Properties
Physical and Chemical
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Physical Properties:
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Monosaccharides/Disaccharides: Crystalline solids, water-soluble, sweet
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Polysaccharides: Amorphous solids or fibrous, mostly insoluble, generally tasteless
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Chemical Properties:
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Monosaccharides:
- Cyclization (forming α and β anomers)
- Oxidation (reducing sugars can reduce other compounds)
- Esterification, Glycoside Formation
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Disaccharides:
- Hydrolyzed by enzymes or acid into monosaccharides
- Can be reducing or non-reducing
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Polysaccharides:
- Hydrolyzed by enzymes or acid into smaller sugars
- Vary in structure (linear vs. branched) and composition (homo- vs. heteropolysaccharides)
Monosaccharides
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Definition: The basic building blocks of carbohydrates; cannot be hydrolyzed into smaller units (also known as simple sugars)
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Examples: Glucose, fructose, galactose, ribose
Physical
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State: Crystalline solids at room temperature
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Solubility: Highly soluble in water due to numerous hydroxyl (-OH) groups that form hydrogen bonds with water molecules
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Taste: Generally sweet, although the degree of sweetness varies (fructose > sucrose > glucose > galactose)
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Optical Activity: Chiral molecules that rotate plane-polarized light; classified as either dextrorotatory (+) or levorotatory (-)
Chemical
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Cyclization:
- Monosaccharides with five or more carbon atoms can cyclize in aqueous solution, forming cyclic hemiacetals (from aldehydes) or hemiketals (from ketones)
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Anomers: The cyclization process creates a new chiral center at the carbonyl carbon (anomeric carbon), resulting in α and β anomers
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Mutarotation: The spontaneous interconversion between α and β anomers in solution until an equilibrium mixture is reached
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Oxidation:
- Monosaccharides with a free aldehyde or ketone group can be oxidized (reducing sugars)
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Reducing Sugars: Glucose, fructose, galactose, lactose, maltose
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Tests for Reducing Sugars:
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Benedict’s Test: Reducing sugars react with copper(II) sulfate in alkaline solution, reducing it to copper(I) oxide, forming a red-brown precipitate
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Fehling’s Test: Similar to Benedict’s test, uses copper(II) tartrate complex
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Reduction: The aldehyde or ketone group of a monosaccharide can be reduced to form a sugar alcohol (alditol)
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Examples
- Glucose is reduced to sorbitol (glucitol)
- Xylose is reduced to xylitol (a sugar substitute)
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Esterification: Hydroxyl groups (-OH) can react with acids to form esters
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Example: Phosphorylation of glucose (e.g., glucose-6-phosphate) is essential in metabolism
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Glycoside Formation: The anomeric hydroxyl group can react with another alcohol to form a glycoside with an O-glycosidic bond
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Glycosidic Bond: The bond that links monosaccharides together to form disaccharides, oligosaccharides, and polysaccharides
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Isomerization: Monosaccharides can be converted into isomers
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Example: Glucose can be converted to fructose via an enediol intermediate
Examples
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Glucose (D-Glucose):
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Also Known As: Dextrose, blood sugar
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Significance:
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Primary Energy Source: The main energy source for most cells in the body
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Central to Metabolism: Central molecule in carbohydrate metabolism; glycolysis, gluconeogenesis, glycogenesis, and glycogenolysis revolve around glucose
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Regulation: Blood glucose levels are tightly regulated by insulin and glucagon
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Fructose (D-Fructose):
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Also Known As: Fruit sugar, levulose
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Significance:
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Sweetest Natural Sugar: Found in fruits, honey, and high-fructose corn syrup
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Metabolism: Metabolized primarily in the liver
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Dietary Concerns: High intake of fructose has been linked to metabolic issues
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Galactose (D-Galactose):
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Significance:
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Part of Lactose: Found in milk as part of the disaccharide lactose
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Metabolism: Converted to glucose in the liver
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Galactosemia: Deficiency in enzymes that metabolize galactose leads to galactosemia
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Ribose (D-Ribose):
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Significance:
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Component of RNA: A crucial component of ribonucleic acid (RNA)
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Component of ATP: Also found in ATP, NADH, and other important coenzymes
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Pentose Phosphate Pathway: Synthesized in the pentose phosphate pathway
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Deoxyribose (2-Deoxy-D-Ribose):
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Significance:
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Component of DNA: A crucial component of deoxyribonucleic acid (DNA). It is a derivative of ribose, having one fewer oxygen atom
Disaccharides
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Definition: Two monosaccharides linked together by a glycosidic bond
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Examples: Sucrose, lactose, maltose
Physical
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State: Crystalline solids at room temperature
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Solubility: Soluble in water due to the presence of hydroxyl groups
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Taste: Sweet, although the degree of sweetness varies
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Optical Activity: Chiral and optically active
Chemical
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Hydrolysis: Disaccharides can be hydrolyzed (broken down) into their constituent monosaccharides by:
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Enzymes: Specific disaccharidases (e.g., sucrase, lactase, maltase)
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Acid: Heating with dilute acid
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Reducing/Non-Reducing:
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Reducing Disaccharides: Have a free anomeric carbon that can undergo oxidation (e.g., maltose, lactose)
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Non-Reducing Disaccharides: Both anomeric carbons are involved in the glycosidic bond and cannot be oxidized (e.g., sucrose)
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Glycosidic Bond Specificity: The properties of a disaccharide depend on the type of glycosidic bond (α or β) and the specific monosaccharides involved
Examples
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Sucrose (Table Sugar):
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Composition: Glucose + Fructose (α-1,2-glycosidic bond)
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Non-reducing: Both anomeric carbons are involved in the glycosidic bond
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Lactose (Milk Sugar):
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Composition: Galactose + Glucose (β-1,4-glycosidic bond)
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Reducing: Glucose has a free anomeric carbon
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Maltose (Malt Sugar):
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Composition: Glucose + Glucose (α-1,4-glycosidic bond)
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Reducing: One glucose has a free anomeric carbon
Polysaccharides
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Definition: Polymers consisting of many monosaccharide units linked together by glycosidic bonds
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Examples: Starch, glycogen, cellulose
Physical
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State: Amorphous solids or fibrous materials
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Solubility: Generally insoluble or form colloidal dispersions in water
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Taste: Generally tasteless
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Molecular Weight: High molecular weights
Chemical
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Hydrolysis: Polysaccharides can be hydrolyzed into smaller oligosaccharides and monosaccharides by:
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Enzymes: Amylases, cellulases, etc
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Acid: Heating with dilute acid
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Structure:
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Homopolysaccharides: Composed of only one type of monosaccharide (e.g., starch, glycogen, cellulose)
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Heteropolysaccharides: Composed of two or more types of monosaccharides (e.g., hyaluronic acid)
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Branching:
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Linear Polysaccharides: Monosaccharides linked in a straight chain (e.g., cellulose)
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Branched Polysaccharides: Chains with branch points (e.g., glycogen, amylopectin)
Examples
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Starch:
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Composition: Polymer of glucose
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Types:
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Amylose: Linear chain of glucose with α-1,4-glycosidic bonds
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Amylopectin: Branched chain of glucose with α-1,4-glycosidic bonds and α-1,6-glycosidic bonds at branch points
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Hydrolysis: Hydrolyzed by amylases to produce glucose, maltose, and dextrins
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Glycogen:
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Composition: Polymer of glucose
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Structure: Highly branched structure with α-1,4-glycosidic bonds and α-1,6-glycosidic bonds at branch points (more branched than amylopectin)
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Function: Storage form of glucose in animals
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Cellulose:
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Composition: Polymer of glucose
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Structure: Linear chain of glucose with β-1,4-glycosidic bonds
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Properties: Forms strong, rigid fibers due to extensive hydrogen bonding between chains. Not digestible by humans (lacks cellulase enzyme)
Key Terms
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Monosaccharide: A simple sugar that cannot be hydrolyzed further
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Disaccharide: Two monosaccharides linked by a glycosidic bond
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Polysaccharide: A polymer of many monosaccharide units
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Reducing Sugar: A sugar that can be oxidized, reducing another substance
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Glycosidic Bond: The bond that links monosaccharides together
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Anomers: Isomeric forms of cyclic monosaccharides that differ only in the position of the hydroxyl group at the anomeric carbon
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Mutarotation: The change in optical rotation resulting from the conversion of one anomer to another
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Hydrolysis: The cleavage of a chemical bond by the addition of water