The shape of the lesson
By the end of the lesson, learners can
- Where food and water are absorbed: Start the sim on one villus.
- Villi and microvilli: Extended.
- Inside a villus: Extended.
- Nutrients: absorbed in the small intestine.
- Water: most in the small intestine, some in the colon.
- Extended: villi and microvilli increase the internal surface area.
- Extended: capillaries take glucose and amino acids; the lacteal takes fatty acids and glycerol.
One picture every idea returns to
The small intestine is the region where nutrients are absorbed. Most water is absorbed from the small intestine, but some is also absorbed from the colon. Extended: villi and microvilli greatly increase the internal surface area of the small intestine; each villus has a one-cell-thick epithelium, a network of capillaries that absorb glucose and amino acids, and a central lacteal that absorbs fatty acids and glycerol.
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, Millions of fingers to grab the food.. Once food is digested into small soluble molecules, it must cross the wall of the gut into the blood. 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: Where food and water are absorbed | Start the sim on one villus. Project tab 5 of the lab (Absorption by villi) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Villi and microvilli | Extended. | Pairs |
| 21 to 30 min | Teach and check: Inside a villus | Extended. Project tab 5 of the lab (Capillaries and lacteal in one villus) and let a student drive it while the class predicts. | Cooperative Concept Mapping |
| 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, 19 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 Inside a villus, 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.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
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 |
|---|---|
| Most water is absorbed in the small intestine, not the colon. | The colon only absorbs some of the water that remains. |
| Fatty acids and glycerol go into the lacteal, not the blood capillaries. | Glucose and amino acids go into the capillaries. Keep the two routes separate. |
Absorption: worksheet
6 questions, 19 marks. Written in the style of the papers; no past paper question is reproduced.
State where in the alimentary canal (a) nutrients are absorbed and (b) water is absorbed.
Extended. Describe the structure of a villus.
Extended. Explain the significance of villi and microvilli in the small intestine.
Extended. Describe the roles of the capillaries and the lacteal in a villus.
Extended. A 1 cm2 area of flat intestine lining is folded into villi, which multiply the area by 10. The epithelial cells carry microvilli, which multiply the area by a further 20. (a) Calculate the final surface area. (b) A section of small intestine has a flat lining area of 2000 cm2. Calculate its absorbing surface area in m2. (1 m2 = 10 000 cm2)
Extended. Coeliac disease damages the villi so that they become short and flattened. Suggest and explain why people with untreated coeliac disease may lose body mass even though they eat a normal diet.
Answer key and marking notes
- (a) The small intestine.
- (b) Most water in the small intestine; some in the colon.
- A finger-like projection (about 1 mm long) of the lining of the small intestine.
- Covered by an epithelium one cell thick, whose cells have microvilli.
- Contains a dense network of blood capillaries.
- With a lacteal (lymph vessel) in the centre.
- Villi and microvilli greatly increase the internal surface area of the small intestine.
- So more molecules of digested food can be absorbed at the same time.
- Absorption is faster and more complete before the food leaves the small intestine.
- Capillaries absorb glucose and amino acids (and water and ions).
- The blood carries them away to the liver, keeping the concentration gradient steep.
- The lacteal absorbs fatty acids and glycerol.
- They are carried away in the lymph, which later drains into the blood.
- (a) 1 × 10 × 20 = 200 cm2.
- (b) 2000 × 200 = 400 000 cm2.
- 400 000 ÷ 10 000 = 40 m2.
- Flattened villi (and fewer microvilli) give a much smaller internal surface area.
- So fewer digested nutrients such as glucose, amino acids, fatty acids and glycerol are absorbed into the blood and lymph.
- Less energy and fewer raw materials reach the cells, so body stores are used up and mass is lost; more food leaves in the faeces.
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.
In which region of the alimentary canal are digested nutrients absorbed?
- ASmall intestine
- BStomach
- CColon
- DOesophagus
Which statement about the absorption of water is correct?
- AMost is absorbed in the small intestine; some is absorbed in the colon
- BAll of it is absorbed in the colon after digestion is complete
- CMost is absorbed in the stomach; the rest is absorbed in the rectum
- DNone is absorbed; it all leaves the body in the faeces
Why are most nutrients absorbed in the small intestine rather than in the stomach?
- AOnly small soluble molecules can be absorbed, and digestion is completed in the small intestine
- BThe stomach lining is too thin to let molecules pass through it
- CNutrients are only soluble after they have been mixed with bile in the colon
- DAbsorption needs acid, which is only found in the small intestine
After absorption, how is the concentration gradient for glucose between the small intestine and the blood kept steep?
- ABlood flow carries absorbed glucose away
- BGlucose is turned back into starch in the gut
- CThe intestine wall becomes thicker after a meal
- DBile adds more glucose to the blood
A person has severe diarrhoea, so food passes through the small intestine and colon very quickly. Which is the most likely effect?
- ALess water and fewer nutrients are absorbed, so the person may become dehydrated
- BMore water is absorbed, so the faeces become hard and dry
- CMore nutrients are absorbed, because food moves past more villi per minute
- DDigestion in the stomach stops, so no protein is digested at all
Extended: the diagram shows a villus. Which row correctly names X, Y and Z?
- AX: epithelium one cell thick; Y: capillary; Z: lacteal
- BX: lacteal; Y: capillary; Z: epithelium one cell thick
- CX: epithelium one cell thick; Y: lacteal; Z: capillary
- DX: microvillus; Y: lacteal; Z: vein to the liver
Extended: what is the role of the lacteal in a villus?
- ATo absorb fatty acids and glycerol and carry them away in the lymph
- BTo absorb glucose and amino acids and carry them to the liver
- CTo secrete maltase onto the surface of the villus
- DTo store bile until it is needed for emulsifying fats
Extended: 1 cm2 of the inner wall of the small intestine is folded into villi, giving 10 cm2 of surface. Microvilli then increase the surface of every cell 20 times. What is the final surface area of this 1 cm2 of wall?
- A200 cm2
- B30 cm2
- C20 cm2
- D2000 cm2
Quiz key
The small intestine is the region where digested nutrients are absorbed into the blood. The stomach digests protein but absorbs very little. The colon absorbs some water but not the bulk of the nutrients. The oesophagus only moves food to the stomach.
Most water, from drinks, food and digestive juices, is absorbed in the small intestine, and some more is absorbed in the colon. Saying all of it is absorbed in the colon is a common misconception. The stomach absorbs very little water and the rectum only stores faeces. Faeces contain some water, but most of the water is absorbed.
Large food molecules must first be digested into small soluble molecules, and this is completed in the small intestine, where they are then absorbed. The stomach wall is thick and muscular, not too thin. Bile acts in the duodenum, not the colon. The small intestine is alkaline, not acidic; acid is found in the stomach.
Flowing blood carries glucose away as soon as it is absorbed, keeping its concentration in the blood low and the gradient steep. Glucose is not changed back into starch in the gut. A thicker wall would increase the diffusion distance and slow absorption. Bile contains no glucose.
When food passes very quickly, there is less time for water and nutrients to be absorbed in the small intestine and colon, so a lot of water is lost in the faeces and the person may become dehydrated. Hard, dry faeces result from slow movement, not fast. Passing more villi per minute does not help when each region has less time to absorb. Stomach digestion is not stopped by diarrhoea.
X is the outer layer, the epithelium one cell thick. Y is the network of blood capillaries looping around the centre. Z is the central lacteal. Naming the outer layer as the lacteal, or swapping the capillary and lacteal, mixes up their positions. Microvilli are far too small to see at this scale, and Z is in the centre of the villus, not a vein.
The lacteal absorbs fatty acids and glycerol and carries them away in the lymph, which later drains into the blood. Glucose and amino acids are absorbed into the blood capillaries and carried to the liver. Maltase is held on the membranes of the epithelial cells, not secreted by the lacteal. Bile is stored in the gall bladder.
The villi give 10 cm2, and microvilli multiply this by 20: 10 × 20 = 200 cm2. Adding the factors (10 + 20) gives 30 cm2, which is wrong because the effects multiply. 20 cm2 ignores the villi. 2000 cm2 multiplies by an extra factor of 10.
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.