The five processes, in order
Ingestion — taking food into the body through the mouth. Digestion — breakdown of large, insoluble molecules into small, soluble ones. Absorption — movement of those small molecules from the small intestine into the blood or lymph. Assimilation — use of the absorbed molecules by body cells (glucose in respiration, amino acids to build proteins). Egestion — removal of undigested food as faeces at the anus.
The nine organs and what each does
Mouth (chewing, salivary amylase); salivary glands (amylase + mucus); oesophagus (peristalsis only, no digestion); stomach (churning, pepsin, hydrochloric acid); small intestine — duodenum receives bile and pancreatic juice, ileum absorbs; pancreas (amylase, protease, lipase into the duodenum); liver (makes bile); gall bladder (stores bile); large intestine (absorbs water, forms faeces). The full function table is in the study note.
Absorption happens mainly in the ileum
Absorption — glucose and amino acids pass into the blood capillaries of each villus (then to the liver); fatty acids and glycerol pass into the lacteal and travel in the lymph. Adaptations of the ileum: many villi and microvilli (very large surface area); a wall one cell thick (short diffusion distance); a rich capillary network (keeps the concentration gradient steep). Water is absorbed in both the small intestine and the large intestine.
Drawn from real examiner reports.
Bile is not an enzyme
Bile is a substance made by the liver, not an enzyme, and it does not chemically digest fat. It emulsifies fat — breaking large globules into many smaller droplets — which increases the surface area for the enzyme lipase to work on. Cambridge awards two marks: (1) bile emulsifies fat / breaks it into smaller droplets; (2) this increases the surface area for lipase.
Flagged s23 P41 Q1b i — "bile digests fat" / "bile is an enzyme"; bile is not an enzyme, it emulsifies fat to increase surface area
Physical digestion ≠ chemical digestion
Physical (mechanical) digestion breaks food into smaller pieces, increasing the surface area for enzymes to act on. The molecules are unchanged — still large and insoluble. Chemical digestion uses enzymes to break chemical bonds, turning large insoluble molecules into small soluble ones. Saying physical digestion produces small soluble molecules scores zero.
Flagged s23 P11/21 Q14-15; w22 P41 Q1a i — physical digestion increases surface area for enzyme action; many stated it breaks large molecules into small ones
Physical digestion is not just chewing
Candidates often state that physical digestion happens only in the mouth. Chewing by the teeth is one site, but churning by the stomach muscles and emulsification of fat by bile in the duodenum are physical digestion too. All three break food into smaller pieces to increase surface area, without breaking any chemical bonds.
Flagged s23 P11/21 Q14 — physical digestion said to occur only in the mouth
Egestion ≠ excretion
Egestion removes undigested food (faeces) from the gut at the anus — that material was never absorbed into the body, so it is not a metabolic waste. Excretion removes the waste products of metabolism made inside cells ( from respiration, urea from the breakdown of excess amino acids). "Faeces are excreted at the anus" scores zero; they are egested.
Flagged s22 P31 Q6a; P32 Q6a — egestion (not excretion) occurs at the anus; many confused the two terms
Liver makes bile, gall bladder stores it
The liver produces bile; the gall bladder stores it and releases it into the duodenum through the bile duct. The pancreas produces none of it. On a diagram the gall bladder is the small pear-shaped sac tucked beneath the liver — candidates have labelled it as the liver itself, or as a kidney.
Flagged s23 P31 Q7a; s22 P33 Q3a — gall bladder / bile duct confused; gall bladder sometimes labelled as kidney or liver
Duodenum digests, ileum absorbs
Both are parts of the small intestine, but they are not interchangeable. The duodenum is the short, C-shaped first part that receives bile and pancreatic juice, so it is the main site of chemical digestion. The ileum is the long coiled part lined with villi, and it is the main site of absorption. The large intestine absorbs water only, not digested food.
Labelling the digestive system diagram
Learn the route: mouth → oesophagus → stomach → duodenum → ileum → large intestine → rectum → anus, with the liver, gall bladder and pancreas alongside. Liver = the large organ upper right; stomach = upper left; pancreas = the gland behind the stomach; gall bladder = the small sac beneath the liver.
Use the three-part scaffold for organ roles
For "describe the role of [organ]": the organ secretes [substance] → which acts on [substrate] → to give [product]. E.g. "the stomach secretes pepsin, a protease, which digests protein into polypeptides." "The stomach digests food" is too vague to score — name the secretion, the substrate and the product.
Name the vessel in absorption answers
Say where the absorbed molecules go, not just that they are absorbed: glucose and amino acids into the blood capillary of a villus, fatty acids and glycerol into the lacteal. For adaptation marks, pair every feature with its benefit — villi give a large surface area, a one-cell-thick wall gives a short diffusion distance.
Two marks for the stomach acid question
The acid in the stomach is hydrochloric acid — not lactic or amino acid. Its role carries two marks and candidates usually give only one: it provides the optimum acidic pH for pepsin, and it kills bacteria in food. Give both.
Ingestion: taking food into the body through the mouth.
Digestion: the breakdown of large, insoluble food molecules into small, soluble molecules. Occurs by two processes:
Absorption: movement of small, soluble molecules from the small intestine into the blood (glucose, amino acids) or lymph (fatty acids, glycerol).
Assimilation: the use of absorbed food molecules by body cells — e.g. glucose is used in respiration to release energy; amino acids are used to synthesise proteins.
Egestion: removal of undigested food (faeces) from the body at the anus. Egestion is NOT excretion — faeces were never absorbed into the body and are not metabolic waste products.
What is the difference between egestion and excretion? Give an example of each.
(a) Describe the role of the stomach in digestion. Include both physical and chemical digestion in your answer. [4 marks]
(b) State the role of hydrochloric acid in the stomach. Give two functions. [2 marks]
