Carbohydrates and Lipids: Structure, Function, and Classification

Carbohydrates

Definition and General Structure

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with the empirical formula (CH2O)n. They serve as a primary source of energy and as structural components in living organisms.

• Monosaccharides: The simplest carbohydrates, also called simple sugars. They are monomers and typically contain 3-7 carbon atoms. Examples: glucose, fructose, galactose.

• Disaccharides: Consist of two monosaccharides joined by a covalent bond called a glycosidic bond. Examples: sucrose (glucose + fructose), lactose (glucose + galactose), maltose (glucose + glucose).

• Polysaccharides: Large macromolecules formed by the polymerization of many monosaccharides. They serve as energy storage or structural materials. Examples: starch, glycogen, cellulose, chitin.

General formula for monosaccharides: (CH2O)n, where n = 3–7.

Classification of Monosaccharides

• Triose: 3 carbons (C3H6O3)

• Tetrose: 4 carbons (C4H8O4)

• Pentose: 5 carbons (C5H10O5)

• Hexose: 6 carbons (C6H12O6)

Monosaccharides are classified by the number of carbon atoms and the presence of functional groups (aldehyde or ketone).

Formation of Disaccharides

Disaccharides are formed by a dehydration reaction (condensation), which joins two monosaccharides and releases a molecule of water.

General equation for disaccharide formation:

$\text{Monosaccharide}_1 + \text{Monosaccharide}_2 \rightarrow \text{Disaccharide} + H_2O$

Polysaccharides: Structure and Function

Polysaccharides are categorized based on their function:

Type of Polysaccharide	Examples	Function	Occurrence
Energy Storage	Starch (plants), Glycogen (animals)	Store glucose for energy	Starch in plant cells, glycogen in animal liver and muscle
Structural Building Block	Cellulose (plants), Chitin (fungi, arthropods)	Provide structural support	Cellulose in plant cell walls, chitin in exoskeletons

Example: Humans can digest starch and glycogen but not cellulose due to differences in glycosidic bond orientation.

Key Terms and Definitions

• Glycosidic bond: Covalent bond joining carbohydrate (sugar) molecules.

• Hydrolysis: The chemical breakdown of a compound due to reaction with water; used to break down polysaccharides into monosaccharides.

• Cellulose: Structural polysaccharide in plant cell walls; composed of β-glucose monomers.

• Chitin: Structural polysaccharide in fungal cell walls and arthropod exoskeletons; contains nitrogen.

Lipids

Definition and General Properties

Lipids are a diverse group of hydrophobic molecules that are not true polymers. They include fats, oils, waxes, phospholipids, and steroids. Lipids are characterized by their insolubility in water due to their nonpolar nature.

• Hydrophobic: Repelled by water; does not dissolve in water.

Types of Lipids

• Triacylglycerol (Triglyceride): Composed of a glycerol backbone and three fatty acid chains joined by ester linkages. Main function is long-term energy storage.

• Phospholipid: Composed of a glycerol backbone, two fatty acid tails, and a phosphate group with an additional polar group. Major component of biological membranes, forming a bilayer with hydrophilic heads and hydrophobic tails.

• Steroid: Characterized by a carbon skeleton of four fused rings. Examples include cholesterol and steroid hormones (e.g., testosterone, estrogen).

Fatty Acids: Saturated vs. Unsaturated

• Saturated fatty acid: No double bonds between carbon atoms; saturated with hydrogen atoms. Solid at room temperature (e.g., butter).

• Unsaturated fatty acid: One or more double bonds in the hydrocarbon chain; causes kinks, making them liquid at room temperature (e.g., olive oil).

Example: An unsaturated fatty acid with a double bond has fewer hydrogen atoms than a saturated fatty acid of the same length.

Phospholipid Bilayer

Phospholipids spontaneously form bilayers in aqueous environments, with hydrophilic heads facing outward and hydrophobic tails facing inward. This structure is fundamental to cell membrane function.

Key Terms and Definitions

• Ester linkage: The bond formed between a glycerol molecule and a fatty acid in fats and phospholipids.

• Amphipathic: Molecule with both hydrophilic and hydrophobic regions (e.g., phospholipids).

• Cholesterol: A steroid that is an essential component of animal cell membranes and a precursor for steroid hormones.

Summary Table: Comparison of Carbohydrates and Lipids

Property	Carbohydrates	Lipids
Monomer	Monosaccharide	Fatty acid, glycerol (not a true polymer)
Main Elements	C, H, O	C, H, O (sometimes P in phospholipids)
Function	Energy storage, structure	Energy storage, membranes, hormones
Solubility	Generally hydrophilic	Hydrophobic

Example Application

• Carbohydrates: Glycogen is stored in animal liver and muscle for rapid energy release during activity.

• Lipids: Phospholipids form the structural basis of all cell membranes, creating a selective barrier for the cell.

Additional info: Nucleic acids and proteins are also mentioned in the source material, but the primary focus here is on carbohydrates and lipids as per the provided content.