3.3.3 Digestion and Absorption

Explain what occurs during digestion. (2 marks)

- Large, insoluble biological molecules are broken down by hydrolysis into smaller, soluble molecules.

- These smaller molecules are able to cross cell membranes and enter the blood.

Describe the digestion of starch in mammals. (3 marks)

- Amylase, produced in the salivary glands and pancreas, hydrolyses starch into maltose.

- Membrane-bound maltase, found on the surface of ileum epithelial cells, hydrolyses maltose into glucose.

- This process involves breaking glycosidic bonds.

Describe the digestion of disaccharides in mammals. (3 marks)

- Membrane-bound disaccharidases hydrolyse disaccharides into two monosaccharides.

- Maltase breaks maltose into two glucose molecules, sucrase breaks sucrose into fructose and glucose, and lactase breaks lactose into galactose and glucose.

- Digestion involves the hydrolysis of glycosidic bonds.

Describe the digestion of lipids in mammals, including the role of bile salts. (4 marks)

- Bile salts, produced in the liver, emulsify Larger lipid droplets into smaller lipid droplets.

- This increases the surface area of the lipids, allowing lipase to act more quickly.

- Lipase, produced in the pancreas, hydrolyses lipids (e.g. triglycerides) into monoglycerides and fatty acids.

- Hydrolysis involves breaking ester bonds.

Describe the digestion of proteins in mammals. (4 marks)

- Endopeptidases hydrolyse peptide bonds within a polypeptide, producing smaller peptides and increasing surface area for exopeptidases by creating more free ends

- Exopeptidases hydrolyse peptide bonds at the ends of polypeptides to release single amino acids.

- Membrane-bound dipeptidases hydrolyse the peptide bond between amino acids in dipeptides to produce two amino acids.

- Protein digestion involves the hydrolysis of peptide bonds.

Explain why membrane-bound enzymes are important in digestion. (2 marks)

- They are located on the membranes of epithelial cells in the ileum, allowing digestion at the site of absorption.

- By hydrolysing molecules at the absorption surface, they help maintain concentration gradients needed for absorption.

Describe the pathway for absorption of the products of digestion in mammals. (2 marks)

- Digested molecules move from the lumen of the ileum.

- Through the epithelial cells lining the ileum and into the blood.

Describe the absorption of amino acids and monosaccharides in mammals via co-transport. (5 marks)

- Sodium ions (Na⁺) are actively transported from epithelial cells into the blood via the Na⁺/K⁺ pump.

- Which creates a concentration gradient of sodium (higher in the lumen than in the epithelial cell).

- Sodium ions diffuse into the epithelial cell down their concentration gradient through a co-transporter protein.

- Bringing a monosaccharide or amino acid into the cell against its concentration gradient.

- The monosaccharide or amino acid then moves into the blood down its concentration gradient by facilitated diffusion.

Describe the absorption of lipids by a mammal. (6 marks)

- Bile salts join with monoglycerides and fatty acids to produce micelles.

- Micelles transport fatty acids and monoglycerides to the epithelial cells of the ileum and release them there.

- Monoglycerides and fatty acids diffuse into epithelial cells as they are lipid-soluble.

- Inside epithelial cells, triglycerides are re-formed and cluster into globules and these globules are then coated with proteins to form chylomicrons, which are packaged into vesicles.

- Vesicles travel to the cell membrane and release chylomicrons by exocytosis.

- Chylomicrons enter lymphatic vessels before eventually returning to the bloodstream.

Describe the role of micelles. (3 marks)

- Micelles make monoglycerides and fatty acids more soluble in water.

- They transport fatty acids and monoglycerides to the epithelial cells of the ileum and release them there.

- This helps maintain a high concentration of these products close to the epithelial cell surface.