Flashcards in Digestion and Absorption Deck (46):
What form of carbohydrates can be absorbed through the intestinal epithelial cells?
Monosaccharides- thus all carbohydrates must be digested to monosaccharides in order to be absorbed by the intestine
Differentiate amylose from amylopectin
Amylose- straight chain polymers of glucose
Amylopectin- branched-chain polymers of glucose
What are the three disaccharides in food?
Trehalose, sucrose and lactose
What is the composition of trehalose?
two monomers of glucose
What is the composition of sucrose?
glucose and fructose
What is the composition of lactose?
Glucose and galactose
Why can cellulose not be absorbed?
Cellulose has 1,4 beta linkages, and we only have alpha amylose to break down carbohydrates, which breaks down alpha 1-4 linkages
The majority of carbohydrate digestion occurs where/by what enzyme?
Small intestine by pancreatic alpha amylase
What is the digestion product of pancreatic amylase?
Dextrins, maltose and maltotriose --> all di/tri or oligosacchardies
What enzyme is responsible for completing the digestion of disaccharides?
brush border enzymes --> alpha dextrinase, maltase and sucrase
Differentiate the digestion of three disaccharides in food vs. starch
Starches require amylase, the three disaccharides (trehalose, lactose and sucrose) do not.
How are glucose and galactose absorbed across the apical membrane of the small intestine?
SGLT1: sodium glucose transporter 1 transports glucose and galactose against their concentration gradients by coupling their transport to sodium
How are glucose and galactose extruded across the basolateral membrane into the blood?
Facilitated diffusion- GLUT2 transporter
How is fructose absorbed?
Fructose is absorbed via facilitated diffusion across both the apical and basolateral membrane
What causes lactose intolerance?
Lack or deficiency of lactase - the brush border enzyme responsible for digesting lactose into glucose and galactose
Is lactose intolerance normal?
Yes- this is the normal developmental decline in the expression of lactase by enterocytes
Where does protein digestion start?
Stomach- by pepsin
What are the two main classes of peptidases in the small intestine?
Endopeptidases and exopeptidases
Where is pepsinogen released from?
Chief cells- in response to a meal
How is pepsinogen converted to pepsin?
How is pepsin inactivated?
Pepsin is inactivated by pancreatic HCO3- in the duodenum
Is pepsin a necessary enzyme?
No- people with their stomach removed still have normal protein digestion
What digests protein in the small intestine?
Combo of pancreatic enzymes and brush border proteases
What form of protein is absorbable?
amino acids, di- and tri-peptides are absorbable
How are amino acids/ di/tri peptides absorbed?
Mostly symporters with Na
What else can peptide transporters take up?
How to amino acids exit the cell and enter the blood stream?
What is the major lipid in the diet?
Why are lipids a challenge to digest?
They are water insoluble and float on the surface of the gastric contents
What is the major contribution of the stomach to lipid digestion?
slow emptying of chyme into the small intestine- allows for adequate time for pancreatic enzyme action
CCK reduces gastric emptying
What is the role of pancreatic lipase?
Hydrolyze triglycerides to monoglycerides and fatty acids
What is the role of phospholipase A2?
Hydrolyze phospholipids such as those present in cell membranes to lysolecithin and fatty acids
What is the role of cholesterol ester hydrolase
Hydrolyzes cholesterol ester to free cholesterol and fatty acids
Is glycerol water soluble?
Briefly describe how lipids are absorbed into the bloodstream from the lumen of the intestine.
Lipids are surrounded by bile salts that form a micelle around them, which then diffuse to the membrane of the brush border epithelium.
Lipids are then released from the micelle and and diffuse into the cell (down their concentration gradient).
Once in the cell, they are modified and packaged into chylomicrons, which leave the cell via exocytosis, are taken up into the lymphatics, and return to circulation via the thoracic duct.
Where is bile primarily reabsorbed?
In the ileum
What proteins are responsible for repackaging the lipids in a cell into chylomicrons?
Where is the majority of water absorbed?
Compare the tight junctions of the colon vs. small intestine.
Small intestine: leaky- permits paracellular transport
Colon: tight- no paracellular transport (even water)
What is the net absorption of salt in the ileum and jejunum?
How are crypts different in function than the majority of cells lining the small intestine?
epithelial cells of the intestinal crypt SECRETE fluid and electrolytes
Describe how Cl- channels in the apical membrane of the intestinal epithelial cells are normally regulated vs. in cholera.
Normal: hormones and neurotransmitters such as ACh and VIP bind to the basolateral receptors and activate adenylyl cyclase which increases cAMP production. cAMP opens the Cl- channels and causes Cl- secretion.
Cholera activates the Cl- channels via a different receptors and overwhelms the absorptive capacity of the villus cells causing life-threatening diarrhea.
What cofactor is required for the absorption of calcium?
The active form of vitamin D: 1,25-dihydroxycholecalciferol
WHat protein binds Ca2+ in the cell and allows it to be pumped across the basolateral membrane?
What cofactor is required for the absorption of vitamin B12?