Digestion and Absorption

Question 1

Explain what happens in digestion:

Answer 1

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

Question 2

Describe the digestion of starch in mammals:

Answer 2

1. Amylase (produced by salivary glands / pancreas) hydrolyses starch to maltose
2. Membrane-bound maltase (attached to cells lining ileum) hydrolyses maltose to glucose
3. Hydrolysis of glycosidic bond

Question 3

Describe the digestion of disaccharides in mammals:

Answer 3

1. Membrane-bound disaccharidases hydrolyse disaccharides to 2 monosaccharides:
   ○ Maltase - maltose → glucose + glucose
   ○ Sucrase - sucrose → fructose + glucose
   ○ Lactase - lactose → galactose + glucose
2. Hydrolysis of glycosidic bond

Question 4

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

Answer 4

1. Bile salts (produced by liver) emulsify lipids causing them to form smaller lipid droplets
2. This increases surface area of lipids for increased / faster lipase activity
3. Lipase (made in pancreas) hydrolyses lipids (eg. triglycerides) → monoglycerides + fatty acids
4. Hydrolysis of ester bond

Question 5

Describe the digestion of proteins by a mammal:

Answer 5

1. Endopeptidases - hydrolyse internal (peptide) bonds
   within a polypeptide → smaller peptides
   ○ So more ends / surface area for exopeptidases
2. Exopeptidases - hydrolyse terminal (peptide) bonds at
   ends of polypeptide → single amino acids
3. Membrane-bound dipeptidases - hydrolyse (peptide)
   bond between a dipeptide → 2 amino acids
4. Hydrolysis of peptide bond

Question 6

Suggest why membrane-bound enzymes are important in digestion:

Answer 6

1. Membrane-bound enzymes are located on cell membranes of epithelial cells lining ileum
2. (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

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 9

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

Answer 9

1. Bile salts combine with monoglycerides and fatty acids to form micelles. M make MGs and fatty acids (more) soluble in water. M carry fatty acids and MGs to cells lining the ileum, where they break down torelease them. This maintains a high conc of fatty acids and MGs near cells lining the ileum
2. MGs / fatty acids are absorbed (into epithelial cell) by diffusion (as they’re lipid soluble).TGs reformed in (epithelial) cells and aggregate into globules
3. Globules coated with proteins forming chylomicrons which are then packaged into vesicles
4. Vesicles move to cell membrane and fuse with it, releasing c via exocytosis. C enter lymphatic vessels and eventually return to blood circulation