Physical vs chemical digestion
Physical digestion breaks food into smaller pieces with no chemical change — chewing in the mouth, churning by the stomach muscles. No bonds break and no new substance forms; the purpose is to increase surface area for enzymes. Chemical digestion uses enzymes to break large, insoluble molecules into small, soluble ones that can be absorbed through the small-intestine wall into the blood — large insoluble molecules cannot be absorbed at all.
(Extended) The three digestive enzymes
Every enzyme name ends in "-ase" and each is specific to one food group: amylase will not touch protein, protease will not touch starch. Amylase: starch → simple (reducing) sugars, e.g. maltose; secreted by the salivary glands and the pancreas. Protease: protein → amino acids; secreted by the stomach (as pepsin) and the pancreas. Lipase: fats/oils → fatty acids and glycerol (both products are needed for the mark); secreted by the pancreas only.
(Extended) Hydrochloric acid and bile
Hydrochloric acid, from the stomach wall, has two jobs: it kills microorganisms in food, and gives the acidic pH stomach protease needs. Bile is made in the liver, stored in the gall bladder and released into the duodenum. It is not an enzyme: being alkaline it neutralises the acid arriving from the stomach so intestinal enzymes reach their optimum pH, and it emulsifies fats into small droplets, raising surface area for lipase.
Drawn from real examiner reports.
(Extended) Amylase's substrate is starch
The mark scheme wants starch. "Carbohydrate" is too vague — sugars are carbohydrates too; "glucose" is a product further down the pathway, not the substrate; "saliva" is the fluid that carries amylase, not the substance it acts on. The product is simple (reducing) sugars, e.g. maltose — not "glucose" alone. Fixed sentence: amylase breaks down starch into simple sugars.
Flagged Nov 2023 P42 Q7c
(Extended) Amylase has TWO secretion sites
Amylase is secreted from two sites: the salivary glands (into the mouth) and the pancreas (into the small intestine). Candidates name only the salivary glands, because "saliva contains amylase" is the familiar fact. Protease likewise has two — stomach and pancreas. Lipase has only one, the pancreas, so never assume every enzyme has the same number of sites.
Flagged Nov 2023 P41 Q7b
(Extended) Bile: liver ≠ gall bladder
Bile is made in the liver and stored in the gall bladder, which releases it into the duodenum. "Where is bile made?" wants the liver; "where is bile stored?" wants the gall bladder. Swapping the two loses the mark, and "bile is made in the gall bladder" is the commonest version of the error. Keep the pairing fixed: liver makes, gall bladder stores.
(Extended) The pancreas absorbs no water
The pancreas is purely secretory here: it releases amylase, protease and lipase into the small intestine. It does not absorb water — water absorption happens in the large intestine. Asked to pick the pancreas's function from a list, candidates often choose the water-absorption option instead.
Flagged Jun 2022 P11 Q6
(Extended) Bile is not an enzyme
Bile breaks no chemical bonds. Emulsification is physical: large fat droplets become many small droplets of the same substance, so surface area rises. The chemical breakdown of fat into fatty acids and glycerol is still done by lipase, which simply works faster once bile has enlarged the surface. "Bile digests fat" scores nothing.
(Extended) HCl and bile do opposite jobs
They have opposite pH effects at different points in the gut. HCl lowers the pH in the stomach, which is what stomach protease needs. Bile raises the pH in the duodenum, neutralising that same acid so the small intestine's enzymes reach their optimum. Describing them as doing the same job, or placing neutralisation inside the stomach, is wrong.
Physical digestion ≠ chemical digestion
Physical digestion changes only the size of the pieces — no new substances, no bonds broken. Chemical digestion uses enzymes to turn large insoluble molecules into small soluble ones, so new substances are formed. Emulsification by bile is physical, not chemical. "Breaking food into smaller pieces" alone is not a definition — "no chemical change" is a separate mark.
(Extended) Use the four-part enzyme scaffold
Answer any enzyme question in four parts: substrate (starch / protein / fats/oils) → enzyme (amylase / protease / lipase) → product (simple sugars / amino acids / fatty acids and glycerol) → site(s), both where it is secreted AND where it acts, which can differ.
Define with both elements, not one
Several definitions here carry two marking elements. Physical digestion needs "smaller pieces" and "no chemical change". Chemical digestion needs "large insoluble into small soluble" and "using enzymes". Lipase's product needs "fatty acids" and "glycerol".
Explain surface area as a chain
For "explain how physical digestion speeds up chemical digestion", write the chain: food broken into many smaller pieces → no bonds broken, molecules unchanged → total surface area greatly increased → more food surface in contact with enzymes → chemical digestion is faster.
(Extended) Give BOTH roles, not one
HCl and bile each have two roles, and the tariff usually pays for both. HCl: kills microbes and gives protease its acidic pH. Bile: neutralises stomach acid and emulsifies fats. Give one role only and you cap yourself at half the marks — and never call bile an enzyme.
Cambridge 0654 spec reference: Section B7 "Human nutrition", sub-topic B7.3 "Digestion". Covers (Core) physical digestion increasing surface area for chemical digestion, and chemical digestion breaking large insoluble molecules into small soluble molecules for absorption; and (Extended) the named substrate/product/secretion-site/action-site of amylase, protease and lipase, plus the roles of hydrochloric acid (HCl) and bile.
Physical digestion -- the breakdown of food into smaller pieces without any chemical change to the food molecules, which increases the surface area available for chemical digestion (e.g. chewing, churning by the stomach).
Chemical digestion -- the breakdown of large, insoluble food molecules into small, soluble molecules, using enzymes, so that they can be absorbed into the blood.
Emulsification -- the physical breakdown of large fat/oil droplets into many smaller droplets by bile, which increases their surface area for lipase to act on; this is not a chemical reaction, since no chemical bonds are broken.
Define "physical digestion".
(a) Define "physical digestion". (2 marks)
(b) Define "chemical digestion". (2 marks)
(c) State ONE way in which physical digestion helps chemical digestion to happen faster. (1 mark)
