Biological Molecules: Structure and Function in General Biology

Biological Molecules

Introduction to Biological Molecules

Biological molecules are essential compounds found in living organisms, primarily composed of carbon, hydrogen, oxygen, nitrogen, and other elements. Aside from water, most biologically related molecules contain carbon, making them organic molecules.

• Organic molecules: Molecules containing carbon. Examples include carbohydrates, proteins, lipids, and nucleic acids.

• Exceptions: Some carbon-containing molecules are not considered organic (e.g., graphite, diamonds, carbon dioxide, carbon monoxide).

• Carbon atoms are versatile building blocks due to their ability to form four covalent bonds, allowing for complex molecular structures.

Functional Groups in Organic Molecules

Definition and Importance

Functional groups are specific groups of atoms within molecules that confer distinct chemical and physical properties. They are often the reactive regions of organic molecules and determine the molecule's behavior and function.

• Functional groups behave consistently from one organic molecule to another.

• They determine the chemical properties and reactivity of organic molecules.

Major Functional Groups

Functional Group	Structure	Properties
Hydroxyl	-OH	Polar; involved in condensation and hydrolysis reactions
Carbonyl	C=O	Polar; found in aldehydes (terminal) and ketones (within chain)
Carboxyl	-COOH	Polar; acidic; can donate a proton; involved in peptide bonds
Amino	-NH2	Polar; acts as a weak base; involved in peptide bonds
Sulfhydryl	-SH	Non-polar; stabilizes protein structure
Phosphate	-PO4	Polar; acidic; important in energy transfer (e.g., ATP)
Methyl	-CH3	Non-polar; increases hydrophobicity

Synthesizing Organic Molecules

Monomers and Polymers

Biological molecules are often constructed from smaller units called monomers, which join to form polymers. Polymers are chains of similar building blocks or monomers.

• Dehydration synthesis (condensation reaction): Forms polymers by removing a water molecule ($\text{Monomer}_1 - OH + \text{Monomer}_2 - H \rightarrow \text{Polymer} + H_2O$).

• Hydrolysis: Breaks polymers into monomers by adding water ($\text{Polymer} + H_2O \rightarrow \text{Monomer}_1 - OH + \text{Monomer}_2 - H$).

Classes of Biological Macromolecules

Overview

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

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

Carbohydrates

Structure and Function

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in the ratio CH2O. They serve as fuel and building material for cells.

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

• Monosaccharides are simple sugars (e.g., glucose).

• Disaccharides are formed by joining two monosaccharides via glycosidic linkage.

• Polysaccharides are polymers of many monosaccharides.

General Structure of Carbohydrates

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

• Monosaccharides with five or more carbons favor ring structures in aqueous solutions.

Disaccharides

Disaccharides consist of two monosaccharides joined by a glycosidic bond (formed by dehydration synthesis).

Disaccharide	Monomers	Common Use
Maltose	Glucose + Glucose	Beer brewing
Lactose	Glucose + Galactose	Milk sugar
Sucrose	Glucose + Fructose	Table sugar

Polysaccharides

Polysaccharides are large macromolecules formed by hundreds or thousands of monosaccharides. They serve storage and structural functions.

• Starch: Storage polysaccharide in plants (polymer of glucose).

• Glycogen: Storage polysaccharide in animals (muscle and liver).

• Cellulose: Structural polysaccharide in plant cell walls; linear, unbranched polymer of glucose; forms strong fibers via hydrogen bonding.

• Chitin: Structural polysaccharide in arthropod exoskeletons and fungal cell walls; polymer of an amino sugar.

Proteins

Structure and Function

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.

• Each protein has a unique sequence of amino acids.

• Proteins can be structural (keratin), catalytic (enzymes), transport (hemoglobin), contractile (muscle), hormonal, or defensive (antibodies).

Amino Acids

Amino acids are the monomer building blocks of proteins. Each consists of:

• A central carbon atom

• A hydrogen atom

• A carboxyl group (-COOH)

• An amino group (-NH2)

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

Classes of amino acids:

• Hydrophobic (nonpolar)

• Hydrophilic (polar)

Peptide Bonds and Protein Structure

Amino acids are joined by peptide bonds (covalent bonds formed by dehydration synthesis between the carboxyl group of one amino acid and the amino group of another).

Proteins have four levels of structure:

1. Primary structure: Sequence of amino acids; determined by genes.

2. Secondary structure: Regular coiling and folding (helices and pleated sheets); stabilized by hydrogen bonds.

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

4. Quaternary structure: Interaction among several polypeptide chains (subunits) in a single protein.

Lipids

Structure and Function

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

• Fats and oils: Composed of fatty acids and glycerol; used for energy storage.

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

• Steroids: Four fused carbon rings; cholesterol is a precursor to steroid hormones.

Fatty acids can be saturated (no double bonds; solid at room temperature) or unsaturated (one or more double bonds; liquid at room temperature).

Type	Bonding	Physical State
Saturated Fatty Acids	No double bonds	Solid at room temperature
Unsaturated Fatty Acids	One or more double bonds	Liquid at room temperature

Nucleic Acids

Structure and Function

Nucleic acids (DNA and RNA) are polymers of nucleotides, which consist of a sugar, a phosphate group, and a nitrogenous base. They store and transmit genetic information.

• DNA: Contains deoxyribose sugar; stores genetic information.

• RNA: Contains ribose sugar; involved in protein synthesis.

• Nucleotides are linked by phosphodiester bonds.

Summary Table: Major Biological Molecules

Macromolecule	Monomer	Bond Type	Main Function
Carbohydrates	Monosaccharide	Glycosidic	Energy, structure
Proteins	Amino acid	Peptide	Catalysis, structure, transport
Lipids	Fatty acid, glycerol	Ester	Energy storage, membranes
Nucleic Acids	Nucleotide	Phosphodiester	Genetic information

Additional info: This study guide expands on the original notes by providing definitions, examples, and context for each major biological molecule and functional group, as well as summarizing key tables and concepts for exam preparation.