digestion of carbs, proteins, lipids- overview Flashcards
what drives the absorption process?
Na+/K+ ATPase
what is the only sugar not dependent on ATPase?
fructose
what sugar is absorbed via secondary active transport?
glucose
what is the effect of insulin on absorption in the small intestine?
None.
Insulin does not affect secondary active transport!
Insulin does not affect glucose transport in:
-stomach, kidney,
Insulin mainly affects in adipose tissue, resting muscles
5 patterns of digestion and absorption
- Glucose-does not require digestion
- Protein-digested in lumen
3.Sucrose-digested by brush border
enzymes before absorption
4.Oligopeptide-directly absorbed by
cell then broken down inside cell
5.TAG-broken down before absorption
cell then resynthesize original
molecule
small intestine overview and secretion
All chemical digestion & nutrients absorption occurs here
Circular folds of mucosa, villi, and microvilli – enhance s/a for absorption
Largest folds –circular folds-mucosa & submucosa-plicae circularis
Secretion
Juice secreted by small intestine does not contain any digestive enzymes
Synthesized enzymes act within brush-border membrane of epithelial cells
Enterokinase
Disaccharidases
Aminopeptidases
Enzymes of/ into small intestine
Peptidase – breaks down peptides into amino acids
Sucrase, maltase, lactase – break down disaccharides into
monosaccharides
Lipase – breaks down fats into fatty acids and glycerol
Enterokinase – converts trypsinogen to trypsin
Somatostatin – hormone that inhibits acid secretion by stomach
Cholescystokinin – hormone that inhibits gastric glands,
stimulates pancreas to release enzymes in pancreatic juice,
stimulates gallbladder to release bile
Secretin – stimulates pancreas to release bicarbonate ions in
pancreatic juice
Small intestine absorptions
80 % ingested water
Electrolytes
Vitamins
Minerals
Carbonates
- active/facilitated transport
- monosaccharides
Proteins
- di-/tripeptides
- amino acids
Lipids
- monoglycerides
- fatty acids
- micelles
- chylomicronsWhat happens to lactose?
Beta Galactosidase, also known as lactase or galactocerebrosidase, turns it into galactose (which becomes glucose) and ceramide.
(brush border enzyme)
What happens to sucrose
scurase/ isomaltase converts it to fructose and glucose
brush border enzyme
What happense to glucosylceramide?
beta glucosidase converts it to ceramide. (brush border enzyme)
Overview of carb digestion and absorption
- Polysaccharides into disaccharides by salivary and pancreatic amylase
- Disaccharides into monosaccarides by disaccharidases in brush borders of S.I. epithelial cells (maltase, lactase and sucrase)
- Glucose/galactose absorbed by Na+ and energy dependent secondary active transport
- Fructose absorbed by passive facilitated diffusion
2 steps of carb digestion
- Intraluminal hydrolysis of starch to oligosaccharides by amylases (salivary & pancreatic)
- Membrane digestion of oligo to mono by brush border disaccharidases (94: lactase, glucoamylase = maltase, sucrase, isomaltase)
2 steps of carb absorption
- Uptake across apical membrane into epithelial cell and
2. coordinated exit across basolateral membrane
Protein digestion and absorption (overview)
Digestion begins in the stomach when pepsin digests proteins to
form polypeptides
In the duodenum and jejunum:
- Endopeptidases cleave peptide bonds in the interior of the polypeptide:
Trypsin
Chymotrypsin
Elastase
- Exopeptidases cleave peptide bonds from the ends of the polypeptide: Carboxypeptidases Aminopeptidase
What continues protein digestion in the small intestine?
trypsin and chymotripsin
What enzymes complete protein digestion in the small intestine?
brush border enzymes:
carboxypeptidase
aminopeptidase
dipeptidase
How are amino acids and polypeptides transported across basolateral membrane of enterocytes?
Amino acids and remaining polypeptides are transported across basolateral membrane of enterocytes by facilated or simple diffusion
Protein digestion (general)
- Proteins hydrolyzed into amino acid and peptides by gastric and pancreatic proteolytic enzymes
- Amino acids absorbed by Na+ and energy dependent secondary active transport.
Various amino acids are transported by specific carriers - Small peptides are broken down into amino acids by aminopeptidases in the epithelial cells’s brush borders or by intracellular peptidases
Can whole proteins be absorbed?
Enterocytes and M cells can
absorb whole proteins
Lysosomal proteases in
enterocytes degrade proteins
H+/ oligopeptide cotransporter PepT1
H+/oligopeptide cotransporter PepT1 moves dipeptides, tripeptides and tetrapeptides into enterocytes across apical membrane
Peptidases hydrolyzes oligopeptides into AA
AA exit across basolateral membrane through Na+ independent AA transporters
Harnup disease
Hartnup disease is an autosomal recessive disorder caused by impaired neutral (ie, mono-amino-monocarboxylic) amino acid transport in the apical brush border membrane of small intestine and proximal tubule of kidney.
Patients present with pellagra like skin eruptions, cerebellar ataxia, and gross aminoaciduria.
Hartnup disease manifests during infancy with variable clinical presentation: failure to thrive, photosensitivity, intermittent ataxia, nystagmus and tremor.
A high-protein diet can overcome the deficient transport of neutral amino acids in most patients.
Poor nutrition leads to more frequent and more severe attacks of the disease, which is otherwise asymptomatic
Hartnup disease and the PEPT1 transporter
The human peptide transporter 1, PEPT1 is primarily responsible for the absorption of dietary di- and tripeptides from the small intestinal lumen.
The physiological role of PEPT1 is the absorption of small peptides from the intestinal tract after their release by enzymatic breakdown of dietary or endogenous proteins.
Thus, PEPT1 also plays a major role in the nitrogen supply to the body.
Presence of PepT1, individual do not become deficient in amino acids
A high-protein diet can overcome the deficient transport of neutral amino acids in most patients
Where do fats go?
fatty acids and monoglicerides enter the lacteals of hte villi and are transported to the systemic circulation via the lymph in the thoracic duct (glycerol and short-chain fatty acids are absorbed into the capillary blood in the villi and transported to the liver via the hepatic portal vein)
Digestion and absorption of lipids
Arrival of lipids in the duodenum serves as a stimulus for secretion of bile
Emulsification:
- bile salts micelles are secreted into duodenum to break up fat droplets
Pancreatic lipase and colipase hydrolyze triglycerides to free fatty acids
and monoglycerides
- colipase coats the emulsification droplets and anchors the
lipase enzyme to them
- forms micelles and move to brush border
Free fatty acids, monoglycerides and lysolecithin leave micelles and
enter into epithelial cells
- resynthesize triglycerides and phospholipids within cell
combine with a protein to form chylomicrons
Secreted into central lacteals
Another card on the digestion and absorption of lipids
Fat is not soluble in water
It undergoes a series of
transformations for digestion
and absorbtion
1. Dietary fat (TGs) emulsified by
bile salts into fat droplets
2. Lipase hydrolyses TGs into
mono-glycerides & free FAs
3. These water insoluble products
are carried in interior of water-
soluble micelles to luminal
surface of S.I. epithelial cells
4. Micelle in epithelial surface,
mono-glycerides & free FAs
leave micelles & passively diffuse
through luminal membranes
5. Mono-glycerides & free FAs
resynthesized into TGs inside
epithelial cells
6. TGs aggregate & coated with
lipoprotein to form water
soluble chylomicrons
7. Chylomicrons enters lacteals
Transport and aborption of lipids
In blood, lipoprotein lipase hydrolyzes triglycerides to free fatty
acids and glycerol for use in cells
Remnants containing cholesterol are taken to the liver:
- form VLDLs which take
glycerides to cells
- once glycerides are removed,
VLDLs are converted to LDLs
LDLs transport cholesterol to organs and blood vessels
HDLs transport excess cholesterol back to liver
Large intestine
The main functions of the large intestine are:
- completion of fluid absorption
most water absorption occurs in the right colon (the cecum,
the ascending colon and the first half of the transverse colon)
- active secretion active secretion of K+ and HCO3- into colonic lumen
- absorption Na+ actively absorbed bile salts vitamins produces by bacteria-organic molecules cofactors or coenzymes
- storage and elimination of fecal waste prior to defecation, fecal waste is stored in left colon (distal half of the transverse colon, descending colon and sigmoid colon)
by the time fecal material reaches the rectum, it consists of a
small volume of K+ rich fluid containing undigested plant fibers,
bacteria, and inorganic material.
