3.3 Digestion and absorption

Question 1

During digestion, large biological molecules are hydrolysed to smaller molecules. Why?

Answer 1

During digestion, large biological molecules are hydrolysed to smaller molecules so that they can be absorbed across cell membranes.

Question 2

Describe the digestion of carbohydrates.

Answer 2

Amylases hydrolyse 1-4 glycosidic bonds in starch, releasing maltose as a product.
Maltose is then digested by the membrane-bound disaccharidase maltase in the ileum epithelial wall, releasing alpha glucose.

Question 3

Describe the digestion of proteins.

Answer 3

Exopeptidases hydrolyse peptide bonds at the terminal ends of a polypeptide.
Endopeptidases hydrolyse internal peptide bonds within a polypeptide.
Endopeptidases make more terminals for exopeptidases.
Membrane-bound dipeptidases hydrolyse the final peptide bonds to release the amino acids.

Question 4

Describe the digestion of lipids.

Answer 4

Lipids don’t form solutions with water, so coalesce into droplets. Lipases (like all enzymes) are globular proteins and therefore water soluble. This means they cannot be dissolved in lipid, so can only act on the surface of the lipid droplet, at the interface between lipid and water. Bile salts, produced by the liver, have both hydrophobic and hydrophilic regions. Bile salts emulsify the lipid droplets (break them up into much smaller droplets) increasing the surface area of the droplets, providing more available surface for the lipases.

Lipases hydrolyse ester bonds, producing glycerol, fatty acids and monoglycerides.

Question 5

Describe the absorption of carbohydrates.

Answer 5

Sodium ions are actively transported out of the ileum epithelial cell into the blood, lowering the concentration of sodium ions in the cell. This forms an electrochemical gradient for sodium ions between the lumen of the ileum, and the epithelial cell.

Sodium ions move down this gradient from lumen of the ileum to the epithelial cell though membrane-bound co-transport proteins in the cell’s plasma membrane. This movement of ions is therefore facilitated diffusion. But, these cotransport proteins also have a binding site for a monosaccharide, and will not let sodium ions across unless this site is occupied. In this way monosaccharides are brought into the epithelial cells with the sodium ions, by co-transport.

Question 6

Describe the absorption of proteins.

Answer 6

Sodium ions are actively transported out of the ileum epithelial cell into the blood, lowering the concentration of sodium ions in the cell. This forms an electrochemical gradient for sodium ions between the lumen of the ileum, and the epithelial cell.

Sodium ions move down this gradient from lumen of the ileum to the epithelial cell though membrane-bound co-transport proteins in the cell’s plasma membrane. This movement of ions is therefore facilitated diffusion. But, these cotransport proteins also have a binding site for an amino acid, and will not let sodium ions across unless this site is occupied. In this way amino acids are brought into the epithelial cells with the sodium ions, by co-transport.

Question 7

Describe the role of micelles in the absorption of fats into the cells lining the ileum.

Answer 7

Micelles contain the fatty acids from the digestion of triglycerides, and the bile salts from the liver. The bile salts allow the fatty acids to be more soluble in the watery content of the ileum lumen.

The micelles carry the fatty acids to the epithelial cells lining the ileum, helping to maintain a higher concentration of fatty acids just outside the epithelial cell than inside. The fatty acids cross the plasma membrane by simple diffusion (being lipid soluble).

Question 8

Describe the absorption of lipids.

Answer 8

Micelles carry the fatty acids to the epithelial cells lining the ileum, helping to maintain a higher concentration of fatty acids just outside the epithelial cell than inside. The fatty acids cross the plasma membrane by simple diffusion (being lipid soluble), and enter the smooth endoplasmic reticulum where they are reformed into triglycerides. These are then sent to the Golgi apparatus in vesicles, where they are packaged into chylomicrons. Chylomicrons are then excreted by exocytosis into the lumen of the villus and enter the lacteal of the lymph system.

Question 9

How is the epithelium of the ileum adapted for absorption?

Answer 9

Epithelium is one cell thick to shorten diffusion distance.

Epithelium is folded into villi to maximise surface area for absorption.

The plasma membrane of the epithelial cells (on the side facing the ileum lumen) is folded into microvilli, further maximising surface area for absorption.

The plasma membrane of the epithelial cells (on the side facing the ileum lumen) is embedded with:
- membrane-bound disaccharidases (maltase, lactase, sucrase)
- membrane-bound dipeptidases
- co-transport carrier proteins
- channel proteins

The plasma membrane of the epithelial cells (on the side facing the lumen of the villus) is embedded with:
- sodium-potassium pumps
- channel proteins

The epithelial cells have large numbers of mitochondria to provide the ATP needed to establish a sodium ion concentration gradient

The epithelial cells have smooth endoplasmic reticulum and Golgi bodies for processing lipids