IB Biology: Organelles & Cell Structure FAQ

You need the structure and function of each named organelle (nucleus, mitochondria, ribosomes, rough and smooth ER, Golgi apparatus, lysosomes, chloroplasts, vacuoles), the prokaryote-eukaryote distinction, and how to interpret electron micrographs. HL students also need compartmentalisation — why separating functions into organelles makes cells more efficient.

Prokaryotic cells (bacteria) lack a membrane-bound nucleus and most organelles — DNA sits free in the cytoplasm as a single circular chromosome, often with plasmids. Eukaryotic cells (animals, plants, fungi, protists) have a true nucleus and membrane-bound organelles like mitochondria and the Golgi apparatus.

Plant cells have a cellulose cell wall, chloroplasts for photosynthesis, and a large permanent central vacuole for turgor pressure and storage. Animal cells instead often have small temporary vacuoles, centrioles (used in cell division), and no cell wall or chloroplasts at all.

Both — and the link between them. IB Biology exam questions almost always use command terms like "state the function" or "outline the role", so you're marked on function as much as structure. Learning shape without purpose loses you marks on nearly every organelle question I've seen in past papers.

SL students need to identify and describe organelle structure and function using light and electron micrographs. HL students go further, covering compartmentalisation — how dividing cell functions into separate membrane-bound organelles increases efficiency — plus more detail on the endomembrane system's interconnected pathways.

Electron microscopes reveal organelle detail invisible under light microscopes because they achieve far higher resolution — roughly 0.5 nanometres versus around 200 nanometres for light microscopes. IB Biology exams often give you a micrograph and ask you to calculate actual organelle size using the magnification formula.

The biggest one is drawing a straight label line that doesn't touch the actual structure — examiners mark strictly on precision. Others include confusing rough and smooth ER, mislabelling the nucleolus as the whole nucleus, and forgetting that ribosomes appear both free in cytoplasm and attached to rough ER.

Expect "identify", "label", "annotate", "describe", "outline", "explain" and "compare", roughly in order of increasing depth required. "Annotate" specifically means adding brief notes about structure or function alongside a diagram label — a step examiners check for carefully and one students routinely underdo.

Group organelles by shared purpose rather than learning them alphabetically — protein-making (ribosomes, rough ER, Golgi), energy (mitochondria, chloroplasts), and storage/waste (vacuoles, lysosomes). This mirrors how questions are actually asked and makes recall far more reliable under exam pressure than a flat list.

Structure your answer around named features — cell wall, vacuole, chloroplasts, centrioles — stating what's present or absent in each cell type for every feature, not two separate unlinked paragraphs. Examiners award marks per valid comparative point, so explicit "whereas" or "but" statements score more reliably than parallel description.

Cell structure underpins nearly every later topic — cell membranes and transport, respiration and photosynthesis, protein synthesis, and even genetics all assume your child already knows what mitochondria, ribosomes and the nucleus actually do. Weak foundations here tend to resurface as confused answers two years later in Paper 2.

It's a genuine step up, not just a repeat. IB Biology expects quantitative work (magnification calculations), comparative reasoning across cell types, and, at HL, an explanation of why compartmentalisation benefits the cell — a level of analysis GCSE and IGCSE biology generally don't require.

Past-paper practice with real electron micrographs matters more than flashcards alone, since so many marks come from diagram interpretation and magnification calculations. On RevisionPrep, students can work through topical worksheets and revision notes built around exactly this kind of exam-style question, then check understanding against mock papers.

Two to three focused sessions of 45 minutes each is usually enough for SL, closer to four for HL given the added compartmentalisation content. The priority isn't re-reading notes — it's timed practice on labelling diagrams and magnification calculations, since that's where marks are actually lost in mocks.