The Human Digestive System & Required Food Tests

1

Carbohydrases & Amylase

Carbohydrases break down carbohydrates into simple sugars. Specifically, amylase catalyses the breakdown of starch (an insoluble polymer) into maltose and simple sugars (glucose). Produced in the salivary glands, pancreas, and small intestine.

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2

Proteases

Enzymes that catalyse the breakdown of proteins into amino acids. Produced in the stomach (where pepsin works at an optimum pH of 2), the pancreas, and the small intestine (where trypsin works at an alkaline pH of ~8).

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3

Lipases

Enzymes that catalyse the breakdown of lipids (fats and oils) into fatty acids and glycerol. Produced in the pancreas and small intestine, acting in the lumen of the small intestine.

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4

Bile & Emulsification

Bile is made in the liver, stored in the gallbladder, and released down the bile duct into the small intestine. It neutralises hydrochloric acid from the stomach (creating an alkaline pH) and physically emulsifies large lipid globules into small droplets, increasing surface area for lipase action.

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5

Benedict's & Iodine Tests (Carbohydrates)

Iodine test for starch: add orange-brown iodine solution; turns blue-black if starch is present. Benedict's test for reducing sugars (e.g., glucose): add blue Benedict's reagent and heat in a 75°C water bath; turns green, yellow, orange, or brick-red precipitate depending on sugar concentration.

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6

Biuret & Lipid Tests (Proteins & Fats)

Biuret test for protein: add blue Biuret solution; turns purple/lilac if protein is present. Sudan III test for lipids: add Sudan III stain to unfiltered food sample; a bright red-stained oil layer separates on top. Alternatively, the Ethanol emulsion test turns cloudy white when poured into water.

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1. Q1

   Explain how bile helps speed up the digestion of lipids in the small intestine without acting as a catalyst itself. [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Bile is alkaline and neutralises the hydrochloric acid coming from the stomach [1 mark]. 2. This creates the optimum alkaline pH (~pH 8) for lipase enzymes to work at their maximum rate without denaturing [1 mark]. 3. Bile also physically emulsifies large fat globules into tiny lipid droplets [1 mark]. 4. This greatly increases the total surface area of the lipid for lipase enzymes to attach to, increasing the rate of breakdown into fatty acids and glycerol [1 mark].

2. Q2

   Describe how a student could safely test a biscuit sample to determine if it contains both starch and reducing sugars. [5 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Grind the biscuit with distilled water using a pestle and mortar to make a liquid suspension [1 mark]. 2. For starch: add a few drops of orange-brown iodine solution to a sample in a spotting tile; a colour change to blue-black confirms starch is present [1 mark]. 3. For reducing sugars: add blue Benedict's solution to a second sample in a test tube [1 mark]. 4. Heat the test tube in a thermostatically controlled water bath at 75°C to 80°C for 5 minutes (wearing safety goggles) [1 mark]. 5. A colour change from blue to green, yellow, orange, or brick-red precipitate confirms reducing sugar is present [1 mark].

3. Q3

   Describe the complete chemical digestion of a meal containing pasta (starch) and chicken (protein) as it travels from the mouth to the small intestine. [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. In the mouth, salivary glands secrete amylase, which begins breaking down starch in the pasta into maltose [1 mark]. 2. In the stomach, protease (pepsin) works in hydrochloric acid (pH 2) to break down proteins in the chicken into peptides/amino acids [1 mark]. 3. The pancreas secretes both amylase and protease into the small intestine, and the wall of the small intestine also produces these enzymes [1 mark]. 4. Starch digestion is completed into glucose, and protein digestion is completed into soluble amino acids, which are absorbed through the villi into the blood [1 mark].

4. Q4

   In an experiment investigating the effect of pH on amylase activity, iodine solution is placed into wells on a spotting tile. Every 30 seconds, a drop of the starch-amylase mixture is added to a well. Explain how the student knows when all the starch has been digested, and state one limitation of this method. [3 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. While starch is still present, the iodine turns blue-black [1 mark]. 2. Digestion is complete when the iodine remains orange-brown (no longer turns blue-black) [1 mark]. 3. Limitation: Testing only every 30 seconds means the exact completion time occurred somewhere within a 30-second interval, so shorter time intervals (e.g., every 10 seconds) are needed for a more accurate rate measurement [1 mark].

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