The shape of the lesson
By the end of the lesson, learners can
- Four kinds of teeth: The sim opens on the lower jaw of an adult.
- Inside a tooth: Switch the sim view to inside a tooth: a molar cut in half.
- Churning and emulsifying: Physical digestion does not stop at the mouth.
- Physical digestion: smaller pieces, no chemical change, larger surface area for enzymes.
- Teeth: incisors cut, canines tear, premolars and molars chew and grind.
- Tooth layers: enamel, dentine, pulp (nerves, blood vessels), cement; embedded in jaw bone and gum.
- Extended: bile emulsifies fat; it is not an enzyme.
One picture every idea returns to
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.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Smaller pieces, more surface.. Before enzymes can do much, food has to be broken up. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Four kinds of teeth | The sim opens on the lower jaw of an adult. Project tab 3 of the lab (Teeth and physical digestion) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Inside a tooth | Switch the sim view to inside a tooth: a molar cut in half. | Pairs |
| 21 to 30 min | Teach and check: Churning and emulsifying | Physical digestion does not stop at the mouth. Project tab 4 of the lab (Churning in the stomach and bile in the duodenum) and let a student drive it while the class predicts. | Quiz-Quiz-Trade |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 18 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Churning and emulsifying, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Quiz-Quiz-Trade
Each student holds a card with a question on the front and the answer on the back. They pair up, quiz each other, coach if needed, trade cards and find a new partner. Three rounds covers the whole set.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Physical digestion does not produce smaller molecules. | Only chemical digestion by enzymes breaks large molecules into small ones. |
| Bile contains no enzymes and does not digest fat. | It emulsifies fat, a physical change that increases the surface area for lipase. |
Physical digestion: worksheet
6 questions, 18 marks. Written in the style of the papers; no past paper question is reproduced.
Describe what is meant by physical digestion and state why it is important.
Name the four types of human teeth and state the function of each.
A diagram shows a vertical section through a molar. Describe the structure of the tooth from the outside of the crown to the centre, and state how the tooth is held in place.
Describe the role of the stomach in physical digestion.
Extended. A drop of oil is a cube of side 2 mm. Bile emulsifies it into cubes of side 0.5 mm. (a) Calculate the surface area of the original drop. (b) Calculate the total surface area of the small droplets. (c) Use your answers to explain why emulsification speeds up fat digestion.
Extended. A student says that bile digests fat. Explain why this is incorrect.
Answer key and marking notes
- 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.
- Incisors: biting and cutting food.
- Canines: tearing and gripping food.
- Premolars: chewing and grinding food.
- Molars: chewing and grinding food into small pieces.
- The crown is covered by hard enamel.
- Under it is dentine, which forms most of the tooth.
- The centre is the pulp cavity, containing nerves and blood vessels.
- The root is covered by cement and embedded in a socket in the jaw bone, with the gum around the neck.
- The muscular wall of the stomach contracts to churn the food.
- This breaks food into smaller pieces (without chemical change).
- And mixes it with gastric juice, increasing the surface area in contact with the enzymes.
- (a) 6 × 2 × 2 = 24 mm2.
- (b) Number of droplets = (2 ÷ 0.5)3 = 64; each has area 6 × 0.5 × 0.5 = 1.5 mm2; total = 64 × 1.5 = 96 mm2 (four times larger).
- (c) Lipase acts on the surface of the fat, so a larger surface area lets more lipase molecules act at once, and fat is digested faster.
- Bile contains no enzymes and does not change the fat molecules; it emulsifies fat into small droplets, which is physical digestion.
- The chemical digestion of fat into fatty acids and glycerol is done by the enzyme lipase.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which statement describes physical digestion?
- AFood is broken into smaller pieces without chemical change to its molecules
- BLarge insoluble molecules are broken into small soluble molecules
- CEnzymes break the bonds in starch, protein and fat molecules
- DSmall molecules pass from the intestine into the blood
Which type of tooth is chisel-shaped and used for biting and cutting food?
- AIncisor
- BCanine
- CPremolar
- DMolar
Which part of a tooth contains nerves and blood vessels?
- APulp cavity
- BEnamel
- CDentine
- DCement
Why does chewing food speed up its chemical digestion?
- ASmaller pieces have a larger surface area for enzymes to act on
- BChewing changes starch molecules into maltose molecules
- CChewing raises the food to the optimum temperature for enzymes
- DSmaller pieces contain more enzyme molecules per gram
Which describes the role of the stomach in physical digestion?
- AIts muscular wall churns food and mixes it with gastric juice
- BIts acid breaks down protein molecules into amino acids
- CIts enzymes break fat drops into tiny droplets
- DIts lining absorbs water to make the food more solid
Extended: what does bile do to fats and oils in the small intestine?
- AEmulsifies them into small droplets, increasing their surface area
- BBreaks them down into fatty acids and glycerol
- CAbsorbs them into the blood capillaries of the villi
- DMakes them acidic so that lipase can act on them
Extended: a fat drop is a cube of side 4 mm. Bile breaks it into cubes of side 1 mm. By how many times does the total surface area increase?
- A4 times
- B16 times
- C64 times
- D2 times
An elderly person has lost all their molars but still has their incisors. Which food would they find hardest to digest physically?
- ARaw carrots and nuts that need grinding
- BSoup and other liquids
- CSoft mashed potato
- DA thin slice of soft bread
Quiz key
Physical digestion breaks food into smaller pieces without changing the food molecules. Breaking large insoluble molecules into small soluble ones, and enzymes breaking bonds in molecules, both describe chemical digestion. Small molecules moving into the blood is absorption.
Incisors are chisel-shaped front teeth with a sharp edge for biting and cutting. Canines are pointed for tearing and gripping. Premolars and molars are broad, with cusps, for chewing and grinding.
The pulp cavity is the soft centre of the tooth containing nerves and blood vessels. Enamel is the hard outer layer of the crown. Dentine is the hard layer beneath the enamel. Cement is a thin bony layer covering the root that helps fix it into the jaw bone.
Breaking food into smaller pieces greatly increases its total surface area, so more enzyme molecules can act on the food at once. Changing starch into maltose is chemical digestion by amylase, not chewing. Chewing does not noticeably change the temperature of food. Food does not supply the digestive enzymes; they are secreted by the body.
Muscles in the stomach wall contract to churn the food, breaking it into smaller pieces and mixing it with gastric juice. Breaking down protein is chemical digestion by protease, not by the acid itself. Breaking fat into droplets is emulsification by bile in the small intestine. The stomach is not a main site of water absorption.
Bile emulsifies fats and oils, breaking large drops into many small droplets with a much larger total surface area for lipase. Breaking fats into fatty acids and glycerol is chemical digestion by lipase. Bile does not absorb fats; fatty acids and glycerol are absorbed into the lacteals. Bile is alkaline, not acidic.
The big cube has area 6 × 4 × 4 = 96 mm2. It makes 4 × 4 × 4 = 64 small cubes of 6 mm2 each, a total of 384 mm2; 384 ÷ 96 = 4 times. 16 times is the area of one face of the big cube, not the change. 64 times is the number of droplets, not the change in area. 2 times underestimates the effect of cutting each side into four.
Molars chew and grind hard food into small pieces, so without them raw carrots and nuts cannot be ground well. Soup needs no chewing. Mashed potato is already in very small pieces. Soft bread can be bitten off with the incisors and is easily broken up by the tongue and by churning in the stomach.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.