Biological Molecules: Structure, Function, and Classification

Biological Molecules

Introduction to Biological Molecules

Biological molecules, also known as biomolecules, are essential compounds found in living organisms. Most biologically related molecules contain carbon, making them organic molecules. The versatility of carbon allows for the formation of a wide variety of complex molecules necessary for life.

• Organic molecules: Molecules containing carbon, except for a few exceptions such as carbon dioxide, carbon monoxide, graphite, and diamonds.

• Living organisms can synthesize organic molecules.

Carbon: The Versatile Building Block

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

• This property enables the creation of diverse and complex molecular structures.

Functional Groups

Functional groups are small characteristic groups of atoms frequently bonded to the carbon skeleton of organic molecules. They determine the chemical properties and reactivity of organic molecules.

• Have specific chemical and physical properties.

• Are regions of organic molecules that are frequently chemically reactive.

• Behave consistently from one organic molecule to another.

There are seven general functional groups found in organic molecules:

1. Hydroxyl Group (-OH): Polar group involved in condensation (dehydration) and hydrolysis reactions.

2. Carbonyl Group (C=O): Polar group; if terminal, forms an aldehyde; if internal, forms a ketone.

3. Carboxyl Group (-COOH): Polar group; acts as an acid by donating a proton; involved in peptide bonds.

4. Amino Group (-NH2): Polar group; acts as a weak base; involved in peptide bonds.

5. Sulfhydryl Group (-SH): Nonpolar group; critical in stabilizing protein structure via disulfide bridges.

6. Phosphate Group (-PO4): Polar group; acts as an acid; important in energy storage and transfer (e.g., ATP); links nucleotides.

7. Methyl Group (-CH3): Nonpolar group; affects gene expression and molecular recognition.

Synthesizing Organic Molecules: A Modular Approach

Monomers and Polymers

Biological molecules are often assembled from smaller subunits called monomers. When monomers are linked together, they form polymers, which are chains of similar building blocks.

• Dehydration synthesis (condensation reaction): The process by which monomers are joined to form polymers, involving the removal of a water molecule.

• Hydrolysis: The process by which polymers are broken down into monomers by the addition of water.

The Four Classes of Biological Macromolecules

There are four major classes of macromolecules in living organisms: carbohydrates, lipids, proteins, and nucleic acids. Each class has distinct monomers, polymers, and biological functions.

Macromolecule Class	Monomers, Dimers, and Polymers	Examples
Carbohydrates	Monosaccharides, Disaccharides, Polysaccharides	Glucose, Sucrose, Starch, Glycogen, Cellulose
Lipids	Fatty acids	Triglycerides (oils, fats), Wax, Phospholipids, Steroids (cholesterol)
Proteins	Amino acids	Keratin, Silk
Nucleic Acids	Nucleotides	DNA, RNA

Carbohydrates

Structure and Classification

Carbohydrates are organic molecules made of sugars and their polymers. They serve as fuel and building material in cells.

• Classified by the number of simple sugars (monosaccharides) they contain.

• Monosaccharides: Simple sugars with the formula (CH2O)n. Glucose is the most common.

• Disaccharides: Consist of two monosaccharides joined by a glycosidic linkage (covalent bond formed by dehydration synthesis).

• Polysaccharides: Polymers of hundreds or thousands of monosaccharides. Serve as energy storage (starch, glycogen) or structural support (cellulose, chitin).

Examples of Disaccharides

Disaccharide	Monomers	Common Use
Maltose	Glucose + Glucose	Important in beer brewing
Lactose	Glucose + Galactose	Sugar present in milk
Sucrose	Glucose + Fructose	Table sugar, most common disaccharide

Storage Polysaccharides

• Starch: Glucose polymer used for energy storage in plants.

• Glycogen: Glucose polymer used for energy storage in animals (muscle and liver).

Structural Polysaccharides

• Cellulose: Linear polymer of glucose; major component of plant cell walls; forms strong fibers via hydrogen bonding; indigestible by most animals.

• Chitin: Polymer of an amino sugar; forms exoskeleton of arthropods and cell walls of fungi.

Proteins

Structure and Function

Proteins are polymers of amino acids arranged in a specific linear sequence and linked by peptide bonds. They perform a wide variety of cellular functions.

• Range in length from a few to more than a thousand amino acids.

• Each protein has a unique linear sequence of amino acids, called its primary structure.

• Proteins are abundant, making up 50% or more of some cells by weight.

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

Amino Acids

• Monomer building blocks of proteins.

• Each amino acid has a central carbon with four groups: hydrogen atom, carboxyl group, amino group, and a variable "R" group (side chain).

• The properties of the side chain determine the characteristics of each amino acid.

Classes of amino acids:

• Hydrophobic (nonpolar)

• Hydrophilic (polar)

Peptide Bonds and Protein Structure

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

• The resulting molecule is called a peptide.

Levels of Protein Structure

1. Primary structure: Sequence of amino acids in a protein, determined by genes.

2. Secondary structure: Regular folding patterns (e.g., alpha helices and beta pleated sheets) stabilized by hydrogen bonds.

3. Tertiary structure: Irregular contortions due to interactions between side chains (R groups), including covalent linkages (disulfide bridges), hydrogen bonds, ionic bonds, and hydrophobic interactions.

4. Quaternary structure: Association of multiple polypeptide chains (if present).

Nucleic Acids

Structure and Function

Nucleic acids (DNA and RNA) are polymers of nucleotides and are responsible for the storage and transmission of genetic information.

• Nucleotides consist of a sugar, a phosphate group, and a nitrogenous base.

• DNA contains deoxyribose sugar; RNA contains ribose sugar.

• The sugar-phosphate backbone forms the structural framework of nucleic acids.

• ATP (adenosine triphosphate) is a nucleotide important for cellular energy transfer.

Lipids

Structure and Types

Lipids are mostly nonpolar, hydrophobic molecules composed mainly of carbon and hydrogen. They are not true polymers but are grouped together due to their insolubility in water.

• Fats and oils: Composed of fatty acids and glycerol; function as energy storage molecules.

• Fatty acids: Hydrocarbon chains with a carboxyl group at one end; can be saturated (no double bonds) or unsaturated (one or more double bonds).

• Triglycerides: Formed by three fatty acids bonded to glycerol via ester linkages.

• Phospholipids: Contain two fatty acids, a phosphate group, and a glycerol backbone; major component of cell membranes.

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

• Waxes: Long-chain fatty acids linked to alcohols; function in waterproofing.

Comparison of Saturated and Unsaturated Fatty Acids

Saturated Fatty Acids	Unsaturated Fatty Acids
Maximum number of hydrogen atoms; no double bonds	One or more double bonds; causes kinks in the chain
Solid at room temperature	Liquid at room temperature
Most animal fats	Most plant oils

Functions of Lipids

• Energy storage (more compact than carbohydrates)

• Structural components of cell membranes (phospholipids)

• Signaling molecules (steroids, hormones)

• Waterproofing (waxes)

Additional info: The notes above are expanded and clarified for academic completeness, with inferred context for some definitions and examples.