The Molecules of Cells: Structure and Function of Biomolecules

Carbon: The Foundation of Organic Molecules

Properties of Carbon

Carbon is the most abundant element in living systems (excluding water) and forms the backbone of organic molecules due to its unique bonding properties.

• Organic molecules: Compounds containing carbon and usually hydrogen.

• Hydrocarbons: Molecules made of only carbon and hydrogen.

• Carbon forms four covalent bonds, allowing for a variety of molecular shapes (linear, branched, ring).

Example: Variations in carbon skeletons include length, branching, double bonds, and ring structures.

Functional Groups

Definition and Importance

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

• Common functional groups in biology include:

Group	Structure	Example
Methyl	-CH3	Methane
Hydroxyl	-OH	Alcohols
Carboxyl	-COOH	Amino acids
Amino	-NH2	Amino acids
Phosphate	-PO4	Nucleotides
Sulfhydryl	-SH	Cysteine

Example: The presence or absence of functional groups determines the reactivity and properties of organic molecules.

Introduction to Biomolecules

Classes of Biomolecules

Biomolecules are essential molecules for life, classified into four major groups:

• Carbohydrates

• Proteins

• Nucleic Acids

• Lipids

Example: Carbohydrates provide energy, proteins perform cellular functions, nucleic acids store genetic information, and lipids form membranes and store energy.

Monomers & Polymers

Polymerization

Most biomolecules are polymers, made by linking monomers through covalent bonds.

• Dehydration synthesis: Joins monomers by removing water.

• Hydrolysis: Breaks polymers into monomers by adding water.

Example: Formation and breakdown of polymers such as carbohydrates, proteins, and nucleic acids.

Carbohydrates

Structure and Classification

Carbohydrates are molecules with the general formula (CH2O)n, containing many hydroxyl (-OH) groups.

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides linked (e.g., sucrose).

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

Example: Starch (plants) and glycogen (animals) are energy storage polysaccharides; cellulose (plants) provides structural support.

Formation of Polysaccharides

• Monosaccharides are linked by glycosidic bonds via dehydration synthesis.

Equation:

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

Lipids

Structure and Types

Lipids are hydrophobic molecules, not true polymers, and include fats, phospholipids, and steroids.

• Fats (triglycerides): Glycerol + 3 fatty acids (energy storage).

• Phospholipids: Glycerol + 2 fatty acids + phosphate group (membranes).

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

Example: Saturated fatty acids have no double bonds; unsaturated fatty acids have one or more double bonds.

Proteins

Structure and Function

Proteins are polymers of amino acids, joined by peptide bonds. They perform structural, enzymatic, and regulatory roles.

• Amino acid structure: Central carbon, amino group, carboxyl group, hydrogen, and R group.

• Levels of protein structure: Primary (sequence), secondary (folding), tertiary (3D shape), quaternary (multiple polypeptides).

Equation:

$\text{Amino Acid}_1 + \text{Amino Acid}_2 \xrightarrow{\text{Dehydration}} \text{Dipeptide} + H_2O$

Denaturation

• Proteins lose function if their shape is altered by heat, pH, or chemicals.

• Chaperone proteins assist in proper folding and refolding of proteins.

Nucleic Acids

Structure and Function

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

• Nucleotide structure: 5-carbon sugar, phosphate group, nitrogenous base.

• DNA: Deoxyribose sugar, bases A, T, C, G; double helix.

• RNA: Ribose sugar, bases A, U, C, G; single-stranded.

Equation:

$\text{Nucleotide}_1 + \text{Nucleotide}_2 \xrightarrow{\text{Dehydration}} \text{Dinucleotide} + H_2O$

Nitrogenous Bases

Type	Bases
Pyrimidines	Cytosine, Thymine, Uracil
Purines	Adenine, Guanine

Base pairing in DNA: A pairs with T, G pairs with C.

DNA vs. RNA

• DNA: Double-stranded, deoxyribose, thymine.

• RNA: Single-stranded, ribose, uracil.

Additional info: These notes cover foundational concepts from "The Molecules of Cells" and related chapters, including the chemical basis of life, organic molecules, and the structure and function of biomolecules.