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, especially 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 up to four covalent bonds.

• Bonding: Carbon forms stable single, double, and triple bonds easily, enabling a diversity of molecular structures.

Example: The image shows a sample of elemental carbon, illustrating its physical form.

Why Carbon?

• Carbon's ability to form four bonds makes it uniquely suited for building complex, flexible molecules.

• Compared to silicon, carbon is lighter and more versatile in forming stable, diverse structures necessary for life.

Additional info: Carbon's tetrahedral geometry allows for three-dimensional molecular diversity, which is essential for biological function.

Organic Chemistry in Biology

Definition and Scope

• Organic Chemistry: The study of carbon-containing compounds, including their structure, properties, and reactions.

• Biological Relevance: Organic molecules include carbohydrates, lipids, proteins, and nucleic acids.

Example: Butane is a simple organic molecule shown in the ball-and-stick model.

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 connected (e.g., H–C–H).

• Ball-and-Stick Model: Visualizes atoms as spheres and bonds as sticks.

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

Molecule	Molecular Formula	Structural Formula	Ball-and-Stick Model	Space-Filling Model
Methane	CH4	H–C–H	Ball-and-stick diagram	Space-filling diagram
Ethane	C2H6	H–C–C–H	Ball-and-stick diagram	Space-filling diagram
Ethene (ethylene)	C2H4	H2C=CH2	Ball-and-stick diagram	Space-filling diagram

Additional info: Skeletal formulas are often used for complex organic molecules, where each vertex represents a carbon atom.

Carbon Skeletons

Structural Diversity

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

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

• Double Bonds: Carbon skeletons may contain 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 is a ring structure with alternating double bonds, important in many biological molecules.

Functional Groups

Definition and Importance

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

• Hydroxyl group (-OH): Found in alcohols (e.g., ethanol, 1-propanol).

• Carboxyl group (-COOH): Found in carboxylic acids.

• Amino group (-NH2): Found in amino acids.

• Phosphate group (-PO42-): Found in nucleotides.

• Methyl group (-CH3): Found in many organic molecules.

Chemical Group	Example
Hydroxyl	Ethanol
Carboxyl	Acetic acid
Amino	Glycine
Phosphate	Adenosine triphosphate (ATP)
Methyl	Methylated DNA

Additional info: Functional groups determine the solubility, acidity, and reactivity of organic molecules.