Biological Molecules: Structure, Function, and Types

Biological Molecules

Introduction to Organic Molecules

Biological molecules are primarily composed of carbon and are essential for life. Aside from water, most biologically related molecules contain carbon, making them organic molecules. Organic molecules are defined as molecules containing carbon, with a few exceptions such as carbon dioxide, carbon monoxide, and diamonds.

• Organic molecules: Molecules containing carbon, often forming the backbone of biological macromolecules.

• Carbon atoms are highly versatile, able to form four covalent bonds, and can create single, double, or triple bonds, resulting in a variety of molecular shapes.

Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic chemical and physical properties. They are responsible for the reactivity and function of organic molecules and are frequently attached to the carbon skeleton.

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

• There are seven general functional groups found in organic molecules:

Functional Group	Structure	Associated Macromolecule
Hydroxyl	-OH	Carbohydrates, Lipids
Carbonyl	-C=O	Lipids
Carboxyl	-COOH	Proteins
Amino	-NH2	Proteins
Sulfhydryl	-SH	Proteins
Phosphate	-PO4	DNA, ATP
Methyl	-CH3	Varied

1) Hydroxyl Group (-OH)

• Consists of a hydrogen atom bonded to an oxygen atom, which is bonded to a carbon skeleton.

• Polar group; found in alcohols (e.g., ethanol, glycerol).

• Involved in condensation (dehydration) and hydrolysis reactions.

2) Carbonyl Group (-C=O)

• Consists of a carbon atom double bonded to an oxygen atom.

• Polar group; can be terminal (aldehyde) or within the chain (ketone).

• Examples: Propanal (aldehyde), Acetone (ketone).

3) Carboxyl Group (-COOH)

• Contains a carbon atom double bonded to an oxygen atom and single bonded to a hydroxyl group.

• Polar group; acts as an acid (can donate a proton, H+).

• Involved in peptide bonds in proteins.

4) Amino Group (-NH2)

• Consists of two hydrogen atoms bonded to a nitrogen atom, which is bonded to a carbon skeleton.

• Polar group; acts as a base (can accept a proton).

• Found in amino acids, which have both amino and carboxyl groups.

5) Sulfhydryl Group (-SH)

• Consists of a hydrogen atom bonded to a sulfur atom, which is bonded to a carbon skeleton.

• Nonpolar group; critical in stabilizing protein structure via disulfide bridges.

6) Phosphate Group (-PO4)

• Phosphorus atom bonded to four oxygen atoms; one oxygen is attached to the carbon skeleton.

• Polar group; similar to phosphoric acid.

• Acts as an acid (can donate protons); important in energy storage and transfer (e.g., ATP).

7) Methyl Group (-CH3)

• Consists of three hydrogen atoms bonded to a carbon atom.

• Nonpolar group; makes molecules more hydrophobic.

• Subtle changes in methyl group position can alter molecular function (e.g., estradiol vs. testosterone).

Synthesizing Organic Molecules: A Modular Approach

Monomers and Polymers

Biological molecules are often assembled from smaller subunits called monomers. When monomers are joined together, they form polymers, which are chains of repeating units. The process of joining monomers to form polymers is essential for building complex biological macromolecules.

• Dehydration synthesis (condensation reaction): The reaction that forms a polymer from monomers by removing a water molecule.

• One monomer loses a hydroxyl group (-OH), and the other loses a hydrogen (-H).

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

• Hydrolysis: The breaking of a covalent bond between two monomers by the addition of water. One monomer gains a hydroxyl group (-OH), and the other gains a hydrogen (-H).

The Principle Types of Biological Molecules

Four Classes of Macromolecules

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

Macromolecule Class	Monomers/Dimers/Polymers	Examples
Carbohydrates	Monosaccharides, Disaccharides, Polysaccharides	Glucose, Sucrose, Starch, Glycogen, Cellulose
Lipids	Fatty acids, Triglycerides, Phospholipids, Steroids	Oils, Fats, 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) they contain.

• Monosaccharides: Simple sugars with the formula (CH2O)n, where n is typically 3-7.

• Major source of nutrients for cells; glucose is the most common monosaccharide.

• Monosaccharides can be joined to form disaccharides and polysaccharides.

General Structure of Carbohydrates

1. Each carbon has a hydroxyl group attached to it except one, which is a carbonyl group.

2. The size of the carbon skeleton varies (commonly 5 or 6 carbons).

3. In aqueous solutions, monosaccharides with 5 or more carbons form ring structures; the equilibrium favors the ring structure over the straight chain.

Disaccharides

Disaccharides are molecules consisting of two monosaccharides joined by a glycosidic linkage, which is a covalent bond formed by a dehydration synthesis reaction.

$\text{Monosaccharide}_1 + \text{Monosaccharide}_2 \xrightarrow{\text{dehydration}} \text{Disaccharide} + H_2O$

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

Key Point: The structure and function of biological molecules are determined by the arrangement of their monomers and the presence of specific functional groups. Subtle changes in functional group position can lead to significant differences in molecular properties and biological activity.