Macromolecules: Carbohydrates and Lipids

This study guide covers the structure, classification, and biological roles of carbohydrates and lipids, two major classes of biological macromolecules essential for life.

Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio. They serve as energy sources and structural components in living organisms.

Classification of Carbohydrates

• Monosaccharides: Simple sugars, the monomers of carbohydrates. Examples: glucose, galactose, fructose.

• Disaccharides: Composed of two monosaccharides joined by a glycosidic linkage. Examples: lactose (glucose + galactose), sucrose (glucose + fructose), maltose (glucose + glucose).

• Polysaccharides: Polymers of many monosaccharides. Examples: starch, glycogen, cellulose.

Monosaccharides

• Monosaccharides are the simplest carbohydrates and cannot be hydrolyzed into smaller sugars.

• General formula:

• Commonly have 3-7 carbon atoms; hexoses (6 carbons) are most common in biology.

• Examples: Glucose (primary energy source), Galactose, Fructose.

Structure of Monosaccharides

• Hexoses such as glucose, galactose, and fructose share the formula but differ in structure (isomers).

• Monosaccharides can exist in linear or ring forms in aqueous solutions.

Disaccharides

• Formed by joining two monosaccharides via a glycosidic linkage (a type of covalent bond).

• Examples:

  • Lactose: glucose + galactose

  • Sucrose: glucose + fructose

  • Maltose: glucose + glucose

• Disaccharide formation is a dehydration reaction (removal of water).

Polysaccharides

• Polymers of hundreds or thousands of monosaccharides joined by glycosidic linkages.

• Two main types: energy storage and structural polysaccharides.

Energy Storage Polysaccharides

• Glycogen: Storage form of glucose in animals; found in liver and muscle cells.

• Starch: Storage form of glucose in plants; major dietary source of carbohydrates for humans.

Structural Polysaccharides

• Cellulose: Major component of plant cell walls; provides structural support.

• Cellulose is difficult for most animals to digest due to its β-glycosidic linkages.

Biochemical Roles of Carbohydrates

• Energy source (immediate and stored)

• Structural molecules (e.g., cellulose in plants)

• Supply carbon for synthesis of other compounds

Lipids

Lipids are a diverse group of hydrophobic molecules that do not form true polymers. They include fats, oils, phospholipids, and steroids.

Types of Lipids

• Fats and oils (triglycerides): Energy storage, insulation, and protection.

• Phospholipids: Major component of cell membranes.

• Steroids: Include cholesterol and hormones.

Fats and Oils (Triglycerides)

• Composed of one glycerol molecule and three fatty acids.

• Formed by dehydration reactions (ester linkages).

• Function as energy storage, insulation, and protection.

Saturated vs. Unsaturated Fatty Acids

• Saturated fatty acids: No double bonds; straight chains; pack tightly; solid at room temperature (e.g., butter).

• Unsaturated fatty acids: One or more double bonds; kinked chains; do not pack tightly; liquid at room temperature (e.g., olive oil).

• Trans fats: Produced by hydrogenation; associated with negative health effects.

Essential Fatty Acids (EFAs)

• Cannot be synthesized by the human body; must be obtained from the diet.

• Important for brain development, vision, and cell membrane structure.

Phospholipids

• Composed of two fatty acids, a glycerol, and a phosphate-containing polar group.

• Amphipathic: have both hydrophobic (fatty acid tails) and hydrophilic (phosphate head) regions.

• Form bilayers in aqueous environments, making up the fundamental structure of cell membranes.

Steroids

• Characterized by a carbon skeleton with four fused rings.

• Cholesterol: Most common steroid in animals; precursor for steroid hormones (e.g., testosterone, estrogen).

Comparison Table: Types of Lipids

Key Vocabulary

• Carbohydrate

• Glycosidic linkage

• Glycogen

• Starch

• Cellulose

• Monosaccharide / Disaccharide / Polysaccharide

• Lipid

• Triglyceride

• Fatty acid

• Trans fat

Sample Equations

• General formula for monosaccharides:

• Dehydration synthesis (formation of a disaccharide): Monosaccharide + Monosaccharide Disaccharide +

Study Questions

• List and describe the four major classes of macromolecules. Name their monomers.

• Explain the difference between saturated and unsaturated fatty acids.

• How do phospholipids differ from triglycerides in structure and function?

• What are the main functions of carbohydrates and lipids in living organisms?

Additional info: Some explanations and context have been expanded for clarity and completeness based on standard biology curriculum.