm that reversing the dilution gives a larger concentration and name the original extract.
Practice 2: Describing a change in allele frequency
In a population, the frequency of allele R is 0.20 in the first sample and 0.30 in a later sample.
Which statement accurately describes the observed change without claiming an unmeasured mechanism?
Select the best answer.
A. Allele R increased by 0.10, or 10 percentage points, from the baseline of 0.20; relative to that baseline, this is a 50% increase.
✅ Correct! 😊 This reports both forms of change and makes the first sample the comparison baseline. It describes the pattern without inventing its cause.
B. Allele R increased by 0.10%, because 0.30 − 0.20 = 0.10.
❌ Not quite 😭 The subtraction gives an absolute frequency change of 0.10, equivalent to 10 percentage points, not 0.10%. The response confuses frequency and percentage notation.
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C. Allele R increased by 10%, and no baseline is needed because the difference alone gives the relative change.
❌ Not quite 😭 Ten percentage points describes the absolute change, but a relative percentage requires division by the starting value. Omitting the baseline makes the comparison ambiguous.
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D. Natural selection caused allele R to rise from 0.20 to 0.30, proving that the allele improves survival.
❌ Not quite 😭 The samples show a frequency pattern but do not identify its mechanism or prove a survival advantage. This conclusion goes beyond the evidence supplied.
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Full explanation
- Step 1Identify the first sample, 0.20, as the comparison baseline.
- Step 2Calculate the absolute change: 0.30 − 0.20 = 0.10, or 10 percentage points.
- Step 3Calculate the relative change from baseline: 0.10 ÷ 0.20 × 100 = 50%.
- Step 4Describe only the observed change because the two frequencies do not establish a biological mechanism.
Key terms for Unit 1: Theme A: Unity and Diversity
Models, uses, and boundaries
- DNA dilution and concentration
- Multiply the measured diluted DNA concentration by total mixture volume divided by extract volume to recover the original concentration. Use when an extract is diluted before its concentration is measured.
- GC content
- Divide the number of guanine and cytosine bases by the total number of bases, then multiply by one hundred. Use when a sequence sample asks for the percentage made up of guanine and cytosine.
- Microscope scale
- Multiply image length by the scale-bar real length divided by the scale-bar image length. Use when a micrograph and scale bar are used to estimate a real cell dimension.
- Surface-area-to-volume ratio
- Divide the surface area of a cube by its volume to compare exchange surface with cell size. Use when explaining how size changes the area available per unit volume.
IB Biology Unit 1 FAQ
How much of IB Biology does Unit 1 carry?
International Baccalaureate does not publish a per-unit weighting for this exam.
What is the first move on a Theme A: Unity and Diversity problem?
Identify the biological object, then name the comparison or mechanism before selecting a calculation or diagram.
Which relationships should I preserve?
A pattern states what changes, while a mechanism explains how a biological process produces that change. A frequency comparison needs a stated baseline before it can describe evolutionary change. A diversity measure must keep richness and evenness distinct when the result is interpreted.
Which representations should I practice?
Practice moving among molecular or cell diagram with a scale bar, sequence, frequency, or diversity table, cladogram or population graph.
What error should I check before submitting an answer?
Check for hiding a scale conversion by dropping units; treating frequency change as percentage-point change without naming the baseline; claiming a mechanism when the evidence shows only a pattern.
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.
- DNA dilution and concentration
- DNA dilution and concentration (continued)
- GC content from a sequence sample
- Microscope scale and cell diameter
- Surface-area-to-volume ratio
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.02Theme B: Form and Function
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 1
Start with the organizing decision
Before solving, restate the decision in operational terms: Connect molecular detail to patterns at cellular, organismal, and ecological scales while keeping the biological interpretation attached to every measurement.. Your first written move should be to Identify the biological object, then name the comparison or mechanism before selecting a calculation or diagram..
Practice the same idea in several representations
Rotate through molecular or cell diagram with a scale bar, sequence, frequency, or diversity table, cladogram or population graph. Use each representation to practice Water, Nucleic Acids, Cell Structure, Diversity of Organisms, Evolution and Speciation, Conservation of Biodiversity, Origins of Cells and Viruses (HL), Classification and Cladistics (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 hiding a scale conversion by dropping units; treating frequency change as percentage-point change without naming the baseline; claiming a mechanism when the evidence shows only a pattern. 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.