The Molecules of Cells: Structure and Function of Biological Macromolecules

The Molecules of Cells

Introduction

Cells are composed of a variety of molecules that perform essential structural and functional roles. The study of these molecules, particularly those based on carbon, is fundamental to understanding biology at the molecular level.

Carbon: The Backbone of Biological Molecules

Properties of Carbon

• Symbol: C

• Atomic Number: 6

• Valence Electrons: 4, allowing carbon to form four covalent bonds with other atoms.

• Bonding Versatility: Carbon can form single, double, or triple bonds, and can bond with many elements, including itself.

Example: The image shows a sample of elemental carbon, highlighting its ability to exist in different forms (allotropes) such as graphite and diamond.

Why Carbon?

• Carbon's tetravalency allows it to form stable, diverse, and complex molecules necessary for life.

• Compared to silicon, carbon forms more flexible and stable bonds, making it ideal for the backbone of biological molecules.

• The tetrahedral geometry of carbon's bonds enables the formation of three-dimensional molecular structures.

Additional info: Silicon, though similar to carbon, forms less stable and less flexible bonds, which is why life is carbon-based.

Organic Chemistry: The Study of Carbon Compounds

Definition and Scope

• Organic Chemistry: The branch of chemistry that studies carbon-containing compounds, especially those found in living organisms.

• Focuses on the structure, properties, and reactions of organic molecules.

Major Classes of Biological Molecules

• Carbohydrates

• Lipids

• Proteins

• Nucleic Acids

Example: Butane (C4H10) is a simple hydrocarbon, illustrating the diversity of carbon compounds.

Representing Molecules

Types of Molecular Representations

• Molecular Formula: Shows the number and type of atoms (e.g., CH4 for methane).

• Structural Formula: Shows how atoms are bonded (e.g., H–C–H).

• Ball-and-Stick Model: 3D representation showing atoms as spheres and bonds as sticks.

• Space-Filling Model: Shows the relative sizes and spatial relationships of atoms.

Additional info: Skeletal formulas are often used for larger organic molecules, where lines represent bonds between carbon atoms, and hydrogens are implied.

Compound	Molecular Formula	Structural Formula	Ball-and-Stick Model	Space-Filling Model
Methane	CH4	H | H–C–H | H	3D tetrahedral	Compact sphere
Ethane	C2H6	H H | | H–C–C–H | | H H	3D model with two connected tetrahedra	Two spheres joined
Ethene (Ethylene)	C2H4	H H \ / C=C / \ H H	Planar double bond	Flattened spheres

Carbon Skeletons

Variations in Carbon Skeletons

• Length: Carbon chains can vary in length (e.g., ethane, propane, butane).

• Branching: Chains may be unbranched (straight) or branched (e.g., isobutane).

• Double Bonds: Carbon skeletons may include double bonds, which can vary in position (e.g., 1-butene, 2-butene).

• Rings: Carbon atoms can form ring structures (e.g., cyclohexane, benzene).

Example: Benzene (C6H6) is a ring structure with alternating double bonds, important in many biological molecules.

Functional Groups

Definition and Importance

• Functional Groups: Specific groups of atoms within molecules that determine the chemical properties and reactions of those molecules.

• Common functional groups include hydroxyl, carbonyl, carboxyl, amino, phosphate, and methyl groups.

Chemical Group	Structure	Example Compound
Hydroxyl	–OH	Alcohols (e.g., ethanol)
Carbonyl	–C=O	Aldehydes, ketones
Carboxyl	–COOH	Carboxylic acids (e.g., acetic acid)
Amino	–NH2	Amines (e.g., glycine)
Phosphate	–OPO32−	ATP, nucleotides
Methyl	–CH3	Methylated compounds

Example: Ethanol (C2H5OH) contains a hydroxyl group, making it an alcohol.