Carbon and Introduction to Macromolecules

Overview

This section introduces the foundational role of carbon in the structure of living things and explores the diversity and function of biological macromolecules. Understanding carbon's unique properties helps explain the complexity and variety of organic molecules essential for life.

Organic Compounds and Carbon's Role

Definition and Importance of Organic Compounds

• Organic compounds are molecules primarily composed of carbon atoms, often bonded with hydrogen, oxygen, nitrogen, and other elements.

• They range from simple molecules (like methane) to large, complex macromolecules (like proteins and nucleic acids).

• Carbon's ability to form four covalent bonds allows for a vast diversity of molecular structures.

Example: Dopamine, a molecule involved in brain signaling, is an organic compound whose structure is based on a carbon skeleton with attached functional groups.

Molecular Shape and Function

Relationship Between Structure and Function

• The size and shape of a molecule are critical to its biological function.

• Molecular shape determines how biological molecules recognize and interact with each other.

• For example, opiates (like morphine) and natural endorphins have similar shapes, allowing them to bind the same brain receptors and produce similar effects.

Key Point: The three-dimensional structure of molecules is essential for their specific biological activity.

Molecular Diversity from Carbon Skeletons

Variation in Carbon Skeletons

• Carbon atoms can form chains of varying length, branching, and ring structures, creating the backbone (skeleton) of organic molecules.

• These skeletons can be:

  • Unbranched or branched

  • Contain double bonds in different positions

  • Arranged in rings

• This diversity allows for a wide range of molecular complexity and function.

Example: Ethane (C2H6) and propane (C3H8) differ in chain length; butane and isobutane differ in branching; cyclohexane and benzene are examples of ring structures.

The Formation of Bonds with Carbon

Covalent Bonding and Valence

• The number of unpaired electrons in carbon's valence shell is four, allowing it to form four covalent bonds.

• This bonding capacity enables carbon to form stable, diverse molecules with hydrogen, oxygen, nitrogen, and other elements.

• The most common bonding partners for carbon in biological molecules are hydrogen (H), oxygen (O), and nitrogen (N).

Polarity, Hydrophobicity, and Hydrophilicity

Understanding Molecular Interactions with Water

• Polar molecules have regions of partial positive and negative charge, allowing them to interact with water (hydrophilic).

• Non-polar molecules lack charged regions and do not interact well with water (hydrophobic).

• Hydrophilic substances are "water-loving" and dissolve easily in water.

• Hydrophobic substances are "water-fearing" and do not dissolve in water.

Example: Methanol (polar) mixes well with water, while ethane (non-polar) does not.

General Relationship: Polarity increases hydrophilicity; non-polarity increases hydrophobicity.

Macromolecules: Carbohydrates and Lipids

Overview of Biological Macromolecules

• Living organisms are primarily composed of four types of macromolecules: carbohydrates, lipids, proteins, and nucleic acids.

• Many biological macromolecules are polymers—long chains of repeating units called monomers.

• Carbohydrates and nucleic acids are classic examples of polymers.

Carbohydrates

• Monomers: Monosaccharides (simple sugars, e.g., glucose)

• Polymers: Polysaccharides (e.g., starch, glycogen, cellulose, chitin)

• Functions:

  • Major fuel source for cells

  • Building material for cell structures

  • Cell surface markers

• Examples:

  • Starch: Storage polysaccharide in plants

  • Glycogen: Storage polysaccharide in animals

  • Cellulose: Structural polysaccharide in plant cell walls

  • Chitin: Structural polysaccharide in fungi and arthropod exoskeletons

Lipids

• Lipids are a diverse group of hydrophobic molecules that do not form true polymers.

• Main types: Fats, phospholipids, steroids

• Functions:

  • Energy storage

  • Membrane structure (phospholipids form cell membranes)

  • Insulation (thermal and electrical)

• Fats: Composed of glycerol and fatty acids; important for long-term energy storage

• Phospholipids: Have hydrophilic heads and hydrophobic tails; form bilayers in water, creating cell membranes

• Steroids: Characterized by a carbon skeleton with four fused rings (e.g., cholesterol, hormones)

Phospholipid Bilayers

• When placed in water, phospholipids self-assemble into bilayers with hydrophilic heads facing outward and hydrophobic tails facing inward.

• This structure forms the fundamental boundary of all cell membranes, separating the cell from its environment.

Summary Tables

Key Concepts: Carbohydrates

Key Concepts: Lipids

Key Equations and Concepts

• General formula for carbohydrates:

• Dehydration reaction (polymer formation): Monomers are joined by the removal of a water molecule.

• Hydrolysis reaction (polymer breakdown): Polymers are broken down by the addition of a water molecule.

Additional info:

• Proteins and nucleic acids, though mentioned, are covered in subsequent classes.

• Understanding the relationship between molecular structure and function is a recurring theme in biology.