The Molecules of Life: Study Guide and Key Concepts

Chapter 3 – The Molecules of Life

Introduction

This study guide covers the fundamental molecules that make up living organisms, focusing on the structure, function, and importance of carbohydrates, lipids, proteins, and nucleic acids. Understanding these macromolecules is essential for grasping the chemical basis of life.

Key Terms and Definitions

• Organic compound: A chemical compound containing carbon and usually hydrogen, often found in living organisms.

• Hydrocarbon: Molecules consisting entirely of carbon and hydrogen atoms.

• Saturated hydrocarbon: Hydrocarbons with only single bonds between carbon atoms (alkanes).

• Unsaturated hydrocarbon: Hydrocarbons containing one or more double or triple bonds between carbon atoms (alkenes, alkynes).

• Functional group: Specific groups of atoms within molecules that determine the chemical properties of those molecules (e.g., hydroxyl, carboxyl, amino).

• Macromolecule: Large, complex molecules such as proteins, nucleic acids, carbohydrates, and lipids.

• Polymer: A large molecule made up of repeating subunits (monomers).

• Monomer: The basic building block of a polymer.

• Dehydration: A chemical reaction that removes a water molecule to form a new bond, joining monomers into polymers.

• Hydrolysis: A chemical reaction that adds a water molecule to break a bond, splitting polymers into monomers.

• Carbohydrate: Organic molecules composed of carbon, hydrogen, and oxygen, serving as energy sources and structural components.

• Monosaccharide: The simplest carbohydrate, or simple sugar (e.g., glucose, fructose).

• Disaccharide: A carbohydrate formed by joining two monosaccharides (e.g., sucrose, lactose).

• Polysaccharide: A complex carbohydrate composed of many monosaccharide units (e.g., starch, glycogen, cellulose).

• Isomer: Molecules with the same molecular formula but different structures and properties.

• Starch: A storage polysaccharide in plants, composed of glucose monomers.

• Glycogen: A storage polysaccharide in animals, composed of glucose monomers.

• Cellulose: A structural polysaccharide in plant cell walls, composed of glucose monomers.

• Hydrophilic: "Water-loving"; molecules that dissolve easily in water.

• Hydrophobic: "Water-fearing"; molecules that do not dissolve easily in water.

• Fat: A lipid composed of glycerol and fatty acids; used for energy storage.

• Fatty acid: A long hydrocarbon chain with a carboxyl group; building block of lipids.

• Triglyceride: A lipid formed from one glycerol and three fatty acids.

• Atherosclerosis: A disease in which plaque builds up inside arteries, often linked to certain fats.

• Hydrogenation: The process of adding hydrogen to unsaturated fats to make them more saturated.

• Trans fat: An unsaturated fat with trans double bonds, often produced by hydrogenation; associated with health risks.

• Steroid: A type of lipid with a characteristic four fused ring structure (e.g., cholesterol, hormones).

• Polypeptide: A polymer of amino acids linked by peptide bonds; forms proteins.

• Protein: A functional biological molecule made of one or more polypeptides folded into a specific shape.

• Amino acid: The monomer of proteins, containing an amino group, carboxyl group, hydrogen atom, and R group attached to a central carbon.

• Peptide bond: The covalent bond joining amino acids in a protein.

• Denaturation: The process in which a protein loses its shape and function due to heat, pH, or chemicals.

• Nucleic acid: Macromolecules (DNA and RNA) that store and transmit genetic information.

• Nucleotide: The monomer of nucleic acids, consisting of a sugar, phosphate group, and nitrogenous base.

• DNA (Deoxyribonucleic acid): The molecule that stores genetic information in cells.

• RNA (Ribonucleic acid): A nucleic acid involved in protein synthesis and gene regulation.

• Chromosome: A structure composed of DNA and proteins that contains genetic information.

• Gene: A segment of DNA that codes for a specific protein or functional RNA.

Polymer Formation and Breakdown

Monomers and Polymers

Monomers are small molecules that can join together to form polymers through chemical reactions. The process of building and breaking down polymers is essential for life.

• Dehydration synthesis: Monomers are joined by covalent bonds through the removal of a water molecule. This reaction builds polymers.

• Hydrolysis: Polymers are broken down into monomers by the addition of a water molecule, breaking the covalent bond.

Example: The formation of a disaccharide from two monosaccharides by dehydration synthesis, and its breakdown by hydrolysis.

Nutritional Importance of Macromolecules

• Carbohydrates: Primary source of energy; provide structural support in plants (cellulose).

• Lipids: Long-term energy storage, insulation, cell membrane structure (phospholipids), and hormones (steroids).

• Proteins: Enzymes, structural support, transport, defense, movement, and regulation of cellular processes.

Carbohydrates

Monosaccharides, Disaccharides, and Polysaccharides

• Monosaccharide: Simple sugar, e.g., glucose (C6H12O6), with a ring structure in aqueous solutions.

• Disaccharide: Two monosaccharides joined by a glycosidic bond (e.g., sucrose = glucose + fructose).

• Polysaccharide: Many monosaccharides linked together (e.g., starch, glycogen, cellulose).

Example: The ring structure of glucose is a six-membered ring with hydroxyl groups attached.

Macromolecules and Their Building Blocks

Macromolecule	Monomer (Building Block)
Carbohydrates	Monosaccharides
Lipids	Glycerol and fatty acids
Proteins	Amino acids
Nucleic acids	Nucleotides

Lipids

Triglycerides and Steroids

• Triglyceride: Composed of one glycerol and three fatty acids, joined by ester bonds.

• Steroid: Characterized by a structure of four fused carbon rings (e.g., cholesterol).

Example: Cholesterol is a steroid with a four-ring structure.

Proteins

Amino Acids and Protein Structure

• Amino acid: Central carbon atom bonded to an amino group (–NH2), carboxyl group (–COOH), hydrogen atom, and R group (side chain).

• General structure: $\mathrm{H_2N{-}CHR{-}COOH}$

Levels of Protein Structure

1. Primary structure: Sequence of amino acids in a polypeptide chain (peptide bonds).

2. Secondary structure: Local folding into alpha-helices and beta-pleated sheets (hydrogen bonds).

3. Tertiary structure: Overall 3D shape of a polypeptide (interactions among R groups: hydrophobic interactions, ionic bonds, disulfide bridges, hydrogen bonds).

4. Quaternary structure: Association of multiple polypeptide chains (subunits) into a functional protein.

Nucleic Acids

Nucleotides and Nitrogenous Bases

• Nucleotide: Consists of a five-carbon sugar (deoxyribose or ribose), a phosphate group, and a nitrogenous base.

• Nitrogenous bases in DNA: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).

• Nitrogenous bases in RNA: Adenine (A), Uracil (U), Cytosine (C), Guanine (G).

Comparison of DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Strands	Double-stranded	Single-stranded
Nitrogenous Bases	A, T, C, G	A, U, C, G
Function	Genetic information storage	Protein synthesis, gene regulation

Structure and Function of a Gene

• Gene: A segment of DNA that contains the instructions for making a specific protein or functional RNA molecule.

• Function: Genes determine the traits of an organism by coding for proteins that perform essential cellular functions.