Physical digestion — pieces, not molecules
Physical digestion is the mechanical breakdown of food into smaller pieces with no chemical change to the molecules. Its purpose is to increase the surface area available for enzyme action, so chemical digestion runs faster. Three sites: the mouth (teeth chew, tongue forms a bolus), the stomach (muscular walls churn food into chyme) and — Extended only — the small intestine (bile emulsifies fat). No enzymes are involved.
Four tooth types, four jobs
Incisors (8) are flat and chisel-shaped for cutting; canines (4) are long and pointed for tearing tough or fibrous food; premolars (8) and molars (12, including wisdom teeth) are broad and cusped for grinding and crushing. Shape explains function — a thin sharp edge concentrates force onto a small area and slices, while a broad bumpy surface gives a large contact area and grinds food to a paste. Together they leave food in small pieces with a large surface area.
(Extended) Bile emulsifies, lipase digests
Bile is an alkaline fluid made in the liver, stored in the gall bladder and delivered to the duodenum by the bile duct. It emulsifies fat — one large globule becomes thousands of tiny droplets. This is physical: the triglyceride molecules are unchanged. Lipase (from the pancreas) works only at a droplet's surface, so the far larger surface area gives faster chemical digestion of fat into fatty acids and glycerol.
Drawn from real examiner reports.
Smaller pieces, not smaller molecules
The most heavily penalised error on this sub-topic. "Physical digestion breaks large molecules into small molecules" describes chemical digestion and scores zero however good the rest of the answer is. Physical digestion changes only the size and shape of food pieces. Say instead: smaller pieces, no chemical change, increased surface area for enzyme action.
Flagged s23 P11/21 Q14–15 · w22 P41 Q1a i
Physical digestion is not just the mouth
Asked where physical digestion happens, most candidates name chewing and stop. The stomach also churns food with its muscular walls, mixing it with gastric juice into chyme — mechanical breakdown too. On Extended, bile emulsifying fat in the duodenum is a third site. If a question asks for a site other than the mouth, give the stomach.
Flagged s23 P11/21 Q14
Bile is not an enzyme
Bile is an alkaline fluid, not a biological catalyst — it catalyses no reaction. "Bile is a digestive enzyme" scores zero and often contaminates the rest of the answer. The enzyme that acts on fat is lipase, secreted by the pancreas. Bile prepares the fat; lipase digests it.
Flagged s23 P41 Q1b i
Liver makes bile; gall bladder stores it
Writing that bile is "made in the gall bladder" loses the mark. The liver produces bile, the gall bladder only stores and concentrates it, and the bile duct delivers it to the duodenum. The same three structures are muddled on labelling questions — the gall bladder is the small sac, the bile duct the tube leading from it.
Flagged s22 P33 Q3a · s23 P31 Q7a
Emulsifying is not digesting fat
"Bile breaks fat into fatty acids and glycerol" is wrong twice over — those are lipase's products, made by chemical digestion. Emulsification produces smaller droplets of the same fat; the triglyceride molecules survive intact. Order matters in the answer: bile first (increases surface area), then lipase (breaks the molecules).
Flagged s23 P41 Q1b i
Physical digestion uses no enzymes
Teeth, stomach muscle and bile need no catalyst — physical digestion is purely mechanical. Answers that have enzymes "helping the teeth break the food up" have merged the two processes. Enzymes belong to chemical digestion only; what physical digestion does is expose more surface so those enzymes work faster.
The three-part definition
Mark schemes reward, in order: (1) mechanical breakdown into smaller pieces; (2) no chemical change; (3) increased surface area for enzyme action. Write all three even on a 2-mark question — a missing surface-area clause is the usual reason a definition falls short.
Tooth answers need shape then function
"Explain how molars are adapted" wants the chain, not the label: broad multi-cusped surface, so a large contact area with food, so it grinds to a fine paste, so more surface area for enzymes. Naming the function alone earns at most half the marks.
(Extended) Bile in three steps
For "describe the role of bile in fat digestion": (1) bile emulsifies fat; (2) large globules become tiny droplets, not molecules; (3) this increases the surface area available for lipase. Never write that bile is an enzyme or that it chemically digests fat.
MCQ rule: molecules or pieces?
Scan each option for what changes. Molecules changing means chemical digestion; a size or shape change only means physical. Reject on sight any option saying bile is an enzyme, that bile alters fat's chemical structure, or that physical digestion happens only in the mouth.
Physical digestion: the mechanical breakdown of food into smaller pieces without any chemical change to the molecules, thereby increasing the surface area available for enzyme action.
Definition trap — the single most common error (s23 Papers 11/21 Q14–15): physical digestion does NOT break large molecules into small molecules. That is chemical digestion. Physical digestion only changes the size and shape of food pieces, leaving the chemical composition of the molecules intact.
State the function of each of the four human tooth types: incisors, canines, premolars, molars.
(a) State what is meant by physical digestion.
(b) State TWO sites in the human body where physical digestion occurs, and for each site name the mechanism involved.
