3.3 - Digestion and absorption

Question 1

Define digestion

Answer 1

hydrolysis of large biological molecules to smaller molecules which can be absorbed across cell membranes

Question 2

Describe the digestion of carbohydrates by amylases and membrane-bound disaccharidases

Answer 2

• amylase: hydrolyses starch molecule glycosidic bond -> disaccharide
• membrane-bound disacharridases: disaccharide -> monosaccharides

Question 3

Describe the digestion of lipids by lipase and bile salt action, as well as micelles in lipid absorption

Answer 3

• bile salts: emulsify lipids into droplets to increase SA for lipase action
• lipase: hydrolyses ester bonds to produce fatty acids + monoglycerides
• micelles: make fatty acids + monoglycerides more soluble in H2O + carry them to ileum lining where they (NOT MICELLES) are absorbed by diffusion
• triglycerides: reformed in cells inside ER + Golgi body
• vesicles: move to and fuse with cell membrane + released by exocytosis to be transported in lymph, which eventually drains into capillary system

Question 4

Describe the digestion of proteins

Answer 4

• endopeptidases: hydrolyse peptide bonds in middle of polypeptide chain
• exopeptidases: hydrolyse peptide bonds at end of polypeptide chain
• dipeptidases: hydrolyse peptide bonds between 2 amino acids

Question 5

What do micelles contain?

Answer 5

bile salts, fatty acids (and monoglycerides)

Question 6

Explain the advantages of lipid droplet and micelle formation

Answer 6

• droplets increase SA of lipids
• so FASTER hydrolysis of lipids (hydrolysis of ester bonds) by lipase
• micelles carry fatty acids + monoglycerides to membrane

Question 7

Where are amylase and lipase produced and secreted?

Answer 7

amylase:
- produced: pancreas + salivary glands
- secreted: mouth + small intestine

lipase:
- produced: pancreas
- secreted: small intestine

Question 8

What do maltases (in the ileum) break down?

Answer 8

monosaccharides

Question 9

Why can monoglycerides and fatty acids easily diffuse across membrane of epithelial cells?

Answer 9

they are non-polar

Question 10

How is the ileum specialised for absorption?

Answer 10

• villi (and microvilli): thin walls, surrounding capillary network
• large SA, reduce diffusion distance + maintain conc gradient

Question 11

How is Golgi apparatus involved in lipid absorption?

Answer 11

• modifies triglycerides
• combines them with proteins
• forms vesicles