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Digestion is both mechanical (chewing, churning, peristalsis) and chemical (enzymes). Salivary amylase begins breaking starch into maltose; in the stomach (pH 1.5–2.5) pepsin breaks proteins into peptides.

The pancreas secretes amylase, protein-digesting enzymes such as trypsin, and lipases, plus bicarbonate that neutralises acidic chyme.

Bile, made in the liver and stored in the gallbladder, emulsifies fat into many small droplets, increasing the surface area for lipases, which produce fatty acids and monoglycerides.

Experiment: How does pH affect amylase?

1. Put single drops of iodine into each dimple of the spotting tile.
2. In a labelled test tube, mix 2 cm³ of amylase with 1 cm³ of one buffer.
3. Add 2 cm³ of starch, mix and start the stopwatch.
4. Every 10 seconds, put one drop of the mixture onto a fresh iodine drop. Blue-black means starch is still present; stop when the iodine stays orange.
5. Record the time to the end point, repeat for each pH, calculate rate = 1 ÷ time (s⁻¹) and plot rate against pH.

pH	Time to end point (s)	Repeat (s)	Mean time (s)	Rate = 1/time (s⁻¹)
4	Predict the pH at which the starch will disappear fastest.
5	Predict the pH at which the starch will disappear fastest.
6	Predict the pH at which the starch will disappear fastest.
7	Predict the pH at which the starch will disappear fastest.
8	Predict the pH at which the starch will disappear fastest.

Questions

1. Copy and complete: starch → (enzyme) → maltose → (enzyme) → ___; protein → (enzyme) → ___; fat → (enzyme) → ___.

2. Explain how the structure of the small intestine is adapted for absorption. Include numbers.

3. Use your results to calculate the rate at each pH and describe the pattern. Which pH was the optimum?

4. Give the main source of uncertainty in your timings and one way to reduce it.

Sources for every fact on this sheet: echoesof.science/school/digestion/sources/

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