Digestive enzymes: Table 1.1

Each enzyme has one food to cut

An enzyme acts on one kind of substance, its substrate, and turns it into products. Table 1.1 lists, region by region, which enzyme does what.

Mouth: salivary amylase turns starch into maltose. Stomach: pepsin turns proteins into peptides.

Small intestine: pancreatic amylase, trypsin, chymotrypsin and lipase continue the work. Then peptidase, dipeptidases, maltase, sucrase, lactase and nucleases finish it, leaving only small molecules.

Enzyme names often give the substrate away: maltase for maltose, sucrase for sucrose, lactase for lactose, lipase for lipids.

Key terms

Enzyme

A protein that speeds up breaking a specific substrate.

Substrate

The substance an enzyme breaks down.

Product

The smaller substance an enzyme produces.

Amylase

Breaks starch into maltose, in the mouth and the small intestine.

Short questions with model answers

1. Q1. Follow a starch molecule until it can be absorbed.

   • To maltose.
   • To glucose.

   Starch, maltose, glucose: amylases then maltase.

2. Q2. Follow a protein molecule until it can be absorbed.

   • To peptides.
   • To amino acids.

   Protein, peptides, amino acids.

Common mistakes

• ✗ Each food is digested in only one place.

  ✓ Starch starts in the mouth and protein in the stomach; both finish in the small intestine.

• ✗ Maltose is the final product of starch digestion.

  ✓ Maltase splits maltose further into glucose.

• ✗ Lipase digests proteins.

  ✓ Lipase digests fats into fatty acids and glycerol.

MCQs

1. 1. In which part are carbohydrates, lipids and proteins all digested?

   1. (a) Oral cavity
   2. (b) Stomach
   3. (c) Small intestine
   4. (d) Large intestine
   Show answer

   (c) Exercise MCQ 7 (p. 10); Table 1.1.

2. 2. Lactase breaks lactose into:

   1. (a) Glucose and fructose
   2. (b) Glucose and galactose
   3. (c) Maltose
   4. (d) Amino acids
   Show answer

   (b) Table 1.1. Glucose and fructose come from sucrose.

3. 3. Which enzyme acts on fats?

   1. (a) Trypsin
   2. (b) Maltase
   3. (c) Pepsin
   4. (d) Lipase
   Show answer

   (d) Table 1.1: fats to fatty acids and glycerol.

4. 4. Which enzyme is NOT listed for the small intestine in Table 1.1?

   1. (a) Pepsin
   2. (b) Trypsin
   3. (c) Sucrase
   4. (d) Nucleases
   Show answer

   (a) Pepsin is listed for the stomach.

5. 5. Nucleases break nucleic acids into:

   1. (a) Peptides
   2. (b) Nucleotides
   3. (c) Glycerol
   4. (d) Maltose
   Show answer

   (b) Table 1.1.

Quick revision

• Each enzyme breaks one substrate into a smaller product.
• Starch: amylases then maltase. Protein: pepsin, trypsin, chymotrypsin, then peptidases. Fat: lipase.
• All three food types are fully digested in the small intestine.