Phys: Digestion of Carbs, proteins, liquids. Flashcards
carbs, proteins, fats digestions
carbs –> monoscacharides
proteins –> AAs
Fats –> glycerol and FA’s, to be absorbed back into lymphatics
what drives entire absorption process?
–>Na+/K+ ATPase
(fructose is only ATPase independent)
- req’d by absorption of AA’s, dipeptides, tripeptides, glucose, galactose
- insulin has no effect on this process (insulin mainly effects adipose tissues and muscles)
5 patterns of digestion and absorption/
- Glucose-does not require digestion: can move straight through epithelium prior to transport
- Protein-digested in lumen
- Sucrose-digested by brush border enzymes before absorption (broken down to fructose and glucose)
- Oligopeptide-directly absorbed by cell then broken down inside cell
- TAG-broken down glycerol and FA’s before absorption. cell then resynthesizes to original molecule triglycerol and transported in lacteals.
Small Intestine
- All chemical digestion & nutrients absorption occurs here
- Circular folds of mucosa, villi, and microvilli – enhance s/a for absorption (plicae circulares)
- secretion: Juice secreted by small intestine does not contain any digestive enzymes. Rather, brush border enzymes (Enterokinase, Disaccharidases, Aminopeptidases) activate the zymogens secreted by pancreas to complete digestion of carbohydrates and proteins.
Peptidase
breaks down peptides into AA’s
Sucrase, Maltase, Lactase
break down disaccharides into monosaccharides
lipase
breaks down fats into FA’s and glycerol
enterokinase
convertest trypsinogen to trypsin
somatostatin
hormone that inhibitits acid secretion by stomach
CCK
hormone that inhibits gastric glands, stimulates pancreas to release enzymes in pancreatic juice, stimulates gallbladder release bile
secretin
stimulates pancreas to release bicarb. ions in pancreatic juice
starch
starch–> maltose –> glucose + glucose
maltose
maltose –> glucose + glucose
digested by brush border maltase of SI
Sucrose
–> glucose and fructose
digested by brush border sucrase of SI
Lactose
- -> glucose+galactose
- digested by brush border lactase
2 steps of carbohydrate digestion and absorption
1.Intraluminal hydrolysis of starch
to oliogosaccharides by amylases
(salivary & pancreatic)
2.Membrane digestion of oligo to mono by brush border disaccharidases (lactase, maltase, sucrase, isomaltase)
- Uptake across apical membrane into epithelial cell
- coordinated exit across basolateral membrane
endopeptidases
- located in brush border of duodenum and jejunum
- cleave bonds in the interior of polypeptide: trypsin, chymotrypsin, elastase
exopeptidases
- located in brush border of duodenum and jejunum
- cleave peptide bonds from the ends of the poplypeptide: carboxypeptidase, aminopeptidase
Pepsinogen
converted to Pepsin via HCl
Trypsinogen
converted to trypsin via enterokinase secreted from intestinal glands
chymotrypsinogen
converted to chymotrypsin via trypsin
procarboxypeptidase
converted to carbodypeptidase via trypsin
5 pancreatic peptidases
- trypsinogen, chymotrypsinogen, prelastase, procarboxypeptiase A and B
- Pepsin from stomach and 5 pancreatic peptidases hydrolyze proteins to single amino acids or oligopeptides
- Occurs in lumen of stomach or sm. Intestine
- Various peptidases at brush borders hydrolyze oligopeptidases to amino acids
- Amino acids are taken up by transporters
Hartnup 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
Protein absorption
- Enterocytes and M cells can absorb whole proteins
- Lysosomal proteases in enterocytes degrade proteins
- H+/oligopeptide cotransporter PepT1 moves dipeptides, tripeptides and tetrapeptides into enterocytes acrossapical membrane
- Peptidases hydrolyzes oligopeptides into AA
- AA exit across basolateral membrane through Na+ independent AA transporters
Digestion and absorption of lipids
- arrival of lipids in duodenum serves as a stimulus for secretion of bile.
- emulsification: bile salt micelles are secreted into duodenum to break up fat droplets
- pancreatic lipase and colipiase hydrolyze triglycerides to FFA’s and monoglycerides.
- Colipase coats the emulsification droplets and anchors the lipase enzyme to them.
- Forms micelles which move to the brush border
- FFA’s and monoglycerides leave micelles and enter into epithelial cells where they are resynthesized into triglycerides and phosophlipids to form chylomicrons.
- chylomicrons secreted into central lacteals
How are lipids transported?
- In blood, lipoprotein lipase hydrolyzes the triglycerides to FFA’s and glycerol for use in cells.
- remnants containing cholesterol are taken to the liver (form VLDLs which take triglycerides to cells. once triglycerides are removed, VLDLs are converted to LDLs)
HDLs transport excess cholesterol back to liver
Note: LDLs transport cholesterol to organs and blood vessels
Where is most water absorbed?
- the ascending colon and first half of transverse colon. fecal waste is stored in the left colon
- large intestine also reabsorbs water, bile salts, and vitamins
Where is most water absorbed?
the ascending colon and first half of transverse colon. fecal waste is stored in the left colon
Digestion/absorption of proteins:
- digestion begins in stomach when pepsin digests proteins to form polypeptides.
- In duodenum and jejunum, brush border enzymes activate trypsin, chymotrypsin, elastase, carboxypeptidase, and aminopeptidase, which result in free amino acids.
- Free AA’s are absorbed by cotransport along with Na+
- Dipeptides and tripeptides are transported by secondary active transport using an H+ gradient.
- free AA’s are then secreted into the blood
PEPT1 and Hartnup disease
- 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.
- In presence of PepT1, individual do not become deficient in amino acids
- can be defective, leading to Hartnup disease
