Understanding Carbohydrates and Lipids: Structure, Properties, and Classification

Understanding Carbohydrates

Carbohydrates are biomolecules primarily composed of carbon, hydrogen, and oxygen. Their atoms are linked to alcohol groups (hydroxyl groups, -OH) and hydrogen atoms (-H). All carbohydrates contain a carbonyl group (C=O), which can be either an aldehyde group (-CHO) or a ketone group (-CO). Therefore, they can be defined as polyhydroxyaldehydes or polyhydroxyketones.

Classification of Carbohydrates

Carbohydrates are classified into several groups:

• Monosaccharides: Simple sugars made of single units.
• Oligosaccharides: Contain between 2 and 10 monosaccharide units.
• Polysaccharides: Formed by multiple repeating units of monosaccharides, further divided into:
  • Homopolysaccharides: Formed by the repetition of a single monomer.
  • Heteropolysaccharides: Composed of more than one type of monomer.
• Glycosides: Complex compounds resulting from the combination of monosaccharides with non-carbohydrate molecules.

Oligosaccharides and O-Glycosidic Bonds

Oligosaccharides are formed by linking up to 10 monosaccharides via O-glycosidic bonds. This bond forms when the hydroxyl group of one monosaccharide reacts with the hydroxyl group of another, resulting in a disaccharide and a water molecule.

If the bond is between two anomeric carbons, it is called a dicarbonyl bond, resulting in a non-reducing disaccharide. Examples of important disaccharides include maltose, isomaltose, cellobiose, lactose, and sucrose.

Polysaccharides

Polysaccharides consist of hundreds or thousands of monosaccharides linked by O-glycosidic bonds. Important polysaccharides include:

• Energy Storage: Starch and glycogen
• Structural Components: Cellulose and chitin

Understanding Lipids

Lipids are a diverse group of chemical compounds characterized by their insolubility in water and solubility in organic solvents or acids.

Key Features of Lipids

• Functions: Protective, regulatory, energy storage, structural, and transport roles.
• Physical Properties: Oily to the touch, have an oily sheen, are less dense than water, and are poor conductors of heat.

Chemical Properties of Lipids

Some key reactions involving lipids include:

• Saponification: Occurs with fatty acids, glycerides, waxes, and phospholipids.
• Esterification: Occurs with steroids.

Fatty Acids

Fatty acids are components of saponifiable lipids. They are organic acids with hydrocarbon chains, typically containing between 12 and 20 carbon atoms.

Fatty acids can be:

• Saturated: Contain no double bonds between carbon atoms.
• Unsaturated: Contain double bonds.

The presence or absence of double bonds affects the molecule's shape (straight or bent), influencing its melting point and whether it is liquid or solid at room temperature.

Glycerides

Glycerides are esters of glycerol and fatty acids, classified as monoacylglycerides, diacylglycerides, or triglycerides.

Saponification of a triacylglycerol involves reacting it with strong bases like sodium hydroxide or potassium hydroxide, yielding glycerol and the corresponding sodium or potassium salts of fatty acids (soap).

Waxes

Waxes are esters of a fatty acid and a long-chain alcohol. They are solids at room temperature due to their long hydrocarbon chains and have low melting points. Waxes are waterproof and provide protective functions, such as in the ear canal.

Phospholipids

Phospholipids are saponifiable lipids composed of fatty acids, an alcohol, and phosphoric acid, often with complex additions. They include phosphoglycerides and sphingolipids.

Phosphoglycerides

Phosphoglycerides are lipids formed from fatty acids, glycerol, phosphoric acid, and an alcohol. The alcohol can be choline, inositol, glycerol, serine, or ethanolamine.

Amphiphilic Lipids

Amphiphilic lipids are structural constituents of biological membranes. They form monolayers in aqueous environments, with the hydrophilic part of the lipid interacting with water.