Cells and organisation · GCSE Biology
Digestion
Explain the human digestive system for GCSE Biology: organs in order, mechanical and chemical digestion, enzymes, bile, peristalsis and absorption in the small intestine.
Mouth → oesophagus → stomach → small intestine → large intestine. Enzymes break large insoluble molecules into small soluble ones. Bile emulsifies fats. Villi absorb in the ileum.
The important bits
What you need to know
- 1
Digestion breaks down large, insoluble food molecules into smaller, soluble molecules that can be absorbed into the blood and used by cells.
- 2
The digestive system is a tube: mouth, oesophagus, stomach, small intestine (duodenum and ileum), large intestine, rectum and anus. Accessory organs include salivary glands, pancreas, liver and gall bladder.
- 3
Mechanical digestion increases surface area: teeth chew food; the stomach churns it. Chemical digestion uses enzymes to break bonds in large molecules.
- 4
Amylase (salivary glands and pancreas) breaks starch into sugars. Protease (stomach and pancreas) breaks proteins into amino acids. Lipase (pancreas) breaks lipids into fatty acids and glycerol.
- 5
The stomach produces protease (pepsin) in acid conditions (pH ~2). Acid kills many bacteria and denatures proteins so enzymes can access them.
- 6
Bile is made in the liver, stored in the gall bladder and released into the duodenum. It is alkaline, neutralising stomach acid, and emulsifies fats into small droplets for faster lipase action.
- 7
Peristalsis is waves of muscular contraction that push food along the gut. It does not depend on gravity — you can swallow upside down.
- 8
In the ileum, villi and microvilli give a huge surface area, thin walls one cell thick, a rich blood supply and lacteals for lipids. Glucose and amino acids are absorbed by diffusion and active transport.
Quotations worth analysing
Short evidence. Real method.
“Digestion is the breakdown of large insoluble molecules into small soluble molecules.”
Both halves matter: large to small, and insoluble to soluble. “Food is broken down” without naming molecule size loses the definition mark.
“Bile emulsifies fats and neutralises stomach acid.”
Emulsification increases surface area for lipase; alkalinity raises duodenal pH for intestinal enzymes. Do not say bile digests fat — lipase does.
“Villi provide a large surface area for the absorption of digested food.”
Link each adaptation to absorption: microvilli, capillaries, lacteals, thin epithelium. “They absorb food” is too vague.
Go deeper
What actually happens in each region?
In the mouth, amylase starts starch digestion and saliva lubricates the bolus. The oesophagus moves it by peristalsis. In the stomach, hydrochloric acid and pepsin begin protein digestion; the thick muscular wall churns food into chyme. The duodenum receives pancreatic enzymes and bile — this is where most chemical digestion finishes. The ileum is mainly for absorption: villi increase surface area so glucose, amino acids, fatty acids and glycerol enter the blood or lacteals. The large intestine absorbs water; undigested material leaves as faeces. In exams, draw a simple flow diagram and attach one enzyme or function to each organ. “Digestion happens in the stomach only” is a common zero.
Go deeper
Why must molecules be small and soluble?
Starch granules and protein chains are too large to cross the epithelium of the gut. Enzymes hydrolyse them into monomers (glucose, amino acids, fatty acids, glycerol) that can diffuse or be actively transported across the one-cell-thick lining into capillaries. Lipids are rebuilt into lipoproteins in the epithelial cells before entering lacteals. If absorption is the question, name the molecule, the process (diffusion or active transport) and the destination (blood or lymph). Villi are the same exchange-surface logic as alveoli: large area, short distance, good supply.
Go deeper
Where do students mix up bile, enzymes and peristalsis?
Bile is not an enzyme. It has no active site and is not a protein catalyst — it emulsifies fat and neutralises acid. Lipase digests lipids. Peristalsis is muscular, not chemical. Amylase in the mouth is often inactivated in the acid stomach, but pancreatic amylase continues in the duodenum. Pepsin works in acid; intestinal enzymes need neutral/alkaline conditions after bile. A six-mark adaptation answer should split mechanical (teeth, churning) from chemical (named enzymes, substrates, products) and finish with absorption in the ileum.
See the idea in action
Explain how the small intestine is adapted for absorption (4 marks). Villi increase surface area so more digested food can cross per unit time. Epithelium is one cell thick, giving a short diffusion distance. Capillaries maintain a blood flow that carries absorbed glucose and amino acids away, keeping a concentration gradient. Lacteals absorb fatty acids and glycerol into the lymph. Microvilli on each epithelial cell further increase surface area for diffusion and active transport.
Exam technique
Turn knowledge into marks
Name the enzyme, substrate and product, then the organ. For bile: emulsifies fats and neutralises acid — not “digests”. For villi, use the same four adaptations as any exchange surface: area, thin wall, blood supply, transport away.
Common mistakes
Do not give these marks away
- 01
Saying bile digests fats, or that digestion only happens in the stomach.
- 02
Forgetting that absorption is mainly in the ileum, not the stomach.
- 03
Describing peristalsis as chemical digestion, or omitting that enzymes produce soluble products.
Where are most digested food molecules absorbed into the blood?
AStomach
BLarge intestine
CSmall intestine (ileum)
DOesophagus
Show the answer
Small intestine (ileum). The ileum is lined with villi adapted for absorption. The stomach mainly digests protein; the large intestine mainly absorbs water.
Quick questions
If this is the bit you searched
What is the role of bile in digestion?
Bile emulsifies fats into smaller droplets for lipase, and neutralises stomach acid so intestinal enzymes work in the correct pH.
What is peristalsis?
Waves of muscular contraction in the gut wall that move food along the alimentary canal without needing gravity.
Why are villi important?
They greatly increase the surface area of the small intestine for absorbing digested glucose, amino acids and other products into the blood and lymph.
What is the difference between mechanical and chemical digestion?
Mechanical digestion physically breaks food into smaller pieces (teeth, churning). Chemical digestion uses enzymes to break chemical bonds in large molecules.