IB Biology Study Guide: HL and SL, Internal Assessment, and the Extended Response Strategy

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.