Salam Academy
Sign inCreate account
Learn7402Paper 1 · Paper 1: Topics 1–4Organisms exchange substances with their environmentDigestion and absorption
Organisms exchange substances with their environment · Subtopic 4 of 5

Digestion and Absorption

Digestion hydrolyses large insoluble molecules into small soluble ones. Carbohydrates: amylase (salivary glands, pancreas) hydrolyses starch to maltose; membrane-bound disaccharidases on the ileum epithelium (maltase, sucrase, lactase) produce monosaccharides. Proteins: endopeptidases cut within chains (more ends); exopeptidases remove terminal amino acids; membrane-bound dipeptidases split dipeptides. Lipids: bile salts emulsify fats into small droplets (larger surface area) and form micelles; lipase hydrolyses ester bonds to monoglycerides and fatty acids. Absorption in the ileum: glucose and amino acids by co-transport with Na⁺ (gradient maintained by the Na⁺/K⁺ pump); micelles carry fatty acids to epithelial cells, which diffuse in, are rebuilt into triglycerides in the SER, packaged with proteins into chylomicrons, exocytosed and enter lacteals (lymph). Villi and microvilli give a huge surface area; thin epithelium; good blood supply.

AQA 7402 Paper 1≈ 45 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe digestion of carbohydrates, proteins and lipids.
  • Explain the roles of bile and micelles.
  • Describe absorption by co-transport and of lipids.
  • Explain adaptations of the ileum.
Pause and think

Why do endopeptidases make exopeptidases more effective?

Section 2 of 14

Core idea

Start hereBreaking food down

A sandwich contains starch, protein and fat, none small enough to enter your blood until enzymes break them down.

The big idea

Hydrolytic enzymes break big molecules into small ones; the ileum absorbs them by co-transport and diffusion across a huge surface.

Lipid digestion and absorption

Fats take a special route.

Worked exampleSurface area

Villi ×10, microvilli ×20.

Goal
Overall factor.
Strategy
Multiply.
  1. 10×2010\times20
    Effects combine.
  2. =200=200
    Evaluate.
Answer×200.
✓ CheckHuge area for absorption ✓.

Summary

  • Carbohydrases, proteases, lipase.
  • Bile and micelles.
  • Co-transport; chylomicrons.
Key words
Micelle
Cluster of bile salts and lipid products.
Chylomicron
Lipoprotein carrying absorbed fat into lymph.
HistoryA hole in the stomach. In the 1820s William Beaumont studied digestion through a permanent hole in the stomach of his patient Alexis St. Martin, who had been shot.
Fun factLactose intolerance. Most adults worldwide stop making much lactase after childhood, so milk sugar passes to the colon where bacteria ferment it.
Keep reading for free

Create a free account to read the rest of this lesson: key formulae, worked examples, explore the graph, common mistakes, exam tip, guided practice and more. Your answers are checked and your progress is saved.

Create free accountSign in