Biological Molecules: Structure, Function, and Classification

Biological Molecules

Introduction to Organic Molecules

Most biologically important molecules are organic, meaning they contain carbon. The versatility of carbon allows for the formation of a wide variety of complex molecules essential for life.

• Organic molecules: Molecules containing carbon, with exceptions such as carbon dioxide, carbon monoxide, graphite, and diamonds.

• Living organisms can synthesize organic molecules.

Carbon: The Versatile Building Block

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

• This property enables the formation 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 confer specific chemical and physical properties and are often the reactive regions of molecules.

• Behave consistently from one organic molecule to another.

• Determine the chemical properties and reactivity of organic molecules.

Seven general functional groups found in organic molecules:

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

2. Carbonyl Group (-C=O): Polar; found in aldehydes (terminal) and ketones (internal).

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

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

5. Sulfhydryl Group (-SH): Nonpolar; important for stabilizing protein structure via disulfide bridges.

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

7. Methyl Group (-CH3): Nonpolar; makes molecules more hydrophobic.

Synthesizing Organic Molecules: A Modular Approach

Monomers and Polymers

Biological molecules are often constructed from smaller subunits called monomers. Chains of monomers form polymers.

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

• Hydrolysis: Breaks covalent bonds between monomers by adding water.

Dehydration synthesis equation:

$\text{Monomer}_1\text{-OH} + \text{Monomer}_2\text{-H} \rightarrow \text{Polymer} + \text{H}_2\text{O}$

Hydrolysis equation:

$\text{Polymer} + \text{H}_2\text{O} \rightarrow \text{Monomer}_1\text{-OH} + \text{Monomer}_2\text{-H}$

Major Classes of Biological Macromolecules

There are four principal classes of macromolecules in living organisms:

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

Carbohydrates

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

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

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

• Produced by photosynthetic organisms from CO2, H2O, and light.

• Can be joined to form disaccharides and polysaccharides.

General Structure of Carbohydrates

• Each carbon has a hydroxyl group except one, which has a carbonyl group.

• The size of the carbon skeleton varies.

• Monosaccharides with 5 or more carbons often form rings in aqueous solutions.

Disaccharides

Disaccharides are molecules consisting of two monosaccharides joined by a glycosidic linkage (covalent bond formed by dehydration synthesis).

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

Polysaccharides

Polysaccharides are macromolecules that are polymers of hundreds or thousands of monosaccharides. They serve various biological functions:

• Energy storage: Starch (plants), Glycogen (animals)

• Structural support: Cellulose (plant cell walls), Chitin (exoskeletons of arthropods)

Storage Polysaccharides

• Starch: Glucose polymer used for storage in plants.

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

Structural Polysaccharides

• Cellulose: Linear, unbranched polymer of glucose; major component of plant cell walls; cannot be digested by most animals.

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

Proteins

Proteins are polymers of amino acids arranged in a specific linear sequence and linked by peptide bonds. They are the molecular tools for most cellular functions.

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

• Each protein has a unique sequence of amino acids.

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

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

Amino Acids

Amino acids are the monomer building blocks of proteins. Each consists of a central carbon with four groups:

1. A hydrogen atom

2. A carboxyl group

3. An amino group

4. A variable "R" group (side chain) that determines the properties of the amino acid

Classes of amino acids:

• Hydrophobic (nonpolar)

• Hydrophilic (polar)

Peptide Bonds

Peptide bonds are covalent bonds 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; dictates all higher levels of structure.

2. Secondary structure: Regular coiling and folding of the polypeptide backbone, stabilized by hydrogen bonds; main types are alpha helices and beta pleated sheets.

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

4. Quaternary structure: Structure resulting from the interaction among several polypeptides (subunits) in a single protein.

Protein function depends on the correct structure at each level.

Denaturation

Denaturation is the process in which a protein loses its native structure due to environmental changes (e.g., pH, temperature), disrupting weak interactions and sometimes covalent bonds. This can render the protein nonfunctional.

Nucleic Acids

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

• Nucleotide: Composed of a sugar, phosphate group, and nitrogenous base.

• Sugars differ between DNA (deoxyribose) and RNA (ribose).

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

• Sequence of nucleotides encodes genetic information.

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

Lipids

Lipids are a diverse group of hydrophobic organic molecules, including fats, oils, phospholipids, and steroids. They are insoluble in water but soluble in nonpolar solvents.

• 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 or unsaturated.

• Triglycerides: Three fatty acids bonded to glycerol via ester linkages.

• Phospholipids: Major component of cell membranes; contain two fatty acids, a phosphate group, and glycerol.

• Steroids: Lipids with a characteristic four-ring structure; cholesterol is a key example.

Functions of lipids: Energy storage, insulation, membrane structure, and signaling.

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard biology textbooks.