Properties

Physical and Chemical

Physical Properties:
- Monosaccharides/Disaccharides: Crystalline solids, water-soluble, sweet
- Polysaccharides: Amorphous solids or fibrous, mostly insoluble, generally tasteless
Chemical Properties:
- Monosaccharides:
  - Cyclization (forming α and β anomers)
  - Oxidation (reducing sugars can reduce other compounds)
  - Esterification, Glycoside Formation
- Disaccharides:
  - Hydrolyzed by enzymes or acid into monosaccharides
  - Can be reducing or non-reducing
- Polysaccharides:
  - Hydrolyzed by enzymes or acid into smaller sugars
  - Vary in structure (linear vs. branched) and composition (homo- vs. heteropolysaccharides)

Monosaccharides

Definition: The basic building blocks of carbohydrates; cannot be hydrolyzed into smaller units (also known as simple sugars)
Examples: Glucose, fructose, galactose, ribose

Physical

State: Crystalline solids at room temperature
Solubility: Highly soluble in water due to numerous hydroxyl (-OH) groups that form hydrogen bonds with water molecules
Taste: Generally sweet, although the degree of sweetness varies (fructose > sucrose > glucose > galactose)
Optical Activity: Chiral molecules that rotate plane-polarized light; classified as either dextrorotatory (+) or levorotatory (-)

Chemical

Cyclization:
- Monosaccharides with five or more carbon atoms can cyclize in aqueous solution, forming cyclic hemiacetals (from aldehydes) or hemiketals (from ketones)
- Anomers: The cyclization process creates a new chiral center at the carbonyl carbon (anomeric carbon), resulting in α and β anomers
- Mutarotation: The spontaneous interconversion between α and β anomers in solution until an equilibrium mixture is reached
Oxidation:
- Monosaccharides with a free aldehyde or ketone group can be oxidized (reducing sugars)
- Reducing Sugars: Glucose, fructose, galactose, lactose, maltose
- Tests for Reducing Sugars:
  - Benedict’s Test: Reducing sugars react with copper(II) sulfate in alkaline solution, reducing it to copper(I) oxide, forming a red-brown precipitate
  - Fehling’s Test: Similar to Benedict’s test, uses copper(II) tartrate complex
Reduction: The aldehyde or ketone group of a monosaccharide can be reduced to form a sugar alcohol (alditol)
- Examples
  - Glucose is reduced to sorbitol (glucitol)
  - Xylose is reduced to xylitol (a sugar substitute)
Esterification: Hydroxyl groups (-OH) can react with acids to form esters
- Example: Phosphorylation of glucose (e.g., glucose-6-phosphate) is essential in metabolism
Glycoside Formation: The anomeric hydroxyl group can react with another alcohol to form a glycoside with an O-glycosidic bond
- Glycosidic Bond: The bond that links monosaccharides together to form disaccharides, oligosaccharides, and polysaccharides
Isomerization: Monosaccharides can be converted into isomers
- Example: Glucose can be converted to fructose via an enediol intermediate

Examples

Glucose (D-Glucose):
- Also Known As: Dextrose, blood sugar
- Significance:
  - Primary Energy Source: The main energy source for most cells in the body
  - Central to Metabolism: Central molecule in carbohydrate metabolism; glycolysis, gluconeogenesis, glycogenesis, and glycogenolysis revolve around glucose
  - Regulation: Blood glucose levels are tightly regulated by insulin and glucagon
Fructose (D-Fructose):
- Also Known As: Fruit sugar, levulose
- Significance:
  - Sweetest Natural Sugar: Found in fruits, honey, and high-fructose corn syrup
  - Metabolism: Metabolized primarily in the liver
  - Dietary Concerns: High intake of fructose has been linked to metabolic issues
Galactose (D-Galactose):
- Significance:
  - Part of Lactose: Found in milk as part of the disaccharide lactose
  - Metabolism: Converted to glucose in the liver
  - Galactosemia: Deficiency in enzymes that metabolize galactose leads to galactosemia
Ribose (D-Ribose):
- Significance:
  - Component of RNA: A crucial component of ribonucleic acid (RNA)
  - Component of ATP: Also found in ATP, NADH, and other important coenzymes
  - Pentose Phosphate Pathway: Synthesized in the pentose phosphate pathway
Deoxyribose (2-Deoxy-D-Ribose):
- Significance:
  - Component of DNA: A crucial component of deoxyribonucleic acid (DNA). It is a derivative of ribose, having one fewer oxygen atom

Disaccharides

Definition: Two monosaccharides linked together by a glycosidic bond
Examples: Sucrose, lactose, maltose

Physical

State: Crystalline solids at room temperature
Solubility: Soluble in water due to the presence of hydroxyl groups
Taste: Sweet, although the degree of sweetness varies
Optical Activity: Chiral and optically active

Chemical

Hydrolysis: Disaccharides can be hydrolyzed (broken down) into their constituent monosaccharides by:
- Enzymes: Specific disaccharidases (e.g., sucrase, lactase, maltase)
- Acid: Heating with dilute acid
Reducing/Non-Reducing:
- Reducing Disaccharides: Have a free anomeric carbon that can undergo oxidation (e.g., maltose, lactose)
- Non-Reducing Disaccharides: Both anomeric carbons are involved in the glycosidic bond and cannot be oxidized (e.g., sucrose)
Glycosidic Bond Specificity: The properties of a disaccharide depend on the type of glycosidic bond (α or β) and the specific monosaccharides involved

Examples

Sucrose (Table Sugar):
- Composition: Glucose + Fructose (α-1,2-glycosidic bond)
- Non-reducing: Both anomeric carbons are involved in the glycosidic bond
Lactose (Milk Sugar):
- Composition: Galactose + Glucose (β-1,4-glycosidic bond)
- Reducing: Glucose has a free anomeric carbon
Maltose (Malt Sugar):
- Composition: Glucose + Glucose (α-1,4-glycosidic bond)
- Reducing: One glucose has a free anomeric carbon

Polysaccharides

Definition: Polymers consisting of many monosaccharide units linked together by glycosidic bonds
Examples: Starch, glycogen, cellulose

Physical

State: Amorphous solids or fibrous materials
Solubility: Generally insoluble or form colloidal dispersions in water
Taste: Generally tasteless
Molecular Weight: High molecular weights

Chemical

Hydrolysis: Polysaccharides can be hydrolyzed into smaller oligosaccharides and monosaccharides by:
- Enzymes: Amylases, cellulases, etc
- Acid: Heating with dilute acid
Structure:
- Homopolysaccharides: Composed of only one type of monosaccharide (e.g., starch, glycogen, cellulose)
- Heteropolysaccharides: Composed of two or more types of monosaccharides (e.g., hyaluronic acid)
Branching:
- Linear Polysaccharides: Monosaccharides linked in a straight chain (e.g., cellulose)
- Branched Polysaccharides: Chains with branch points (e.g., glycogen, amylopectin)

Examples

Starch:
- Composition: Polymer of glucose
- Types:
  - Amylose: Linear chain of glucose with α-1,4-glycosidic bonds
  - Amylopectin: Branched chain of glucose with α-1,4-glycosidic bonds and α-1,6-glycosidic bonds at branch points
- Hydrolysis: Hydrolyzed by amylases to produce glucose, maltose, and dextrins
Glycogen:
- Composition: Polymer of glucose
- Structure: Highly branched structure with α-1,4-glycosidic bonds and α-1,6-glycosidic bonds at branch points (more branched than amylopectin)
- Function: Storage form of glucose in animals
Cellulose:
- Composition: Polymer of glucose
- Structure: Linear chain of glucose with β-1,4-glycosidic bonds
- Properties: Forms strong, rigid fibers due to extensive hydrogen bonding between chains. Not digestible by humans (lacks cellulase enzyme)

Key Terms

Monosaccharide: A simple sugar that cannot be hydrolyzed further
Disaccharide: Two monosaccharides linked by a glycosidic bond
Polysaccharide: A polymer of many monosaccharide units
Reducing Sugar: A sugar that can be oxidized, reducing another substance
Glycosidic Bond: The bond that links monosaccharides together
Anomers: Isomeric forms of cyclic monosaccharides that differ only in the position of the hydroxyl group at the anomeric carbon
Mutarotation: The change in optical rotation resulting from the conversion of one anomer to another
Hydrolysis: The cleavage of a chemical bond by the addition of water

Physiology

Procedures