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Questions and Answers
What is the primary end product of carbohydrate digestion in the small intestine?
Which enzyme is NOT directly involved in the digestion of carbohydrates?
How are glucose and galactose absorbed by the intestinal epithelial cells?
Which disaccharide is specifically digested by the enzyme lactase?
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What happens to starch digestion in the stomach?
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What type of transport mechanism is used for the extrusion of glucose and galactose into the bloodstream?
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Which of the following is not a product of carbohydrate digestion?
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What is the role of intestinal brush-border enzymes in carbohydrate digestion?
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Where does the digestion of proteins begin?
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What is the primary function of endopeptidases?
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Which of the following is an example of an exopeptidase?
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At what pH range is pepsin most effective?
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Which pancreatic enzyme is secreted as an inactive precursor?
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What happens to pepsin at pH levels above 5?
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What type of protease hydrolyzes amino acids one at a time from the C-terminus?
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Which of the following proteases is NOT an endopeptidase?
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What primary function do bile salts serve in the small intestine?
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What is the role of colipase in the digestion of lipids?
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Which enzyme initiates lipid digestion in the stomach?
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What percentage of triglycerides is hydrolyzed in the stomach?
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How does gastric emptying affect lipid digestion?
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Which of the following enzymes is activated in the intestinal lumen?
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What problem does pancreatic lipase encounter during lipid digestion?
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What is the main output of the action of pancreatic lipase on triglycerides?
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What primarily causes the osmotic diarrhea in lactase deficiency?
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Which of the following mechanisms is involved in secretory diarrhea caused by cholera toxin?
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What effect does overgrowth of enteropathic bacteria have on fluid secretion in the intestine?
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How does the cholera toxin facilitate fluid secretion in the intestines?
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What is a potential outcome of undigested lactose in the intestine?
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What is the primary transport mechanism for fat-soluble vitamins in the intestinal cells?
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Which vitamin requires intrinsic factor for its absorption?
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What role do R proteins play in the absorption of vitamin B12?
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In which part of the digestive system does the absorption of vitamin B12 begin?
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Which of the following vitamins is NOT water-soluble?
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What is the function of pancreatic proteases in vitamin B12 absorption?
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Which of the following is a water-soluble vitamin absorbed via Na+-dependent cotransport?
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What process occurs after fat-soluble vitamins diffuse into intestinal cells?
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Which structural feature of the small intestine is responsible for increasing absorptive surface area significantly?
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What is the role of transporters in the absorption of nutrients in the small intestine?
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Which path of absorption involves crossing tight junctions between intestinal epithelial cells?
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What is the primary function of villi in the small intestine?
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Where in the digestive system does the majority of carbohydrate digestion and absorption occur?
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Study Notes
Carbohydrate Digestion
- Digestion of carbohydrates starts in the mouth with the action of salivary α-amylase.
- The low pH of the stomach inactivates salivary α-amylase.
- Pancreatic amylase breaks down starch into α-limit dextrins, maltose, and maltotriose.
- Intestinal brush-border enzymes like α-dextrinase, maltase, and sucrase further digest disaccharides into monosaccharides.
- The end products are glucose, galactose, and fructose, all of which are monosaccharides that are absorbed by the intestinal epithelial cells.
- Disaccharides like trehalose, lactose, and sucrose are digested into two monosaccharides by trehalase, lactase, and sucrase respectively.
Carbohydrate Absorption
- Glucose and galactose are absorbed via secondary active transport mechanisms across the apical membrane of intestinal cells.
- Glucose and galactose are then extruded from the cell into the blood by facilitated diffusion through GLUT2 protein on the basolateral membrane.
- Fructose absorption is different from glucose and galactose.
Protein Digestion
- Protein digestion begins in the stomach with the action of pepsin.
- Stomach pH between 1 and 3 is ideal for pepsin activity.
- Pepsin is inactivated above pH 5.
- Protein digestion is completed in the small intestine with the combined action of pancreatic and brush-border proteases.
- There are two types of proteases:
- Endopeptidases that hydrolyze interior peptide bonds in proteins.
- Exopeptidases that hydrolyze one amino acid at a time from the C-terminus of proteins and peptides.
- Major pancreatic proteases secreted as inactive precursors are:
- Trypsinogen
- Chymotrypsinogen
- Proelastase
- Procarboxypeptidase A
- Procarboxypeptidase B
- Brush-border proteases include aminopeptidase and dipeptidases.
- Carboxypeptidases A and B are the exopeptidases of the gastrointestinal tract.
Lipid Digestion
- Lipid digestion is initiated in the stomach by lingual and gastric lipases, hydrolyzing about 10% of ingested triglycerides into glycerol and free fatty acids.
- Stomach also emulsifies lipids by breaking them into small droplets, increasing their surface area for enzymatic digestion.
- The primary role of the stomach in lipid digestion is to slowly empty chyme into the small intestine to allow enough time for pancreatic enzymes to act on lipids.
- Cholecystokinin (CCK) regulates gastric emptying when dietary lipids appear in the small intestine.
- Bile salts, lysolecithin, and digestion products of lipids emulsify dietary lipids in the small intestine.
- Pancreatic lipase, cholesterol ester hydrolase, and phospholipase A2 are secreted into the small intestine to digest lipids.
- Pancreatic lipase hydrolyzes triglycerides into monoglyceride and two fatty acids.
- Colipase, activated by trypsin, is necessary for pancreatic lipase to function as it is inhibited by bile salts.
Fat-Soluble Vitamin Absorption
- Fat-soluble vitamins (A, D, E, and K) are absorbed similarly to lipids:
- Vitamins are incorporated into micelles.
- Micelles transport vitamins to the apical membrane of intestinal cells.
- Vitamins diffuse across the membrane into the cells.
- They are then incorporated into chylomicrons and transported to the general circulation via lymph.
Water-Soluble Vitamin Absorption
- Water-soluble vitamins are absorbed in the small intestine via Na+-dependent cotransport systems.
- Vitamin B12 absorption is an exception and requires intrinsic factor for absorption.
Vitamin B12 Absorption Steps:
- Dietary vitamin B12 is released from food by pepsin in the stomach.
- Vitamin B12 binds to R proteins in salivary juices.
- Pancreatic proteases degrade R proteins in the duodenum, allowing Vitamin B12 to bind to intrinsic factor.
- Intrinsic factor, a glycoprotein secreted by gastric parietal cells, facilitates vitamin B12 absorption.
Lactose intolerance
- Lactose intolerance occurs when lactase, an enzyme that digests lactose, is deficient.
- Lactose remains undigested in the intestinal lumen, causing osmotic diarrhea due to water retention.
- Bacteria in the intestine can further worsen the problem by degrading lactose into osmotically active solute particles.
Secretory Diarrhea
- Caused by excessive secretion of fluid by crypt cells.
- Enteropathic bacteria like Vibrio cholerae and Escherichia coli cause secretory diarrhea.
- Cholera toxin can enter intestinal crypt cells, detach and move across the cell to the basolateral membrane, where it activates adenylyl cyclase.
- This leads to increased cAMP levels, which keeps Cl− channels open in the apical membrane, causing chloride, sodium, and water secretion.
- Excessive fluid secretion overwhelms the absorptive mechanisms of the small intestine and colon, leading to massive diarrhea.
Digestion and Absorption
- Digestion is the chemical breakdown of food into absorbable molecules.
- Digestive enzymes are secreted in saliva, gastric juice, and pancreatic juice; they are also present on the surface of intestinal epithelial cells.
- Absorption is the movement of nutrients, water, and electrolytes from the intestinal lumen into the blood.
- There are two pathways for absorption: cellular and paracellular.
- Cellular pathway: Substances cross the apical membrane, enter the epithelial cell, and are extruded across the basolateral membrane into the blood.
- Paracellular pathway: Substances move through tight junctions between intestinal epithelial cells and into the blood.
- The small intestine has structural features that increase its surface area for digestion and absorption.
- Longitudinal folds (folds of Kerckring)
- Finger-like villi that project from the folds.
- Microvilli on the surface of epithelial cells (enterocytes) create a brush border appearance.
- This increases the total surface area by 600-fold.
Carbohydrate Digestion and Absorption
- Carbohydrates are digested into monosaccharides for absorption.
- Starch digestion begins with salivary α-amylase in the mouth and continues with pancreatic amylase in the small intestine.
- Disaccharides (maltose, maltotriose, α-limit dextrins) are further digested to monosaccharides by intestinal brush-border enzymes (α-dextrinase, maltase, sucrase, trehalase, lactase, sucrase).
- The end products of carbohydrate digestion are glucose, galactose, and fructose.
- Glucose and galactose are absorbed via secondary active transport across the apical membrane.
- Glucose and galactose are extruded from the cell into the blood by facilitated diffusion (GLUT2).
- Fructose is handled differently and is absorbed by facilitated diffusion (GLUT5).
Fat-Soluble Vitamin Absorption
- Absorption of fat-soluble vitamins (A, D, E, K) is similar to the absorption of dietary lipids.
- They are incorporated into micelles and transported to the apical membrane of intestinal cells.
- They diffuse across the apical membrane and are incorporated into chylomicrons.
- Chylomicrons are then extruded into lymph, which delivers them to the general circulation.
Water-Soluble Vitamin Absorption
- Most water-soluble vitamins (B1, B2, B6, B12, C, biotin, folic acid, nicotinic acid, pantothenic acid) are absorbed via Na+-dependent cotransport in the small intestine.
- Vitamin B12 absorption requires intrinsic factor and occurs in several steps:
- Vitamin B12 is released from food by pepsin in the stomach.
- Free vitamin B12 binds to R proteins in saliva.
- Pancreatic proteases degrade R proteins in the duodenum, causing vitamin B12 to bind to intrinsic factor.
- The vitamin B12-intrinsic factor complex is absorbed in the ileum by receptor-mediated endocytosis.
Water Secretion and Absorption
- Water follows the movement of electrolytes.
- Net water absorption occurs in the small intestine and the colon.
- Crypt cells secrete electrolytes and water into the lumen of the intestinal tract.
- Villus cells absorb electrolytes and water from the lumen.
- Water secretion is driven by Cl− secretion, which is regulated by cAMP levels.
- Cl− channels are typically closed but open upon binding of hormones or neurotransmitters (ACh, VIP) to basolateral membrane receptors, leading to Cl− secretion and subsequent Na+ and water secretion.
- In diseases where adenylyl cyclase is maximally stimulated (e.g., cholera), fluid secretion overwhelms absorptive capacity and causes diarrhea.
Secretory Diarrhea
- Occurs due to excessive fluid secretion by crypt cells.
- Major cause is pathogenic bacteria such as Vibrio cholerae or Escherichia coli.
- Cholera toxin enters crypt cells and activates adenylyl cyclase, leading to increased cAMP levels.
- This opens Cl− channels in the apical membrane and triggers secretion of Cl−, Na+, and water.
- Fluid secretion exceeds absorptive capacity, causing massive diarrhea.
Lactase Deficiency
- Lactase deficiency impairs the breakdown of lactose into glucose and galactose.
- Undigested lactose retains water in the intestinal lumen and causes osmotic diarrhea.
- Bacterial degradation of lactose can further increase the osmotic load, exacerbating diarrhea.
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Description
This quiz covers the processes involved in carbohydrate digestion and absorption, including the roles of various enzymes and transport mechanisms. It details how carbohydrates are broken down into monosaccharides and subsequently absorbed in the intestines. Test your knowledge of these essential biological processes!
