HBG 5 ( Carbohydrate Chemistry and Importance)

• The chemical nature of dietary carbohydrates is complex and includes monosaccharides, disaccharides, and polysaccharides.
• Dietary carbohydrates are a source of glucose, which is important for various organs in the body.
• Glucose is utilised in various organs such as the heart, brain, and kidney.
• Glucose undergoes oxidation, a process that is important in certain tissues.
• Glycolysis, glycogen metabolism, gluconeogenesis, and the pentose phosphate pathway are important in carbohydrate metabolism.
• The term "carbohydrates" is derived from their general formula [C(H2O)] n that makes them seem to be "hydrates of carbon".
• The terms carbohydrate and saccharide are closely related.
• The most abundant carbohydrates are polysaccharides.
• Glucose is the most important carbohydrate in the body.
• Saccharide comes from the word for table sugar in several languages: sarkara in Sanskrit, sakcharon in Greek, saccharum in Latin.
• Carbohydrates are distributed widely in nature, are key intermediates of metabolism (glucose), structural components of plants (cellulose), and are a key component of food sources: sugars, flour, vegetable fiber.
• A carbohydrate is a polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives these compounds on hydrolysis.
• There are 3 irreversible steps in glycolysis: Phosphoenolpyruvate (PEP) to pyruvate, fructose - 6 - phosphate to fructose - 1,6 - bisphosphate, and glucose to glucose - 6 - phosphate.
• During degradation, several molecules of glucose can be released simultaneously, one from each end of a branch.
• Glycogen is similar to starch except in the degree of branching.
• The Pentose Phosphate Pathway (PPP) generates reducing equivalents and ribose for nucleic acid synthesis.
• Monosaccharides are the simplest form of carbohydrates with a general formula: C n H 2n O n, n varies from 3 to 8.
• The reversal of these 3 steps in gluconeogenesis is reversal of these three steps, but by different reactions and using different enzymes.
• Gluconeogenesis is not the exact reversal of glycolysis because pyruvate to glucose does not occur by reversing the steps of glucose to pyruvate.
• Glycogen provides a quick supply of glucose to the glycolytic pathway.
• The Pentose Phosphate Pathway produces NADPH for cell biosynthesis and precursor (ribose - 5 - phosphate) for nucleic acid synthesis.
• Amino acids, lactate, glycerol are substrates in gluconeogenesis.
• Monosaccharides are classified by their carbononyl group and number of carbon atoms.
• There are two trioses (3-carbon monosaccharides) Glyceraldehyde (an aldo trioses) and dihydroxyacetone (a keto trioses).
• Glucose is the most important physiological and biomedical monosaccharide.
• Kidney medulla, testis, leukocytes and white muscle fibers have relatively few mitochondria and so almost depend totally on glycolysis for ATP.
• RBCs lack mitochondria and thus cannot convert pyruvate to CO2 and H2O, so end product of glycolysis is lactate (through anaerobic glycolysis).
• Glycolysis provides precursors for fatty acids, amino acids and five-carbon sugar phosphates (pentose phosphate pathway).
• Glucose is utilised in various organs in the human body, such as brain, heart, liver, kidney, muscle, intestine, adipose, and VLDL.
• Cornea, lens and retina have a limited blood supply and also lack mitochondria, thus they depend on glycolysis as the major mechanism for ATP production.
• The Glycolytic Pathway (Glycolysis) is the most important carbohydrate metabolic pathway, present in all cells of the body.
• Glucose, mannose, fructose, galactose enter the glycolitic pathway through different points.
• Gluconeogenesis is the biosynthesis of glucose from metabolic intermediates.
• In some cells, glucose is the sole/major source of energy, such as brain, RBC.
• The importance of glycolysis can be seen in tissues that lack of mitochondria, such as RBCs, cornea, lens and retina.
• Glucose is converted to pyruvate in the liver and muscle, and then to acetyl CoA in the TCA cycle.
• Glycolysis is the catabolism of glucose, a series of 10 enzyme-catalyzed reactions by which glucose is oxidized to two molecules of pyruvate, with net conversion of 2ADP to 2ATP.
• Glycogenesis is the synthesis of glycogen from glucose, the carbohydrate storage in liver and muscle cells.
• Carbohydrate metabolism is important as it involves catabolic processes of carbohydrates, especially glucose.
• All organisms obtain energy from oxidative breakdown of glucose and other carbohydrates.