Before enzymes can do much, food has to be broken up. Teeth cut, tear and grind it, the stomach churns it and, in the small intestine, bile breaks fat into tiny droplets. None of this changes the food molecules; it just gives enzymes far more surface to work on. The role of bile is Extended content.
Physical digestion is the breakdown of food into smaller pieces without chemical change to the food molecules. It increases the surface area of the food for the action of enzymes in chemical digestion. Teeth start it, the muscular stomach wall continues it by churning and (Extended) bile emulsifies fats into small droplets.
Fig. 1: section through a molar: enamel covers the crown, dentine forms most of the tooth, the pulp holds nerves and blood vessels, and cement fixes the root into the jaw bone
Section 01
Four kinds of teeth.
The sim opens on the lower jaw of an adult. Click each kind of tooth to see its job. On each side there are two incisors, one canine, two premolars and three molars.
Tooth
Position and shape
Function in physical digestion
Incisors
front; chisel-shaped, sharp edge
biting and cutting food
Canines
beside the incisors; pointed
tearing and gripping food
Premolars
behind the canines; broad with two cusps
chewing and grinding
Molars
back; largest, several cusps
chewing and grinding food into small pieces
No chemical change: a chewed piece of bread is still starch, just in smaller pieces.
Why it matters: smaller pieces have a much larger total surface area, so enzymes such as amylase can act on more of the food at once and digestion is faster.
Examiner trap
Physical digestion does not produce smaller molecules. Only chemical digestion by enzymes breaks large molecules into small ones.
Section 02
Inside a tooth.
Switch the sim view to inside a tooth: a molar cut in half. Click each layer from the outside in.
Enamel: the hardest substance in the body; covers the crown and resists wear while chewing.
Dentine: hard, bone-like tissue under the enamel that forms most of the tooth.
Pulp (pulp cavity): soft centre containing nerves, which detect pressure, heat and pain, and blood vessels, which bring oxygen and nutrients to the living cells.
Cement: thin bony layer on the root that, with fibres, anchors the tooth in its socket.
Embedded: the root sits in a socket in the jaw bone, and the gum covers the bone around the neck of the tooth.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the jaw view of the sim, how many molars are on each side of the lower jaw?
Quick check+10 XP
Which kind of tooth does the sim describe as pointed, for tearing and gripping food?
Quick check+10 XP
In the inside-a-tooth view, which layer contains the nerves and blood vessels?
Quick check+10 XP
Which layer does the sim call the hardest substance in the body?
Section 03
Churning and emulsifying.
Physical digestion does not stop at the mouth. In the chemical digestion sim choose the stomach and press Run digestion: the mixture is churned. Then choose the duodenum and watch the large yellow fat drop break into many small droplets.
Stomach: muscles in the stomach wall contract to churn the food, breaking it into smaller pieces and mixing it with gastric juice, so the enzymes reach more of the food.
Extended.Bile emulsifies fats and oils: it breaks large fat drops into many tiny droplets. This is physical: the fat molecules are not changed.
Extended.Why: many tiny droplets have a far larger total surface area than one big drop, so lipase can act on the fat much faster.
Examiner trap
Bile contains no enzymes and does not digest fat. It emulsifies fat, a physical change that increases the surface area for lipase.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which statement describes physical digestion?
Exam style+20 XP
Why does chewing food help chemical digestion?
Exam style+20 XP
Extended: what is the role of bile in the digestion of fats?
Exam style+20 XP
Which tissue fixes the root of a tooth into the jaw bone?
Skill unlocked
Physical digestion, mastered.
This skill is now recorded on your profile. Keep the chain going.