General Biology: Chapters 4 & 5 – Organic Molecules and Biological Macromolecules

Organic Chemistry in Biology

CHNOPS Elements

Living organisms are primarily composed of six elements, often remembered by the acronym CHNOPS: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. These elements form the basis of organic molecules essential for life.

• Organic compounds contain at least carbon and hydrogen.

• Inorganic compounds may lack carbon and/or hydrogen (e.g., CO2, H2O).

Carbon: The Foundation of Life

Carbon is central to the chemistry of life due to its ability to form four covalent bonds, allowing for diverse and complex molecules.

• Carbon forms long chains, branched chains, and ring structures.

• The number of covalent bonds an atom can form is determined by its valence shell electrons.

Element	Number of Covalent Bonds
C (Carbon)	4
H (Hydrogen)	1
N (Nitrogen)	3
O (Oxygen)	2

Functional Groups

Functional groups are specific groups of atoms within molecules that confer characteristic chemical properties and reactivity.

Functional Group	Structure
Hydroxyl	R–OH
Carboxyl	R–COOH
Amino	R–NH2
Phosphate	R–PO4

Example: The structure of glucose contains multiple hydroxyl groups.

Macromolecules: Monomers and Polymers

Monomers and Polymers

Organic compounds often exist as monomers (small building blocks) that join to form polymers (long chains).

• Monomer: Small molecule that can join with others to form a polymer.

• Polymer: Large molecule made from repeating monomer units.

Chemical Reactions in Biology

• Anabolic reactions: Build larger molecules from smaller ones (e.g., dehydration synthesis).

• Catabolic reactions: Break down larger molecules into smaller ones (e.g., hydrolysis).

Dehydration synthesis removes water to form a new bond; hydrolysis adds water to break a bond.

Classes of Organic Molecules

There are four main classes: Carbohydrates, Lipids, Proteins, and Nucleic Acids.

1. Carbohydrates

Carbohydrates are energy sources and structural components. They are composed of carbon, hydrogen, and oxygen, typically in a 1:2:1 ratio (CH2O).

• Monosaccharides: Simple sugars (e.g., glucose, fructose). Can exist as straight chains or rings.

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

• Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose).

Type	Example	Function
Monosaccharide	Glucose	Primary energy source
Disaccharide	Sucrose	Transported sugar in plants
Polysaccharide	Starch	Energy storage in plants
Polysaccharide	Glycogen	Energy storage in animals
Polysaccharide	Cellulose	Structural support in plants

Example: Glycogen is stored in animal liver and muscle cells for energy.

2. Lipids

Lipids are hydrophobic molecules important for energy storage, insulation, and cell membranes. They include fats, oils, phospholipids, and steroids.

• Triglycerides: Composed of glycerol and three fatty acids. Can be saturated (no double bonds) or unsaturated (one or more double bonds).

• Phospholipids: Contain a glycerol, two fatty acids, and a phosphate group. Major component of cell membranes.

• Steroids: Four fused carbon rings (e.g., cholesterol).

Lipid Type	Structure	Function
Triglyceride	Glycerol + 3 fatty acids	Energy storage
Phospholipid	Glycerol + 2 fatty acids + phosphate	Cell membrane structure
Steroid	4 fused carbon rings	Hormones, membrane stability

Example: Phospholipids form the bilayer of cell membranes.

3. Proteins

Proteins are polymers of amino acids and perform a wide variety of functions, including catalysis (enzymes), structure, transport, and signaling.

• Amino acids: Building blocks of proteins. Each has an amino group (–NH2), carboxyl group (–COOH), hydrogen atom, and a unique R group.

• Peptide bond: Covalent bond linking amino acids.

• Polypeptide: Chain of amino acids; folds into a functional protein.

Protein structure has four levels:

1. Primary: Sequence of amino acids.

2. Secondary: Local folding (α-helix, β-pleated sheet) via hydrogen bonds.

3. Tertiary: 3D folding due to side chain interactions.

4. Quaternary: Multiple polypeptides forming a functional protein.

Denaturation is the loss of protein structure and function due to heat, pH, or chemicals.

Example: Enzymes are proteins that catalyze biochemical reactions.

4. Nucleic Acids

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

• DNA: Deoxyribonucleic acid; double helix; bases are adenine (A), thymine (T), cytosine (C), guanine (G).

• RNA: Ribonucleic acid; single-stranded; bases are adenine (A), uracil (U), cytosine (C), guanine (G).

Nucleic Acid	Sugar	Bases
DNA	Deoxyribose	A, T, C, G
RNA	Ribose	A, U, C, G

Base pairing: In DNA, A pairs with T, and C pairs with G. In RNA, A pairs with U.

Example: mRNA carries genetic instructions from DNA to ribosomes for protein synthesis.

Key Terms and Concepts

• Reactant: Molecule that starts a chemical reaction.

• Product: Molecule produced by a chemical reaction.

• Metabolism: All chemical reactions in a living organism.

• Hydrophobic: Repels water (e.g., lipids).

• Hydrophilic: Attracts water (e.g., carbohydrates).

Sample Equations

• Dehydration synthesis: $\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + H_2O$

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

Summary Table: Major Macromolecules

Macromolecule	Monomer	Bond Type	Main Function
Carbohydrate	Monosaccharide	Glycosidic	Energy, structure
Lipid	Fatty acid, glycerol	Ester	Energy storage, membranes
Protein	Amino acid	Peptide	Catalysis, structure, transport
Nucleic Acid	Nucleotide	Phosphodiester	Genetic information

Additional info:

• Some context and terminology were expanded for clarity and completeness.

• Figures referenced (e.g., Figure 5.2a) are not included but their content is described in the notes.