Digestion and Absorption Flashcards
What is digestion?
the chemical and mechanical breakdown of ingested food into absorbable molecules
What is absorption?
movement of macronutrients, water, electrolytes from intestinal lumen into blood
What are the two pathways for absorption?
- transcellular
- paracellular
What is transcellular absorption?
cross luminal membrane via passive diffusion or transporters
What is paracellular absorption?
move across tight junctions between intestinal epithelial cells
What is the structure of intestinal mucosa?
- surface arranged in transverse folds
- villi line lumen of small intestine
The folds, villi, and microvilli of small intestine increase the surface area by how much?
600 fold
The surface of epithelial cells in the small intestinal lumen are line with microvilli, which is known as?
brush border
The villi of the small intestinal lumen are lined with what?
epithelial cells and goblet cells
Villi are longest in what section of small intestine?
duodenum
Villi are shortest in what section of small intestine?
ileum
What are the transverse folds of the small intestine?
folds of Kerckring with fingerlike villi
Give an example of monosaccharide.
glucose
Give an example of disaccharide.
sucrose
Give an eexample of polysaccharide.
amylose
What is the primary source of energy for most cells?
glucose
How is glucose supplied in diet?
- monosaccharides (simple sugars)
- disaccharides
- polysaccharides (complex chain of sugars)
All ingested carbohydrates must be digested to what for absorption by enterocytes?
monosaccharides
What enzymes are used for digestion of starch?
- alpha amylase
- glucoamylase
- alpha dextrinase
- maltase
- sucrase
- lactase
What are the monosaccharides for absorption?
- glucose
- galactose
- fructose
What are the brush border enzymes?
- sucrase
- lactase
- maltase
- alpha dextrinase
There are no mammalian enzymes to digest cellulose. What is used instead?
microbial enzymes
What transporter is used for glucose and galactose to get from intestinal lumen into enterocyte?
SGLT 1 (a Na+/glucose co-transporter)
What transporter is used for only fructose to get from intestinal lumen to enterocyte?
GLUT 5 (facilitated diffusion)
What transporter is used for glucose, galactose, and fructose to get from enterocyte into blood?
GLUT 2 (facilitated diffusion)
What enzyme will cleave bonds of oligosaccharides released from starch?
alpha dextrinase
A disorder characterized by failure to digest carbohydrates to absorbable form will cause what?
carbohydrates will remain in lumen and “hold” water to remain isosmotic which leads to osmotic diarrhea
Lactose intolerance is caused by what?
lactase deficiency
Lactose intolerance leads to what?
osmotic diarrhea
Which species have a natural diet high in cellulose, a complex polysaccharide?
- horses
- cattle
- sheep
Which species are usually fed diets high in grain (starch)?
- chickens
- pigs
Which species are omnivores and do well on diets of starch and soluble fiber?
- dogs
Which species is usually fed some starch in pet food diets but is naturally on a prey diet?
- cats
How many essential amino acids are there?
10
List the 10 essential amino acids. (Hint: PVT TIM HALL)
- phenylanine
- valine
- threonine
- tryptophan
- isoleucine
- methionine
- histidine
- arginine
- leucine
- lysine
Amino acids are usually provided in what form?
protein
Proteins must be digested to absorable forms via what enzymes?
proteases
What is the absorbable form of proteins?
amino acids and short peptides
Digestion of protein is ultimately completed by?
- endopeptidases
- exopeptidases
In young ruminants, chief cells of the abomasum produce what? Why?
rennin (NO, NOT RENIN)
an ezyme that coagulates milk and reduces passage rate
Protein digestion begins where?
stomach
What causes the protein to unfold and expose peptide bonds to pepsin?
HCl
What enzyme is activated to pepsin at a low pH?
pepsinogen
Digestion of proteins in the small intestine uses what?
pancreatic and brush border proteases
As digesta enters the small intestine, what is released?
CCK
The release of CCK as a result of digesta entering SI causes release of what?
zymogens (inactive enzymes)
What is the first enzyme to be activated in the SI?
trypsinogen
Trypsinogen is activated by what to form trypsin?
enterokinase
What are the zymogens in the SI?
- trypsinogen
- chymotrypsinogen
- proelastase
- procarboxypeptidase A and B
What is responsible for activating all other enzymes in the SI?
trypsin
What are the endopeptidases?
- trypsin
- chymotrypsin
- elastase
What are the exopeptidases?
- carboxypeptidases A and B
What is the final digestion step of proteins in the SI?
amino peptidases at the brush border break down proteins into single amino acids
Amino acids move across the basolateral membrane (enterocyte to blood) via?
facilitated diffusion
Enterocyte can absorb what?
- amino acids
- depeptides
- tripeptides
there are four co-transporters in the luminal membrane for amino acids using what?
the Na+ gradient
Di- and tri-peptides using the H+ dependent co transporters are cleaved to amino acids inside the cell by?
aminopeptidase
In pancreatitis, prematurely activated enzymes can lead to local damage of exocrine pancreas and cause what?
edema, inflammation
What is exocrine pancreatic insufficiency?
insufficient production and secretion of digestive enzymes
What general disorder of protein digestion and absorption can be caused secondary to chronic pancreatitis?
exocrine pancreatic insufficiency
What is cystinuria?
defect or absence of a Na+/amino acid co transporter that can contribute to crystal formation and calculi, leading to urinary blockages
Triglycerides are made up of what?
glycerol and 3 fatty acids
What supplies dietary lipids?
- triglycerides
- cholesterol
- phospholipids
Lipids must be what for digestion and absorption?
solubilized in the SI
What is the significance of churning for lipid digestion?
Breaks lipids into small droplets to increase surface area
What is repsonsible for churning and mxiing lipids to initiate enzymatic digestion?
stomach
What enzyme hydrolyzes about 10% of ingested TAG into glycerol and FFAs?
gastric lipase
The stomach slowly empties chyme into the SI, which allows?
plenty of time for pancreatic lipase to digest
What are the two portions of SI digestion of lipids?
- bile salts emulsify lipids –> surround small lipid droplets to increase surface area
- pancreatic enzymes digest different lipids –> lipase, colipase, cholesterol ester hydrolase, phospholipase A2
Lipase needs colipase. Why?
Colipase binds to lipase and allows it to digest at lipid water interface
What hydrolyzes cholesterol ester into free cholesterol and FA, and releases glycerol from triglycerides?
cholesterol ester hydrolase
Phospholipase A2 is activated by?
trypsin
What hydrolyzes phospholipids into lysolecithin and FA?
phospholipase A2
The final products of lipid digestion must be solubilized into what for absorption using bile salts?
micelles
Bile salts solubilize products within SI lumen as micelles, except what?
glycerol
What are the five steps to absorption of lipids?
- bile salts solubilize products within SI lumen as micelles
- micelles diffues to brush border membrane of enterocyte, products are released and diffused into cell (NOT bile salts)
- products are re-esterified inside enterocyte to form original compounds
- re-esterified lipids are packaged with apoproteins to form chylomicrons
- Chylomicrons packaged in secretory vesicles and exocytosed across basolateral membrane into lymphatic capillaries
Why do chylomicrons exocytose into lymphatic capillaries?
too big for vascular capillaries –> will enter blood at thoracic duct
A problem at any step of digestion or absorption of lipids can result in what?
steatorrhea
A lack of apoprotein B causes what?
abetealipoproteinemia (chylomicrons do not form or cannot be transported)
Fat soluble vitamins within SI lumen are incorporated into what for absorption? Transport?
micelles for absorption
chylomicrons for transport
Most water soluble vitamins are absorbed via what in SI?
Na+ dependent co-transport
What water soluble vitamin requires intrinsic factor for absorption?
B12 (cobalamin)
What are the four steps for B12 absorption?
- B12 released from food by pepsin
- B12 binds to R proteins from salivary secretions
- Proteases in SI degrade R proteins and release B12, which then binds intrinsic factor.
- B12/IF complex resists proteases, travels to ileum for absorption
What active vitamin do you need to absorb calcium in SI?
vitamin D
How is vitamin D activated?
Dietary vitamin D3 (cholecalciferol)
–> 25-hydroxycholecalciferol in liver
–> 1,25-dihydroxycholecalciferol in kidney (active form)
The lack of vitamin D or calcium results in what?
rickets and osteomalacia
Active vitamin D promotes synthesis of what?
calbindin D-28 K
What ion can be asorbed by both active and passive mechanisms?
calcium
Iron can occur as?
free iron or heme iron
What is heme iron?
iron bound to hemoglobin or myoglobin
Iron absorption occurs by what transporter?
DMT-1 transporter in luminal membrane
Heme iron is digested into free iron by what?
lysosomal enzymes inside enterocytes
Free iron binds to apoferritin inside enterocytes for transport into blood via?
ferroportin
Iron circulates bound to transferrin and is stored as what?
ferritin
Once free iron is transported into blood via ferroportin it binds to what in plasma?
transferrin
