Digestion in Mammals
Biology: AQA A Level, topic 3.3.2
Carbohydrates
Carbohydrates are a staple in the mammalian diet. The digestion of carbohydrates begins in the mouth and involves the enzyme amylase.
Amylase
- Amylase is an enzyme that is produced in the salivary glands and the pancreas.
- Amylase catalyses the hydrolysis of starch (carbohydrate) into maltose (disaccharide).
- Hydrolysis of carbohydrates breaks bonds called glycosidic bonds.
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Membrane-bound disaccharidases
- Membrane-bound disaccharidases are enzymes in the cell membranes of the epithelial cells in the ileum (the small intestine).
- Membrane-bound disaccharidases catalyse the hydrolysation of disaccharides (e.g. maltose) into monosaccharides (e.g. glucose).
Transporter proteins
- Monosaccharides are transported into the epithelial cells in the ileum using transporter proteins.
- Glucose and galactose are transported by co-transporter proteins.
- Fructose is absorbed by facilitated diffusion.
- Once inside the cells, monosaccharides can be used in respiration.
Lipids
Lipids are a staple in the mammalian diet. Most of the digestion of lipids takes place in the small intestine and involves the enzyme lipase.
Lipase
- Lipase is an enzyme that is produced by the pancreas and is released into the small intestine.
- Lipase hydrolyses lipids into fatty acids and monoglycerides.
- Hydrolysis of lipids breaks bonds called ester bonds.
Bile salts
- Bile salts are produced by the liver.
- Bile salts also help digest lipids by forming small lipid droplets called micelles.
- The process of forming micelles is called emulsification.
- The many micelles have a larger surface area than a single larger micelle. This allows the lipids to be hydrolysed by lipase more quickly.
Micelles
- The micelles allow the monoglycerides and fatty acids to be absorbed by the epithelial cells in the ileum.
- The monoglycerides and fatty acids diffuse through the cell surface membrane into the cell cytoplasm.
Proteins
Proteins are a staple in the mammalian diet. Most of the digestion of proteins takes place in the small intestine and involves different enzymes called proteases. Some digestion occurs in the stomach.
Endopeptidases
- Endopeptidases are enzymes that catalyse the hydrolysis of bonds within a protein to amino acids.
- Hydrolysis of proteins breaks bonds called peptide bonds.
- Examples of endopeptidases include trypsin, chymotrypsin and pepsin.
- Trypsin and chymotrypsin are released by the pancreas into the small intestine.
- Pepsin is released into the stomach.
Exopeptidases
- Exopeptidases are enzymes that catalyse the hydrolysis of peptide bonds at the ends of proteins.
- E.g. Dipeptidases hydrolyse the peptide bond between two amino acids.
Co-transporter proteins
- Amino acids are transported into the epithelial cells in the ileum using co-transporter proteins.
Test yourself on Digestion in Mammals
Which of the following statements about digestion of carbohydrates are true?
How is fructose absorbed by the epithelial cells in the ileum?
Steps in Digestion of Carbohydrates
Which of the following statements about the digestion of lipids are true?
What are the stages of digestion of lipids?