Digestion of starch
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of enzyme action and the structure of carbohydrates. You can test your knowledge on these below.
What is an enzyme and how does it aid digestion?
An enzyme is a biological catalyst (made of protein) that speeds up the breakdown of large molecules into smaller, soluble molecules which can be absorbed into the bloodstream.
Which enzyme breaks down proteins and what is the product?
Protease (e.g., pepsin, trypsin) breaks down proteins into amino acids.
What are the products of lipid digestion?
Lipase breaks down lipids into monoglycerides and fatty acids.
Topic Explainer Video
What is Starch?
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Starch is a large polysaccharide made of many α-glucose monomers.
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It is found in foods such as potatoes, bread, and rice.
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Starch must be broken down into single monosaccharides (glucose) for absorption in the ileum of the small intestine.
Enzymes Involved in Starch Digestion
1. Amylase
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Produced by: Salivary glands & pancreas.
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Site of action: Mouth & small intestine.
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Reaction: Breaks starch into maltose by hydrolysing glycosidic bonds.
Begins digestion in the mouth and continues in the small intestine.
2. Maltase
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Produced by: Epithelial cells of the small intestine. This is a membrane-bound disaccharidase (found embedded in the membranes of the epithelial cells lining the villi).
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Site of action: Small intestine.
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Reaction: Breaks maltose into two glucose molecules by hydrolysing glycosidic bonds.
Process of Starch Digestion
1. In the Mouth
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Salivary amylase begins starch digestion → Produces maltose.
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pH maintained by saliva (contains mineral salts) to keep amylase active.
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Amylase is denatured by stomach acid, stopping starch digestion in the stomach.
2. In the Small Intestine
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Pancreatic amylase continues starch breakdown → Produces maltose.
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Alkaline conditions maintained by bile released into the small intestine.
3. At the Epithelial Lining of the Small Intestine
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Maltase hydrolyses maltose → Produces glucose.
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Glucose absorbed by co-transport with sodium ions.
Key Terms
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Amylase: Enzyme that breaks starch into maltose.
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Maltase: Enzyme that breaks maltose into glucose.
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Glycosidic bond: Bond between monosaccharides in carbohydrates.
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Membrane-bound enzyme: Enzyme attached to epithelial cells of the small intestine.
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Co-transport: Process of glucose absorption with sodium ions in the ileum.
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Exam Tip
Remember to use the key term ‘hydrolysis’ instead of just saying ‘breaks down’ and remember to name the glycosidic bonds.
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Describe how starch is digested in the human digestive system. (4 marks)
- Salivary amylase is released by the salivary glands in the mouth and hydrolyses starch into maltose.
- By breaking glycosidic bonds.
- Pancreatic amylase continues hydrolysis of starch into maltose in the small intestine.
- Maltase, a membrane-bound disaccharidase on the epithelial lining of the small intestine, hydrolyses maltose into two glucose molecules.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!