Digestion and Absorption
Hydrolysis of large food molecules, enzyme action in the digestive system and absorption through a specialised intestinal epithelium.
How to study A-level Biology
Move between scales deliberately: identify the biological process, trace matter or information, use quantitative evidence, then evaluate the method and conclusion.
Core concepts
Concept 1
Extracellular digestion uses specific hydrolytic enzymes to produce small molecules that can cross cell membranes.
Exam cue: Match substrate, enzyme, bond broken and absorbable product.
Concept 2
The ileum combines a large surface area, short diffusion pathway, transport proteins and blood or lymph flow for efficient absorption.
Exam cue: Trace a molecule from the gut lumen through epithelial membranes to blood or lymph.
Concept 3
Glucose and amino-acid uptake may combine sodium-dependent cotransport, facilitated diffusion and active maintenance of ion gradients.
Exam cue: Identify which step directly uses ATP and which uses an ion gradient established indirectly by ATP.
Risk pitfalls and guardrails
Calling digestion the same process as absorption.
Guardrail: Do not replace a mechanism with a vague purpose statement, or infer causation from an association without experimental support.
Saying bile is an enzyme.
Guardrail: Do not replace a mechanism with a vague purpose statement, or infer causation from an association without experimental support.
Treating every molecule as entering blood capillaries by simple diffusion.
Guardrail: Do not replace a mechanism with a vague purpose statement, or infer causation from an association without experimental support.
Memory anchors
Digestion
Digestion hydrolyses large biological molecules into smaller absorbable products.
Amylase
Amylase hydrolyses starch glycosidic bonds to smaller sugars.
Protease
Proteases hydrolyse peptide bonds in proteins and peptides.
Bile
Bile emulsifies lipids and helps neutralise acidic chyme; it is not an enzyme.
Cotransport
Cotransport uses one solute's gradient to move another solute through the same carrier.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
Why is digestion required before most proteins can be absorbed?
Which enzyme–substrate pairing is correct?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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