IB Biology Complete Study Guide
IB Diploma Program Biology · 2027
IB Biology Complete Study Guide
- SL and HL
- 4 syllabus themes
- Paper 1: MCQ and data analysis
- Paper 2: short and extended responses
IB Biology organizes content through four themes: Unity and Diversity, Form and Function, Interaction and Interdependence, and Continuity and Change. The guide connects each theme to biological diagrams, data tables, calculations, mechanisms, experimental reasoning, and precise written conclusions.
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. Study every theme and use the published paper weights only for planning practice across question formats.
- Name the biological object. Identify the organism, cell, molecule, population, or measured system before choosing a relationship.
- Keep units and assumptions visible. Show conversions, rounding, comparison baselines, and the conditions that support a model.
- Separate pattern from mechanism. State what the evidence shows, explain a supported cause, and stop where the data stop.
What IB Biology covers
The four themes appear in the same order as the student pages. Each route joins subject knowledge to representations, calculations, common errors, and newly written worked examples. International Baccalaureate does not publish a per-unit weighting for this exam.
Paper 1, Paper 2, and the scientific investigation
Both levels use Paper 1 and Paper 2. Paper 1 combines Part 1A multiple-choice questions with Part 1B data-based questions that can address all themes. Paper 2 combines data-based and short-answer questions with extended responses. HL gives more time and expects greater depth.
| Level and component | Time | Weight | What students do |
|---|---|---|---|
| SL Paper 1 | 90 minutes | 36% | Part 1A multiple choice and Part 1B with four data-based questions |
| SL Paper 2 | 90 minutes | 44% | Data-based, short-answer, and extended-response questions |
| SL Internal Assessment | 10 recommended hours | 20% | Open-ended scientific investigation reported in no more than 3,000 words |
| HL Paper 1 | 120 minutes | 36% | Part 1A multiple choice and Part 1B with four data-based questions |
| HL Paper 2 | 150 minutes | 44% | Data-based, short-answer, and extended-response questions |
| HL Internal Assessment | 10 recommended hours | 20% | Open-ended scientific investigation reported in no more than 3,000 words |
The available subject brief does not state whether a formula or reference sheet is supplied. Students should check current instructions from their school. Review the official IB Biology subject brief for later changes. These details are stated as of 2026-10-09.
Recover DNA concentration after dilution
Read each question as it appears on the exam paper, choose your answer, then tap it to see the feedback.
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. Use 1.80 ÷ 0.20 = 9 as the dilution factor, then calculate 18.0 × 9 = 162 ng/mL.
❌ Not quite 😭 This treats the buffer volume as the total diluted volume. The dilution factor must use 2.00 mL, so the calculation underestimates the original concentration.
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B. Use (0.20 + 1.80) ÷ 0.20 = 10.0 as the dilution factor, then calculate 18.0 × 10.0 = 180 ng/mL.
✅ Correct! 😊 This correctly compares total mixture volume with extract volume and reverses the dilution by multiplication. It also retains ng/mL in the result.
C. Use (0.20 + 1.80) ÷ 0.20 = 10.0, then calculate 18.0 ÷ 10.0 = 1.80 ng/mL.
❌ Not quite 😭 The dilution factor is correct, but dividing applies the dilution again. The original extract must be more concentrated than the diluted mixture.
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D. Add the buffer volume to the reading and report 18.0 + 1.80 = 19.8 ng/mL.
❌ Not quite 😭 This adds quantities with incompatible dimensions and does not account for the extract's tenfold dilution. Volume cannot be added directly to concentration.
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Full explanation
- Step 1Find the total diluted volume: 0.20 mL + 1.80 mL = 2.00 mL.
- Step 2Calculate the dilution factor from total volume ÷ extract volume: 2.00 ÷ 0.20 = 10.0.
- Step 3Reverse the dilution: 18.0 ng/mL × 10.0 = 180 ng/mL.
- Step 4Check that the original is more concentrated than the diluted mixture and state the result in ng/mL.
An investigation measured mean enzyme rate at different temperatures: 10 °C, 1.2 units/min; 20 °C, 2.4 units/min; 30 °C, 4.1 units/min; 40 °C, 3.0 units/min.
(a) Describe the relationship between temperature and mean enzyme rate. (b) Suggest a biological explanation for the change from 30 °C to 40 °C.
Answer both parts using the data provided.
A. The rate rises from 1.2 to 4.1 units/min between 10 °C and 30 °C, then falls to 3.0 units/min at 40 °C; altered enzyme structure may reduce active-site function at the higher temperature.
✅ Correct! 😊 This quantifies the pattern before offering a cautious mechanism. The wording keeps the observation separate from the explanation and does not claim the data prove denaturation.
B. Temperature makes the enzyme rate increase, so the highest temperature always produces the fastest reaction.
❌ Not quite 😭 This ignores the fall from 4.1 units/min at 30 °C to 3.0 units/min at 40 °C. Its absolute claim conflicts with the supplied data.
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C. The enzyme is completely denatured at 40 °C because the rate is lower than at 30 °C.
❌ Not quite 😭 A non-zero rate remains at 40 °C, and the measurements alone do not establish complete denaturation. This turns a possible mechanism into an unsupported certainty.
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D. The rate changes by 1.2 units/min from 10 °C to 20 °C, proving that substrate concentration increased.
❌ Not quite 😭 The numerical difference is correct, but no substrate measurements are given. A change in enzyme rate cannot by itself prove that substrate concentration changed.
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Full explanation
- Step 1Read the independent variable, measured response, values, and units before proposing a cause.
- Step 2Describe the full pattern: the rate increases to 30 °C and then decreases at 40 °C.
- Step 3Support the description with values, including the peak of 4.1 units/min and the fall to 3.0 units/min.
- Step 4Offer altered enzyme structure as a possible mechanism, using cautious language because the data do not directly measure structure.
Fifteen terms to use precisely
These definitions come from the guide's appendix. Connect each term to a diagram, table, calculation, or written explanation so the term does real work in a response.
- nucleic acid
- a polymer whose nucleotide sequence stores or transmits biological information
- scale bar
- a marked image length paired with its real-world distance
- allele frequency
- the share of gene copies in a population represented by one allele
- species richness
- the number of species recorded in a defined sample
- enzyme rate
- product formed or substrate used per unit time under stated conditions
- cardiac output
- blood volume pumped by one side of the heart per unit time
- specific force
- force normalized to a stated muscle cross-sectional area
- niche overlap
- the degree to which organisms use the same resource categories
- respiratory quotient
- carbon dioxide produced divided by oxygen consumed
- logistic growth
- density-dependent growth that slows as abundance approaches carrying capacity
- prevalence
- the proportion of a population that has a condition at a stated time
- mark–release–recapture
- population estimation using the fraction of marked organisms found in a later catch
- Hardy–Weinberg equilibrium
- a null model relating allele and genotype frequencies under stated population assumptions
- mitotic index
- the fraction of counted cells observed in mitosis
- independent assortment
- separation of allele pairs independently when loci are unlinked
What to expect and how to use this guide
Are these practice values taken from past IB questions?
No. The public material used here contains no released questions or scoring records. Every practice setting and numerical value in this guide is independently written.
Does the available subject brief say that a formula sheet is supplied?
The available subject brief does not state whether a formula or reference sheet is supplied. Students should check current instructions from their school.
How should a calculation be presented?
Write the relationship, substitute values with units, show conversion and rounding, then state the biological meaning of the result.
Is this guide produced or endorsed by International Baccalaureate?
No. This is an independent AskSia study guide. AskSia is not affiliated with or endorsed by the International Baccalaureate Organization. IB and International Baccalaureate are registered trademarks of the International Baccalaureate Organization.
What is the official IB Biology exam structure?
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%). These figures come from the current International Baccalaureate specification; where International Baccalaureate does not publish a per-unit weighting, this guide does not invent one.
Build a repeatable route from evidence to biological meaning
Pass 1 — map the four themes. For each theme, list its major biological objects and the scale at which a prompt is asking you to reason. Practice moving from molecule to cell, organism, population, and ecosystem without letting evidence at one scale stand in for a mechanism at another.
Pass 2 — reproduce complete examples. Work from problem to calculation to answer. Write the relationship first, place units beside every substituted value, show conversion and rounding where they occur, and finish with a sentence that states what the number means in the measured system.
Pass 3 — read data before explaining it. Identify axes, variables, units, sample size, uncertainty, and the comparison baseline. Describe the pattern separately from the proposed mechanism. When the design does not support a causal claim, state the limitation rather than filling the gap with subject knowledge.
Pass 4 — practice both paper styles. For Paper 1A, predict the relationship before reading options and reject choices that reverse cause and effect or change scale. For Paper 1B and Paper 2, annotate data, show working, and use a claim-evidence-reasoning structure for explanations and evaluations.
Keep SL and HL expectations clear. Both levels use Paper 1, including Part 1A multiple choice and Part 1B data, plus Paper 2. HL papers are longer and require deeper treatment, so HL practice should add linked mechanisms and more demanding interpretation without skipping the core SL foundations.
Use mixed review. International Baccalaureate does not publish a percentage for each theme. Alternate focused repair with mixed sets that require diagrams, tables, calculations, experimental evaluation, and extended explanation. Keep an error log that records the first decision that should have been made.
AskSia is not affiliated with or endorsed by the International Baccalaureate Organization. IB and International Baccalaureate are registered trademarks of the International Baccalaureate Organization. Exam structure is based on official International Baccalaureate information as of 2026-10-09; every practice setting and numerical value in this guide is independently written.