3.3 Digestion And Absorption

Question 1

Describe the process of digestion of carbohydrates

Answer 1

• Salivary amylase catalyses the hydrolysis of starch into maltose
• Membrane-bound disaccharidases in the membranes of epithelial cells in the ileum hydrolyses disaccharides (e.g. maltose) into monosaccharides (e.g. glucose.)

Question 2

Describe the process of the digestion of lipids

Answer 2

• Lipase hydrolyses lipids into monoglycerides and fatty acids in the small intestine
• Lipids are emulsified into micelles by bile salts which are produced in the liver. This increases the surface area for lipase action.

Question 3

Describe the process of the digestion of proteins

Answer 3

• Endopeptodases catalyse the hydrolysis of peptide bonds within a protein
• Exoptodases catalyse the hydrolysis of peptide bonds on the end of a polypeptide chain
• Dipeptidases catalyse the hydrolysis of peptide bonds between two amino acids

Question 4

Describe the process by which amino acids and glucose are absorbed

Answer 4

• Sodium ions are actively transported out of the ileum by the sodium-potassium pump into the blood.
• This maintains a higher concentration of sodium ions in the ileum than in the epithelial cells.
• Sodium ions diffuse into the epithelial cells via carrier proteins.
• As the sodium ions diffuse into the epithelial cells, they carry with them amino acid molecules or glucose molecules.

Question 5

Describe the process by which lipids are absorbed

Answer 5

• Micelles come into contact with epithelial cells and break down releasing the monoglycerides and fatty acids.
• As these are non-polar molecules, they diffuse directly through the membrane.
• Once inside, they are transported to the endoplasmic reticulum and recombined to form triglycerides.
• In the Golgi apparatus, the triglycerides associate with cholesterol and lipoproteins to form chylomicrons.
• Chylomicrons leave the epithelial cells by exocytosis into the lacteals.