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 membranes into blood

Question 2

Describe the digestion of starch in mammals

Answer 2

● Amylase (produced by salivary glands / pancreas) hydrolyses starch to maltose
● Membrane-bound maltase (attached to cells lining ileum) hydrolyses 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

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

Answer 4

● 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 5

Describe the digestion of proteins by a mammal

Answer 5

● 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 6

Suggest why membrane-bound enzymes are important in digestion

Answer 6

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 7

Describe the pathway for absorption of products of digestion in mammals

Answer 7

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

Question 8

Describe the absorption of amino acids and monosaccharides in mammals

Answer 8

Co-transport:
● 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)
● Na + enters epithelial cell down
its concentration gradient with
monosaccharide or amino
acid against its concentration
gradient
● Via a co-transporter protein
● Monosaccharide or amino
acid moves down a
concentration gradient into
blood via facilitated diffusion

Question 9

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

Answer 9

● 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
*fatty acids are absorbed (into epithelial cell) by diffusion (as they’re lipid soluble)
● Triglycerides reformed in (epithelial) cells and turn into globules
● Globules 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