Digestion And Absorption Flashcards
What is the definition of digestion?
The mechanical, chemical and microbial breakdown of large, insoluble food molecules into simple absorbable compounds.
What is the definition of absorption?
The process by which these simple compounds are taken across the intestinal membrane into the blood
What is the basic function of alpha amylases?
Alpha amylases catalyse the hydrolysis of starch into sugars
What is the optimum pH of salivary alpha amylase?
6.6-6.8
What is the function of salivary alpha amylase?
To START carbohydrate suggestion and act on starch
Where is pancreatic alpha amylase secreted from?
Secreted from the exocrine pancreas
What is pancreatic alpha amylase secreted from?
Secreted from the exocrine pancreas
What is the function of pancreatic alpha amylase?
To END carbohydrate digestion and act on complex carbohydrates.
How do alpha amylases work?
They attack alpha-1,4 glycosidic bonds in the middle of a CHO chain.
What are the 2 phases of carbohydrate digestion?
1) Luminal phase
2) Membranous phase
What happens during the luminal phase of carbohydrate digestion?
- Starch and glycogen are degraded into compounds containing 2-9 glucose units
- Remember that the products of luminal degradation cannot be absorbed by epithelial cells.
What happens during the membranous phase of carbohydrate digestion?
Di, tri and aligosacchrides broken down into monosaccharides by enzymes bound to the apical membrane of epithelial cells.
Where does the luminal phase of carbohydrate digestion occur?
Luminal phase happens mostly in the duodenum as this is where the ducts of the pancreas empty
What are enterocytes?
Columnar epithelial cells found at the surface of the villi
What 3 types of vessel can be found within each of the individual villi
Artery, vein, lymphatic vessel
What is the life span for an enterocyte?
2-3 days; when they die they will either be passed out with faeces/ degraded and reabsorbed
What are the 4 different mechanisms for absorption?
- Passive diffusion
- Facilitated diffusion
- Active transport
- Endocytosis
What are the products of carbohydrate digestion?
Glucose, fructose and galactose
How is fructose absorbed?
Fructose absorbed by carrier mediated (facilitated diffusion)
How is glucose transported from the lumen of the GI tract into the blood?
- Glucose and sodium are cotransported together from the lumen into the intestinal cell
- The concentration of sodium and glucose is higher inside the cell than in the blood
- Glucose is moved into the blood via facilitated diffusion
- Sodium pumps into the blood and potassium is pumped out of the blood (needs energy)
What acs as a competitive inhibitor for glucose?
Galactose (can also be inhibited by glucose analogues)
What is the general function of proteases?
Proteases break down proteins and polypeptides
How do proteases work?
They cause cleavage of the peptide bonds by hydrolysis
What are zymogens?
Zymogens are inactive precursors of proteases
What are the two groups of gastrointestinal proteases?
- Endopeptidases (these attack and cleave in the middle of amino acid chains)
- Exopeptidases (these split off amino acids from the end of the chain)
Where are endopeptidases and exopeptidases produced?
- Endopeptidases are secreted by the stomach and the pancreas
- Exopeptidases are secreted by the pancreas and the glands of the small intestine
What are pepsins?
Pepsins are gastric proteases
What is the function of pepsin and where is it produced?
- Pepsin is alive at a low pH (<3)
- Pepsin attacks all proteins (except mucin and keratin)
- Pepsin is made from pepsinogen which is secreted by chief cells
What is secreted simultaneously with pepsinogen?
H+ (secreted by parietal cells)
How is HCl secreted by parietal cells?
By carrier mediated active transport from the parietal cell into the lumen
Why is HCl needed?
- HCl activates pepsinogen to make activated pepsinogen
- Activated pepsinogen loses 44 amino acids from its chain to make pepsin
How does pepsin make a positive feedback loop?
When pepsin is made, it stimulates more activated pepsinogen to become pepsin.
What are the 3 endopeptidases produced by the pancreas?
- Trypsinogen (trypsin)
- Chymotrypsinogen (chymotrypsin)
- Pro-elastase (elastase)
Why do pancreatic endopeptidases have an alkaline optimum pH?
Because carbonate HCO3- ions are also secreted from the pancreas
What are the two types of pancreatic Exopeptidase?
- Carboxypeptidases (split amino acids from the carboxy terminal)
- Aminopeptidases (split amino acids from the N terminal)
Complete the sentence;
Proteases are both stores and secreted as ….
Zymogens
What is the function of lipases?
Lipases digest fats by remaining fatty acids from triglycerides
Where is lingual lipase active?
In the mouth and stomach
Where is pancreatic lipase active?
In the small intestine (requires phospholipids and bile acids for activation)
When is bile released?
In response to CCK
What happens to bile after it has been used?
5% is excreted in faeces, 95% returns to the liver via enterohepatic circulation
What does a micelle look like?
- Small, spherical structures with hydrophilic heads on the outside and hydrophobic tails on the inside
What does gastric lipase do?
Acts on fat droplets to start breaking them down into smaller droplets
After pancreatic lipase had bound to the fat droplet, what else much be involved for fat digestion to start?
Colipase
What happens once fatty acids and monoglycerides diffuse into micelles
They merge with the brush borders and are absorbed into the enterocytes
How are chylomicrons formed?
Triacylglycerol is conjugated with proteins and phospholipids to form chylomicrons
What else happens to T.A.G after the formation of chylomicrons?
Some of the TAG is broken down and fatty acids released into tissues
How are vitamins and minerals absorbed into the small intestine?
By passive diffusion, carrier-mediated transport and active transport.
Where does the absorption of water occur?
In the small and large intestine
