Macromolecules in Biology

Overview of Macromolecules

Macromolecules are large, complex molecules essential for life. The four major classes are carbohydrates, lipids, proteins, and nucleic acids. This guide focuses on carbohydrates and lipids, their structures, types, and biological functions.

• Carbohydrates: Energy storage and structural support

• Lipids: Energy storage, membrane structure, and signaling

Carbohydrates

Classification of Carbohydrates

Carbohydrates are classified based on the number of sugar units:

• Monosaccharides: Single sugar units (simple carbohydrates)

• Disaccharides: Two monosaccharides joined together

• Polysaccharides: Many monosaccharide units (complex carbohydrates)

Monosaccharides: The Monomer

Monosaccharides are the simplest carbohydrates, serving as building blocks for larger molecules.

• Contain 3 to 7 carbon atoms

• Common examples: Glucose, Galactose, Fructose

• General formula:

• Ratio of carbon:hydrogen:oxygen is typically 1:2:1

Example: Glucose () is a hexose (six-carbon sugar) and a primary energy source for cells.

Structure of Monosaccharides

Monosaccharides can exist in linear or ring forms. The most common hexoses are:

• Glucose

• Galactose

• Fructose

Example: Glucose and galactose are aldoses (contain an aldehyde group), while fructose is a ketose (contains a ketone group).

Disaccharides: Formation and Examples

Disaccharides are formed by joining two monosaccharides via a dehydration reaction, creating a glycosidic linkage.

• Glycosidic linkage: Covalent bond between two monosaccharides

• Common disaccharides:

  • Lactose = Glucose + Galactose

  • Sucrose = Glucose + Fructose

  • Maltose = Glucose + Glucose

Example: Lactose intolerance results from inability to digest lactose due to lack of the enzyme lactase.

Polysaccharides: Structure and Function

Polysaccharides are long chains of monosaccharide units, serving various roles in organisms.

• Energy storage polysaccharides:

  • Glycogen: Storage form of glucose in animals

  • Starch: Storage form of glucose in plants

• Structural polysaccharides:

  • Cellulose: Major component of plant cell walls; difficult for mammals to digest

Example: Humans consume cellulose as dietary fiber, but cannot break it down for energy.

Biochemical Roles of Carbohydrates

Carbohydrates serve four major biochemical roles:

• Energy: Immediate and stored energy source

• Structural molecules: Provide shape to organisms (e.g., cellulose in plants)

• Supply carbon: For synthesis of other compounds

• Cell recognition: Surface carbohydrates involved in cell signaling

Lipids

Overview of Lipids

Lipids are hydrophobic molecules that do not form polymers. They are diverse in structure and function.

• Fats and oils (triglycerides)

• Phospholipids

• Steroids

Fats and Oils (Triglycerides)

Triglycerides are composed of one glycerol molecule joined to three fatty acids via dehydration reactions.

• Function: Energy storage, insulation, and protection

• Can be solid (fat) or liquid (oil) at room temperature

• Fatty acids can be saturated (no double bonds) or unsaturated (one or more double bonds)

Example: Saturated fats are typically solid at room temperature (e.g., butter), while unsaturated fats are liquid (e.g., olive oil).

Triglyceride Synthesis

Triglycerides are formed by joining fatty acids to glycerol via dehydration reactions.

• Hydrophobic interactions cause fats to aggregate in water

• Individual fat molecules are not covalently bonded together

Saturated vs. Unsaturated Fatty Acids

Saturated fatty acids have no double bonds, resulting in straight chains that pack tightly. Unsaturated fatty acids have one or more double bonds, causing kinks that prevent tight packing.

• Saturated fatty acids: Solid at room temperature

• Unsaturated fatty acids: Liquid at room temperature

Example: Animal fats are usually saturated; plant oils are usually unsaturated.

Essential Fatty Acids (EFAs)

Essential fatty acids cannot be synthesized by the human body and must be obtained from the diet.

• Required for proper growth and development

• Examples: Omega-3 and omega-6 fatty acids

Saturated vs. Unsaturated Fats in the Diet

Dietary fats impact health. Saturated fats are associated with increased cholesterol and cardiovascular risk. Unsaturated fats are considered healthier.

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

Phospholipids

Phospholipids are major components of cell membranes. They consist of two fatty acids, a glycerol, and a phosphate group.

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

• Form bilayer structures in water, critical for membrane formation

Example: The phospholipid bilayer is the fundamental structure of biological membranes.

Steroids

Steroids are lipids with a characteristic four-ring structure. Cholesterol is the most common steroid in humans and serves as a precursor for steroid hormones.

• Cholesterol: Maintains membrane fluidity and is a precursor for hormones such as testosterone and estrogen

Key Vocabulary

• Carbohydrate

• Glycosidic linkage

• Glycogen

• Starch

• Cellulose

• Mono-/Polysaccharide

• Lipid

• Triglyceride

• Fatty acid

• Trans fat

Summary Table: Carbohydrate Types

Summary Table: Lipid Types

Key Equations

• General formula for monosaccharides:

• Dehydration reaction (formation of disaccharide):

Additional info:

• Cellulose is indigestible to humans due to lack of cellulase enzyme.

• Phospholipids form bilayers because their amphipathic nature causes hydrophobic tails to avoid water and hydrophilic heads to interact with water.

• Trans fats are artificially produced and associated with increased risk of heart disease.