School › Digestion › Ages 15–16
Enzymes, digestion and absorption
Mechanical and chemical digestion, the enzymes that digest carbohydrates, proteins and fats, bile and emulsification, the surface area of the small intestine and how nutrients are absorbed. With an amylase and pH investigation.
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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.[4][2]
The pancreas secretes amylase, protein-digesting enzymes such as trypsin, and lipases, plus bicarbonate that neutralises acidic chyme.[4][2]
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.[4][1]
Enzymes fixed to the microvilli (maltase, sucrase, lactase) split disaccharides into monosaccharides and finish protein digestion.[4][3]
The small intestine (about 3 m long in life) has circular folds, 20–40 villi per mm² and about 200 million microvilli per mm², increasing its absorptive surface more than 600-fold, to about 200 m².[3]
Sugars and amino acids enter the blood and go to the liver; fats are packed into chylomicrons that enter lymph capillaries. Chyme takes about 3–5 hours to leave the small intestine.[1][4][3]
Residue spends about 12–24 hours in the large intestine, which absorbs most of the remaining water; its trillions of bacteria make vitamins such as vitamin K. Hormones and nerves in the gut wall control digestion.[3][1]
Try it: How does pH affect amylase?
You need
- 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.
What to do
- Put single drops of iodine into each dimple of the spotting tile.
- In a labelled test tube, mix 2 cm³ of amylase with 1 cm³ of one buffer.
- Add 2 cm³ of starch, mix and start the stopwatch.
- 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.
- Record the time to the end point, repeat for each pH, calculate rate = 1 ÷ time (s⁻¹) and plot rate against pH.
What happens, and why?
Amylase breaks starch into maltose, so the iodine stops turning blue-black when the starch is gone. The reaction is fastest near the enzyme's optimum pH (about pH 6 here; around 60 s), and slower at pH values further away. Because you sample every 10 s, each time has an uncertainty of up to about 10 s.[5][4]
Play: Carbohydrate, protein or fat?
Sort each enzyme, substance or product: is it part of carbohydrate, protein or fat digestion?
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Quiz
Other ages: Ages 6–7 · Ages 10–11