3.3 Digestion and absorption

What happens in digestion?

Large (insoluble) molecules are hydrolysed into smaller (soluble) molecules that are small enough to be absorbed across the cell membrane and into the blood

Describe the digestion of starch in animals

• Amylase hydrolyses glycosidic bonds in starch to maltose

• Membrane-bound maltase hydrolyses glycosidic bonds in maltose to glucose

Describe the digestion of disaccharides in animals

• Membrane-bound disaccharides hydrolyse disaccharides into 2 monosaccharides

• Maltase hydrolyses maltose into glucose + glucose

• Sucrase hydrolyses sucrose into fructose + glucose

• Lactase hydrolyses lactose into galactose + glucose

Describe the digestion of lipids in animals

• Bile salts emulsify (break down) lipids, causing them to form smaller lipid droplets

• This increases surface area of lipids for increased/faster lipase activity

• Lipase hydrolyses lipids into monoglycerides and fatty acids - hydrolysis of ester bond

Describe the digestion of proteins by an animal

• Endopeptidases hydrolyse internal peptide binds within a polypeptide to form smaller peptides so more ends/ surface area for exopepidases

• Exopeptidases hydrolyse terminal peptide binds at the ends of polypeptides to form single amino acids

• Membrane-bound dipeptidases hydrolyse peptide binds between a dipeptide to form two amino acids

• Hydrolysis of peptide bonds

Where is amylase produced?

By salivary glands and pancreas

When are membrane-bound maltase found?

Attached to cells lining the ileum

Where are bile salts produced?

By the liver

Where is lipase made?

In the pancreas

Why are membrane-bound enzymes important in digestion?

• Membrane-bound enzymes located on cell membranes of epithelial cells lining ileum

• They maintain concentration gradients for absorption

Describe the pathway for absorption of products of digestion in animals

Lumen (inside) of ileum → cells lining the ileum (part of small intestine) → blood

Describe the absorption of amino acids and monosaccharides in animals

1. Na+ actively transported from epithelial cells lining ileum to blood via sodium potassium pump, establishing a concentration gradient of Na+ (higher in lumen than epithelial cel)

2. Na+ enters epithelial cell down its concentration gradient with glucose against its concentration gradient via a co-transporter protein

3. Glucose moves down a concentration gradient into blood via facilitated diffusion

Describe the absorption of lipids in animals

• Micelles contain bile salts, monoglycerides and fatty acids - Make monoglycerides and fatty acids more soluble in water, carry/release fatty acids and monoglycerides to lining of ileum and maintain high concentration gradient of fatty acids to cell/lining

• Monoglycerides and fatty acids absorbed into epithelial cell by diffusion

• Triglycerides reformed in epithelial cell and aggregate (collect) into globules

• Globules coated with proteins, forming chylomicrons which are packaged into vesicles

• Vesicles move to cell membrane and leave via exocytosis. They enter lymphatic vessels and eventually return to blood circulation