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GI Digestion and Absorption
Digestion and Absorption in the GI Tract
51
Physiology
Graduate
03/08/2012

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Term
What are the results of digestion that are capable of being absorbed for the following: carbohydrates, fats, proteins.
Definition
1. monosaccharides
2. small peptides and amino acids
3. monoglycerides and FFAs
Term
What is involved with luminal digestion? How about membrane or contact digestion?
Definition
1. digestion occurs in the GI tract with enzymes from the stomach, pancreas, etc
2. digestion occurs as nutrients make contact with the brush border of villi and the enzymes in the enterocytes break down the nutrients
Term
What digestive enzymes are secreted by the salivary glands? The stomach?
Definition
1. α-amylase and lingual lipase
2. pepsin
Term
What digestive enzymes are secreted by the pancreas?
Definition
amylase, trypsin, chymotrypsin, carboxypeptidase, elastase, lipase-colipase, phospholipase A2, and cholesterol esterase
Term
What digestive enzymes are secreted by the intestinal mucosa?
Definition
enterokinase, sucrase, maltase, lactase, trehalase, α-dextrinase (isomaltase), amino-oligopeptidase, and dipeptidase
Term
What is the total surface areas of the small intestine? What contribute to the surface area?
Definition
1. 250-400 m^2
2. the length, the folds of Kerckring, the villi, and the microvilli
Term
What does senescent mean?
Definition
something has reached the end of its division/growth power; this is used to refer to enterocytes that migrate up the crypts to the microvilli and cross the border beyond which they cannot differentiate
Term
What is the function of trehalase?
Definition
it digests three-carbon sugars
Term
What is the shape of the cells in the crypts of Lieberkühn? How about the cells of the villi?
Definition
1. cuboidal
2. columnar
Term
What happens to enterocytes when they reach the top of the villi and die?
Definition
they are shed into the GI tract and become digested and absorbed
Term
What are the cell types in a villus?
Definition
secretory, endocrine, goblet, and mature absorptive epithelial
Term
What four mechanisms are important in transporting substances across the intestinal cell?
Definition
a. active transport (primary and secondary)
b. passive diffusion
c. facilitated diffusion
d. endocytosis
Term
What is the most important process in SI absorption? How is it achieved? Why is it so important?
Definition
1. the absorption of Na+
2. by the activity of Na+/K+ ATPases on the basolateral side of the membrane
3. it provides the energy for secondary transport for many other substances and provides the shift in osmotic pressure that allows absorption of a lot of water
Term
What substances are absorbed in the stomach? The duodenum and jejunum?
Definition
1. ethanol and NSAIDs
2. fats, carbs, amino acids, vitamins, ions, water, and electrolytes
Term
What substances are absorbed in the ileum? The colon? The rectum?
Definition
1. bile salts and vitamin B12
2. water and electrolytes
3. drugs such as steroids and salicylates
Term
What barrier must a nutrient cross to be absorbed by the blood or lymph?
Definition
the unstirred layer, glycocalyx, apical cell membrane, cytoplasm, basolateral membrane, intercellular space, basement membrane, and capillary wall or lymph vessel wall
Term
Where does carbohydrate digestion begin? What is the rate-limiting step of carbohydrate absorption?
Definition
1. in the mouth with saliva; it continues in the stomach with acid and in the SI with pancreatic amylase; lactose and sucrose, however, are only digested by contact digestion
2. assimilation
Term
What do the following break down into: starch, maltose, lastose, sucrose
Definition
1. maltose and then glucose
2. glucose
3. glucose and galactose
4. fructose and glucose
Term
How are the following transported across the apical membrane: glucose, galactose, fructose.
Definition
1. secondary active transport with Na+ in SGLUT-1 using energy from Na+/K+ ATPase
2. same as 1
3 facilitated diffusion using GLUT-5
Term
What is a hexose transporter? Which ones transport by facilitated diffusion? Which ones transport by secondary active transport?
Definition
1. a transporter of a 6-carbon ring structure (sugars)
2. GLUTs 1-5
3. SGLUT-1
Term
What is lactose intolerance? What are its symptoms? How is it diagnosed?
Definition
1. a lack of brush border lactase
2. abdominal cramps, bloating, diarrhea, and flatulence
3. feed the patient lactose and look for glucose in the plasma
Term
What is the consequence of absent SGLUT-1?
Definition
the patient will have to be fed fructose to get his carbohydrates
Term
Where does protein digestion begin?
Definition
Stomach, followed by SI via endo- and exopeptidases, brush border oligopeptidases and dipeptidases, and cytoplasmmic dipeptidases
Term
What is the activation/deactivation mechanism for proteolytic enzymes?
Definition
enterokinase activates trysinogen, making trypsin; trypsin autocatalyzes and also activates other proenzymes; proteolytic enzymes digest themselves
Term
How are protein break-down products absorbed?
Definition
amino acids are absorbed by secondary active transport and facilitated diffusion; di- and tri- peptides are absorbed by different carrier systems, but are absorbed faster
Term
What is pancreatic insufficiency? What causes it?
Definition
1. inability or decreased ability of the pancreas to produce digestive enzymes
2. pancreatitis or CF
Term
What are possible problems a person might have that would prevent them from assimilating protein?
Definition
pancreatic insufficiency, congenital absence of trypsin, Hartnup's disease
Term
What is the consequence of a congenital absence of trypsin?
Definition
if there is no trypsin, none of the other pancreatic enzymes will be activated
Term
What is Hartnup's disease? What are its consequences?
Definition
1. an inability to absorb neutral amino acids
2. the patient has reduced ability to absorb some amino acids, but can still get neutral amino acids through di- and tri- peptides
Term
What are the most common sources of fiber? What is the difference between these fibers?
Definition
1. wheat and pectin
2. wheat decreases transit time of food and pectin increases transit time of food
Term
After absorption, how is fructose processed?
Definition
in the liver, it is turned into fru-1-phosphate, then into either glyceraldehyde or dihydroxyacetone; glyceraldehyde turns into GBP (glucose bisphosphate) and can enter gluconeogenesis to make glycogen or to make glycerol to start TG synthesis in the liver
Term
How does trypsin work? Chymotrypsin? Carboxypeptidase?
Definition
1. cleaves peptide bonds on the carboxy side of a basic amino acid
2. cleaves on the amino side
3. cleaves the amino acids off the carboxy side
Term
Define: sprue and tropical.
Definition
1. not being able to absorb something in your SI
2. typically bacteria or parasite; non-tropical is often idiopathic
Term
What are the main processes that triglycerides must undergo to by absorbed?
Definition
emulsification, enzymatic digestion into glyceride and FFAs, and reconstitution into TGs and chylomicron formation
Term
What is a micelle? Steatorrhea?
Definition
1. a spherical formation of FFAs
2. fat in the stool
NOTE: steatorrhea can be an indicator of sprue is the fat is in the form of FFA (in which case it has been digested but not aborbed)
Term
List the break-down products of the following: triglyceride, cholesterol-ester, and phospholipid.
Definition
1. either 1 2-mono-glyceride and 2 FFAs or 1 glycerol and 3 FFAs
2. 1 cholesterol and 1 FFA
3. 1 lysolecithin and 1 FFA
Term
What breaks down a 2-monoglyceride?
Definition
cholesterol ester hydrolase
Term
What is malabsorption? What can cause it?
Definition
1. inability to absorb something, usually fat
2. motility disorders causing to rapid peristalsis, digestion disorders (pancreatitis or CF), or an absorption disorder (tropical or nontropical sprue)
Term
Give examples of nontropical sprue. What is another name for nontropical sprue? What does it cause?
Definition
1. allergy to gluten (wheat, rye)
2. celiac disease
3. destruction of the microvilli and sometimes to villi as well
Term
On average, what is the volume of fluid entering the gut? How much is absorbed? How much is excreted?
Definition
1. about 9 L
2. about 8.8-8.9 L
3. 100-200 mL
Term
What is the tonicity of chyme throughout the GI tract?
Definition
isotonic except in some of the large intestine, where water is greatly reabsorbed
Term
How is the majority of water and electrolytes absorbed?
Definition
Basolateral Na+/K+ ATPases cause a Na+ gradient, so Na+ is reabsorbed by diffusion through water-filled channels, co-transport with amino acids or glucose, co-transport with Cl-, or counter-transport with H+, and Cl- and water follow the Na+
Term
What hormone greatly increases Na+ absorption in the gut?
Definition
aldosterone
Term
About how much total Na+ is absorbed daily? How much is typically taken in in the diet? How much is secreted in the small intestine? How much is absorbed from the intestine? How much is excreted in the feces?
Definition
1. 15% of the total body Na+
2. 5-8 g/day
3. 20-30 g/day
4. 25-35 g/day
5. 0.1 g/day
Term
How is water secreted into the lumen?
Definition
increased osmotic pressure resulting from digestion of food and/or crypt cells actively secrete Cl- (whose channel is activated by cAMP, which will be followed by Na+, which will pull water into the lumen
Term
What is the third leading cause of death by disease worldwide? What are some common causes of it?
Definition
1. diarrhea
2. infectious diarrhea, ulcerative colitis, drug-related diarrhea, or psychogenic diarrhea
Term
What is cholera? What can it cause? What is its mechanism of action? What is the prognosis for it? What is the treatment for it?
Definition
1. vibrio cholerae infection
2. cholera toxin stimulates secretion of water and electrolytes from crypt cells; patients may lose up to 10 L per day
3. the cholera toxin increases the cAMP to the CFTR, causing increased Cl- loss and increased Na+ and water loss in turn
4. untreated, 50% will die; treated, <1% would die
5. oral rehydration (glucose and electrolytes)
Term
What stimulates net secretion in the following: mucosa epithelial cells, lamina propria cells, enteric neurons, blood?
Definition
1. serotonin, CCR, gastrin, and neurotensin
2. aracadonic acid, NO, bradykinins
3. ACh, serotonin, VIP, and substance P
4. VIP and prostaglandins
Term
What stimulates a net absorption in the following: mucosa epithelial cells, lamina propria cells, enteric neurons, blood?
Definition
1. somatostatin
2. none listed
3. norepinephrine and neuropeptide Y
4. epinephrine, glucocorticoids, and aldosterone
Term
What is lecithin? What does it do?
Definition
a phospholipid secreted in the bile that emulsifies fats to assist in their absorption
Term
What is feces composed of? Why is feces brown?
Definition
1. about 3/4 water and 1/4 solid matter; the solid matter has dead bacteria, fat, inorganic matter, protein, fiber, dried constituents of digestive juices, bile pigment, and epithelial cells
2. it contains breakdown products of bilirubin, which are brown
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