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 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:
1 ● Na+ actively transported from epithelial cells lining ileum toblood (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 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
● monoglycerides / fatty acids are absorbed (into epithelial cell) by diffusion (as they’re lipid soluble)
● triglycerides reformed in (epithelial) cells and aggregate 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