Major Organic Molecules and Functional Groups in Biology

Major Organic Molecules in Biology

Overview

Organic molecules are the chemical building blocks of life, composed primarily of carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Understanding their structure and function is essential for studying biological systems.

Definitions and Examples

Functional Group

Functional groups are specific groups of atoms attached to a carbon skeleton that give molecules distinct chemical properties.

• Example: The hydroxyl group (–OH) makes alcohols polar and soluble in water.

Amino Group (–NH2)

An amino group consists of a nitrogen atom bonded to two hydrogens. It acts as a base and is found in amino acids.

• Example: Found in glycine, an amino acid.

Monosaccharide

A monosaccharide is a single sugar unit, serving as a quick energy source.

• Example: Glucose, fructose.

• Function: Quick energy source.

Disaccharide

Disaccharides are two monosaccharides joined by a glycosidic bond.

• Example: Sucrose (glucose + fructose), lactose.

• Function: Short-term energy.

Polysaccharide

Polysaccharides are many monosaccharides linked together, serving as energy storage or structural molecules.

• Example: Starch, glycogen, cellulose.

• Function: Energy storage or structure.

Monomer

A monomer is a small, repeating unit that makes up polymers.

• Example: Amino acids (monomers) make up proteins (polymers); glucose (monomer) makes up starch (polymer).

Polymer

A polymer is a large molecule made of many monomers joined together.

• Example: Proteins, starch, DNA.

Carbohydrate

Carbohydrates are made of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio ($C_xH_{2x}O_x$).

• Function: Energy storage and structure.

• Example: Glucose, starch, cellulose.

Protein

Proteins are made of amino acids linked by peptide bonds. They contain carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur.

• Function: Structure, enzymes, transport.

• Example: Hemoglobin, enzymes.

Lipid

Lipids are mostly composed of carbon and hydrogen, with few oxygen atoms. They are nonpolar and hydrophobic.

• Function: Energy storage, insulation, membranes.

• Example: Fats, oils, phospholipids.

• Structure tip: Long hydrocarbon chains or rings.

Nucleic Acid

Nucleic acids are made of nucleotides (phosphate + sugar + base) and store and transmit genetic information.

• Example: DNA, RNA.

• Elements: C, H, O, N, P.

Common Functional Groups to Recognize

Functional groups determine the chemical reactivity and properties of organic molecules. The table below summarizes key groups:

Functional Group	Structure	Found In	Properties / Notes
Hydroxyl (–OH)	Alcohols	Carbohydrates	Polar, hydrophilic
Carboxyl (–COOH)	Organic acids	Amino acids, fatty acids	Acidic
Amino (–NH2)	Amines	Proteins	Basic
Carbonyl (–C=O)	Aldehydes/ketones	Sugars	Polar
Phosphate (–PO4)	Phosphates	Nucleic acids	Polar, energy transfer
Methyl (–CH3)	Alkanes	Lipids, gene regulation	Nonpolar, hydrophobic
Sulfhydryl (–SH)	Thiols	Proteins	Forms disulfide bridges (structure)

Determining Major Organic Molecule Type

To identify the type of organic molecule, look for key structural features and functional groups:

If you see...	It's likely a...	Reason
C, H, O in 1:2:1 ratio	Carbohydrate	Classic sugar ratio
NH2 and COOH groups	Protein / Amino Acid	Key protein functional groups
Long C–H chains or glycerol + fatty acids	Lipid	Energy-rich hydrocarbon chains
Phosphate + nitrogen base + sugar	Nucleic Acid	Nucleotide structure

Example Practice

Molecule Shown	Clues	Answer
Ring with OH on each carbon	Polar, 1:2:1 ratio	Carbohydrate (sucrose)
Long chain of C–H, ends with COOH	Hydrophobic tail	Lipid (fatty acid)
N, C, H, O with NH2 and COOH	Both acidic & basic groups	Protein (amino acid)
Phosphate–Sugar–Base unit	Contains P and N	Nucleic acid (nucleotide)