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Questions and Answers
What is the primary function of salivary α-amylase in the mouth?
What is the product of pancreatic α-amylase digestion in the intestine?
What is the function of oligo 1,6 glucosidase (α-dextrinase) in the intestine?
What is the byproduct of lactase digestion in the intestine?
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Why do indigestible polysaccharides like cellulose pass through the intestine into the faeces?
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What is the function of maltase (α-glucosidase) in the intestine?
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What is the main cause of lactose intolerance?
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Where is glucose absorbed through portal blood?
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What is the normal range of fasting blood glucose level?
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What is the end product of glycolysis under anaerobic conditions?
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Where does glycolysis occur in the cell?
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What is the importance of glycolysis under aerobic conditions?
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Study Notes
Carbohydrate Metabolism
- Major dietary carbohydrates include starch, sucrose, and lactose, with small amounts of free glucose and fructose.
- Cellulose is an indigestible polysaccharide that passes through the intestine and is part of dietary fiber.
Digestion and Absorption
- Salivary α-amylase breaks down starch and glycogen in the mouth, producing α-dextrins.
- Pancreatic α-amylase breaks down α-1,4 linkages between glucose residues, producing maltose, maltotriose, and small oligosaccharides.
- Intestinal enzymes, such as oligo 1,6 glucosidase, sucrase, lactase, and maltase, break down oligosaccharides, sucrose, lactose, and maltose into glucose and other simple sugars.
- Lactose intolerance is caused by a lack of lactase enzyme in the small intestine, leading to symptoms such as abdominal pain, bloating, and diarrhea.
Fates of Absorbed Glucose
- Glucose is absorbed through portal blood to the liver, where it is converted to glycogen or oxidized by glycolysis for acetyl CoA and lipid synthesis.
- Fructose and galactose are converted to glucose in the liver.
- Glucose is used in oxidation, conversion to biologically active substances, storage, and excretion.
Oxidation of Glucose
- Glucose is oxidized through two main pathways: glycolysis and citric acid cycle (CAC) for energy production.
- Minor pathways for oxidation include hexose monophosphate shunt (HMS) and uronic acid pathway.
Conversion to Biologically Active Substances
- Galactose is essential for the formation of lactose, glycolipids, and mucopolysaccharides.
- Fructose is needed for the nutrition of sperms.
- Glucose is converted to non-essential amino acids, fatty acids, ribose-5-P, and glucuronic acid.
Storage of Glucose
- Glucose is stored as glycogen in the liver and muscles, and as triglycerides (TG) in adipose tissues.
Excretion of Glucose
- Glucose is excreted in urine when blood glucose levels exceed 180 mg/dl, a condition known as glucosuria.
Glycolysis
- Glycolysis is the degradation of glucose to generate ATP and provide intermediates for other synthetic and metabolic pathways.
- It occurs in the cell cytosol of all tissues and ends with pyruvate under aerobic conditions and lactate under anaerobic conditions.
- Phosphoglycerate kinase and pyruvate kinase are examples of phosphorylation at the substrate level.
- Two molecules of ATP are liberated in each step, with a total of 8 ATP produced per glucose molecule.
Importance of Glycolysis
- Glycolysis produces energy under aerobic and anaerobic conditions.
- It is a preparatory step for complete oxidation in tissues that have mitochondria.
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Description
This quiz covers the digestion and absorption of dietary carbohydrates, including the breakdown of starch, sucrose, and lactose by enzymes in the mouth and small intestine.
