DigestionAQA A-Level Biology: Revision notes

Large biological molecules such as starch, triglycerides and proteins are too big to cross cell membranes. During digestion they are hydrolysed (broken down by adding water) by enzymes into smaller molecules (monosaccharides, fatty acids and monoglycerides, and amino acids) that can be absorbed across the cell surface membranes of the epithelial cells of the small intestine.

Enzymes work in the gut lumen (extracellular enzymes) or are membrane-bound on the surface of the epithelial cells.

• Amylase (in saliva from the salivary glands and in pancreatic juice) hydrolyses the glycosidic bonds in starch to maltose
• Membrane-bound disaccharidases on the epithelial cell surface hydrolyse disaccharides to monosaccharides: maltase (maltose to glucose + glucose), sucrase (sucrose to glucose + fructose) and lactase (lactose to glucose + galactose)

Salivary amylase is inactivated by the acid in the stomach, so most starch digestion occurs in the small intestine, using pancreatic amylase.

• Lipase (made in the pancreas) hydrolyses the ester bonds in triglycerides to form monoglycerides and fatty acids
• Bile salts (made in the liver, stored in the gall bladder and released into the small intestine) emulsify lipids, breaking large droplets into smaller ones

Emulsification increases the surface area of lipid, so lipase can hydrolyse it faster. Bile salts are not enzymes and do not hydrolyse the lipid.

Fatty acids lower the pH, which is how the rate of lipase action can be followed.

Proteins are hydrolysed by three types of peptidase.

• Endopeptidases hydrolyse peptide bonds within the polypeptide, forming shorter polypeptides (for example pepsin in the stomach)
• Exopeptidases hydrolyse peptide bonds at the ends of polypeptides, removing single amino acids and dipeptides
• Membrane-bound dipeptidases on the epithelial cell surface hydrolyse dipeptides into amino acids

Endopeptidases speed up exopeptidases because they create more ends for the exopeptidases to act on.

Calculating a rate: rate = change in pH ÷ time. If pH falls from 8.0 to 6.0 in 4 minutes, the rate is 2.0 ÷ 4 = 0.50 pH units min⁻¹.

Counting ends: if an endopeptidase cuts a polypeptide in 5 places, 6 fragments form. Each has 2 ends, so there are 12 ends for exopeptidases, instead of 2.

In application questions, link the enzyme to the substrate and the product, and link missing enzymes or bile to the symptoms (for example fatty stools when lipase or bile is lacking).