The Molecules of Life: Structure and Function of Biological Macromolecules

Chapter 3: The Molecules of Life

Introduction

This chapter explores the fundamental molecules that make up living organisms, focusing on their structure, function, and importance in health and nutrition. Understanding these molecules is essential for comprehending biological processes and making informed dietary choices.

Organic Molecules of Living Organisms

Carbon: The Building Block of Life

• Carbon is the central element in organic molecules, making up about 18% of the human body by weight.

• It can form four covalent bonds, allowing for a diversity of stable structures, including single and double bonds.

• Carbon's bonding versatility enables the formation of both small molecules (micromolecules) and large molecules (macromolecules).

• Examples of carbon-based molecules include carbohydrates, lipids, proteins, and nucleic acids.

Example: The structural formulas shown in the images represent carbon atoms bonded to hydrogen, illustrating single and double bonds between carbon atoms.

Macromolecules of Life

Overview of Macromolecules

• The four major classes of biological macromolecules are carbohydrates, lipids, proteins, and nucleic acids.

• These macromolecules are polymers, constructed by linking smaller subunits called monomers.

• Cells build polymers through dehydration synthesis (removal of water) and break them down via hydrolysis (addition of water).

Polymerization Reactions

• Dehydration Synthesis: Joins monomers by removing a water molecule, requiring energy input.

• Hydrolysis: Breaks polymers into monomers by adding a water molecule, releasing energy.

Carbohydrates

Structure and Function

• Composed of carbon, hydrogen, and oxygen in a 1:2:1 ratio (C:H:O).

• Serve as energy sources and provide structural support in plants.

• Monomers are monosaccharides (simple sugars); polymers are polysaccharides (complex carbohydrates).

Types of Carbohydrates

• Monosaccharides: Single sugar units (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides joined by dehydration synthesis (e.g., sucrose = glucose + fructose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).

Examples and Applications

• Starch: Energy storage in plants.

• Glycogen: Energy storage in animals.

• Cellulose: Structural support in plant cell walls.

Lipids

Structure and Function

• Composed mainly of carbon and hydrogen; hydrophobic (insoluble in water).

• Major types include triglycerides (fats), phospholipids (membranes), and steroids (hormones).

Triglycerides

• Formed from glycerol and three fatty acids.

• Function as energy storage, insulation, and protection of organs.

Phospholipids

• Contain a glycerol backbone, two fatty acids, and a phosphate group.

• Form the bilayer structure of cell membranes, with hydrophilic heads and hydrophobic tails.

Steroids

• Characterized by four fused carbon rings.

• Function as chemical messengers (e.g., hormones like testosterone and estrogen).

Proteins

Structure and Function

• Polymers of amino acids (20 different types).

• Each amino acid has a central carbon, an amino group, a carboxyl group, a hydrogen atom, and a variable R group.

• Linked by peptide bonds formed through dehydration synthesis.

• Function in catalysis (enzymes), structure, transport, signaling, and defense.

Protein Structure

• Proteins fold into specific 3D shapes essential for their function.

• Shape is determined by the sequence of amino acids and can be affected by environmental factors (e.g., temperature, pH).

• Denaturation is the loss of protein structure and function due to unfavorable conditions.

Nucleic Acids

Structure and Function

• Polymers of nucleotides, each consisting of a five-carbon sugar, a phosphate group, and a nitrogenous base.

• Store and transmit genetic information.

• Two main types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

DNA

• Double helix structure with complementary base pairing (A with T, G with C).

• Stores genetic instructions for protein synthesis.

RNA

• Single-stranded; involved in protein synthesis and gene regulation.

Summary Table: Major Macromolecules

Macromolecule	Monomer	Polymer	Main Function	Example
Carbohydrate	Monosaccharide	Polysaccharide	Energy storage, structure	Starch, glycogen, cellulose
Lipid	Glycerol & Fatty Acids	Triglyceride, Phospholipid	Energy storage, membranes, hormones	Fats, oils, steroids
Protein	Amino Acid	Polypeptide	Catalysis, structure, transport	Enzymes, hemoglobin
Nucleic Acid	Nucleotide	DNA, RNA	Genetic information storage	DNA, RNA

Additional info: The images provided illustrate the importance of nutrition, the role of carbon in organic molecules, and the structural diversity enabled by carbon bonding. The notes also emphasize the relevance of dietary choices to health, as well as the molecular basis for these effects.