Biological Molecules: Structure, Function, and Importance in Life

Chapter 3: Biological Molecules

Introduction

Biological molecules are essential for life, forming the basis of cellular structure and function. This chapter explores the chemistry of life, focusing on the major classes of biological molecules, their synthesis, and their roles in living organisms.

Why Is Carbon So Important in Biological Molecules?

Organic vs. Inorganic Molecules

• Biological molecules are defined as all molecules produced by living things.

• Nearly all biological molecules are organic, meaning they contain carbon and usually hydrogen and oxygen.

• Organic molecules are typically more complex than inorganic molecules, which generally lack carbon atoms.

• Organic molecules allow cells to acquire nutrients, eliminate wastes, grow, and reproduce.

Bonding Properties of Carbon

• The bonding properties of carbon are key to the complexity of organic molecules.

• Carbon has four outer shell electrons and tends to form four covalent bonds, allowing for a variety of molecular structures.

• Atoms with partially filled outer electron shells tend to react with one another, sharing electrons to fill vacancies.

• Common elements in biological molecules include carbon, hydrogen, oxygen, and nitrogen.

Functional Groups

• Functional groups are specific groups of atoms attached to the carbon backbone of organic molecules.

• They are less stable than the carbon backbone and more likely to participate in chemical reactions.

• Examples include hydroxyl (-OH), carboxyl (-COOH), amino (-NH2), and phosphate (-PO4).

How Are Large Biological Molecules Synthesized?

Monomers and Polymers

• Large biological molecules are formed by joining smaller subunits called monomers.

• Chains of monomers are called polymers.

Dehydration Synthesis and Hydrolysis

• Dehydration synthesis is the process of joining monomers by removing water.

• One monomer loses a hydrogen ion (H+), and another loses a hydroxyl ion (OH-), forming a covalent bond and releasing water.

• Hydrolysis is the reverse reaction, breaking polymers into monomers by adding water.

• These reactions are essential for building and breaking down biological molecules in living organisms.

Principal Classes of Biological Molecules

• Carbohydrates

• Proteins

• Nucleotides/Nucleic Acids

• Lipids

What Are Carbohydrates?

Structure and Types

• Composed of carbon, hydrogen, and oxygen in the approximate ratio of 1C:2H:1O.

• Types include monosaccharides (single sugars), disaccharides (two sugars), and polysaccharides (many sugars).

• Monosaccharides have a backbone of three to seven carbon atoms, most with both H+ and OH- attached.

• Examples: Glucose (hexose, 6 carbons), Ribose (pentose, 5 carbons).

• General formula: $C_nH_{2n}O_n$

Disaccharides and Polysaccharides

• Disaccharides are formed by dehydration synthesis of two monosaccharides.

• Examples: Sucrose (glucose + fructose), Lactose (glucose + galactose), Maltose (glucose + glucose).

• Polysaccharides are chains of monosaccharides, often used for energy storage or structural support.

• Examples: Starch (energy storage in plants), Glycogen (energy storage in animals), Cellulose (structural support in plants), Chitin (exoskeletons of arthropods and cell walls of fungi).

What Are Proteins?

Structure and Function

• Proteins are polymers made of amino acid subunits.

• Functions include structural support, movement, defense, storage, signaling, and catalysis (enzymes).

Function	Example(s)
Structural	Keratin (hair, nails), silk (webs, cocoons)
Movement	Actin and myosin (muscle contraction)
Defense	Antibodies, venoms
Storage	Albumin (egg white)
Signaling	Insulin (regulates glucose)
Catalysis	Amylase (digests carbohydrates)

Amino Acids and Protein Structure

• Each amino acid has a central carbon bonded to a hydrogen atom, an amino group (-NH2), a carboxylic acid group (-COOH), and a variable R group.

• There are 20 different amino acids, distinguished by their R groups, which can be hydrophilic or hydrophobic.

• Amino acids are joined by peptide bonds (dehydration synthesis).

Levels of Protein Structure

• Primary structure: Sequence of amino acids.

• Secondary structure: Hydrogen bonding forms alpha-helices or beta-pleated sheets.

• Tertiary structure: 3D globular shape, maintained by hydrogen, disulfide, and hydrophobic interactions.

• Quaternary structure: Multiple polypeptide chains joined together (e.g., hemoglobin).

Protein Denaturation

• Denaturation occurs when a protein loses its three-dimensional structure, often due to heat or chemicals, resulting in loss of function.

• Primary structure usually remains intact, but secondary, tertiary, and quaternary structures are disrupted.

What Are Nucleotides and Nucleic Acids?

Structure and Types

• Nucleic acids include DNA and RNA, polymers of nucleotide monomers.

• Each nucleotide consists of a five-carbon sugar (ribose or deoxyribose), one or more phosphate groups, and a nitrogen-containing base.

• Bases are classified as single-ring (thymine, uracil, cytosine) or double-ring (adenine, guanine).

Functions of Nucleotides

• Energy carriers (e.g., ATP)

• Intracellular messengers

• Subunits of nucleic acids

ATP: The Energy Carrier

• Adenosine triphosphate (ATP) is a ribose nucleotide with adenine base and three phosphate groups.

• ATP stores energy in the bonds between phosphate groups; energy is released when these bonds are broken.

• Equation for ATP hydrolysis: $ATP + H_2O \rightarrow ADP + P_i + \text{energy}$

DNA and RNA

• DNA (deoxyribonucleic acid) is a double-stranded helix containing millions of nucleotides; it stores genetic information.

• RNA (ribonucleic acid) is usually single-stranded and involved in protein synthesis.

• Nucleotides are joined by covalent bonds between the phosphate group of one and the sugar of another.

What Are Lipids?

Structure and Types

• Lipids are diverse molecules with hydrophobic regions composed mostly of hydrogen and carbon.

• They are insoluble in water and not formed by linking monomers into polymers.

• Major groups: fats, oils, waxes; phospholipids; steroids.

Fats, Oils, and Waxes

• Composed of fatty acid chains attached to glycerol (triglycerides).

• Fats are solid at room temperature (saturated fatty acids); oils are liquid (unsaturated fatty acids).

• Waxes are highly saturated, solid, and water-repellent; found in plant leaves, stems, bird feathers, and honeycomb.

Phospholipids

• Phospholipids have a polar, water-soluble head (phosphate group) and two nonpolar, water-insoluble tails (fatty acids).

• They form bilayers in cell membranes, with hydrophobic tails facing inward and hydrophilic heads facing outward.

• Phospholipids are amphiphilic (both hydrophobic and hydrophilic).

Steroids

• Steroids are composed of four fused carbon rings with various functional groups attached.

• Examples: Cholesterol (membrane component), Estrogen and Testosterone (sex hormones).

• Synthetic steroids (anabolic steroids) can have significant physiological effects.

Summary Table: Four Principal Classes of Biological Molecules

Class	Subunits	Examples	Main Functions
Carbohydrates	Monosaccharides	Glucose, starch, cellulose	Energy storage, structural support
Proteins	Amino acids	Enzymes, keratin, hemoglobin	Catalysis, structure, movement, defense
Nucleic Acids	Nucleotides	DNA, RNA, ATP	Genetic information, energy transfer
Lipids	Fatty acids, glycerol	Fats, oils, phospholipids, steroids	Energy storage, membranes, hormones

Additional info: Prions are infectious proteins that cause diseases such as mad cow disease by inducing abnormal folding in normal brain proteins. This highlights the importance of protein structure in biological function and disease.