Study Notes: Biological Molecules in General Biology

Chapter 3: Biological Molecules

Introduction to Biological Molecules

Biological molecules are essential compounds that form the basis of life. Most biologically relevant molecules contain carbon and are known as organic molecules. These molecules are often large and complex, enabling the diversity of life forms and functions.

• Organic molecules: Molecules containing carbon (except for simple forms like CO2, carbonates, and carbides).

• Living organisms can create organic molecules.

Carbon atoms are the most versatile building blocks of molecules due to their ability to form four covalent bonds, allowing for a variety of structures.

• Carbon has a valence of 4.

• Can bond to 2, 3, 4, or other atoms.

Functional Groups in Organic Molecules

Definition and Importance

Functional groups are specific groups of atoms attached to the carbon skeleton of organic molecules. They determine the chemical properties and reactivity of molecules.

• Have specific chemical and physical properties.

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

• Can determine the chemical properties of the organic molecule in which they are located.

Major Functional Groups

• Hydroxyl Group (-OH): Polar, involved in condensation and hydrolysis reactions.

• Carbonyl Group (C=O): Polar, found in aldehydes and ketones.

• Carboxyl Group (-COOH): Polar, acidic, involved in peptide bonds.

• Amino Group (-NH2): Polar, acts as a base, found in amino acids and peptide bonds.

• Sulfhydryl Group (-SH): Nonpolar, critical in stabilizing protein structure.

• Phosphate Group (-PO4): Polar, acidic, important in energy transfer (e.g., ATP).

• Methyl Group (-CH3): Nonpolar, makes molecules more hydrophobic.

Synthesizing Organic Molecules: A Modular Approach

Monomers and Polymers

Biological molecules are often assembled from smaller subunits called monomers. Monomers join to form polymers, which are chains of similar building blocks.

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

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

Classes of Biological Macromolecules

Overview

There are four major classes of biological macromolecules, each with distinct monomers and functions.

Macromolecule Class	Monomers, Dimers, and Polymers	Examples
Carbohydrates	Monosaccharides, Disaccharides, Polysaccharides	Glucose, Sucrose, Starch, Cellulose
Lipids	Fatty acids	Triglycerides, Oils, Waxes, Phospholipids, Steroids
Proteins	Amino acids	Keratin, Silk
Nucleic Acids	Nucleotides	DNA, RNA

Carbohydrates

Structure and Function

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

• Monosaccharides: Simple sugars (e.g., glucose) with the formula (CH2O)n.

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

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

Polysaccharides

• Starch: Storage polysaccharide in plants.

• Glycogen: Storage polysaccharide in animals.

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

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

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, including catalysis, transport, structure, and defense.

• Each protein has a unique sequence of amino acids.

• Proteins can be structural, enzymatic, transport, hormonal, or defensive.

Amino Acids

• Monomer building blocks of proteins.

• Consist of a central carbon with four groups: hydrogen atom, carboxyl group, amino group, and a variable side chain (R group).

• Classified as hydrophobic (nonpolar) or hydrophilic (polar).

Protein Structure

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

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

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

• Quaternary structure: Structure resulting from the interaction among several polypeptide chains.

Protein Denaturation

Denaturation is the unfolding of a protein due to changes in temperature, pH, or other environmental factors, disrupting its structure and function. Sometimes, proteins can refold to their original conformation, but not always.

Nucleic Acids

Information Storage and Transmission

Nucleic acids (DNA and RNA) are polymers of nucleotides linked by dehydration synthesis. They store and transmit genetic information.

• Nucleotide: Composed of a sugar (ribose or deoxyribose), a phosphate group, and a nitrogenous base.

• DNA and RNA differ in their sugar and base composition.

• Nucleotides also form energy carriers (e.g., ATP) and coenzymes.

Lipids

Structure and Types

Lipids are mostly nonpolar, hydrophobic molecules composed mainly of carbon and hydrogen. They are insoluble in water but soluble in nonpolar solvents.

• Fats, oils, and waxes: Energy storage and insulation.

• Phospholipids: Major component of cell membranes.

• Steroids: Include hormones and cholesterol.

Summary Table: Major Classes of Biological Molecules

Class	Monomer	Polymer	Main Function
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structure
Proteins	Amino acid	Polypeptide	Catalysis, structure, transport, defense
Nucleic Acids	Nucleotide	DNA/RNA	Information storage and transfer
Lipids	Fatty acid, glycerol	Triglyceride, phospholipid, steroid	Energy storage, membranes, hormones

Key Equations and Concepts

• Dehydration Synthesis: $\text{Monomer}_1 - OH + H - \text{Monomer}_2 \rightarrow \text{Polymer} + H_2O$

• Hydrolysis: $\text{Polymer} + H_2O \rightarrow \text{Monomer}_1 - OH + H - \text{Monomer}_2$

Additional info: This guide expands on the provided notes with definitions, examples, and context for each class of biological molecule, as well as the structure and function of functional groups, to ensure a comprehensive understanding suitable for college-level General Biology.