CARBOHYDRATES

CARBOHYDRATES

Definition

• Carbohydrates: Refers to polyhydroxy aldehydes, polyhydroxy ketones, or compounds yielding these via hydrolysis.

• Examples: Glucose (polyhydroxy aldehyde), Fructose (polyhydroxy ketone).

• Characterized by multiple hydroxyl groups and either an aldehyde or ketone functional group.

Importance and Composition

• Commonly known as sugars, the most abundant biomolecules on Earth.

• Composed of Carbon (C), Hydrogen (H), and Oxygen (O) with a general formula of (CH₂O)ₙ.

• Critical roles: Energy metabolism, components of nucleic acids (RNA & DNA), attached to proteins (glycoproteins).

Classification of Carbohydrates

• Simple Carbohydrates:

  • Monosaccharides: Simplest form; single polyhydroxy aldehyde or ketone units.

    • Examples: Glucose, Fructose.

  • Disaccharides: Two monosaccharides linked.

    • Examples: Sucrose, Lactose.

  • Oligosaccharides: Three to ten monosaccharides.

  • Polysaccharides: Large polymers of monosaccharides.

    • Examples: Cellulose, Starch, Glycogen.

Monosaccharides Classification

• Based on Carbonyl Group:

  • Aldoses: Monosaccharides with aldehyde groups.

  • Ketoses: Monosaccharides with ketone groups.

• Based on Number of Carbons:

  • Trioses (3), Tetroses (4), Pentoses (5), Hexoses (6), Heptoses (7).

SUGARS

Characteristics of Sugars

• Sugars are polyhydroxy aldehydes or polyhydroxy ketones.

• Aldehyde sugars: Aldoses (e.g., ribose); Ketone sugars: Ketoses (e.g., ribulose).

• Sugars exhibit chemical properties of both alcohols and aldehydes/ketones.

STABILITY OF MONOSACCHARIDES

Chirality and Isomers

• Chiral Molecule: Non-superimposable on its mirror image; has chiral centers (asymmetric carbons).

• Enantiomers: Non-superimposable mirror images (e.g., D-Glucose and L-Glucose).

• Diastereomers: Stereoisomers that are not mirror images.

• Epimers: Diastereomers differing at only one carbon.

Fischer Projection Formula

• Used to represent sugar structures on flat paper; bonds orientation is important.

• Can also represent as perspective formulas based on bond direction.

CYCLIZATION OF SUGARS

Haworth Projection

• Sugars cyclize to form pyranose (six-membered) and furanose (five-membered) rings.

• Anomeric carbon: carbon that becomes a new chiral center during cyclization.

• Alpha (α) and beta (β) isomers differ in the orientation of the OH group on the anomeric carbon.

Common Monosaccharides

• D-Glucose: Most abundant monosaccharide; critical energy source; regulated in blood.

• D-Galactose: Component of lactose; key for brain function and blood type determination.

• D-Fructose: Sweetest monosaccharide; primarily metabolized in the liver.

• D-Ribose: Essential in nucleic acids; forms backbone of RNA; vital for energy molecules like ATP.

REACTIONS OF MONOSACCHARIDES

Oxidation and Reduction

• Oxidation: Conversion of aldehyde to carboxylic acid (sugar acid).

• Reduction: Formation of sugar alcohols from aldehydes.

Glycoside Formation

• Monosaccharides can form glycosides through condensation reactions with alcohols.

• Important in forming nucleosides (i.e., ribonucleosides) from ribose.

Phosphate Ester Formation

• Formation of esters from hydroxyl groups in monosaccharides via reaction with oxyacids.

Amino Sugar Formation

• Replacement of hydroxyl group with an amino group leads to amino sugars (e.g., Glucosamine).

Disaccharide Formation

• Formed from monosaccharides through dehydration reactions forming glycosidic bonds.

• Hydrolysis of disaccharides requires water to revert to monosaccharides.

DISACCHARIDES

Examples

• Sucrose: Composed of glucose and fructose.

• Lactose: Composed of glucose and galactose.

• Maltose: Composed of two glucose units with α-linkage.

POLYSACCHARIDES

Structures and Functions

• Cellulose: Most abundant organic molecule; structural component in plants; resistant to breakdown.

• Starch: Storage polysaccharide in plants; comprises amylose and amylopectin.

• Glycogen: Storage polysaccharide in animals; allows rapid glucose mobilization.

• Chitin: Structural polysaccharide in arthropods and fungi.

• Peptidoglycan: Component of bacterial cell walls, confers rigidity.