Carbohydrates: Classification and Structure

Classification of Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, and are classified based on the number of sugar units they contain.

• Monosaccharides: Single sugar units (e.g., glucose, fructose, galactose).

• Disaccharides: Two monosaccharides joined by a glycosidic bond (e.g., sucrose, lactose).

• Oligosaccharides: Short chains (3-10) of monosaccharide units.

• Polysaccharides: Long chains (hundreds to thousands) of monosaccharide units (e.g., starch, cellulose).

Carbohydrates can also be classified as soluble or insoluble fibers:

• Soluble fibers: Dissolve in water; can help lower blood cholesterol (e.g., pectin).

• Insoluble fibers: Do not dissolve in water; aid in digestion (e.g., cellulose).

Organic Functional Groups in Monosaccharides

Monosaccharides contain several functional groups:

• Alcohols: Hydroxyl groups (-OH) attached to carbon atoms. Classified as primary, secondary, or tertiary based on the carbon's connectivity.

• Aldehyde: Carbonyl group at the end of the carbon chain (aldoses).

• Ketone: Carbonyl group within the carbon chain (ketoses).

Structural Properties of Monosaccharides

Monosaccharides exhibit stereoisomerism and can exist in different forms:

• D- and L-stereoisomers: Refers to the orientation of the hydroxyl group on the chiral carbon furthest from the carbonyl group.

• Enantiomers: Non-superimposable mirror images.

• Epimers: Differ at only one chiral center.

• Diastereomers: Stereoisomers that are not mirror images.

Common monosaccharides include:

• Glucose: An aldohexose, primary energy source.

• Fructose: A ketohexose, found in fruits.

• Galactose: An aldohexose, component of lactose.

Reactions at the Anomeric Carbon

The anomeric carbon is the carbonyl carbon that becomes a new chiral center when a monosaccharide cyclizes.

• α and β Anomers: Differ in the position of the hydroxyl group at the anomeric carbon.

• Oxidation: Aldoses can be oxidized to form acids.

• Reduction: Carbonyl groups can be reduced to alcohols.

Glycosidic Bonds in Disaccharides

Disaccharides are formed by condensation reactions between two monosaccharides, creating a glycosidic bond.

• Condensation: Removal of water to form a bond.

• Hydrolysis: Addition of water to break a bond.

• Glycosidic bond: The linkage between two sugar units, named by the carbons involved (e.g., 1→4).

Common disaccharides include:

• Sucrose: Glucose + fructose.

• Lactose: Glucose + galactose.

• Maltose: Glucose + glucose.

The sweetness scale compares the relative sweetness of sugars and sweeteners.

Polysaccharides

Polysaccharides are distinguished by their glycosidic bonds and sugar subunits.

• Starch: α(1→4) and α(1→6) glycosidic bonds; energy storage in plants.

• Cellulose: β(1→4) glycosidic bonds; structural component in plants.

• Glycogen: Highly branched, α(1→4) and α(1→6) bonds; energy storage in animals.

ABO Blood Group Compatibility

Blood groups are determined by specific carbohydrate structures on red blood cells. Compatibility depends on the presence or absence of A and B antigens.

• Type A: Has A antigen.

• Type B: Has B antigen.

• Type AB: Has both A and B antigens.

• Type O: Has neither antigen.

Transfusion compatibility is based on matching antigens to avoid immune reactions.

States of Matter and Their Attractive Forces

Gas Laws

Gas laws describe the relationships between pressure, volume, temperature, and amount of gas.

• Pressure Unit Conversion: Common units include atm, mmHg, torr, and Pa.

• Kinetic Molecular Theory: Explains gas behavior based on particle motion.

• Boyle’s Law: (at constant temperature)

• Charles’s Law: (at constant pressure)

• Gay-Lussac’s Law: (at constant volume)

• Combined Gas Law:

• Ideal Gas Law:

Attractive Forces in Compounds

Attractive forces determine physical properties such as boiling point and vapor pressure.

• Types of Attractive Forces:

  • London dispersion forces

  • Dipole-dipole interactions

  • Hydrogen bonding

  • Ion-dipole interactions

  • Ionic bonds

• Hydrogen Bonds: Occur between molecules with N-H, O-H, or F-H bonds.

• Boiling Point: Increases with stronger attractive forces.

• Vapor Pressure: Decreases with stronger attractive forces.

Solubility in Water

Solubility depends on the nature of attractive forces between solute and solvent.

• Golden Rule of Solubility: "Like dissolves like"—polar substances dissolve in polar solvents, nonpolar in nonpolar.

• Amphipathic Molecules: Contain both polar and nonpolar regions (e.g., fatty acids).

• Emulsifier: Allows mixing of polar and nonpolar substances by stabilizing the interface.

• Micelle: Spherical arrangement of amphipathic molecules in water, with nonpolar tails inward and polar heads outward.

Fats vs. Oils

Fats and oils are triglycerides distinguished by their physical properties and attractive forces.

• Fats: Solid at room temperature; saturated fatty acids; higher melting points.

• Oils: Liquid at room temperature; unsaturated fatty acids; lower melting points.

• Triglyceride: Ester formed from glycerol and three fatty acids.

Cell Membrane Structure

The cell membrane is composed of a phospholipid bilayer, providing structural integrity and selective permeability.

• Phospholipid Bilayer: Two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails inward.

• Polar Regions: Hydrophilic heads interact with water.

• Nonpolar Regions: Hydrophobic tails form the interior.

• Cholesterol: Embedded in the membrane, modulates fluidity.

Example: Water (H2O) exhibits hydrogen bonding, resulting in high boiling point and solubility for polar substances.

Example: Cell membranes are composed of phospholipids, cholesterol, and proteins, with amphipathic molecules forming micelles in aqueous environments.