1.4 Digestion and absorption

Question 1

Define digestion

Answer 1

Mechanical, chemical and microbial breakdown of large insoluble food molecules into simple absorbable compounds

Question 2

Define absorption

Answer 2

Process by which these simple compounds are taken across the intestinal membranes into the bloodstream

Question 3

What are 𝛂-amylases

Answer 3

Enzyme that catalyses hydrolysis of starch into sugars
Salivary = pH optimum 6.6-6.8, inactivates at stomach pH 3
Pancreatic = digests most carbs in small intestine, secreted from exocrine pancreas

Question 4

What are the biological functions of the 𝛂-amylases?

Answer 4

1. Attack 𝛂-1,4 glycosidic links only

2. Attack only in the middle of the chain: endoglycosidases

Question 5

What is an enterocyte?

Answer 5

Absorptive functional unit of the digestive tract

Question 6

Describe the life cycle of an enterocyte

Answer 6

Lumen is toxic so instead of repairing every insult, damaged enterocytes die by apoptosis and are removed from the epithelial layer and continuously replaced by new ones

Question 7

Describe the progression of nutrients over the brush border into cells

Answer 7

Cross over the baso-latera membrane and enter the bloodstream
Directed to the hepatic portal vein to the liver and beyond or indirectly via the lymphatic system

Question 8

What characteristics of glucose transport allow the mechanism to be discovered?

Answer 8

• saturable
• competitively inhibited by galactose
• inhibited by glucose analogues e.g., phlorizin
• sodium dependant
• inhibited by sodium/potassium ATPase inhibitor: ouabain
• requires energy

Question 9

Define proteases

Answer 9

Enzymes that break down proteins and peptides

- cleavage of peptide bonds within proteins by hydrolysis

Question 10

What is a zymogens?

Answer 10

Secreted as inactive precursors - activated immediately before use

Question 11

What are the differences between endo and exopeptidases?

Answer 11

Endopeptidases attack and cleave in the middle of an amino acid chain unlike exopeptidases which spilt off amino acids from the end of the chain
Endopeptidases are secreted by the stomach & pancreas whereas exopeptidases are secreted by the pancreas and the small intestine glands

Question 12

What are pepsins?

Answer 12

Gastric proteases

- pepsin secreted as pepsinogen by chief cells

Question 13

What ion is always produced simultaneously with pepsinogen?

Answer 13

H+, secreted by parietal cells

Question 14

What are pancreatic endopeptidases?

Answer 14

Produced from pepsin digestion in the stomach pass into the small intestine

• zymogen
• trypsinogen
• chymotrypsinogen
• pro-elastase

Question 15

What are the two subtypes of pancreatic exopeptidases?

Answer 15

Carboxypeptidases + Aminopeptidases

Question 16

What are carboxypeptidases?

Answer 16

Split as aas from the carboxy terminal are secreted by the pancreas as zymogens

Question 17

What are aminopeptidases?

Answer 17

Spilt as aas from the N terminal, are made in the SI glands and are found on the intestinal cell membrane

Question 18

What are lipases?

Answer 18

Digest lipases, removing fatty acids from TAG

Question 19

What is the main fat digesting enzyme?

Answer 19

Pancreatic lipase - active in the small intestine

Question 20

What is bile?

Answer 20

Greenish-brown alkaline fluid which aids digestion and is secreted by the liver and stored in the gall bladder

Question 21

What is the function of bile?

Answer 21

Bile acids aid in the digestion and absorption of fat

Also excretes excess cholesterol and the toxic breakdown products of haem (bile pigments)

Question 22

What hormone initiates bile release?

Answer 22

Cholecystokinin

Question 23

What does emulsification entail?

Answer 23

1. Forcing (spraying) the stomachs contents through the pyloric sphincter into duedenum
2. Bile salts stabilise/coat the resulting lipid droplets (emulsification) and prevent them from coalescing
3. Peristalsis helps further stabilise the emulsion droplets and keep them in suspension

Question 24

What are the products of fat digestion?

Answer 24

Free fatty acids
2-monoacylglycerol
1-monoacylglycerol
Glycerol

Question 25

What are chylomicrons?

Answer 25

Small structures containing triglycerides 
Surface proteins = apolipoprotein C 
Surface lipids 
Core containing mostly TAG 
Intergral protein apolipoprotein B

Question 26

How are chylomicron remnants formed?

Answer 26

Some TAG is broken down by lipoprotein lipase on the surface of capillary endothelial cells which are then released from the endothelium and taken up by the liver and further metabolised to other lipid particles e.g., HDL, LDL, VLDL