120 - Digestion and Absorption Flashcards
Major sites of digestion
Duodenum, upper jejunum
Polysaccharides that humans can digest
Only really glucose polymers
Role of non-glucose-polymer polysaccharides
Substrate for bacterial digestion
Disaccharides digested by humans
Lactose
Sucrose
Human dietary monosaccharides
Glucose and fructose
Enzyme in mouth that digests saccharides
Alpha-amylase
Effect of alpha amylase
Hydrolyses 1:4α linkages between glucose molecules linear or branched chain
How is alpha-amylase inactivated?
Inactivated at low pH in stomach
Activation and deactivation of salivary amylase
1
2
3
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
What stimulates pancreatic alpha-amylase release?
CCK release from duodenal mucosa
Pancreatic amylase path
1
2
– Enters duodenum via pancreatic duct
– Pushed back to pylorus by retropulsion
Polysaccharides that alpha amylases have no effect on
Both salivary and pancreatic α-amylase are ineffective at 1:6α linkages, thus leaving a variety of oligosaccharides intact
Polysaccharide digestion that isn’t mediated by amylases
Digested by enzymes on brush border membrane
Enzymes that can digest 1:6α linkages in polysaccharides
Those in the brush border membrane.
Examples of brush border enzymes that digest polysaccharides
1
2
3
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
How are glucose and fructose absorbed?
Rapidly absorbed across mucosal epithelium in duodenum and jejunum (predominantly at tips of villi)
Upon what does glucose transport largely depend at epithelial surface?
Na+
Name for glucose and Na+ cotransporter
Sodium-dependent glucose transporter (SGLT1)
No homology to the GLUT family of glucose transporters
What transports fructose across gut epithelium?
1
2
GLUT5 (facilitated diffusion) into epithelial cells (across apical membrane).
GLUT2 transports from epithelial cell into interstitium (across basolateral surface)
Outcome of having sweet taste receptors on jejunal villi
Stimulated by nutrisweet, etc.
Enhances glucose transport across epithelium
Where does digestion of proteins begin?
In stomach with pepsin
Pepsin secretion 1 2 3 4
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
Pepsin protein digestion
Hydrolyses bonds between aromatic amino acids (phenylalanine, tyrosine) and a second amino acid.
What is pepsin important for in food digestion?
Digesting proteins in cell-cell adhesions
CCK role in protein degradation
CCK release (caused by amino acids) triggers secretion of pancreatic proteases in form of inactive proenzymes
Activation of pancreatic proteases
1
2
3
1) Enterokinase in epithelial membrane phosphorylates trypsinogen
2) Trypsinogen activated to trypsin
3) This leads to a cascade of activation of other zymogens
Endopeptidases in duodenum
Trypsin, elastase, chymotrypsins
Types of polypeptides formed by trypsin, elastase, chymotrypsins
Short polypeptides
Proteases that produce free amino acids in the gut
Pancreastic carboxypeptidases
Peptidases at the brush border membrane
Mix of aminopeptidases, carboxypeptidases, endopeptidases and dipeptidases
How can proteins enter gut epithelial cells?
Either as free amino acids, or as di- or tri-peptides, which are broken into free amino acids within enterocytes
Number of transporters for free amino acids
Seven or more.
Co-transported molecules in amino acid transporters
Five require Na+
Two require Cl-
Where does fat digestion start?
In the mouth with a lingual lipase (very minor effect, maybe there for taste)
Stomach lipase role
Very minor role.
Maybe there to release enough fatty acids to trigger CCK secretion from duodenum
CCK effect on duodenal fat digestion
1
2
3
1) CCK stimulates secretion of pancreatic lipases
2) Lipase secreted in inactive form, activated by colipase.
3) Colipase is activated by trypsin
How are cholesterol esters digested in the duodenum?
Cholesterol esterase is released by pancreas.
This is activated by bile.
How are lipids emulsified?
Bile salts, lecithin are emulsifiers, are vigorously mixed.
Bile salts form micelles with lecithin and monoglycerides
Fat absorption in the gut
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)
Where do epithelial cells reform monoglycerides and monglycerides to triglycerides?
In smooth ER
Where are chylomicrons formed?
In enterocytes
Chylomicron formation
Once enterocytes have formed triglycerides in their smooth ER, triglycerides are coated with apolipoproteins, formed into chylomicrons
Where are chylomicrons secreted?
Secreted via exocytosis into the lymphatics (lacteal)
Short chain fatty acid generation in gut
1 a, b
2
3 a
• 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
