Digestion and Absorption Flashcards
What must happen prior to the absorption of carbohydrates?
The must be digested into monosaccharides before being absorbed
Describe the digestion of carbohydrates prior to absorption
Starches conveted to malt-ose -triose and dextrins via pancreatic/salivary amylases
Brush border membrane enzymes hydrolyze these oligosaccharides to glucose
Sucrose, lactose, and trehalose also cleaved by brush border enzymes
What monosaccarhides are absorbed by the intestine?
Glucose
Galactose
Fructose
How are glucose and galactose absorbed?
Common Na-dependent active transport system (SLGT-1)
Almost all glucose/galactose absorbed and subsequently transported to blood via facilitated diffusion
How is fructose absorbed?
Exclusively by facilitated diffusion
Cannot go against concentration gradient
What is lactose intolerance?
Deficiency in lactase, causing lactose to remain in GI tract as unabsorbed solute
Undigested lactose metabolized by colonic bacteria
What is sucrase-isomaltase deficiency?
Inherited disorder resulting in an inability to digest sucrose and isomaltose
What is glucose/galactose malabsorption?
Deficiency in SLGT-1
What are the two sources of protein in the GI tract?
Endogenous - secretory proteins and cells shed into GI tract lumen
Exogenous - dietary protein
What are endopeptidases, exopeptidases, and brush border peptidases
Endopeptidases - hydrolyze interior peptide bonds
Exopeptidases - hydrolyze one amino acid at a time from C terminus
BB Peptidases - cleave peptides produced by pancreatic proteases to oligopeptides and AAs
Describe the process of pancreatic enzyme secretion and protein digestion
Pancreatic enzymes secreted as inactive precursors
Trypsinogen –> trypsin via enterokinase
Trypsin catalyzes conversion of trypsinogen, Chymotrypsinogen, and proelastase
How are amino acids absorbed?
Na-dependent cotransport
Separate transporters for neutral, acidic, basic, and imino acids
Transported to blood via facilitated diffusion
How are di- and tri-peptides absorbed?
H-dependent cotransporter peptide transporter 1 (PEPT1)
Following a meal, most protein is absorbed in form of di- and tri-peptides
Absorbed peptides are hydrolyzed in the enterocyte to AA’s
What is cystinuria?
Affects uptake of basic amino acids
What is Hartnup disease?
Affects uptake of neurtral amino acids
What is familial iminoglycinuria?
Affects uptake of proline and hydroxyproline
How are lipids digested in the stomach?
Mixing in stomach breaks lipids into droplets to increase total surface area
gastric lipase hydrolyzes triglycerides to diglycerides and FFA’s
CCK slows gastric emptying to allow sufficient time for digestion and absorption in duodenum
What is the role of bile salts and lecithin in the small intestine?
Emulsify fats
Emulsification requires neutral or slightly alkaline environment
What is the function of pancreatic lipase?
Cleaves fatty acids from 1 and 3 positions of triglycerides
What is the function of cholesterol ester hydrolase?
Cleaves fatty acid from cholesterol esters
Produces free cholesterol and gatty acid
What is the function of Phospholipase A2?
Releases fatty acids from 2 position of phospholipids
produces lysophospholipids and FFAs
What is colipase?
Non-enzymatic protein secreted as inactive precursor by pancreas
Activated by trypsin
Prevents inhibition of pancreatic lipase by bile salts, which displace enzyme from surface of emulsion droplet
How are lipids absorbed?
Micelles bring products of lipolysis through the unstirred water layer directly to the lumenal membrane
Lipid products diffuse directly into the enterocyte
What is the monoglyceride acylation pathway?
Triglycerides synthesized from 2 monoglycerides and CoA-activated fatty acids
Long-chain reesterified in smooth ER
Medium chain enter bloodstream without resynthesis into triglyceride
What is the phosphatidic acid pathway?
Formation of phospholipids from ingested fats
How is cholesterol absorbed?
Absorbed in free form
Significant portion re-esterified with fatty acids in enterocyte
All transported in chylomicrons
How are chylomicrons formed and transported?
Resynthesized triglycerides, cholesterol and phospholipids
Transported out via exocytosis and enter lymphatic lacteals
Reach bloodstream via thoracic duct
What can cause lipid malabsorption?
Failure to digest fat - unsecreted or inactivated pancreatic enzymes
Absence of bile salts
Decrease number of absorbing cells
Failure to synthesize apoproteins
What can cause the absence of bile salts?
Liver disease
Common bile duct obstruction
Bacterial overgrowth of small intestine (leads to deconjugation)
Increased duodenal acidity
What is gluten enteropathy (celiac sprue)?
Allergic reaction to gluten in wheat products
Leads to loss of villi in small intestine
Also causes malabsorption of carbs and proteins
What is abetalipoproteinemia?
Failure to synthesize ApoB
Results in lack of formation of chylomicrons
How is heme iron absorbed?
Most easily absorbed form
Taken up by receptor-mediated endocytosis or transporter protein HCP1
Heme then broken down by oxygenase to release free iron
How is free iron absorbed?
Gastric acid dissolves iron and permits it to form soluble complexes with ascorbic acid
This reduces it to ferrous iron, which is more easily absorbed
Ferrous iron transported across brush border by DMT1
Binds to apoferritin for storage
What is the function of duodenal cytochrome b?
Dcytb reduces ferric to ferrous iron, allowing it to be absorbed
What is ferroportin?
Protein that transports iron out of the enterocyte into plasma
How is iron absorption regulated?
Hepcidin regulates entery of iron into plasma by binding to ferroportin
This leads to internalization and degradation of ferroportin
This blocks cellular iron export and reduces plasma iron
What regulation occurs in low and high iron states?
Low iron - low hepcidin, causing increased iron absoprtion and elevated iron release from enterocytes
High iron - high hepcidin, decrease iron export from enterocytes, enterocytes containing iron lost to intestine
What is hemochromatosis?
Chronic absorption of too much iron
Defect in HFE gene causes hepcidin levels to drop
Excess iron collects in liver, leading to cirrhosis and cancer
Can also lead to diabetes and coronary disease
How does aging affect the mouth?
Lose taste buds, chewing weakens, may lose teeth
How does aging affect the esophagus?
Swallowing becomes more difficult, LES function declines
How does aging affect the stomach?
Lose parietal cells
Increase in ulcers because increased use of NSAIDs
How does aging affect the small intestine?
Decrease in motility
Increase in malabsorption
How does aging affect the colon?
Decrease in motility
Increase in polyps
