A grade 7 in IB Biology requires both deep content knowledge and genuine scientific reasoning — the exams reward applying your understanding to unfamiliar data and experimental design, not recall. Spread continuous active revision across the eight core topics (plus the AHL material), treat the Internal Assessment (20%) as a real quantitative investigation with a statistical test and enough data points to support it, and drill the data-based questions on past papers until analysing an unfamiliar graph is automatic.
IB Biology is one of the most demanding science subjects in the International Baccalaureate Diploma Programme — it combines a large content base (covering cell biology, molecular biology, genetics, ecology, evolution, and human physiology) with a requirement for genuine scientific reasoning and experimental design skills. The students who achieve grade 7 understand both the content and the process of biological inquiry.
The most important study philosophy: IB Biology is not about memorising facts about biology. It is about understanding biological processes well enough to reason about unfamiliar experimental data, design investigations, and communicate findings with precision. The extended response questions and the Internal Assessment both test this reasoning capacity directly.
Core molecular biology: the foundation
The central dogma: DNA → (transcription) → mRNA → (translation) → protein. Know every step in molecular detail: transcription occurs in the nucleus (RNA polymerase unwinds DNA, adds complementary RNA nucleotides, produces pre-mRNA); RNA processing (introns excised, exons spliced together, 5' cap and poly-A tail added to produce mature mRNA); translation occurs at ribosomes (mRNA codons read by tRNA anticodons, amino acids assembled according to the genetic code, peptide bonds formed).
DNA replication: Semi-conservative (each daughter DNA has one original strand). Key enzymes: helicase (unwinds the double helix), DNA polymerase (adds nucleotides 5'→3', needs a primer), DNA ligase (joins Okazaki fragments on the lagging strand). Meselson and Stahl experiment — evidence for semi-conservative replication.
Cell respiration: Three stages: glycolysis (cytoplasm, glucose → pyruvate, 2 ATP net), link reaction (matrix, pyruvate → acetyl CoA + CO₂), Krebs cycle (matrix, acetyl CoA → CO₂, NADH, FADH₂, 1 ATP per turn), oxidative phosphorylation (inner mitochondrial membrane, electron transport chain uses NADH and FADH₂ to create proton gradient, ATP synthase makes ATP). Anaerobic respiration: fermentation (lactate in animals; ethanol + CO₂ in yeast) regenerates NAD⁺ for continued glycolysis.
Photosynthesis: Light-dependent reactions (thylakoid membrane): chlorophyll absorbs light, water is split (photolysis) releasing O₂, ATP and NADPH produced. Light-independent reactions / Calvin cycle (stroma): CO₂ fixed by RuBisCO onto RuBP (5C → 6C → 2× GP), GP reduced to G3P using ATP and NADPH, RuBP regenerated.
Use the Spaced Repetition Flashcard Tool for each metabolic pathway step — one card per step (reactants, products, location, enzyme where relevant).
Genetics and inheritance
Meiosis vs mitosis: Meiosis produces haploid gametes (half the chromosome number) through two divisions (meiosis I separates homologous chromosomes, meiosis II separates sister chromatids). Crossing over (between non-sister chromatids of homologous pairs at chiasmata during prophase I) creates new combinations of alleles.
Mendel's laws: Law of segregation (alleles separate into different gametes), law of independent assortment (alleles for different genes assort independently — only holds for genes on different chromosomes or distant genes on the same chromosome). Dihybrid crosses: use punnet squares or probability calculations for independent assortment (9:3:3:1 ratio for two heterozygous parents).
Non-Mendelian inheritance: Codominance (both alleles expressed, e.g., blood type AB), incomplete dominance (heterozygote shows intermediate phenotype), sex-linkage (gene on X chromosome — males are hemizygous, show all alleles), polygenic inheritance (multiple genes contribute to a continuous trait, e.g., height, skin colour).
Chi-squared test for genetics: Used to test whether observed ratios deviate significantly from expected Mendelian ratios. χ² = Σ[(O−E)²/E]. Compare to critical value at appropriate degrees of freedom and significance level. If χ² < critical value, the difference from expected ratio is not statistically significant.
Ecology: populations and ecosystems
Population dynamics: Logistic growth (S-shaped curve) — population grows exponentially when small, slows as it approaches carrying capacity K (due to limiting factors — food, disease, predation). Exponential growth model: dN/dt = rN. Logistic model: dN/dt = rN(1 − N/K).
Energy flow through ecosystems: Gross productivity (total photosynthesis), net productivity (gross − respiration = energy available to higher trophic levels). Energy efficiency between trophic levels ≈ 10% (rest lost as heat). Bioaccumulation: persistent pollutants (DDT, PCBs) accumulate at higher concentrations at higher trophic levels (biomagnification).
Nitrogen cycle: Nitrogen fixation (N₂ → NH₃/NH₄⁺ — by Rhizobium in root nodules), nitrification (NH₄⁺ → NO₂⁻ → NO₃⁻ by nitrifying bacteria), assimilation (plants absorb NO₃⁻), denitrification (NO₃⁻ → N₂ by anaerobic bacteria — returns nitrogen to atmosphere).
Internal Assessment: the 20% you control
The IA is the assessment component where students have the most control over their mark. A well-designed IA with good data and thorough analysis consistently scores 7 or 8 out of 10, which is the equivalent of a grade 7 contribution.
Research question design: Specific (names the independent variable, dependent variable, and organism/system studied), quantitative (produces numerical data amenable to statistical analysis), feasible (achievable with school resources in the available time). Bad: "How does temperature affect enzyme activity?" Good: "How does temperature (10°C, 20°C, 30°C, 40°C, 50°C) affect the rate of catecholase-catalysed oxidation of catechol, measured as absorbance at 440 nm using spectrophotometry?"
Statistical analysis: The IA requires at least one statistical analysis. For comparing two groups: t-test (compare means with variance, provides p-value). For more than two groups: ANOVA or multiple t-tests with correction. For comparing frequencies: chi-squared. For correlation: Pearson's r or Spearman's rho. State the statistical test, the result (t-value or chi-squared statistic), degrees of freedom, p-value, and conclusion.
Evaluation: Assess your conclusion against your hypothesis. Identify the most significant sources of error and explain their effect on the results (random vs systematic errors). Suggest specific, feasible improvements (not generic "more repetitions" but "add three additional replicates per temperature group to reduce the standard error of the mean below 5% of the mean value"). Identify extensions: "A natural follow-up investigation would examine whether the same temperature response curve holds for the enzyme at non-physiological pH values."
The Cornell Notes Tool is ideal for structuring IA write-up planning — each section (exploration, analysis, evaluation) is a topic in the main column. See the IB Chemistry study guide for parallel strategies on the other IB science.
Topics
Frequently asked questions
What are the core topics in IB Biology?
IB Biology (both HL and SL) covers eight core topics: Cell Biology (cell theory, ultrastructure, membrane structure and function, cell division); Molecular Biology (metabolism, water, carbohydrates, lipids, proteins, DNA structure and replication, transcription and translation, cellular respiration, photosynthesis); Genetics (chromosomes, meiosis, inheritance patterns, genetic modification); Ecology (species, communities, populations, evolution of populations, classification); Evolution and Biodiversity (evidence for evolution, natural selection, classification systems); Human Physiology (digestion, blood system, defence against disease, gas exchange, neurons and synapses, hormones); Nucleic Acids (DNA structure, transcription, translation — extended for HL); and Metabolism, Cell Respiration, and Photosynthesis (detailed biochemical pathways — at greater depth than core). HL students additionally cover four Additional Higher Level (AHL) topics.
How is IB Biology assessed?
IB Biology assessment consists of external examinations (80% of the grade at HL, 80% at SL) and internal assessment (20%). External: Paper 1 (multiple choice — 30 questions SL, 40 questions HL, 1 hour/1 hour 15 minutes); Paper 2 (data-based and extended response — 2 hours 15 minutes SL, 2 hours 30 minutes HL); Paper 3 (short answer and extended response on options topic — 1 hour SL, 1 hour 15 minutes HL). Internal Assessment: an individual investigation (10 hours, reported in 6–12 pages). The IA is marked by the teacher and moderated by the IB. Note: the IB periodically revises the assessment model — confirm the current structure for your exam year from your IB coordinator.
What are the most difficult topics in IB Biology HL?
IB Biology HL students consistently find the following AHL topics most challenging: Nucleic Acids (the molecular details of transcription, translation, and gene expression regulation including introns, exons, and RNA processing); Metabolism and enzyme regulation (allosteric enzymes, feedback inhibition, metabolic pathways as integrated systems); Genetics (linkage, crossing over and recombination, Chi-squared testing of inheritance ratios, polygenic inheritance); Animal Physiology (the nephron and kidney ultrafiltration, the cardiac cycle in detail, the immune system's cellular and molecular mechanisms). The difficulty is largely from the requirement to understand processes at a molecular level and to apply that understanding to unfamiliar experimental data.
How do I write a high-scoring IB Biology Internal Assessment?
The IB Biology Internal Assessment is a self-designed investigation worth 20% of the total grade. It is marked against five criteria: Personal Engagement (evidence of genuine interest and initiative); Exploration (background information, research question, hypothesis, variables, methodology); Analysis (data presentation including graphs with appropriate error bars, processing, statistical analysis); Evaluation (conclusion in relation to hypothesis, evaluation of method and improvements, extension suggestions); Communication (clarity, structure, use of scientific language, bibliography). The most common failing is insufficient quantitative data analysis — use at least one statistical test (t-test, chi-squared, standard deviation) and explain what the test result means in the context of your research question. Collect enough data points for the statistics to be meaningful (minimum 10 data points per group for a t-test).
What data-based questions appear in IB Biology Paper 2 and how should I approach them?
Data-based questions (DBQs) appear in Section A of Paper 2 and present unfamiliar experimental data (graphs, tables, images) for analysis. These questions test scientific reasoning skills more than content recall. Strategy: read the graph axis labels carefully before answering; describe data precisely by quoting specific values; when asked to explain, use biological mechanisms rather than restating what the graph shows; for 'suggest' questions, there is no single correct answer — demonstrate biological reasoning with any supported explanation; for statistical analysis questions (comparing means with error bars), know that overlapping error bars do not necessarily indicate a non-significant difference (depends on the overlap distance). Practice DBQs using past IB Biology Paper 2 questions — the more varied the data sets you encounter, the more flexible your analytical approach becomes.
Build your IB Diploma study system
Use the Cornell Notes Tool for Internal Assessment planning, the Spaced Repetition Flashcard Tool to retain content across HL subjects, and the Active Recall course to develop the retrieval practice habits the IB rewards.
More on IB Diploma Study Guides