Biological Molecules: Structure, Function, and Types

CHAPTER 3: BIOLOGICAL MOLECULES

Introduction to Organic Molecules

Most biologically important molecules are organic, meaning they contain carbon. Living organisms synthesize organic molecules, which are essential for life. Carbon's unique properties make it the most versatile element for building biological molecules.

• Organic molecules: Molecules containing carbon (except for a few, such as CO2, carbonates, graphite, and diamonds).

• Carbon atoms have a valence of 4, allowing them to form up to four covalent bonds with other atoms, leading to a diversity of molecular structures.

Functional Groups in Organic Molecules

Functional groups are specific groups of atoms within molecules that have characteristic properties and reactivities. They are responsible for the chemical behavior of organic molecules.

• Hydroxyl Group (-OH): Polar, forms hydrogen bonds, found in alcohols.

• Carboxyl Group (-COOH): Acts as an acid, found in amino acids and fatty acids.

• Amino Group (-NH2): Acts as a base, found in amino acids.

• Sulfhydryl Group (-SH): Forms disulfide bonds, found in some amino acids.

• Phosphate Group (-PO4): Involved in energy transfer (e.g., ATP), found in nucleic acids.

• Methyl Group (-CH3): Nonpolar, affects gene expression.

Table: Major Functional Groups in Organic Molecules

Functional Group	Structure	Properties	Example
Hydroxyl	-OH	Polar, forms hydrogen bonds	Alcohols
Carboxyl	-COOH	Acidic, donates H+	Amino acids, fatty acids
Amino	-NH2	Basic, accepts H+	Amino acids
Sulfhydryl	-SH	Forms disulfide bonds	Cysteine
Phosphate	-PO4	Energy transfer, acidic	ATP, nucleic acids
Methyl	-CH3	Nonpolar, gene regulation	Methylated DNA

Synthesizing Organic Molecules: Monomers and Polymers

Biological macromolecules are often polymers, made by joining smaller subunits called monomers. The process of joining monomers involves dehydration synthesis, while breaking them apart involves hydrolysis.

• Dehydration synthesis (condensation reaction): Joins monomers by removing a water molecule.

• Hydrolysis: Breaks polymers into monomers by adding water.

FOUR CLASSES OF MACROMOLECULES

Macromolecule Class	Monomers/Polymers	Examples
Carbohydrates	Monosaccharides, Disaccharides, Polysaccharides	Glucose, Sucrose, Starch, Glycogen, Cellulose
Lipids	Fatty acids, Triglycerides, Phospholipids, Steroids	Oils, Fats, Cholesterol, Plant cuticle
Proteins	Amino acids (polypeptides)	Keratin, Silk, Hemoglobin
Nucleic Acids	Nucleotides	DNA, RNA

Carbohydrates

Carbohydrates are organic molecules made of sugars and their polymers. They serve as energy sources and structural materials.

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

• Disaccharides: Two monosaccharides joined by a glycosidic linkage (e.g., sucrose, maltose).

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

General formula for monosaccharides:

$C_nH_{2n}O_n$

Table: Examples of Disaccharides

Disaccharide	Monomers	Common Use
Maltose	Glucose + Glucose	Imparted in beer brewing
Lactose	Glucose + Galactose	Sugar present in milk
Sucrose	Glucose + Fructose	Table sugar

Polysaccharides

• Starch: Storage polysaccharide in plants.

• Glycogen: Storage polysaccharide in animals.

• Cellulose: Structural polysaccharide in plant cell walls; not digestible by humans.

• Chitin: Structural polysaccharide in arthropod exoskeletons and fungal cell walls.

Proteins

Proteins are polymers of amino acids and perform most cellular functions. Each protein has a unique sequence of amino acids, which determines its structure and function.

• Functions: Catalysis (enzymes), structural support, movement, transport, signaling, defense.

• Amino acids: Building blocks of proteins, each with a central carbon, amino group, carboxyl group, hydrogen atom, and variable "R" group (side chain).

Levels of Protein Structure

1. Primary structure: Sequence of amino acids.

2. Secondary structure: Local folding (α-helix, β-sheet) stabilized by hydrogen bonds.

3. Tertiary structure: Overall 3D shape due to interactions among side chains.

4. Quaternary structure: Association of multiple polypeptide chains.

Table: Classes of Amino Acid Side Chains

Class	Properties
Hydrophobic (nonpolar)	Do not interact with water
Hydrophilic (polar)	Interact with water

Peptide Bonds and Protein Conformation

• Peptide bond: Covalent bond formed by dehydration synthesis between the carboxyl group of one amino acid and the amino group of another.

• Protein conformation: Determined by primary structure and stabilized by various interactions (hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions).

• Denaturation: Loss of protein structure due to environmental changes (e.g., pH, temperature).

Nucleic Acids

Nucleic acids store and transmit genetic information. They are polymers of nucleotides, each consisting of a sugar, phosphate group, and nitrogenous base.

• DNA (deoxyribonucleic acid): Stores genetic information; sugar is deoxyribose.

• RNA (ribonucleic acid): Involved in protein synthesis; sugar is ribose.

• Nucleotide structure: Each nucleotide contains a pentose sugar, a phosphate group, and a nitrogenous base.

• ATP (adenosine triphosphate): Nucleotide used for energy transfer in cells.

Lipids

Lipids are hydrophobic molecules composed mainly of carbon and hydrogen. They include fats, oils, phospholipids, and steroids.

• Fats (triglycerides): Composed of glycerol and three fatty acids; used for energy storage.

• Phospholipids: Major component of cell membranes; contain a hydrophilic head and hydrophobic tails.

• Steroids: Lipids with a four-ring structure (e.g., cholesterol, hormones).

Table: Types of Lipids and Their Functions

Lipid Type	Structure	Function
Triglycerides	Glycerol + 3 fatty acids	Energy storage
Phospholipids	Glycerol + 2 fatty acids + phosphate group	Cell membrane structure
Steroids	Four fused carbon rings	Hormones, membrane stability

Summary

• Biological molecules are primarily composed of carbon and are essential for life.

• Four major classes: carbohydrates, lipids, proteins, and nucleic acids.

• Each class has unique monomers, structures, and functions in living organisms.