Macromolecules: Carbohydrates and Lipids

Introduction to Macromolecules

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

Carbohydrates

Classification and Structure

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.

• Monosaccharides: The simplest carbohydrates, also known as simple sugars. They are the monomers of larger carbohydrate polymers.

• Disaccharides: Formed by joining two monosaccharides via a glycosidic linkage.

• Polysaccharides: Long chains of monosaccharide units; can serve as energy storage or structural molecules.

Monosaccharides

Monosaccharides are the building blocks of carbohydrates. They vary in carbon number (commonly 3-7 carbons) and are classified by the number of carbons:

• Triose: 3 carbons

• Pentose: 5 carbons

• Hexose: 6 carbons (e.g., glucose, galactose, fructose)

The general formula for a monosaccharide is:

For example, glucose has the formula .

Examples of Hexose Monosaccharides

Disaccharides

Disaccharides are formed when two monosaccharides are joined by a dehydration reaction, creating a glycosidic linkage. Common disaccharides include:

• Lactose: Glucose + Galactose

• Sucrose: Glucose + Fructose

• Maltose: Glucose + Glucose

Disaccharides are important for energy transport and storage in organisms.

Polysaccharides

Polysaccharides are large polymers of monosaccharides. Their function depends on their structure and the type of monomers involved.

• Energy Storage Polysaccharides:

  • Glycogen: Storage form of glucose in animals; highly branched for rapid energy release.

  • Starch: Storage form of glucose in plants; less branched than glycogen.

• Structural Polysaccharides:

  • Cellulose: Major component of plant cell walls; forms strong fibers. Difficult for most animals to digest due to the type of glycosidic bonds.

Biochemical Roles of Carbohydrates

• Energy: Primary source of energy for cells.

• Storage: Stored as glycogen in animals and starch in plants.

• Structural Molecules: Provide shape and support (e.g., cellulose in plants).

• Supply Carbon: Serve as a source of carbon for synthesis of other compounds.

Lipids

Introduction to Lipids

Lipids are a diverse group of hydrophobic molecules, including fats, oils, phospholipids, and steroids. Unlike other macromolecules, lipids do not form true polymers.

• Fats and Oils (Triglycerides): Energy storage, insulation, and protection.

• Phospholipids: Major component of cell membranes.

• Steroids: Include hormones and cholesterol.

Fats and Oils (Triglycerides)

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

• Saturated Fatty Acids: No double bonds; straight chains; solid at room temperature.

• Unsaturated Fatty Acids: One or more double bonds; kinked chains; liquid at room temperature.

Fatty acids can be a mixture of saturated and unsaturated types in natural fats.

Comparison of Saturated and Unsaturated Fatty Acids

Essential Fatty Acids (EFAs)

EFAs are fatty acids that cannot be synthesized by the human body and must be obtained from the diet. They are important for brain development, vision, and cell membrane structure.

Trans Fats

Trans fats are artificially produced by hydrogenating unsaturated fats. They are associated with negative health effects, such as increased risk of heart disease.

Phospholipids

Phospholipids are composed of two fatty acids, a glycerol backbone, and a phosphate group. They are amphipathic, having both hydrophobic (fatty acid tails) and hydrophilic (phosphate head) regions.

• Major component of cell membranes, forming a bilayer structure.

• Self-assemble in water to form membranes due to their amphipathic nature.

Steroids

Steroids are lipids with a structure of four fused carbon rings. Cholesterol is the most common steroid in animals and serves as a precursor for steroid hormones (e.g., testosterone, estrogen).

Key Vocabulary

• Carbohydrate

• Glycosidic linkage

• Glycogen

• Starch

• Cellulose

• Monosaccharide / Disaccharide / Polysaccharide

• Lipid

• Triglyceride

• Fatty acid

• Trans fat

Study Guide Questions

Carbohydrates

1. List and name the four major classes of macromolecules. Name their monomers.

2. Describe the names and structures of various monosaccharides, disaccharides, and polysaccharides.

3. Which different polysaccharides are discussed in lecture? How do they differ in function?

4. What roles do carbohydrates play in living organisms?

5. How do plants store carbohydrates for energy? Animals?

Lipids

1. What are the 3 primary types of lipids? Name cellular functions of each.

2. How does the structure of a phospholipid differ from a triglyceride? How does this affect their role in cells?

3. What is the most common steroid in the human body? What is its function?

4. What is the difference between a fat and an oil?

5. Distinguish between saturated and unsaturated fatty acids. Which is more likely to be solid at room temperature?

6. What are trans fats? What are their effects in both structure and health?