Chapter 3: Carbon and the Molecular Diversity of Life

Introduction

This chapter explores the unique properties of carbon that allow it to form a vast array of complex molecules essential for life. The focus is on two major classes of biological macromolecules: carbohydrates and lipids. Understanding their structure and function is fundamental to the study of biology.

Atoms and Elements: The Basis of Carbon Compounds

Structure of the Carbon Atom

• Carbon (C) has an atomic number of 6, meaning it contains 6 protons, 6 neutrons, and 6 electrons.

• The atomic mass number of carbon is 12.

• Protons and neutrons are located in the nucleus, while electrons orbit the nucleus.

Key Point: Carbon's ability to form four covalent bonds makes it uniquely suited to form large, complex molecules.

Overview of Biological Macromolecules

• Macromolecules are large molecules composed of smaller subunits called monomers.

• The four major classes of macromolecules are: carbohydrates, proteins, nucleic acids, and lipids (though lipids are not true polymers).

Carbohydrates: Fuel and Building Material

Introduction to Carbohydrates

Carbohydrates are organic molecules that serve as a primary energy source and as structural components in cells. They include simple sugars and their polymers.

• Monosaccharides: Simple sugars, monomers (e.g., glucose, fructose).

• Disaccharides: Composed of two monosaccharides (e.g., sucrose, lactose).

• Polysaccharides: Polymers of many monosaccharides (e.g., starch, cellulose).

Example: Table sugar is sucrose (a disaccharide), while plant cell walls are made of cellulose (a polysaccharide).

Storage Polysaccharides

Storage polysaccharides are large carbohydrate molecules that store energy for later use.

• Starch: Storage polysaccharide in plants, composed entirely of glucose monomers connected by alpha (α) bonds.

• Glycogen: Storage polysaccharide in animals, also made of glucose monomers with α-bonds; stored mainly in the liver and muscle cells.

• Most animals have enzymes that can hydrolyze plant starch, releasing glucose for energy.

Key Point: Both starch and glycogen are energy reserves, but their structure and storage locations differ between plants and animals.

Structural Polysaccharides

Some polysaccharides provide structural support rather than energy storage.

• Cellulose: Major component of plant cell walls; composed of glucose monomers linked by beta (β) bonds, making it indigestible to most animals.

• Chitin: Found in the exoskeletons of arthropods and in fungal cell walls; an amide derivative of glucose.

Example: Cotton and paper are primarily made of cellulose.

Additional info: Some herbivores have symbiotic microbes that help digest cellulose.

Lipids: Diverse Hydrophobic Molecules

Introduction to Lipids

Lipids are a diverse group of hydrophobic molecules that do not form true polymers. They are important for energy storage, cell membrane structure, and signaling.

• Major types: fats (triglycerides), phospholipids, and steroids.

• Lipids are hydrophobic due to their high proportion of nonpolar hydrocarbon bonds.

Fats (Triglycerides)

Fats are constructed from two types of smaller molecules: glycerol and fatty acids.

• Formed by three dehydration reactions, creating ester linkages between glycerol and fatty acids.

• Main function: long-term energy storage.

Types of Fats:

• Saturated fats: No double bonds between carbon atoms; solid at room temperature (e.g., butter, lard).

• Unsaturated fats: One or more double bonds; liquid at room temperature (e.g., plant oils).

Saturated vs. Unsaturated Fats

• Saturated fats have the maximum number of hydrogen atoms and no double bonds.

• Unsaturated fats have one or more double bonds, causing kinks in the fatty acid chains.

• Trans fats are artificially created by hydrogenating unsaturated fats, increasing shelf life but raising health risks.

Health Note: Consumption of trans fats is linked to increased risk of heart disease and other health problems.

Phospholipids

Phospholipids are major components of cell membranes, forming bilayers that separate the cell from its environment.

• Structure: Glycerol + phosphate group + 2 fatty acids.

• Amphipathic: Hydrophilic (water-attracting) head and hydrophobic (water-repelling) tails.

• In water, phospholipids self-assemble into bilayers, a fundamental structure of biological membranes.

Steroids

Steroids are lipids characterized by a carbon skeleton with four fused rings.

• Cholesterol: Essential component of animal cell membranes; precursor for steroid hormones and vitamin D.

• High cholesterol levels in blood can contribute to atherosclerosis.

• Steroid hormones (e.g., estrogen, testosterone) regulate various physiological processes.

Additional info: The brain synthesizes its own cholesterol, which is vital for nerve function and synapse formation.

Summary Table: Types of Carbohydrates

Summary Table: Types of Lipids

Key Equations

• General formula for monosaccharides:

• Dehydration synthesis (formation of a disaccharide):

• Hydrolysis (breakdown of a disaccharide):