Digestion & absorption

Question 1

What happens in digestion?

Answer 1

large biological molecules are hydrolysed into smaller ones that can be absorbed across cell membranes into blood

Question 2

How does the digestion of starch in mammals occur?

Answer 2

• amylase hydrolyses starch to maltose
• membrane-bound maltase hydrolyses maltose into alpha-glucose
• involves the hydrolysis of glycosidic bonds

Question 3

How does the digestion of disaccharides in mammals occur?

Answer 3

• membrane-bound disaccharidases hydrolyse disaccharides e.g. maltose, sucrose & lactose to monosaccharides
• involves the hydrolysis of glycosidic bonds

Question 4

How does the digestion of lipids in mammals, including the action of bile salts occur?

Answer 4

• bile salts emulsify lipids to form smaller lipid droplets which increases the surface area for faster lipase activity
• lipase hydrolyses lipids into monoglycerides & fatty acids
• involves hydrolysis of ester bonds

Question 5

How does the digestion of proteins by a mammal occur?

Answer 5

• endopeptidases hydrolyse internal peptide bonds within the polypeptide into smaller peptides which increases the surface area for exopeptidases
• exopeptidases hydrolyse terminal bonds at the ends of polypeptide chains to form single amino acids
• membrane-bound dipeptidase hydrolyses peptide bonds between a dipeptide to form 2 amino acids

Question 6

Why are membrane-bound enzymes important in digestion?

Answer 6

• membrane-bound enzymes are located on cell membranes of epithelial cells lining the ileum
• they maintain concentration gradients for absorption

Question 7

What is the pathway for absorption of products of digestion in mammals?

Answer 7

lumen of ileum –> cells lining ileum –> blood

Question 8

How does the absorption of amino acids & monosaccharides in mammals occur?

Answer 8

co-transport:
- Na+ is actively transported from epithelial cells lining ileum into blood which establishes conc gradient of higher NA+ in lumen than epithelial cells
- Na+ enters epithelial cells down the conc gradient with a monosaccharide/amino acid against its conc gradient using a co-transporter protein
- monosaccharide/amino acid moves down a conc gradient from epithelial cell into blood by facilitated diffusion

Question 9

How does the absorption of lipids by a mammal, including the role of micelles occur?

Answer 9

• bile salts combine with monoglycerides & fatty acids to form micelles which make them soluble in water & carry monoglycerides + fatty acids into cells lining ileum where they break down to release them maintaining high conc gradient
• monoglycerides/ fatty acids are absorbed by diffusion
• triglycerides reformed in cells
• globules coated with proteins forming chlyomicrons which are then packages into vesicles
• vesicles fuse with cell membrane and release chlyomicrons out of cell by exocytosis
• chylomicrons enter lymphatic vessels