IB Biology key terms 2026: The essential vocabulary every student must know
IB Biology key terms are the precise scientific words and definitions students need to interpret questions correctly, communicate biological reasoning, and earn marks consistently. Important vocabulary spans cell biology, molecular biology, genetics, human physiology, and ecology, with terms such as selectively permeable, denaturation, genotype, homeostasis, and trophic level appearing frequently in written responses. In IB Biology, understanding a concept is not always enough; students must also express it using accurate, syllabus-aligned terminology.
This guide explains the most important IB Biology key terms, their meanings, and how students can use them effectively in exams.
- Key cell biology and molecular biology terms in IB Biology explained
- Key genetics and evolution terms in IB Biology explained
- Key human physiology terms in IB Biology explained
- Key ecology and environmental terms in IB Biology explained
- How precise use of key terms earns marks in IB Biology exams
- How to build and revise your IB Biology key term vocabulary
- Frequently asked questions
Key cell biology and molecular biology terms in IB Biology explained
Cell biology forms the bedrock of the entire IB Biology [1] syllabus, and the terminology in this section carries enormous weight across Paper 1, Paper 2, and Paper 3 assessments. Getting these cell biology terms IB examiners expect right can be the difference between a band descriptor 3 and a band descriptor 6 response.
Selectively permeable is the correct term for the property of cell membranes that controls which molecules pass through. A common mistake we see at Times Edu is students writing “semi-permeable,” which is technically acceptable in some curricula but is considered imprecise in IB Biology mark schemes. Always write “selectively permeable” when describing membrane properties.
Osmolarity refers to the concentration of a solution expressed as the total number of solute particles per litre. Students frequently confuse osmolarity with osmotic pressure or water potential. Understanding which term the question demands requires careful reading of the command term used.
Osmosis definition IB: One critical detail often overlooked is that osmosis must be defined as the movement of water molecules across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration, down a water potential gradient. Simply writing “water moves from low to high concentration” loses marks because it is ambiguous about which concentration is being referenced.
Endocytosis and exocytosis must both be described as active processes, meaning they require ATP. Endocytosis involves the membrane folding inward to engulf substances, while exocytosis involves vesicles fusing with the membrane to release substances. Do not describe either process without referencing the role of the membrane.
Differentiation is the process by which unspecialised cells become specialised by selectively expressing specific genes. Students lose marks by writing that cells “change” or “grow” rather than expressing the gene regulation mechanism underlying differentiation.
Now, shifting to molecular biology, the terms related to DNA, protein synthesis, and enzymes are among the highest-frequency topics in IB Biology exams.
Transcription and translation IB: Transcription is the synthesis of mRNA from a DNA template inside the nucleus, while translation is the assembly of a polypeptide chain at the ribosome using the mRNA sequence as a code. A common mistake we see is students reversing these two processes or failing to specify the location of each event. Transcription occurs in the nucleus (or nucleoid in prokaryotes); translation occurs at ribosomes in the cytoplasm.
Denaturation describes the permanent structural change in a protein caused by extreme heat or pH, which destroys the three-dimensional shape of the active site. The word “destroyed” or “died” is never acceptable in IB Biology written answers when describing an enzyme. Always write “denatured.”
Complementary base pairing requires students to name the specific pairs: Adenine with thymine (in DNA) or uracil (in RNA), and cytosine with guanine, held together by hydrogen bonds. Omitting “hydrogen bonds” in structural questions typically costs a mark.
| Casual phrasing (avoid) | IB Biology precise term (use) |
|---|---|
| Semi-permeable | Selectively permeable |
| Enzyme broke down / died | Enzyme denatured |
| Water-loving / water-hating | Hydrophilic / hydrophobic |
| Things moving in or out of cells | Passive diffusion / active transport |
| Breathing in cells | Cell respiration |
| The graph goes up and down | The variable increased, peaked, and decreased |
>>> Read more: IB Biology time management 2026: How to pace yourself across every paper
Key genetics and evolution terms in IB Biology explained
Genetics terms in IB Biology carry some of the highest mark allocations in Paper 2 extended-response questions. Drawing on years of experience at Times Edu supporting IB students across SL and HL, we know that genetics vocabulary is where imprecise language most frequently derails otherwise strong students.
Genotype and phenotype in IB Biology are two of the most frequently tested concepts in the genetics unit. The genotype is the specific combination of alleles an organism carries for a given gene or set of genes. The phenotype is the observable characteristic that results from the interaction of the genotype with the environment. Students often define phenotype as simply “what you can see,” which is incomplete; IB mark schemes expect reference to the role of both genotype and environmental factors.
Homozygous and heterozygous must be defined with reference to alleles, not genes in general. Homozygous means carrying two identical alleles at a given locus; heterozygous means carrying two different alleles at a given locus.
Locus is the specific physical position of a gene on a chromosome. This term appears frequently in pedigree chart questions and linkage questions at HL. Students who write “the place where the gene is” instead of “the locus” signal to the examiner that they are operating below the required vocabulary level.
Carrier must be defined as an individual who carries one copy of a recessive allele for a disorder but does not express the phenotype associated with that disorder. The definition must reference the recessive nature of the allele and the absence of the phenotype.
Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate correctly during meiosis, resulting in gametes with an abnormal chromosome number. Students must specify whether non-disjunction occurs in meiosis I or meiosis II when answering HL questions, as the resulting gametes differ in each case.
| Term | Precise IB definition |
|---|---|
| Genotype | The specific allele combination an organism carries for a gene or genes |
| Phenotype | The observable characteristics resulting from the genotype and environmental interaction |
| Homozygous | Carrying two identical alleles at a given locus |
| Heterozygous | Carrying two different alleles at a given locus |
| Locus | The specific position of a gene on a chromosome |
| Carrier | An individual heterozygous for a recessive allele who does not express the associated phenotype |
| Non-disjunction | Failure of chromosomes to separate correctly during meiosis |
>>> Read more: IB Biology Paper 2 2026: Everything you need to know to perform at your best
Key human physiology terms in IB Biology explained
Physiology vocabulary in IB Biology spans the nervous system, the immune system, the digestive system, and the respiratory system. These topics appear heavily in Paper 2 and HL Paper 3 data-based questions.
Homeostasis is the maintenance of a stable internal environment despite fluctuations in external conditions. Students must be able to link this definition to specific examples: Blood glucose regulation, thermoregulation, and osmoregulation. Answers that define homeostasis without providing a mechanism of negative feedback will not achieve the top mark band.
Action potential describes the rapid depolarisation and repolarisation of the neuronal membrane caused by the sequential opening and closing of sodium and potassium ion channels. In our experience working with international students, this is one of the most commonly misrepresented processes in physiology; students describe nerve impulses as “electrical signals travelling along a wire,” which demonstrates a fundamental misunderstanding of the ionic basis of the action potential.
Antigen and antibody are a paired pair of terms that must be defined together and separately. An antigen is a molecule (often a protein) on the surface of a foreign cell or pathogen that triggers an immune response. An antibody is a Y-shaped glycoprotein produced by plasma cells (B lymphocytes) that binds specifically to a complementary antigen.
Ventilation is the physical movement of air into and out of the lungs, maintaining a concentration gradient for gas exchange in the alveoli. One critical detail often overlooked is that ventilation must never be conflated with cell respiration. Ventilation is a mechanical process; cell respiration is a biochemical one. Examiners penalise students who use the term “respiration” when they mean “ventilation.”
Pathogen is defined as any disease-causing organism, virus, or prion. Students are expected to identify specific examples by pathogen type: Bacterial, viral, fungal, or prion-based.
>>> Read more: IB Biology Paper 1 2026: Everything you need to know to perform at your best
Key ecology and environmental terms in IB Biology explained
Ecology terms in IB Biology are tested extensively in the Option chapters and in core Paper 2 questions on energy flow, biodiversity, and conservation. The precision required in ecology answers is often underestimated.
Trophic level refers to the feeding position an organism occupies in a food chain or food web. Producers occupy trophic level 1; primary consumers occupy trophic level 2, and so on. Students should avoid writing “level in the food chain” as a substitute for the precise term.
Saprotroph describes organisms, typically fungi and bacteria, that obtain nutrients by secreting digestive enzymes externally onto dead organic matter and then absorbing the soluble products. The key distinction between a saprotroph and a predator or parasite is the external digestion mechanism. This term is frequently missed by students who have studied IGCSE Biology, where the term “decomposer” is more commonly used.
Mesocosm is an enclosed, experimental ecosystem designed to model ecological interactions under controlled conditions. This term is primarily tested in Internal Assessment and experimental design questions at HL.
Clade is a grouping on a phylogenetic tree that includes a single common ancestor and all of its descendants. Clade-based classification aligns with cladistics, which is distinct from classical taxonomy. Students must understand that clades are defined by shared derived characteristics, not overall similarity.
Homologous structures are anatomical features in different species that share a common evolutionary origin but may serve different functions, such as the pentadactyl limb found in mammals. These provide evidence for adaptive radiation and common ancestry.
>>> Read more: IB Biology SL exam tips 2026: How to maximise your score across both main papers
How precise use of key terms earns marks in IB Biology exams
IB Biology mark schemes are constructed around command terms and accepted response phrases. The marking is point-based for data-based questions and band-descriptor-based for extended essays. In both formats, the presence or absence of key vocabulary directly influences the mark awarded.
Drawing on years of experience at Times Edu reviewing student scripts and mark schemes, the single most consistent differentiator between students scoring 5 and students scoring 7 is the quality of biological vocabulary. A student can describe the correct process but receive zero marks if the phrasing used does not align with the accepted answer.
The IB Biology guide explicitly states that students should “use appropriate scientific vocabulary.” Examiners are instructed to award marks for the concept only when it is expressed with sufficient precision. Vague language such as “the cell does something to move particles” will not attract marks, even when the underlying logic is correct.
Three strategies for using precision terminology in IB Biology exams:
- Read the command term in every question before writing. “Define,” “outline,” “explain,” and “evaluate” each require a different level of terminology depth.
- Use the exact term from the syllabus, not a synonym, when answering definition-based questions.
- After writing any answer, check whether you have used at least one subject-specific term per mark allocated.
>>> Read more: IB Biology HL exam tips 2026: How to maximise your score across all three papers
How to build and revise your IB Biology key term vocabulary
Building IB Biology vocabulary is not a passive process. Reading textbooks alone is insufficient for exam performance. The following revision system is what our tutors at Times Edu implement with students from Year 1 of their IB Diploma programme.
Step 1: Create a topic-sorted glossary from day one. Divide your glossary by the core themes: Cell biology, molecular biology, genetics, ecology, and physiology. Add each new term as it is introduced in class, not at the end of the unit.
Step 2: Write definitions in your own words, then compare. After writing your own definition, compare it against the IB Biology guide definition and mark scheme phrasing. Identify the exact words that appear in the official answer and incorporate them into your version.
Step 3: Use active recall, not passive reading. Flashcard systems, particularly spaced repetition tools, are highly effective for vocabulary retention. Write the term on one side and the IB-standard definition on the other. Test yourself daily in the weeks before your exam.
Step 4: Practise using terms in full sentences under timed conditions. Knowing a definition in isolation is not enough. You must be able to deploy it accurately in a 4-mark or 6-mark written response within a limited time.
Step 5: Review past mark schemes for your exact wording. For every past paper question you attempt, compare your answer word-by-word against the mark scheme. This is the most direct feedback loop available to IB Biology students.
>>> Read more: IB Biology grade boundaries 2026: How they work and what they mean for your result
Frequently asked questions
What biological vocabulary must you know for IB Biology?
The IB Biology syllabus covers five core areas: Cell biology, molecular biology, genetics, ecology, and human physiology. Within each area, there are between 20 and 40 high-priority terms that appear repeatedly across past papers. The terms outlined in this article represent the highest-frequency vocabulary across all five domains.
Why does using precise terminology matter in IB Biology written answers?
IB examiners are trained to mark against specific accepted phrases found in the mark scheme. Imprecise language, even when conceptually correct, does not attract marks. Precision terminology in IB Biology is not about formality; it is a direct scoring mechanism.
What are the most commonly misused terms in IB Biology?
The most frequently misused terms include: “semi-permeable” (should be “selectively permeable”), “respiration” when referring to ventilation, “denatured” versus “destroyed,” and confusing genotype with phenotype. Students also commonly misdefine osmosis by failing to reference water potential.
What is the difference between transcription and translation in IB Biology?
Transcription is the process by which a complementary mRNA strand is synthesised from a DNA template in the nucleus. Translation is the process by which the mRNA sequence is read at the ribosome to assemble a polypeptide chain. Both processes together constitute gene expression.
How do you define osmosis correctly for IB Biology?
Osmosis is the passive movement of water molecules across a selectively permeable membrane from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration). The definition must include reference to water potential and the selectively permeable membrane to be complete.
What is the difference between genotype and phenotype in IB Biology?
The genotype is the genetic information an organism carries, specifically the combination of alleles present at one or more loci. The phenotype is the observable trait that results from the expression of the genotype in combination with environmental influences.
How do IB Biology key terms differ from IGCSE Biology terminology?
IB Biology vs IGCSE terminology differences are most apparent in three areas. First, IB uses “selectively permeable” where IGCSE accepts “semi-permeable.” Second, IB requires “saprotroph” where IGCSE uses “decomposer.” Third, IB assessment demands full mechanistic definitions (referencing ATP, ion channels, or enzyme kinetics) where IGCSE answers can be more descriptive. Students transitioning from IGCSE to IB must actively upgrade their vocabulary, not simply transfer it.
Conclusion
At Times Edu, our 1-on-1 IB Biology tutors work with students from their first unit to their final revision week, building both conceptual depth and the precision terminology IB Biology demands. If your child is preparing for IB Biology and you want a personalised academic roadmap that targets vocabulary, exam technique, and syllabus coverage systematically, contact Times Edu today to book a consultation. A single session can identify the exact gaps standing between your student and their target score.
