120 - Digestion and Absorption

Question 1

Major sites of digestion

Answer 1

Duodenum, upper jejunum

Question 2

Polysaccharides that humans can digest

Answer 2

Only really glucose polymers

Question 3

Role of non-glucose-polymer polysaccharides

Answer 3

Substrate for bacterial digestion

Question 4

Disaccharides digested by humans

Answer 4

Lactose

Sucrose

Question 5

Human dietary monosaccharides

Answer 5

Glucose and fructose

Question 6

Enzyme in mouth that digests saccharides

Answer 6

Alpha-amylase

Question 7

Effect of alpha amylase

Answer 7

Hydrolyses 1:4α linkages between glucose molecules linear or branched chain

Question 8

How is alpha-amylase inactivated?

Answer 8

Inactivated at low pH in stomach

Question 9

Activation and deactivation of salivary amylase
1
2
3

Answer 9

1) Active in mouth.
2) Deactivated at low pH in stomach.
3) Reactivated in duodenum with neutralisation of acid by bicarbonate in pancreatic juice and secreted from duodenal epithelium

Question 10

What stimulates pancreatic alpha-amylase release?

Answer 10

CCK release from duodenal mucosa

Question 11

Pancreatic amylase path
1
2

Answer 11

– Enters duodenum via pancreatic duct

– Pushed back to pylorus by retropulsion

Question 12

Polysaccharides that alpha amylases have no effect on

Answer 12

Both salivary and pancreatic α-amylase are ineffective at 1:6α linkages, thus leaving a variety of oligosaccharides intact

Question 13

Polysaccharide digestion that isn’t mediated by amylases

Answer 13

Digested by enzymes on brush border membrane

Question 14

Enzymes that can digest 1:6α linkages in polysaccharides

Answer 14

Those in the brush border membrane.

Question 15

Examples of brush border enzymes that digest polysaccharides
1
2
3

Answer 15

1) Isomaltase breaks 1:6α linkages, acts with sucrase and maltase to break down maltotriose and maltose
2) Sucrase breaks sucrose into glucose and fructose
3) Maltase and sucrase are synthesised as a single large glycoprotein, inserted into brush border membrane and then separated and activated by pancreatic proteases

Question 16

How are glucose and fructose absorbed?

Answer 16

Rapidly absorbed across mucosal epithelium in duodenum and jejunum (predominantly at tips of villi)

Question 17

Upon what does glucose transport largely depend at epithelial surface?

Answer 17

Na+

Question 18

Name for glucose and Na+ cotransporter

Answer 18

Sodium-dependent glucose transporter (SGLT1)

No homology to the GLUT family of glucose transporters

Question 19

What transports fructose across gut epithelium?
1
2

Answer 19

GLUT5 (facilitated diffusion) into epithelial cells (across apical membrane).

GLUT2 transports from epithelial cell into interstitium (across basolateral surface)

Question 20

Outcome of having sweet taste receptors on jejunal villi

Answer 20

Stimulated by nutrisweet, etc.

Enhances glucose transport across epithelium

Question 21

Where does digestion of proteins begin?

Answer 21

In stomach with pepsin

Question 22

Pepsin secretion
1
2
3
4

Answer 22

1) Secreted from chief cells as pepsinogens
2) Activated by gastric acid
3) Pepsinogen I secreted in acid-secreting regions
4) Pepsinogen II secreted close to pylorus

Question 23

Pepsin protein digestion

Answer 23

Hydrolyses bonds between aromatic amino acids (phenylalanine, tyrosine) and a second amino acid.

Question 24

What is pepsin important for in food digestion?

Answer 24

Digesting proteins in cell-cell adhesions

Question 25

CCK role in protein degradation

Answer 25

CCK release (caused by amino acids) triggers secretion of pancreatic proteases in form of inactive proenzymes

Question 26

Activation of pancreatic proteases
1
2
3

Answer 26

1) Enterokinase in epithelial membrane phosphorylates trypsinogen
2) Trypsinogen activated to trypsin
3) This leads to a cascade of activation of other zymogens

Question 27

Endopeptidases in duodenum

Answer 27

Trypsin, elastase, chymotrypsins

Question 28

Types of polypeptides formed by trypsin, elastase, chymotrypsins

Answer 28

Short polypeptides

Question 29

Proteases that produce free amino acids in the gut

Answer 29

Pancreastic carboxypeptidases

Question 30

Peptidases at the brush border membrane

Answer 30

Mix of aminopeptidases, carboxypeptidases, endopeptidases and dipeptidases

Question 31

How can proteins enter gut epithelial cells?

Answer 31

Either as free amino acids, or as di- or tri-peptides, which are broken into free amino acids within enterocytes

Question 32

Number of transporters for free amino acids

Answer 32

Seven or more.

Question 33

Co-transported molecules in amino acid transporters

Answer 33

Five require Na+

Two require Cl-

Question 34

Where does fat digestion start?

Answer 34

In the mouth with a lingual lipase (very minor effect, maybe there for taste)

Question 35

Stomach lipase role

Answer 35

Very minor role.

Maybe there to release enough fatty acids to trigger CCK secretion from duodenum

Question 36

CCK effect on duodenal fat digestion
1
2
3

Answer 36

1) CCK stimulates secretion of pancreatic lipases
2) Lipase secreted in inactive form, activated by colipase.
3) Colipase is activated by trypsin

Question 37

How are cholesterol esters digested in the duodenum?

Answer 37

Cholesterol esterase is released by pancreas.

This is activated by bile.

Question 38

How are lipids emulsified?

Answer 38

Bile salts, lecithin are emulsifiers, are vigorously mixed.

Bile salts form micelles with lecithin and monoglycerides

Question 39

Fat absorption in the gut

Answer 39

Lipids in micelles brought to apical surface of epithelial cells at tips of villi.
When in contact with membrane fat soluble lipids dissolve in membrane, enter cells (some long-chain fatty acids might have transporters)

Question 40

Where do epithelial cells reform monoglycerides and monglycerides to triglycerides?

Answer 40

In smooth ER

Question 41

Where are chylomicrons formed?

Answer 41

In enterocytes

Question 42

Chylomicron formation

Answer 42

Once enterocytes have formed triglycerides in their smooth ER, triglycerides are coated with apolipoproteins, formed into chylomicrons

Question 43

Where are chylomicrons secreted?

Answer 43

Secreted via exocytosis into the lymphatics (lacteal)

Question 44

Short chain fatty acid generation in gut
1 a, b
2
3 a

Answer 44

• Short chain fatty acids produced in proximal colon by fermentation of dietary fibre
– Acetate (60%), proprionate (25%), butyrate (15%)
– Concentration in lumen is quite high ~80 mM
• Absorbed in distal small bowel and proximal part of colon via H+ dependent mechanism
• Contribute significantly to total energy intake
– Critical nutrient source in many herbivores