Carbohydrates: Metabolism and Digestion Flashcards

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Carbohydrates

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Energy-contributing nutrients that provide about 4 kcal per gram. They are the major energy source in the human diet, supplying roughly half of dietary calories with about equal parts complex and simple carbohydrates. Chemically, they are composed of the elements $C$, $H$, and $O$.

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Monosaccharides

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The simplest sugars that cannot be hydrolyzed further. Major examples include glucose, fructose, and galactose, which are readily absorbed in the small intestine and feed into energy pathways as glucose derivatives.

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Disaccharides

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Sugars formed by linking two monosaccharides via a glycosidic bond. Lactose (galactose + glucose), sucrose (fructose + glucose), and maltose (glucose + glucose) are common examples. They must be hydrolyzed to monosaccharides before absorption.

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Polysaccharides

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Long chains or branched polymers of monosaccharides. Major types include starch (amylose and amylopectin) for plant energy storage, cellulose for structural support, and glycogen for animal storage. Digestibility depends on glycosidic linkages; cellulose is resistant to human enzymes.

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Amylose

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A linear polymer of glucose connected by α-1,4 glycosidic bonds. It constitutes about 15–20% of polysaccharides in starch. Digestion is performed by amylases that cleave the α-1,4 bonds.

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Amylopectin

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A branched polymer of glucose with α-1,4 linkages and α-1,6 branches. It accounts for the majority of polysaccharides in starch and is rapidly digested due to its branched structure, with enzymes such as amylase and isomaltase acting on the α-1,6 bonds.

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Glycogen

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Branched storage form of glucose in animals, primarily in liver and muscle. Its extensive branching allows rapid synthesis and mobilization of glucose to maintain blood glucose and fuel muscle activity. Branch points enhance solubility and enzyme accessibility.

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Cellulose

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Linear polymer of glucose linked by β-1,4 glycosidic bonds. Humans cannot hydrolyze β-1,4 linkages, so cellulose remains largely unabsorbed and contributes to dietary fiber.

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Glycogenesis

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The synthesis of glycogen from glucose for storage. It occurs in liver and muscle, starting from glucose-6-phosphate and UDP-glucose, with enzymes such as hexokinase (muscle) and glucokinase (liver) driving the process; energy is consumed.

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Glycogenolysis

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Breakdown of glycogen to glucose-1-phosphate by glycogen phosphorylase, which is then isomerized to glucose-6-phosphate. In liver and kidney, glucose-6-phosphatase converts it to free glucose; in muscle, glucose-6-phosphate proceeds to glycolysis for energy.

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Glycolysis

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Glucose is converted to pyruvate in the cytoplasm, generating ATP and NADH. Key steps include phosphorylation of glucose to $G-6-P$ by hexokinase/glucokinase, isomerization to $F-6-P$, and phosphorylation to $F-1,6$-bis-$P$ by $PFK$, followed by formation of glyceraldehyde-3-phosphate and DHAP, yielding ATP and NADH.

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Gluconeogenesis

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Production of glucose from non-CHO sources such as amino acids, glycerol, pyruvate, and lactate. It largely mirrors glycolysis but uses different enzymes at non-reversible steps; pyruvate to PEP involves a mitochondrial malate shuttle and cytosolic PEP carboxykinase. It is important during hypoglycemia or strenuous exercise.

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PDC

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Pyruvate Dehydrogenase Complex converts pyruvate to acetyl-$CoA$, feeding the Krebs cycle. It requires vitamins B1 (Thiamin), B2 (Riboflavin), B3 (Niacin), and B5 (Pantothenic Acid) and operates in the mitochondria to generate NADH for the electron transport chain.

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NADH

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Reduced nicotinamide adenine dinucleotide produced in glycolysis and the Krebs cycle. It donates electrons to the electron transport system to drive ATP production, with about 3 ATP generated per NADH via oxidative phosphorylation.

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ETS

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Electron Transport System and oxidative phosphorylation where NADH and FADH$_2$ donate electrons through a chain of complexes, pumping protons to create a proton gradient that drives ATP synthase. Complex I accepts electrons from NADH and Complex II from FADH$_2$; oxygen is the final electron acceptor.

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Krebs Cycle

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Also called the citric acid cycle, it occurs in the mitochondria and oxidizes acetyl CoA to CO$_2$. It produces NADH, FADH$_2$, and GTP; the cycle is a major source of energy and provides reducing equivalents for the electron transport system.

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Fructose

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Sweetest natural sugar, found in fruit and honey and in high-fructose corn syrup. It is absorbed more slowly than glucose and is converted to glucose in the liver, where it can contribute to energy production or lipogenesis.

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Lactose Intolerance

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Lactase enzyme deficiency leading to lactose intolerance. Symptoms include flatulence, bloating, cramps, diarrhea, and abdominal pain, more common in certain ethnic groups. Management includes restricting lactose and using fermented dairy or lactase supplements.

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Glycemic Index

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AUC of the blood glucose response after consuming a test carbohydrate relative to a reference food, over about 2 hours. Categories: low ≤55, moderate 55–70, high >70. Glycemic Load adjusts GI by the carbohydrate content of a serving.

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Glycemic Load

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Glycemic load = GI × grams of carbohydrate per serving. It better reflects the actual postprandial glucose response than GI alone and helps classify foods by expected glycemic impact.

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Dietary Fiber

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Non-digestible plant polysaccharides with β-1,4 linkages that humans cannot digest. Total fiber includes dietary fiber and functional fiber added to foods for health benefits.

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Soluble Fiber

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Fiber that dissolves in water to form viscous solutions, with high water-holding capacity and the ability to bind bile acids and cholesterol. It is more fermentable by gut bacteria and can slow gastric emptying and nutrient absorption.

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Insoluble Fiber

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Fiber that does not dissolve in water; increases fecal bulk and accelerates intestinal transit. It is less fermentable than soluble fiber and supports colon health.

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Probiotics

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Foods or supplements containing live beneficial bacteria that may support gut microbiota balance and digestion. Common sources include yogurt and kefir.

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Prebiotics

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Non-digestible food ingredients that promote growth and activity of beneficial gut bacteria, such as inulin and fructooligosaccharides.

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Bile Acids

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Compounds synthesized from cholesterol in the liver and stored in the gallbladder; emulsify fats in the intestine and form micelles for digestion, then are reabsorbed.

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Micelle

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Small aggregates of bile salts, phospholipids, and lipids formed in the intestinal lumen that transport lipids to enterocytes for absorption; display hydrophobic cores with hydrophilic surfaces.

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Portal Circulation

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Nutrients absorbed from the gastrointestinal tract enter the portal vein and travel to the liver, where they may be metabolized or released into systemic circulation.

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Digestive Tract

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The alimentary canal and accessory organs involved in digestion, from the oral cavity through the esophagus, stomach, small and large intestines to the rectum, with several organs (liver, gallbladder, pancreas) contributing enzymes and secretions.

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Brush Border Enzymes

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Digestive enzymes located on the microvilli of enterocytes in the small intestine, including lactase, sucrase, and maltase, which break disaccharides into monosaccharides for absorption.