Digestion and Absorption in the GI tract Flashcards
Where does carbohydrate digestion begin?
in the mouth - ptyalin, an alpha amylase
How much carbohydrate digestion is done by ptyalin?
<5% in the mouth, but it will continue to work in the stomach –> leading to 30-40% of starch digestion to maltose
How is carbohydrate digested in the stomach?
The action of pytalin continues for about an hour, before it’s inhibits by the gastric acid
Where/how is the majority of the carbohydrate digested?
In the duodenum, by pancreatic enzymes
- pancreatic alpha amylase is very similar to the salivary one, but much more powerful
- will digest all the starch
How is carbohydrate absorbed?
After digestion by pancreatic amylase, the intestinal epithelial enzymes @ the brush border will break it down disaccharides and small glucose to monosaccharides
What are the breakdown products of lactose, sucrose, and maltose?
Lactose = galactose + glucose
Sucrose = fructose + glucose
Maltose = glucose
Where does protein digestion begin?
It begins in the stomach.
What happens in the stomach re: protein digestion?
Pepsin –> breaks down the collagen fibers, so proteolytic enzymes can penetrate
Where is the majority of the protein digested?
Duodenum, for pancreatic proteolytic enzymes
What are the actions of pancreatic proteolytic enzymes?
Breaks protein down to di- , tri- peptides and some larger peptides
Where does the majority of the protein gets broken down into amino acids?
At the brush border, this is done by peptidases in the enterocytes that line the villi
What are some pancreatic proteolytic enzymes and their actions?
- trypsin & chymotrypsin: split in to small polypeptides
- carboxypeptidases: cleaves amino acids at the carboxyl tail of polypeptides
- proelastase: elastase, digests elastin fibers that hold meat together
What happens to protein at the enterocytes?
- brush border: aminopolypeptidase and dipeptidases –> split protein into tri-, di-peptides and some amino acids –> then transported into the enterocytes
- inside the enterocytes: amino acid linkage specific enzymes breakdown protein into amino acid, then it enters blood
Where is fat digested?
in the duodenum - needs emulsification by bile acid and lecithin
What’s emulsification?
Process which fat globules are broken into smaller pieces by detergent (bile acids, esp lecithin) –> increases the total surface area
- lipases = water soluble, can only attack fat at their surface
How is triglycerides digested?
triglyceride is digested by pancreatic lipase
- can be digested into free fatty acids within minutes after emulsification
What’s the role of micelle in fat digestion?
Formation of micelle from bile acids accelerates fatty acid digestion.
- hydrolysis of triglycerides = highly reversible, so if there is free fatty acids in the proximity, the reaction will stop
- so micelle packs the fatty acids and monoglycerides to the brush borders of enterocytes
How does the absorptive surface of the small intestine increase?
- fold of kerckring (x3)
- villi (x10)
- microvilli (x100)
How is water absorbed in the intestines?
- by osmosis
- Na+ in the paracellular space creates osmotic pool –> water absorbed in the tight junctions in the apical borders, then into circulating blood of the villi
How is Na+ absorbed?
It is actively transported (Na+/K+ pump) from inside the enterocyte to the paracellular space in the basolateral membranes
- this provides a steep electrochemical gradient for Na+ from chyme to move through the brush border into the epithelial cell cytoplasm
How does aldosterone influence water absorption?
Aldosterone (stimulated by dehydration, secreted by the adrenal gland) enhances water absorption, along with other ions such as Cl-
- most prominent at the colon
What happens in diarrhea?
In the large intestines, toxins from cholera and other bacteria can stimulate secretion of large amounts of NaCl and water
How is calcium absorbed?
It’s actively absorbed in the intestine, with the aid of vitamin D
- parathyroid hormone will activate vitamin D at the kidneys; activated vitamin D enhances Ca2+ absorption
How is carbohydrate absorbed?
once it’s broken down into monosaccharides –> most abundant = glucose
- glucose: active co-transport with Na+ via the SGLT1 from lumen into the enterocyte, then it’s facilitated diffusion via GLUT2 from inside the cell through the basolateral membranes
- galactose is absorbed the same way as glucsoe
- fructose has it’s own facilitated diffusion via GLUT5 from lumen into the cell, but also uses GLUT2 to exit into the paracellular space
- some fructose is converted to glucose in the cytoplasm
How is protein absorbed?
Protein, once is broken down to di- , tri-peptide and free amino acids, uses the same mechanism as glucose transport
- Na+ co-transport
- a few amino acids has their own special membrane transport similar to fructose
How is fat absorbed into the enterocyte?
Fats = soluble in the enterocyte border
- some are recombined into triglycerides in cytoplasm
- a few are further digested into glycerol and fatty acids
- triglycerides cannot freely pass through the enterocytes
How is fat absorbed into the body?
Triglycerides will enter the Golgi apparatus –> aggregates as globules (cholesterol and phospholipids)
- re-package themselves so the fatty heads are in the middle with the polar tails on the surface –> makes the globules miscible with water
- then globules released from the Golgi apparatus and exocytosed into the basolateral membrane
- enter the lymphatic system via central lacteal of villi
- once in the lymphatics, it’s call a chylomicron
- drains into the thoracic duct
