Digestion And Absorption Flashcards
When does digestion start?
Digestion starts with the mouth and continues on in the stomach and small intestine.
What is absorption ?
Absorption of all macronutrients takes place in the intestinal mucosal cel
Where are carbs digested?
Carbohydrates (CHO) are digested in the mouth and small intestine.
Carbohydrates provide mainly glucose, galactose and fructose for energy metabolism. Glucose can be stored as glycogen
Where are proteins digested?
Proteins are digested
in the stomach and small intestine
Dietary proteins provide essential amino acids for synthesis of proteins and neurotransmitters.
When are lipids digested?
Lipids are digested
in the stomach and small intestine.
Dietary lipids provide essential fatty acids for membranes and eicosanoids. Fatty acids are used for energy metabolism or are stored in fat cells
When are nucleic acids digested?
Nucleic acids are digested in the small intestine.
Nucleic acids
Ribose, deoxyribose and pyrimidine bases are taken up into the liver. Purine bases are degraded in intestinal cells to uric acid and released in urine
Summarize the big picture if orotein digestion and absorption
- Mouth: proteins are not degraded.
- Stomach: pepsin degrades proteins to polypeptides and amino acids.
- Small intestine: pancreatic proteases form oligopeptides and amino acids.
- Brush border cells: aminopeptidases form peptides and amino acids.
- Intestinal mucosal cells: release of amino acids into the portal vein for
transport to the live
How is trypsinogen activated by enteropeptidase?
Trypsinogen is released by the pancreas and is activated to trypsin only after it reaches the lumen of the duodenum.
This local separation prevents pancreatic damage as trypsin is a powerful protease and activates all other pancreatic zymogens
Explain the proteolytic activation of pancreatic zymogend
Once trypsinogen is activated by enteropeptidase, trypsin activates the following zymogens for protein digestion:
- trypsinogen to trypsin
- chymotrypsinogen to chymotrypsin
- proelastase to elastase
- procarboxypeptidases to carboxypeptidases
Trypsin also activates for lipid digestion:
- prophospholipase A2 to phospholipase A2 - procolipase to colipase (protein
How pancreatic proteases cleave proteins?
Pancreatic proteases cleave proteins after specific amino acid residues
Trypsin: - after Arg or Lys residues.
Chymotrypsin: - after bulky and aromatic residues.
Elastase: - after Gly, Ala, Ser residues.
Carboxypeptidases A or B (exo-peptidases) cleave amino acids from
the carboxyl-end.
What pancreatic proteases ends the. Cascade?
The different proteases act simultaneously
on the same proteins. Carboxypeptidases finish the job.
The digestive proteases cleave the dietary proteins. After dietary proteins
are cleaved the pancreatic proteases cleave the digestive enzymes.
This leads to an uptake of more amino acids than provided by the diet
Summarize the absorption of amino acids
The uptake of dietary amino acids is mainly performed by secondary active transport with cotransport of sodium ions.
The transporters are specific for a group of amino acids which can be overlapping.
The release into the portal vein is by facilitated transpor
Summarize the big picture of carbohydrate digestion and absorption
- Mouth: salivary a-amylase degrades starch and glycogen to dextrins, isomaltose and maltose.
Humans cannot digest cellulose (lost in feces). - Stomach: carbohydrate digestion stops.
- Small intestine: pancreatic a-amylase forms isomaltose and maltose.
- Brush border cells: disaccharidases cleave lactose, sucrose, isomaltose and maltose.
- Intestinal mucosal cells: release of glucose, fructose and galactose into the portal
vein for transport to the liver
Describe the absorption of dietary monosaccharides
Dietary fructose enters the intestinal mucosal
cells via facilitated transport by GLUT-5.
Dietary glucose and galactose
enter the intestinal mucosal cells via SGLT-1 which uses a secondary active transport and cotransport with sodium ions.
All three monosaccharides
are released by facilitated transport by GLUT-2 into the portal vein.
Summarize lipid digestion and absorption
- Mouth: lingual lipase is mainly swallowed.
- Stomach: lingual and gastric lipase degrade milk TAGs
with medium-chain FA. - Small intestine: Bile is needed for lipid degradation
and absorption. - Brush border cells: uptake of free fatty acids, MAG, free cholesterol and lipid-soluble
vitamins needs mixed micelles with bile salts. - Intestinal mucosal cells: TAGs and cholesteryl esters are formed and dietary lipids are released into the lymph inside of chylomicrons.
What is bile?
Bile is an alkaline fluid
that is released by hepatocytes into the bile ducts.
Bile contains: bile salts, phosphatidylcholine, free cholesterol and other.
Bile can be stored and concentrated in the gall bladder
What are the functions of bile?
- Transportation of free cholesterol and conjugated bilirubin in the bile from the liver to the duodenum. Bile salts increase solubility of free cholesterol in bile.
- Emulsification of dietary lipids in the small intestine for digestion by pancreatic lipase and cholesteryl esterase.
- Uptake of digested lipids into intestinal mucosal cells by forming mixed micelles.
What is the purpose of pancreatic lipase?
The protein colipase pushes the bile salts
away from TAGs and anchors pancreatic lipase to the lipid droplet.
The purpose of pancreatic lipase is to form molecules that can enter the intestinal mucosal cells.
This is achieved with MAGs which are not further degraded.
What is the clinical aaplication of pancreatic lipase?
Pancreatic lipase is inhibited by the drug Orlistat which is used for weight loss treatment
Summarize special TAG synthesis in mucosal cells
Only intestinal mucosal cells can start TAG synthesis with MAG formed by pancreatic lipase.
This special MAG pathway of TAG synthesis uses dietary MAG and fatty acids
What is the fate of chylomicrons?
Chylomicrons filled with dietary lipids are released into the lymph and reach the blood.
The dietary TAGs in chylomicrons are cleaved by lipoprotein lipases (LPL) mostly in capillaries of the heart and fat cells. The formed fatty acids are used for energy metabolism in heart and muscle or
are stored in fat cells as TAGs.
Only the chylomicron remnants containing less TAGs, mainly cholesteryl ester and the lipid-soluble vitamins are taken up by the liver by endocytosis.
Describe the enterogepatic circulation of bile salts
Hepatocytes release bile salts into bile canaliculi and the bile reaches the duodenum.
Bile salts are needed for: 1. Cholesterol transport into duodenum. 2. Emulsification for digestion and 3. Absorption of dietary lipids in the duodenum, jejunum and ileum.
95% of bile acids/salts are reabsorbed at the terminal ileum and reach the liver via the hepatic portal vein. They are transported in the blood bound to albumin.
The normal bile salt pool is about 3-5 g and about
0.5 g of bile acids/salts are lost per day into feces
What is enterohepatic circulation?
The enterohepatic circulation describes the formation of bile salts in the liver, their release into the bile ducts, the eventual reabsorption of bile salts/acids from the intestine and finally their transport back into the liver via the portal vein
The liver conjugates primary and secondary bile acids with glycine or taurine and releases bile salts and free cholesterol into the bile. Bile reaches the duodenum and the bile salts are now used for emulsification of dietary lipids mainly in the duodenum, jejunum and ileum. Bacteria from the large intestine are present in the terminal ileum and deconjugate bile salts to bile acids and glycine or taurine. Some primary bile acids are modified by these intestinal bacteria to secondary bile acids. A mixture of primary and secondary bile acids is taken up and reach the liver via the portal circulation. Hepatocytes conjugate primary and secondary bile acids for another cycle of the enterohepatic circulation. Only 5% of the released bile salts are excreted in feces
The small loss of bile acids is refilled in the liver by synthesis of primary bile acids using free cholesterol
