Teacher sheet · Ages 15–16 · about 45–50 minutes
Enzymes, digestion and absorption
Lesson page: echoesof.science/school/digestion/ages-15-16/ · Pupil worksheet: echoesof.science/school/digestion/ages-15-16/print/
Learning aims
- Distinguish mechanical and chemical digestion and name the main enzymes, substrates and products.
- Explain emulsification and the adaptations of the small intestine for absorption.
- Carry out the amylase–pH practical, calculate rates and evaluate uncertainty.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: why can't a starch molecule pass into the blood? |
| 10 min | Read the text; enzyme-substrate-product table. |
| 25 min | Amylase and pH practical. |
| 5 min | Rate calculations and graph. |
| 5 min | Quiz or sorting game. |
Materials for the experiment
- 1% amylase solution (prepared by the technician)
- 1% starch solution, freshly made
- Buffer solutions (for example pH 4, 5, 6, 7, 8)
- 0.01 M iodine solution
- Spotting (dimple) tile, test tubes, 1 cm³ and 5 cm³ syringes or pipettes
- Stopwatch
- Eye protection
Safety: Wear eye protection: iodine irritates the eyes and universal buffer is an irritant. Use amylase only as a dilute solution prepared by the technician (the powder is harmful). Wash off any splashes with water. Follow your school's risk assessment.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Digestion only happens in the stomach. | Digestion starts in the mouth, and most chemical digestion and absorption happens in the small intestine. |
| Bile is an enzyme that digests fat. | Bile emulsifies fat; lipases digest it. |
| Enzymes are used up in the reaction. | Enzymes are catalysts: the same enzyme molecule can act on many substrate molecules. |
Worksheet answers
- Starch → amylase → maltose → maltase → glucose (monosaccharides); protein → pepsin/trypsin → peptides (and finally amino acids); fat → lipase → fatty acids and monoglycerides (glycerides).
- Long (about 3 m in life), with circular folds, villi (20–40 per mm²) and microvilli (about 200 million per mm²) giving a surface more than 600 times larger, about 200 m²; this lets nutrients pass into blood and lymph quickly.
- Example: pH 4 about 180 s (0.0056 s⁻¹), pH 6 about 60 s (0.017 s⁻¹), pH 8 about 150 s (0.0067 s⁻¹): the rate peaks near pH 6 and falls on either side.
- Sampling every 10 s gives up to about ±10 s uncertainty, and judging 'orange' is subjective; sample more often (e.g. every 5 s), use a colour chart, repeat and take means.
Quiz answers
- What does salivary amylase produce from starch? Maltose. Amylase breaks starch into the disaccharide maltose.[4]
- Why does bile speed up fat digestion? It emulsifies fat, increasing the surface area for lipase. Bile breaks fat into many small droplets.[4]
- What neutralises acidic chyme in the small intestine? Bicarbonate from the pancreas. The pancreas secretes bicarbonate.[2]
- By roughly how much do folds, villi and microvilli increase the absorptive surface? More than 600-fold. The total surface is about 200 m².[3]
- How do digested fats mainly leave the small intestine? As chylomicrons into lymph capillaries. Fats enter lymph; sugars and amino acids enter the blood.[4][1]
- In the amylase experiment, the end point is reached when the iodine… stays orange. Orange means no starch is left.[5]
Going further
Repeat the investigation at different temperatures with the same pH. Explain why the rate falls at high temperatures, and how you would control the pH.
Sources used in this lesson
- [1] Your Digestive System & How It Works (National Institute of Diabetes and Digestive and Kidney Diseases (NIH, USA)): reliability High
- [2] Concepts of Biology, 16.2 Digestive System (OpenStax, Rice University): reliability High
- [3] Anatomy and Physiology, 23.5 The Small and Large Intestines (OpenStax, Rice University): reliability High
- [4] Biology for AP Courses, 25.3 Digestive System Processes (OpenStax, Rice University): reliability High
- [5] Investigating the effect of pH on amylase activity (Practical Biology (Royal Society of Biology, with Nuffield Foundation and CLEAPSS)): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/digestion/sources/
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