Carbohydrates Overview

Overview

This lecture covers the major types of biochemically important carbohydrates, including their structures, functions, and dietary considerations.

Glucose (dextrose) is the main blood sugar and primary energy source for cells.
Blood glucose normally ranges from 70–100 mg/dL, rising after eating.
Hospital IVs often use 5% glucose solutions for nourishment.
Galactose is synthesized from glucose and is vital for brain glycoproteins and blood markers.
Fructose (fruit sugar) is found in fruits, honey, and is sweeter than glucose; used in high-fructose corn syrup (HFCS).
Ribose is a component of RNA, DNA, and ATP.

Disaccharides result from condensation of two monosaccharides via glycosidic linkages.
Maltose: two glucose units with an α(1→4) linkage; produced during starch digestion.
Cellobiose: two glucose units with a β(1→4) linkage; produced from cellulose hydrolysis, not digestible by humans.
Lactose: galactose and glucose with a β(1→4) linkage; main sugar in milk; intolerance results from lactase enzyme deficiency.
Sucrose: glucose and fructose with an α,β(1→2) linkage; common table sugar, non-reducing, hydrolysis yields invert sugar.
Oligosaccharides (3–10 units) like raffinose and stachyose exist in beans and cause gas due to bacterial fermentation.
Blood types are determined by specific oligosaccharides on red blood cells.

Saccharin: oldest, 300x sweeter than sucrose, non-caloric, heat unstable.
Aspartame: 180x sweeter, not heat stable, contains phenylalanine (risk for PKU).
Sucralose: 600x sweeter, heat stable, non-caloric, not digested.
Neotame: 7,000x sweeter, heat stable, negligible calories.

Polysaccharides are large, usually insoluble carbohydrates that may serve storage or structural roles.
Homopolysaccharides have one type of repeating unit; heteropolysaccharides have more.
Starch (plants): consists of amylose (unbranched, α(1→4)) and amylopectin (branched, α(1→6)), main energy reserve in plants.
Glycogen (animals): similar to amylopectin but more highly branched, energy storage in liver and muscles.
Cellulose: structural component of plants; β(1→4) linked glucose, indigestible by humans.
Chitin: similar to cellulose but with N-acetylglucosamine, forms exoskeletons and fungal cell walls.

Hyaluronic acid: viscous, lubricates joints and eyes, composed of alternating uronic acids and aminosugars.
Heparin: sulfated, acts as a natural blood anticoagulant, found in mast cells.

Monosaccharide — single sugar unit, e.g., glucose.
Disaccharide — two monosaccharides joined by glycosidic linkage.
Glycosidic linkage — bond formed between monosaccharides via condensation.
Hydrolysis — breaking of glycosidic linkage by adding water.
Polysaccharide — long chain polymer of monosaccharides.
Homopolysaccharide — polysaccharide with identical monosaccharide units.
Heteropolysaccharide — polysaccharide with different monosaccharide units.
Lactose intolerance — inability to digest lactose due to lactase deficiency.
Invert sugar — mixture of glucose and fructose from sucrose hydrolysis.