A-level and IB · Sciences

IB Biology: themes and key ideas

IB Biology flashcards across the four themes: unity and diversity, form and function, interaction and interdependence, continuity and change.

37 cards5 topicsAbout 15 minutes a roundIB Diploma Programme Biology (SL and HL) style content, comparable to Grade 12 biology plus first-year university cell biology and ecology

Independent study material written by StudyBrix, not produced or reviewed by any exam board or by the International Baccalaureate Organization. Check your own specification or subject guide.

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About this deck

This deck follows the structure of the IB Diploma Biology course first assessed in 2025, which arranges content into four themes (unity and diversity, form and function, interaction and interdependence, continuity and change) studied at the levels of molecules, cells, organisms and ecosystems. Cards are tagged by theme so you can revise the way your teacher is likely to teach, and a final tag covers practical skills and assessment. It suits SL and HL students; some cards go into HL depth.

Assessment for this course uses two written papers (Paper 1 with multiple-choice and data-based sections, Paper 2 with short and extended responses) plus an internally assessed scientific investigation worth 20%. If you are comparing with A-level, note that specifications differ between exam boards (AQA, Edexcel, OCR) and from the IB, so readers should check their own course guide or specification.

You should know Grade 10 or MYP science: cells, basic chemistry of water and bonds, and simple genetics. Study by explaining each card's idea in two or three linked sentences, then look for a link to another theme, since IB questions reward connections across the course. Work every numerical example yourself.

Common mistakes: stating that evolution happens to individuals rather than populations; confusing the number of chromosomes with the number of chromatids; writing that plants respire only at night; and forgetting units or uncertainty in data questions. In extended responses, name the structure and then say what it does.

Before you start: MYP or Grade 10 science, including cells, basic chemistry (atoms, bonds, pH) and simple inheritance.

All 37 cards

Cover the definitions with a sheet of paper and test yourself, or use the study modes above.

Unity and diversity

Hydrogen bonding in water: Weak attractions between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of another; they give water cohesion and a high specific heat capacity.Example: Cohesion lets water be pulled up xylem; high specific heat keeps lake temperatures steady.
Cell theory: The idea that all living things are made of cells, the cell is the smallest unit of life, and cells come from pre-existing cells.Example: Aseptate fungal hyphae and striated muscle fibres are often discussed as exceptions to typical cell structure.
Endosymbiosis: The theory that mitochondria and chloroplasts came from free-living prokaryotes taken in by an ancestral cell.Example: Evidence: both organelles have their own circular DNA, 70S ribosomes and a double membrane.
Virus structure: A non-cellular infectious particle: genetic material (DNA or RNA) in a protein capsid, sometimes with a lipid envelope; it replicates only inside a host cell.Example: Lytic cycle: a bacteriophage injects DNA, the host makes new phages, and the cell bursts.
Biological species concept: Defines a species as a group of organisms that can interbreed and produce fertile offspring.Example: Horses and donkeys produce mules, which are infertile, so they are separate species.
Cladistics: Classifying organisms into clades, groups of an ancestor and all its descendants, using shared derived characteristics and base sequences.Example: A cladogram where birds and crocodiles share a node shows they are more closely related to each other than to lizards.
Speciation by reproductive isolation: New species form when populations are separated geographically or behaviourally, then diverge through natural selection and genetic drift.Example: Different mating calls can isolate frog populations in the same pond.

Form and function

Polysaccharide structure and role: Long glucose chains: branched glycogen and amylopectin store energy compactly, while cellulose chains of beta glucose form strong fibres for cell walls.Example: Animals store glycogen in liver; plants store starch in tubers and build walls from cellulose.
Phospholipid bilayer: Phospholipids arrange with hydrophilic heads facing water and hydrophobic tails inside, forming the basis of all membranes.Example: Small non-polar molecules such as O2 diffuse straight through; ions need protein channels.
Protein denaturation: Loss of a protein's 3D shape when heat or extreme pH breaks the bonds holding it; the amino acid sequence remains but function is lost.Example: Pepsin works near pH 2 but is inactive at pH 8 because its active site changes shape.
Compartmentalization: Membranes divide eukaryotic cells into organelles, so different reactions can run at different conditions and enzymes are concentrated.Example: Lysosomes keep digestive enzymes away from the cytoplasm.
Gas exchange surfaces: Surfaces adapted for diffusion: large area, thin, moist, with a steep concentration gradient kept by ventilation and blood flow.Example: Alveoli walls are one cell thick and surrounded by capillaries.
Adaptations of capillaries: Narrow vessels with walls one cell thick and pores that let exchange happen between blood and tissue fluid.Example: Fenestrated capillaries in the kidney allow fast filtration.
Ecological niche: The role and position of a species in its ecosystem, including habitat, feeding and interactions with other species.Example: Two species of warbler feed in different parts of the same tree, so they share the habitat with less competition.

Interaction and interdependence

Activation energy: The minimum energy needed to start a reaction. Enzymes lower it by binding the substrate in a way that strains the bonds to be broken.Example: Catalase speeds the breakdown of hydrogen peroxide many times over at body temperature.
Anaerobic respiration: Releasing energy from glucose without oxygen, making only a little ATP from glycolysis; lactate forms in humans and ethanol plus CO2 in yeast.Example: During a sprint, lactate builds up in muscles.
Limiting factors in photosynthesis: The rate is set by whichever factor is in shortest supply: light intensity, CO2 concentration or temperature.Example: In a greenhouse on a bright winter day, extra heating may raise the rate more than extra light.
Chromatography Rf value: The distance a pigment moves divided by the distance the solvent front moves, used to identify photosynthetic pigments.Example: A spot travelling 3.2 cm with the solvent at 8.0 cm has Rf = 0.40.
Neural signalling: Neurones carry messages as action potentials along axons, and chemical transmitters cross synapses to the next cell.Example: Myelinated axons conduct faster because impulses jump between nodes (saltatory conduction).
Specific immune response: Lymphocytes recognize specific antigens; B cells become plasma cells making antibodies and memory cells give lasting immunity.Example: A vaccine exposes the body to antigens without causing illness, so memory cells form.
Lincoln index: An estimate of population size from capture, mark, release and recapture: N = (number marked first time * total caught second time)/marked recaptured.Example: 40 beetles marked, 50 caught later of which 8 were marked: N = 40 * 50/8 = 250.
Energy loss between trophic levels: Only a small fraction of energy passes to the next level because much is lost as heat from respiration and in uneaten or undigested material.Example: If producers fix 10000 kJ and about 10% passes on, primary consumers get about 1000 kJ.
Carbon cycle: Carbon moves between the atmosphere, organisms, oceans and fossil stores through photosynthesis, respiration, feeding, decomposition and combustion.Example: Peat stores carbon when waterlogged soil slows decomposition.

Continuity and change

DNA replication enzymes: Helicase unwinds the double helix, DNA polymerase adds nucleotides to the 3' end of a growing strand, and ligase joins fragments.Example: On the lagging strand, short Okazaki fragments are made and then sealed by ligase.
Gene expression: The process by which information in a gene is used to make a functional product, through transcription and translation, and regulated at many steps.Example: Lactose switches on lac genes in E. coli; methylation of DNA can switch genes off.
Point mutation: A change in a single base. It may be silent, change one amino acid, or create a stop codon.Example: Sickle cell: GAG becomes GTG, so glutamic acid is replaced by valine in beta globin.
Gene editing with CRISPR: A guide RNA leads the Cas9 enzyme to a matching DNA sequence where it cuts both strands, letting genes be disabled or altered.Example: Researchers have used it to correct disease alleles in cultured cells.
Meiosis and variation: Reduction division forming haploid gametes; crossing over and random orientation of chromosome pairs produce new allele combinations.Example: Nondisjunction in meiosis can cause trisomy 21.
Codominance: Both alleles are fully expressed in the heterozygote's phenotype.Example: Blood group AB: the I(A) and I(B) alleles both produce their antigen.
Chi-squared test in genetics: A statistical test comparing observed with expected counts; a value above the critical value suggests the difference is unlikely to be chance.Example: Expected 75:25, observed 70:30: 25/75 + 25/25 = 1.33, below 3.84 at 1 degree of freedom, so the 3:1 ratio is supported.
Hardy-Weinberg principle: In a large, randomly mating population with no selection, mutation or migration, allele frequencies stay constant: p^2 + 2pq + q^2 = 1.Example: If 9% of a population is homozygous recessive, q = 0.3 and p = 0.7, so 42% are heterozygous.
Homeostasis: Keeping internal conditions such as blood glucose, temperature and water balance within narrow limits through negative feedback.Example: When body temperature rises, sweating and vasodilation increase heat loss.
Evidence for evolution: Support from fossils, homologous structures, base and amino acid sequences, and observed change in populations.Example: The pentadactyl limb in bats, whales and humans suggests a shared ancestor.
Keystone species: A species with a large effect on its community relative to its numbers, so its loss changes the whole ecosystem.Example: Sea otters eat urchins; when otters disappear, urchins overgraze kelp forests.

Skills and assessment

Magnification: Image size divided by actual size, both in the same unit. Scale bars on micrographs give the actual length.Example: A 30 mm image of a 15 micrometre cell: 30000/15 = 2000 times.
Scientific investigation: The internally assessed component: an open-ended investigation in which students form their own research question, collect and analyse data and write a report.Example: Choose a question with a measurable dependent variable, such as rate of oxygen release at five light intensities.
Percentage change: (New value - original value)/original value times 100; used to compare results that start from different values.Example: A potato cylinder going from 2.50 g to 2.30 g changes by -8.0%.