Lesson overview
The gut and physical digestion: Why do we chew our food?
Cut one cube of egg white into eight, mash another into crumbs and put them in protease beside a whole cube, then time how long each takes to go. Work out the surface area, match the tools to your teeth and follow food along the gut.
Suggested sequence
- Lesson 1 · Why we chew. Physical digestion and why smaller pieces of food are digested faster by enzymes. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Teeth. The four types of human teeth, the structure of a tooth and what each type does. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- Lesson 3 · Along the gut. The organs of the digestive system, the five processes of nutrition, the churning stomach and bile. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- End of module · final check. 10 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 10 questions, 15 min, lesson closed.
Learning goals
Part 1: Why we chew
- Describe physical digestion as the breakdown of food into smaller pieces without chemical change to the food molecules (7.3.1)
- State that physical digestion increases the surface area of food for the action of enzymes in chemical digestion (7.3.2)
Part 2: Teeth
- Identify in diagrams and images the types of human teeth: incisors, canines, premolars and molars (7.3.3)
- Describe the structure of human teeth: enamel, dentine, pulp, nerves, blood vessels and cement, embedded in the jaw bone and the gums (7.3.4)
- Describe the functions of the types of human teeth in physical digestion of food (7.3.5)
Part 3: Along the gut
- Identify in diagrams and images the main organs of the digestive system: the alimentary canal and the associated organs (7.2.1)
- Describe the functions of the organs of the digestive system in ingestion, digestion, absorption, assimilation and egestion (7.2.2)
- Describe the function of the stomach in physical digestion (7.3.6)
- Supplement: outline the role of bile in emulsifying fats and oils to increase the surface area for chemical digestion (S7.3.7)
Misconceptions to listen for
- “Physical digestion makes new, smaller molecules.” — Only enzymes break molecules down; that is chemical digestion. Physical digestion only makes the pieces smaller.
- “The eight small cubes have the same surface as the whole cube, because it is the same egg white.” — The mass is the same, but every cut makes new faces, so the total surface area grows.
- “The crumbs were lighter.” — All three samples had the same mass: one cube of egg white each.
- “Mashing changed the protein into a different substance.” — Mashing is physical digestion: the crumbs are made of the same protein molecules as before.
- “A change in how food looks is a chemical change.” — A change of shape or size is physical. A chemical change makes new substances.
- “Cutting a cube into 27 pieces makes 27 times as much surface.” — Only the new faces are added. Here the surface becomes 3 times larger: the cubes are one third as wide.
- “Churning in the stomach is chemical, because it happens inside the body.” — Where it happens does not decide it. Churning only breaks food into smaller pieces, so it is physical.
- “The small cubes are digested faster because the protease gets inside them.” — The protease cannot get inside a solid piece: it works on the surface, and the small cubes have more of it.
- “Mashed food has more nutrients in it.” — Mashing does not add nutrients: it only makes the pieces smaller, so enzymes digest them faster.
- “A control is not needed, because everybody knows water does not digest egg white.” — A fair test checks it: the control shows that the change is caused by the enzyme and not by something else in the tube.
Scope
The organs of the digestive system and the associated organs (7.2.1) and the five processes of ingestion, digestion, absorption, assimilation and egestion (7.2.2); physical digestion and surface area (7.3.1, 7.3.2) through the egg-white practical; the types, structure and functions of human teeth (7.3.3–7.3.5); the churning stomach (7.3.6) and bile emulsifying fats (Supplement 7.3.7). Named digestive enzymes and their products are in the chemical digestion module, villi and absorption in a later module; tooth decay is not covered. Times in the experiment come from a teaching model.