The shape of the lesson
By the end of the lesson, learners can
- Large insoluble to small soluble: Choose a region of the gut in the sim and press Run digestion.
- Where enzymes are made and where they act: An enzyme is often secreted in one place and acts in another: pancreatic juice is made in the pancreas but works in the duodenum.
- Maltase, pepsin, trypsin and bile: Extended.
- Amylase: starch to simple reducing sugars (maltose); salivary glands and pancreas.
- Proteases: protein to amino acids; stomach and pancreas. Lipase: fats to fatty acids and glycerol; pancreas.
- HCl: kills microorganisms and gives the acidic optimum pH for the stomach protease.
- Extended: pepsin in acid, trypsin in alkali; maltase on the epithelium; alkaline bile neutralises acid.
One picture every idea returns to
Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules, which can then be absorbed. Amylase breaks down starch to simple reducing sugars, proteases break down protein to amino acids and lipase breaks down fats and oils to fatty acids and glycerol. Hydrochloric acid in gastric juice kills harmful microorganisms in food and provides an acidic pH for the stomach protease.
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, Enzymes cut big molecules small.. Starch, proteins and fats are far too large to cross the wall of the gut. 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: Large insoluble to small soluble | Choose a region of the gut in the sim and press Run digestion. Project tab 4 of the lab (Chemical digestion along the gut) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Where enzymes are made and where they act | An enzyme is often secreted in one place and acts in another: pancreatic juice is made in the pancreas but works in the duodenum. | Pairs |
| 21 to 30 min | Teach and check: Maltase, pepsin, trypsin and bile | Extended. | Anticipation / Reaction Guide |
| 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, 21 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 Maltase, pepsin, trypsin and bile, 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.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
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 |
|---|---|
| Say "proteases break down protein to amino acids ", not "to peptides" or "to glucose". | And amylase produces simple reducing sugars (maltose), not glucose directly. |
| Hydrochloric acid is not an enzyme and does not digest protein itself. | Its jobs are to kill microorganisms and to provide the optimum pH for the protease. |
| Pepsin and trypsin are both proteases, but each has a different optimum pH. | Pepsin would be denatured or inactive in the alkaline duodenum, and trypsin in the acid stomach. |
Chemical digestion: worksheet
6 questions, 21 marks. Written in the style of the papers; no past paper question is reproduced.
Describe what is meant by chemical digestion and state why it is needed.
Copy and complete a table for amylase, protease and lipase, giving for each the substrate, the products, one place where it is secreted and one place where it acts.
Describe the functions of hydrochloric acid in gastric juice.
The activity of two proteases was measured at different pH values.
| pH | 1 | 2 | 4 | 6 | 8 | 10 |
|---|---|---|---|---|---|---|
| Protease X activity / % | 70 | 100 | 45 | 5 | 0 | 0 |
| Protease Y activity / % | 0 | 0 | 10 | 55 | 100 | 60 |
Extended. (a) State the optimum pH of each enzyme. (b) Identify which enzyme is pepsin and which is trypsin, and explain your choice.
Extended. Describe the complete digestion of starch to glucose in the alimentary canal.
Extended. A person has their gall bladder removed, so bile is released into the duodenum only in small amounts. Suggest and explain how this affects digestion in the small intestine.
Answer key and marking notes
- The breakdown of large insoluble molecules into small soluble molecules.
- By enzymes.
- So that the small soluble molecules can be absorbed through the wall of the small intestine into the blood.
- Amylase: starch to simple reducing sugars (maltose).
- Amylase: secreted by the salivary glands (or pancreas); acts in the mouth (or duodenum).
- Protease: protein to amino acids.
- Protease: secreted by the stomach (or pancreas); acts in the stomach (or small intestine).
- Lipase: fats and oils to fatty acids and glycerol.
- Lipase: secreted by the pancreas; acts in the small intestine (duodenum).
- It kills harmful microorganisms in the food.
- It provides an acidic pH, the optimum for the protease (pepsin) in the stomach.
- (a) X: pH 2. Y: pH 8.
- (b) X is pepsin: its optimum is acidic, matching the stomach, where hydrochloric acid gives about pH 2.
- Y is trypsin: its optimum is alkaline, matching the small intestine.
- There the stomach acid has been neutralised by alkaline bile (and pancreatic juice).
- Amylase from the salivary glands (in the mouth) and from the pancreas (in the duodenum) breaks starch down to maltose.
- Maltase breaks maltose down to glucose.
- Maltase acts on the membranes of the epithelium lining the small intestine.
- Less bile to neutralise the acidic mixture arriving from the stomach, so the pH in the duodenum is less suitable and the enzymes there (trypsin, amylase, lipase) work more slowly.
- Fats are less well emulsified, so there is a smaller surface area for lipase.
- So fat digestion in particular is slower and less complete (fatty meals may cause discomfort).
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.
What are the products of the digestion of fats and oils by lipase?
- AFatty acids and glycerol
- BAmino acids
- CSimple reducing sugars
- DSmall fat droplets
Where is amylase secreted?
- ASalivary glands and pancreas
- BStomach and liver
- CLiver and gall bladder
- DStomach and pancreas
Which are both functions of the hydrochloric acid in gastric juice?
- AKilling harmful microorganisms and giving an acidic pH for the protease
- BBreaking down protein and emulsifying fats
- CNeutralising bile and killing harmful microorganisms
- DBreaking down starch and giving an acidic pH for amylase
Why must protein be chemically digested before the body can use it?
- AProtein molecules are too large and insoluble to be absorbed
- BProtein molecules are too small to be held in the gut
- CProtein is poisonous until it is broken into amino acids
- DProtein can only be absorbed in the stomach after digestion
A student mixed starch with water, with amylase, and with boiled amylase, kept the tubes at 37 °C for 10 minutes and then tested them. What do the results show?
- AAmylase digests starch to reducing sugar, but not after boiling
- BAmylase turns reducing sugar into starch, even after boiling
- CWater alone digests starch to reducing sugar at 37 °C
- DBoiled amylase digests starch faster than active amylase
Extended: the graph shows the activity of two proteases, P and Q, at different pH values. Which row identifies them correctly?
- AP is pepsin from the stomach; Q is trypsin from the pancreas
- BP is trypsin from the pancreas; Q is pepsin from the stomach
- CP is pepsin from the pancreas; Q is trypsin from the stomach
- DP is amylase from the mouth; Q is lipase from the pancreas
Extended: where does maltase act on maltose?
- AOn the membranes of the epithelium lining the small intestine
- BIn the saliva, as soon as food enters the mouth
- CIn the gastric juice in the stomach
- DInside the lacteals of the villi
Extended: why is it important that bile is alkaline?
- AIt neutralises stomach acid, giving a suitable pH for enzymes in the duodenum
- BIt makes the stomach contents more acidic so that pepsin works faster
- CIt kills harmful microorganisms that survive the stomach acid
- DIt breaks down fats chemically into fatty acids and glycerol
Quiz key
Lipase breaks down fats and oils to fatty acids and glycerol. Amino acids come from protein digested by proteases. Simple reducing sugars come from starch digested by amylase. Small fat droplets are the result of emulsification by bile, which is physical and does not change the molecules.
Amylase is secreted by the salivary glands (into the mouth) and the pancreas (into the duodenum). The stomach secretes protease and acid, not amylase. The liver makes bile and the gall bladder stores it; bile contains no enzymes. The stomach and pancreas both secrete protease, so that pair belongs to protein digestion.
Hydrochloric acid kills harmful microorganisms in food and provides the acidic pH that is the optimum for the stomach protease. The acid does not digest protein itself, and emulsifying fats is done by bile. It does not neutralise bile; bile neutralises the acid later, in the duodenum. Amylase does not work well at acidic pH, and the acid does not break down starch.
Protein molecules are large and insoluble, so they cannot pass through the wall of the small intestine; digestion to small soluble amino acids allows absorption. Protein molecules are large, not small. Protein in food is not poisonous. Amino acids are absorbed in the small intestine, not the stomach.
Only tube 2 (active amylase) has lost its starch (iodine stays orange-brown) and gained reducing sugar (Benedict's brick red), so amylase digests starch to reducing sugar. In tube 3 the boiled enzyme was denatured, so the starch remained. The reaction goes from starch to sugar, not the reverse. Tube 1 shows that water alone does nothing. Boiled amylase did not digest any starch.
P has its optimum near pH 2, matching the acidic stomach, so it is pepsin, secreted by the stomach. Q has its optimum near pH 8, matching the alkaline small intestine, so it is trypsin, secreted by the pancreas. Swapping them ignores the optimum pH values. Pepsin is not made by the pancreas, nor trypsin by the stomach. Both enzymes are proteases, so amylase and lipase are wrong.
Maltase is held on the membranes of the epithelial cells lining the small intestine, where it breaks maltose into glucose, which is then absorbed straight away. Saliva contains amylase, which makes maltose rather than breaking it down. Gastric juice contains pepsin, a protease. Lacteals carry fatty acids and glycerol away in the lymph; no digestion happens there.
The mixture of food and gastric juice entering the duodenum is acidic; alkaline bile neutralises it so that the small intestine enzymes, such as trypsin and lipase, work at a suitable pH. Bile does not act in the stomach, and it makes conditions less acidic, not more. Killing microorganisms is a job of the stomach acid. Breaking fats into fatty acids and glycerol is done by lipase; bile only emulsifies them.
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.