Chapter 30: Digestion and Absorption of Nutrients
What is the process by which food is broken down into components to be absorbed in the intestine?
What is the uptake of the products of digestion by intestinal cells from the gut lumen?
What results from impaired breakdown of nutrients to their absorbable products?
What results from defective absorption, uptake, and transport of nutrients?
What are the key clinical signs of signs of malabsorption and/or maldigestion? (5)
- weight loss of
- failure to thrive
What can result from disruption of the handling of electrolytes and water by the GI tract?
What acid-base disorder can result from loss of electrolytes due to vomiting?
Prolonged vomiting results in loss of H+, Cl−, and K+, leading to metabolic alkalosis.
What acid-base disorder can result from loss of electrolytes due to diarrhea?
Severe diarrhea results in loss of HCO3 −, Na+, and K+, leading to metabolic acidosis.
What is the driving force for transport processes in the enterocytes?
What glucose cotransporter is present on the luminal membrane of enterocytes?
Na+/glucose linked transport-1 (SGLT-1)
What glucose cotransporter is present on the basolateral membrane of enterocytes?
What is the clinical significance of glucose being linked to Na+ and Cl− transport?
Glucose improves absorption of sodium chloride in dehydrated patients with diarrhea.
What electroneutral sodium transporter is located on the luminal and basolateral enterocyte membranes?
sodium/hydrogen exchanger (NHE)
What electrogenic sodium transporter is located on the luminal and basolateral enterocyte membranes?
epithelial sodium channels (ENaCs)
What transporter on the enterocyte membrane is responsible for luminal secretion of chloride?
cystic fibrosis transmembrane conductance regulator (CFTR)
What signaling pathway controls the function of CFTR?
G-protein–cAMP-protein kinase A (PKA)
What transporter on the enterocyte membrane is responsible for basolateral uptake of chloride?
Na+ K+ Cl− cotransporter (NKCC1)
What disease is caused by loss-of-function mutations of CFTR, inhibiting chloride transport?
What is the inheritance pattern of cystic fibrosis?
What are the complications of cystic fibrosis? (6)
- decreased airway mucus (infections)
- intestinal obstruction
- meconium ileus
- biliary cirrhosis
- chronic cholelithiasis
What transporter mediates potassium absorption on the luminal enterocyte membrane?
What cotransporter mediates potassium transport on the basolateral enterocyte membrane?
K+/Cl− cotransporter (KCC1)
What transporter mediates bicarbonate secretion on the luminal enterocyte membrane?
Cl−/HCO3 − exchanger
What ion channels mediate water reabsorption in the colon?
Which component of digestion is responsible to breaking down food and mixing it with saliva?
What drives the movement of food from the mouth to the stomach, where it is further digested?
What process, triggered by digest in the stomach, helps mixing and stimulates digestive secretions?
What are the invaginations on the surface of the epithelium in the small intestine called?
What are the invaginations on the membranes of the epithelial cells in the small intestine called?
Which cells in the mucosal wall of the stomach secrete pepsinogen, the precursor of pepsin?
Which cells generates H+ through the action of carbonic anhydrase, pumping it into the lumen of the stomach?
What stimulates parietal cell activity by binding to H2 receptors on their membranes?
What hormone secreted by G cells stimulates parietal cell activity?
What peptide secreted by parietal cell facilitates absorption of vitamin B12?
What substance secreted by epithelial cells protects the stomach lining from the effects of the strong acid?
What disease results from damage to the lining of the stomach or duodenum?
What bacterial infection is associated with most cases of peptic ulcer disease?
What is the mechanism of action of antacids in treatment of peptic ulcer disease?
They act by neutralizing the pH.
What is the mechanism of action of cimetidine or ranitidine in treatment of peptic ulcer disease?
They are H2 antagonists, which block the stimulatory effect of histamine on parietal cells.
What is the mechanism of action of omeprazole in treatment of peptic ulcer disease?
It is a proton pump inhibitor, which blocks H+ secretion by the parietal cells.
What does the indigestible components of ingested material constitute?
What are the eight general processes involved in digestion?
- lubrication and homogenization of food with fluids secreted by glands
- secretion of enzymes that break down macromolecules
- secretion of hydrogen ion and bicarbonate to optimize enzymic hydrolysis
- secretion of bile acids to emulsify dietary lipid, facilitating hydrolysis and absorption
- further hydrolysis of oligomers and dimers by membrane-bound enzymes
- uptake of digested material into enterocytes and transfer to blood or lymph
- recycling of bile acids and absorption of the SCFAs produced by colonic bacteria
- reabsorption of water and electrolytes
Why is it that minor functional loss of the GI tract may go unnoticed?
Because there is considerable functional reserve in all aspects of digestion and absorption.
What are three explanations for the considerable functional reserve of digestion?
- digestive organs have the capacity to increase their activity several-fold
- digestion of particular nutrients takes place at several points in the GI tract
- the GI tract can accommodate loss of function of one constituent organ
What are the inactive precursors of digestive enzymes secreted into the gut lumen called?
What type of secretion is demonstrated by release of zymogens into the gut lumen?
What are three ways in which zymogens are converted to their active form?
- they can be activated by active forms
- they can be activated at a specific pH
- they can be activated by enteropeptidases
What type of enzyme are all digestive enzymes?
What are carbohydrates hydrolyzed into?
What are proteins hydrolyzed into?
What are lipids hydrolyzed into?
- fatty acids
What are the three most common dietary polysaccarides?
- starch (amylose)
- glycogen (amylopectin)
What are the three most common dietary monosaccarides?
What are the three most common dietary monosaccarides?
- sucrose (fructose + glucose)
- lactose (glucose + galactose)
- maltose (glucose + glucose)
What two types of bonds join linear chains of glucose molecules in starch and glycogen?
- α-1,4-glycosidic bonds (amylose)
- α-1,6 glycosidic bonds (amylopectin)
What enzyme hydrolyzes α-1,4-glycosidic linkages of glucose residues not serving as branching units?
What three products are produced by hydrolysis of starch by amylase?
- maltose (disaccharide)
- maltotriose (trisaccharide)
- α-limit dextrin
What two enzymes hydrolyze α-1,4-glycosidic linkages of maltose, maltotriose, and α-limit dextrin?
What enzyme hydrolyzes α-1,6-glycosidic bonds of α-limit dextrin?
What is the rate-limiting step in the absorption of all dietary disaccharides except lactose?
Since most disaccharidases are inducible, the rate-limiting step is the transport of monosaccharides.
What is the rate-limiting step in the absorption of lactose?
Lactase is the a noninducible disaccharidase, therefore the rate-limiting step in lactose absorption is hydrolysis.
What type of diarrhea is caused by nonabsorbable solutes present in the gut such as in malabsorption?
What type of diarrhea is caused by secretory agonists such as in infections or Zollinger–Ellison syndrome?
How can secretory diarrhea be distinguished clinically from osmotic diarrhea?
Characteristically, secretory diarrhea, but not osmotic, persists on fasting.
What type of diarrhea is caused by impaired absorption and increased secretion due to inflammation?
What is the main cause of inflammatory diarrhea?
inflammatory bowel disease (IBD)
Which form of IBD only affects the large bowel and rarely causes malabsorption?
Which form of IBD can affect any part of the GI tract and is more likely to cause malabsorption?
What are the three primary monosaccharides generated by digestion of dietary carbohydrates?
What cotransporter transports glucose or galactose along with Na+ across the brush-border membrane?
What transporter transports fructose across the brush-border membrane?
What transporter transports transfers monosaccharides out of the enterocyte into the circulation?
What are produced by the colonic bacteria from incompletely digested carbohydrates?
short-chain fatty acids (SCFAs)
What are the carbohydrates digestable by colonic bacteria into SCFAs refered to as?
fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs)
What type of diet is recognized as beneficial in the management of irritable bowel syndrome?
What autoimmune condition precipitated by sensitivity to gluten results in inflammation of the small intestine and atrophy of the villi, leading to in malabsorption?
What autoantibodies are tested for in cases of celiac disease?
tissue transglutaminase antibodies
What substance released by the gallbladder aids in solubilization of dietary lipids?
What are the small lipid aggregates produced by the action of bile acids on lipid emulsion droplets?
What hormones stimulates secretion of bile salts from the gallbladder?
What is the major enzyme secreted by the pancreas?
What enzyme secreted by the pancreas prevents inhibition of pancreatic lipase by bile slats?
What is the main product of triacylglycerol metabolism by pancreatic lipase?
2-monoacyl glycerols (2-MAG)
What type of fatty acid chains can pass directly through the enterocytes into the hepatic portal system?
medium and short chain (less than 10 carbon atoms)
What type of fatty acid chains must bind to a fatty acid–binding protein to enter the enterocytes?
long chain (more than 12 carbon atoms)
What enymes functions in activation of fatty acids for triacylglycerol synthesis from long-chain fatty acids?
What are triacylglycerols assembled into before being secreted into the intercellular space?
What life-threatening illness can be caused by gallstones blocking the pancreatic duct?
What is the most important biochemical marker of pancreatitis?
increased serum amylase
What illness can be caused by long-term inflammation of the pancreas, which leads to malnutrition?
What is an important clinical finding in chronic pancreatitis caused by loss of exocrine function?
What are the two distinct functional roles of the pancreas?
- exocrine (secretion of digestive enzymes)
- endocrine (secretion of hormones)
Where do exocrine secretions of the pancreas flow into?
What hormone triggers the secretion of bicarbonate-rich pancreatic fluid, which neutralizes the acidity in the duodenum?
Which pancreatic enzyme digests carbohydrates?
Which pancreatic enzyme digests triacylglycerols?
Which pancreatic enzyme digests proteins?
How does the pancreas prevent autodigestion from the powerful proteases it produces and secretes?
Proteolytic enzymes are released from the pancreas as inactive zymogens (proenzymes).
What enzymes hydrolyze the peptide bonds of dietary proteins?
Which type of peptidases cleave internal peptide bonds of the protein molecule?
Which type of peptidases cleave amino acids from the ends of the protein molecule?
Which type of exopeptidases remove amino acids from the amino terminus?
Which type of exopeptidases remove amino acids from the carboxy terminus?
What occurs to proteins in the low pH of the stomach, where their digestion begins?
What proteolytic enzymes released from the pancreas activates all of the other zymogens for peptidases?
What are the four main pancreatic proteases?
Which pancreatic protease cleaves proteins at arginine and lysine residues?
Which pancreatic protease cleaves proteins at aromatic amino acids?
Which pancreatic protease cleaves proteins at hydrophobic amino acids?
What is secreted by the pancreas to neutralize the stomach contents as they passes into the duodenum, thus promoting pancreatic protease activity?
What three enzymes complete digestion of oligopeptides and dipeptides in the small intestine?
What inherited disorder of intestinal amino acid absorption results in urinary loss of amino acids?