General Biology: Structure and Function of Biological Molecules

Structure and Function of Biological Molecules

Introduction to Organic Molecules

Organic molecules are the foundation of life, built primarily from carbon atoms. The unique properties of carbon allow for the formation of a wide variety of complex molecules essential for biological processes.

• Carbon's Bonding Ability: Carbon can form four covalent bonds, enabling the construction of large and diverse organic compounds.

• Carbon Skeletons: The backbone of most organic molecules, which can vary in length, branching, and ring formation.

• Isomers: Molecules with the same molecular formula but different structures.

• Hydrocarbons: Compounds composed only of carbon and hydrogen, serving as major energy sources.

Example: Ethane (C2H6) and propane (C3H8) are simple hydrocarbons.

Functional Groups and Chemical Properties

Certain groups of atoms, called functional groups, confer specific chemical properties to organic molecules. These groups influence the molecule's reactivity and interactions with other molecules.

• Hydrophilic Groups: Attract water and increase solubility (e.g., hydroxyl, carboxyl, amino, phosphate).

• Hydrophobic Groups: Repel water (e.g., methyl group).

Table: Six Important Functional Groups

Functional Group	Structure	Properties
Hydroxyl	-OH	Polar, forms hydrogen bonds
Carbonyl	>C=O	Found in sugars (aldehydes, ketones)
Carboxyl	-COOH	Acidic, donates H+
Amino	-NH2	Basic, accepts H+
Sulfhydryl	-SH	Forms disulfide bonds in proteins
Phosphate	-OPO32-	Contributes negative charge, energy transfer

Macromolecules: Structure and Function

Overview of Macromolecules

Cells contain four major classes of large biological molecules, known as macromolecules: carbohydrates, lipids, proteins, and nucleic acids. These are often polymers, made by linking smaller units called monomers.

• Macromolecules: Large molecules essential for life.

• Polymers: Chains of monomers joined by covalent bonds.

• Monomers: The repeating subunits that make up polymers.

• Dehydration Reaction: Joins monomers by removing water.

• Hydrolysis: Breaks polymers into monomers by adding water.

• Enzymes: Biological catalysts that mediate these reactions.

Carbohydrates

Carbohydrates are sugars and their polymers, serving as energy sources and structural materials.

• Monosaccharides: Simple sugars (e.g., glucose, C6H12O6), basic building blocks of carbohydrates.

• Disaccharides: Formed by joining two monosaccharides via a dehydration reaction (e.g., sucrose).

• Polysaccharides: Long chains of monosaccharide units; serve as energy storage (starch in plants, glycogen in animals) or structural support (cellulose in plants, chitin in fungi and arthropods).

Example: The formula for a monosaccharide is typically (CH2O)n.

Lipids

Lipids are hydrophobic molecules, including fats, phospholipids, and steroids. They are important for energy storage, membrane structure, and signaling.

• Fats (Triglycerides): Composed of glycerol and three fatty acids. Serve as energy storage molecules.

• Saturated Fatty Acids: No double bonds; solid at room temperature; found in animal fats.

• Unsaturated Fatty Acids: One or more double bonds; liquid at room temperature; found in plant oils.

• Trans Fats: Unsaturated fats that have been hydrogenated; associated with health risks.

• Phospholipids: Major components of cell membranes; have hydrophilic heads and hydrophobic tails.

• Steroids: Lipids with a carbon skeleton of four fused rings (e.g., cholesterol, sex hormones).

Example: Cholesterol is a precursor for steroid hormones.

Proteins

Proteins are polymers of amino acids and perform a vast array of functions in the cell, including catalysis, transport, structure, and signaling.

• Amino Acids: 20 different types, each with a central carbon, amino group, carboxyl group, hydrogen atom, and R group (side chain).

• Peptide Bond: Covalent bond linking amino acids in a protein.

• Polypeptide: A chain of amino acids.

• 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 R groups.

  4. Quaternary Structure: Association of multiple polypeptide chains.

• Denaturation: Loss of protein structure and function due to environmental changes.

Example: Enzymes are proteins that catalyze biochemical reactions.

Nucleic Acids

Nucleic acids store and transmit genetic information. The two main types are DNA and RNA.

• Nucleotides: Monomers of nucleic acids, each consisting of a sugar, phosphate group, and nitrogenous base.

• DNA (Deoxyribonucleic Acid): Double helix; stores genetic information; bases are adenine (A), guanine (G), cytosine (C), and thymine (T).

• RNA (Ribonucleic Acid): Single-stranded; involved in protein synthesis; bases are adenine (A), guanine (G), cytosine (C), and uracil (U).

• Base Pairing: In DNA, A pairs with T, and G pairs with C.

Example: Mutations in DNA can affect traits such as lactose tolerance.

Summary Table: Major Biological Molecules

Class	Monomer	Polymer	Function
Carbohydrates	Monosaccharide	Polysaccharide	Energy, structure
Lipids	Glycerol, fatty acids	Triglyceride, phospholipid	Energy storage, membranes
Proteins	Amino acid	Polypeptide	Catalysis, structure, transport
Nucleic Acids	Nucleotide	DNA, RNA	Genetic information

Key Equations and Concepts

• Dehydration Reaction: $\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + \text{H}_2\text{O}$

• Hydrolysis: $\text{Polymer} + \text{H}_2\text{O} \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

• General Formula for Monosaccharides: $\text{C}_n(\text{H}_2\text{O})_n$

Additional info:

• Recent research links high sugar intake to adverse health effects, supporting dietary guidelines to limit added sugars.

• Trans fats are associated with increased health risks compared to saturated fats.

• Lactose tolerance in humans is due to mutations that keep the lactase gene active into adulthood.