3.3.3 - Digestion and absorption

Question 1

Explain what happens in digestion

Answer 1

● Large (insoluble) biological molecules hydrolysed to smaller (soluble) molecules
● That are small enough be absorbed across cell surface membranes into blood

Question 2

Describe the digestion of starch in mammals

Answer 2

● Amylase (produced by salivary glands / pancreas) hydrolyses the glycosidic bonds in starch to form maltose

● Membrane-bound maltase (attached to cells lining ileum) hydrolyses the glycosidic bonds in maltose to glucose

● Hydrolysis of glycosidic bond

Question 3

Describe the digestion of disaccharides in mammals

Answer 3

● Membrane-bound disaccharidases hydrolyse disaccharides to 2 monosaccharides:

○ Maltase - maltose → glucose + glucose
○ Sucrase - sucrose → fructose + glucose
○ Lactase - lactose → galactose + glucose

● Hydrolysis of glycosidic bond

Question 4

Maltose

Answer 4

Glucose + Glucose

Question 5

Sucrose

Answer 5

Fructose + Glucose

Question 6

Lactose

Answer 6

Galactose + Glucose

Question 7

Describe the digestion of lipids in mammals, including action of bile salts

Answer 7

● Bile salts (produced by liver) emulsify lipids causing them to form smaller lipid droplets

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

● Lipase (made in pancreas) hydrolyses lipids (eg. triglycerides) → monoglycerides + fatty acids

● Hydrolysis of ester bond

Question 8

Describe the digestion of proteins by a mammal

Answer 8

● Endopeptidases - hydrolyse internal (peptide) bonds
within a polypeptide → smaller peptides
○ So more ends / surface area for exopeptidases

● Exopeptidases - hydrolyse terminal (peptide) bonds at
ends of polypeptide → single amino acids

● Membrane-bound dipeptidases - hydrolyse (peptide)
bond between a dipeptide → 2 amino acids

● Hydrolysis of peptide bond

Question 9

Suggest why membrane-bound enzymes are important in digestion

Answer 9

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

● (By hydrolysing molecules at the site of absorption they) maintain concentration gradients for absorption

Question 10

Describe the pathway for absorption of products of digestion in mammals

Answer 10

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

Question 11

Describe the absorption of amino acids and monosaccharides in mammals
(Co-transport:)

Answer 11

1 ● Na+ actively transported from
epithelial cells lining ileum to blood (by Na+/K+ pump)
● Establishing a concentration
gradient of Na+
(higher in
lumen than epithelial cell)

2 ● Na+ enters epithelial cell down its concentration gradient with monosaccharide or amino acid against its concentration
gradient
● Via a co-transporter protein

3 ● Monosaccharide or amino acid moves down a
concentration gradient into blood via facilitated diffusion

Question 12

Describe the absorption of lipids by a mammal, including the role of micelles

Answer 12

● Bile salts combine with monoglycerides and fatty acids to form micelles
○ Micelles make monoglycerides and fatty acids (more) soluble in water
○ Micelles carry fatty acids and monoglycerides to cells lining the ileum, where they break down to
release them
○ This maintains a high concentration of fatty acids and monoglycerides near cells lining the ileum

● Monoglycerides / fatty acids are absorbed (into epithelial cell) by diffusion across the cell surface membrane of the epithelial cell (as they’re lipid soluble)

● Triglycerides reformed in (epithelial) cells and aggregate into globules

● Globules are coated with proteins forming chylomicrons which are then packaged into vesicles

● Vesicles move to cell membrane and fuse with it, releasing chylomicrons via exocytosis
○ Chylomicrons enter lymphatic vessels and eventually return to blood circulation.