IB Biology: Digestion & Absorption FAQ

This topic covers how food is mechanically and chemically broken down along the gut, the role of digestive enzymes (amylase, protease, lipase), and how the small intestine's villi and microvilli maximise surface area for absorbing glucose, amino acids and fatty acids into the blood and lymph.

It's tested across all three papers: Paper 1 uses multiple-choice on enzyme action and gut structure; Paper 2 has structured short-answer and extended-response questions asking you to link villus structure to function; Paper 3 sometimes uses digestion data (enzyme rate graphs) as an unseen data-based question.

It's core content, so every SL and HL student studies the same material on gut structure, enzymes and villus absorption. The difference isn't extra digestion content at HL — it's that HL papers demand more synthesis, linking digestion to enzyme kinetics and cell specialisation taught elsewhere in the course.

Digestion is the chemical and mechanical breakdown of large food molecules into smaller, soluble units — starch into glucose, protein into amino acids. Absorption is the separate process of moving those small molecules across the gut epithelium into the bloodstream or lymph, mainly via villi in the small intestine.

The most frequent command terms are "describe" (state structural features), "explain" (link structure to function with a mechanism), and "outline" (give a brief account, usually of the digestion pathway for one nutrient). Higher-level papers sometimes use "discuss" when comparing digestion of different macromolecules.

The top three: confusing digestion with absorption, forgetting to name the specific enzyme and substrate (writing "protein is digested" instead of "protease breaks peptide bonds in protein"), and describing villi structure without connecting it to the actual absorption mechanism being asked about.

There's no fixed percentage published by the IB, but digestion and absorption is a reliably examined sub-topic within Theme B — it typically appears as one structured question worth 4-8 marks on Paper 2 most sessions, plus occasional Paper 1 items on enzymes or villus structure.

Work through it in three passes: first learn the pathway (mouth to small intestine) with the enzyme and substrate at each stage, then master villus structure with a labelled diagram, then practise structured past-paper questions that force you to link structure to function under time pressure.

Take starch digestion: salivary amylase hydrolyses starch into maltose in the mouth; pancreatic amylase continues this in the duodenum; membrane-bound maltase on microvilli finishes the job, converting maltose to glucose right at the absorbing surface, so glucose enters enterocytes by facilitated diffusion and active co-transport with sodium.

Yes — past IB Biology Paper 2 papers reliably include structured questions on gut structure and enzyme function, and the RevisionPrep question bank has topical worksheets organised by sub-topic so you can drill digestion and absorption specifically before moving to mixed past papers.

It can — some students choose an IA investigating enzyme activity (e.g. amylase or lipase rate under different pH or temperature), which links directly to the digestion topic, though it's assessed against the general IA criteria (Personal Engagement, Exploration, Analysis, Evaluation, Communication), not topic-specific marks.

The most useful thing you can do is ask your child to explain a diagram out loud — say, a villus cross-section — without notes. If they can narrate structure-to-function links fluently, they're ready; if they hesitate, that's exactly where marks get lost on the actual exam.