Podcast
Questions and Answers
What is digestion primarily defined as?
Which type of carbohydrate does not require digestion?
Where does the digestion of carbohydrates begin?
What stops the action of salivary amylase in the stomach?
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Which enzyme is responsible for carbohydrate digestion in the mouth?
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What is the role of pancreatic amylase in carbohydrate digestion?
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What ion is required for the activation of salivary amylase?
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What is the primary type of carbohydrate that undergoes digestion?
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Why can't humans digest cellulose?
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What is the end product of carbohydrate digestion?
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What is necessary for the activity of pancreatic amylase?
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Which mechanism is NOT responsible for the absorption of sugars?
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What role does cellulose play in human digestion?
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Which of the following sugars is absorbed through active transport?
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Which statement about intestinal brush border enzymes is accurate?
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What does active transport of glucose and galactose require?
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What process involves the simultaneous transport of glucose and sodium into the cell?
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What is the role of the sodium pump in glucose transport?
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What condition arises from a deficiency of the enzyme lactase?
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What happens to lactose in the absence of lactase in the gut?
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Which glucose transporter is primarily known for its role in the uptake of glucose in various tissues?
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What initiates the emulsification of lipids in the stomach?
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Which lipase acts on short chain triglycerides in the stomach?
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Which of the following glucose transporters is NOT mentioned as being important for glucose uptake?
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What is the primary product of the action of lingual lipase on triglycerides?
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Which enzyme is responsible for the hydrolysis of cholesteryl esters?
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What role do bile salts play in lipid digestion?
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Which end products result from the complete digestion of triglycerides by pancreatic lipase?
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What is the primary function of phospholipase A2 during lipid digestion?
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What is the significance of emulsification in lipid digestion?
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How does Orlistat function in relation to lipid digestion?
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Which of the following is NOT a major end product of triglyceride digestion?
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What composes the structure of mixed micelles?
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Which type of fatty acids do not require bile salts for absorption?
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Where are short and medium chain fatty acids absorbed into the bloodstream?
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What happens to absorbed free glycerol in the intestine?
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What condition is characterized by the loss of lipids in the feces?
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What can cause bile salt deficiency?
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What is the main outcome of pancreatic enzyme deficiency?
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What is the end product when long chain fatty acids combine with 2-MAG?
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Study Notes
Digestion and Absorption
- Digestion is the process of breaking down food into molecules small enough to be absorbed.
- Absorption is the process of moving nutrients from the gastrointestinal (GI) tract into the bloodstream.
- Transport is the process of moving nutrients throughout the body using the circulatory and lymph systems.
- Elimination is the excretion of undigested food through feces
Digestion of Carbohydrates
- Carbohydrates broken down include:
- Polysaccharides: starch, glycogen, and cellulose
- Disaccharides: sucrose, maltose, and lactose
- Monosaccharides: glucose and fructose
- Monosaccharides do not need digestion.
- Carbohydrate digestion begins in the mouth with the help of salivary amylase (ptyalin) which:
- Hydrolyzes (1-4) glycosidic bonds in starch and glycogen
- Produces dextrins and maltose
- Requires Cl- ions for activation and a pH of 6.7
- Digestive action stops in the stomach due to its high acidity, but resumes in the small intestine.
- The small intestine has two stages of carbohydrate digestion:
- Pancreatic Amylase: hydrolyzes dextrins to a mixture of maltose, isomaltose, and limit dextrin, requiring Cl- ions for activity with an optimal pH of 7.1
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Intestinal Brush Border Enzymes: responsible for final carbohydrate digestion, including:
- Lactase: hydrolyzes lactose into glucose and galactose.
- Sucrase: hydrolyzes sucrose into glucose and fructose.
- Maltase: hydrolyzes maltose into two glucose molecules.
- Isomaltase: hydrolyzes isomaltose into two glucose molecules.
Why Cellulose is Not Digested by Humans?
- Cellulose is a polysaccharide found in plants containing (1-4) glycosidic bonds in its structure.
- Humans lack the enzyme needed to break these bonds.
- Cellulose provides fiber that aids in intestinal motility and stool softening.
Absorption of Carbohydrates
- The final products of carbohydrate digestion are glucose, fructose, and galactose.
- Three mechanisms are responsible for absorption of these sugars:
- Passive Diffusion: relies on concentration gradients between intestinal lumen and mucosal cells.
- Facilitated Diffusion: requires carrier proteins and is sensitive to concentration gradients.
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Active Transport: requires energy, a transport protein, and sodium ions.
- Glucose and galactose are absorbed via this mechanism using a sodium-dependent glucose transporter (SGLT-1).
- This process is called cotransport or symport.
- Energy is provided by ATP-linked sodium pumps.
Glucose Transporters
- There are 7 important glucose transporters (GLUT-1 to GLUT-7) for uptake of glucose into specific cells.
- GLUT-2 is found in the intestines, liver, and kidneys, and is a critical transporter for glucose uptake.
- GLUT-5 is found in the intestines and transports fructose.
Disorders of Digestion and Absorption of Carbohydrates
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Lactose Intolerance: deficiency of the enzyme lactase results in lactose accumulation in the gut.
- Lactase hydrolyzes lactose into glucose and galactose.
- Undigested lactose is fermented by bacteria in the large intestine, producing gases (H2 and CO2) and acids (acetic acid), causing:
- Abdominal cramps
- Flatulence
- Diarrhea
- Dehydration due to water drawn into the lumen
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Causes of Lactose Intolerance:
- Congenital: present at birth
- Acquired: develops later in life (primary or secondary)
Digestion of Lipids
- Digestion begins in the stomach where heat liquidizes lipids.
-
Emulsification: dispersion of lipids into small droplets to reduce surface tension.
- This process is essential for lipid digestion.
- Emulsification is achieved by:
- Bile salts
- Peristaltic contractions
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Enzymatic Action on Lipids:
- Lingual lipase and gastric lipase act on short-chain triglycerides (SCTs).
- Pancreatic lipase, cholesteryl esterase, and phospholipase-A2 are essential enzymes for lipid digestion.
- Pancreatic Lipase: hydrolyzes fatty acids in the 1 and 3 positions of triglycerides, producing 2-monoacylglycerol (2-MAG) and two free fatty acids.
- Cholesteryl Esterase: hydrolyzes cholesteryl esters into cholesterol and free fatty acids.
- Phospholipase-A2: hydrolyzes fatty acids from the 2-position of glycerophospholipids, producing lysophospholipids.
- Orlistat: an anti-obesity drug, inhibits gastric and pancreatic lipases, reducing fat absorption and weight loss.
Absorption of Lipids
-
Mixed Micelles: spherical particles with hydrophilic exteriors and hydrophobic interiors.
- Formed by free fatty acids, 2-MAG, free cholesterol, lysophospholipids, and bile salts.
- Help in absorbing lipids.
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Absorption:
- All digested products passively diffuse into mucosal cells.
- Short and medium-chain fatty acids do not require bile salts for absorption.
- They are absorbed directly into intestinal cells and enter the portal blood.
- They are transported to the liver bound to serum albumin.
- Fat-Soluble Vitamins (A, D, E, and K): packaged in micelles and absorbed.
Synthesis of Lipids in Intestinal Cells
- Long-chain fatty acids are activated by thiokinase to fatty acyl CoA.
- Acyl CoA combines with 2-MAG to form triglycerides.
- Absorbed cholesterol and lysophospholipids are reactivated to form cholesteryl esters and phospholipids.
- Free glycerol released in intestinal lumen is not reutilized but passes directly to the portal vein.
- Glycerol-3-phosphate, formed from glucose within intestinal cells, can be used for triglyceride synthesis.
Abnormalities in Lipid Digestion and Absorption
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Lipid Malabsorption: loss of lipid in the feces.
- Can lead to loss of fat-soluble vitamins and essential fatty acids.
- Occurs in conditions such as:
- Bile salt deficiency: liver disease or bile duct obstruction.
- Pancreatic enzyme deficiency: pancreatitis or cystic fibrosis.
- Defective chylomicron synthesis.
- Steatorrhea: condition characterized by lipid loss in feces.
Digestion of Proteins
- Whole proteins are not absorbed.
- Pepsinogen, secreted by the stomach, is activated by its own acid to become the active enzyme pepsin.
- Pancreatic enzymes further break down proteins into peptides.
- Intestinal brush border enzymes break down peptides into amino acids, which are then absorbed.
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Description
Test your knowledge on the processes of digestion and absorption of nutrients. This quiz covers key concepts of carbohydrate digestion, including the types of carbohydrates and the role of enzymes. Enhance your understanding of how food is broken down and absorbed in the body.
