Water: Structure and Hydrogen Bonding

The Unique Structure of Water

Water (H2O) is a polar molecule, meaning it has regions of partial positive and negative charge due to the unequal sharing of electrons between oxygen and hydrogen atoms. This polarity allows water molecules to form hydrogen bonds with each other, which are weak attractions between the partially positive hydrogen of one molecule and the partially negative oxygen of another.

• Polarity: Oxygen is more electronegative than hydrogen, resulting in a partial negative charge (δ-) near the oxygen and a partial positive charge (δ+) near the hydrogens.

• Hydrogen Bonds: Each water molecule can form up to four hydrogen bonds with neighboring water molecules.

• Example: The diagram shows water molecules oriented so that the hydrogen of one is near the oxygen of another, forming a hydrogen bond.

Emergent Properties of Water

How Water Supports Life on Earth

Water's unique properties make it essential for life. These properties arise from its structure and ability to form hydrogen bonds.

• Ability to Moderate Temperature: Water can absorb or release large amounts of heat with only a slight change in its own temperature, helping to stabilize environmental and organismal temperatures.

• Cohesive Behavior: Water molecules stick together due to hydrogen bonding, contributing to phenomena like surface tension and the transport of water in plants.

• Expansion Upon Freezing: Water expands as it freezes, making ice less dense than liquid water. This allows ice to float, insulating aquatic life in winter.

• Versatility as a Solvent: Water can dissolve a wide variety of substances, making it the "universal solvent" and facilitating chemical reactions in cells.

Water: The Solvent of Life

Solutions, Solvents, and Solutes

A solution is a homogeneous mixture of two or more substances. The solvent is the dissolving agent (often water in biological systems), and the solute is the substance that is dissolved.

• Example: In a saltwater solution, water is the solvent and sodium chloride (NaCl) is the solute.

• Aqueous Solution: A solution in which water is the solvent.

How Water Dissolves Substances

When an ionic compound (like NaCl) dissolves in water, each ion becomes surrounded by water molecules, forming a hydration shell. Water's polarity allows it to interact with and stabilize ions and polar molecules.

• Hydration Shell: The sphere of water molecules surrounding each dissolved ion.

• Not Limited to Ions: Water can also dissolve other polar molecules, such as sugars and proteins, by forming hydrogen bonds with them.

• Example: The diagram shows Na+ and Cl- ions surrounded by water molecules, each oriented according to charge.

Hydrophilic and Hydrophobic Substances

Substances that interact well with water are called hydrophilic ("water-loving"), while those that do not are hydrophobic ("water-fearing").

• Hydrophilic: Polar or charged molecules that dissolve easily in water (e.g., salts, sugars).

• Hydrophobic: Nonpolar molecules that do not dissolve in water (e.g., oils, fats).

• Example: When oil is added to water, it aggregates rather than dissolving, demonstrating hydrophobic behavior.

Acids, Bases, and the pH Scale

Acids, Bases, and Ionization of Water

Water can dissociate into hydrogen ions (H+) and hydroxide ions (OH-):

• Acid: A substance that increases the concentration of H+ in a solution.

• Base: A substance that reduces the concentration of H+, often by increasing OH-.

• Pure Water: Has equal concentrations of H+ and OH- (neutral).

The product of [H+] and [OH-] in pure water at 25°C is:

The pH Scale

The pH of a solution is a measure of its hydrogen ion concentration:

• pOH:

• Relationship:

• Acidic Solution: pH < 7

• Neutral Solution: pH = 7

• Basic Solution: pH > 7

• Example: A solution with has (acidic).

pH Scale Table

The pH scale is logarithmic; each unit represents a tenfold difference in H+ concentration.

Carbon and the Molecular Diversity of Life

Why Carbon is Special

Carbon atoms can form four covalent bonds, allowing for a diversity of stable organic molecules with various shapes and functions.

• Bonding: Carbon can form single, double, or triple bonds with other atoms, including other carbon atoms.

• Versatility: Carbon chains can be straight, branched, or arranged in rings.

• Example: Hydrocarbons are molecules consisting only of carbon and hydrogen (e.g., methane, ethane, propane).

Hydrocarbons

Hydrocarbons are the simplest organic molecules and serve as the backbone for more complex molecules.

• Alkanes: Saturated hydrocarbons with single bonds (e.g., methane, ethane, propane).

• Alkenes: Unsaturated hydrocarbons with at least one double bond.

• Alkynes: Unsaturated hydrocarbons with at least one triple bond.

• Example: Octane (C8H18) is a straight-chain alkane found in gasoline.

Isomers

Isomers are compounds with the same molecular formula but different structures and properties.

• Structural Isomers: Differ in the covalent arrangement of atoms.

• Cis-Trans (Geometric) Isomers: Differ in spatial arrangement around a double bond.

• Enantiomers: Mirror-image isomers, important in biology (e.g., L- and D-amino acids).

• Example: 2-methylbutane and pentane are structural isomers.

Functional Groups

Key Functional Groups in Organic Molecules

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

• Hydroxyl (-OH): Found in alcohols; makes molecules polar.

• Carbonyl (C=O): Found in aldehydes and ketones.

• Carboxyl (-COOH): Found in carboxylic acids; acts as an acid.

• Amino (-NH2): Found in amines; acts as a base.

• Sulfhydryl (-SH): Found in thiols; can form disulfide bonds.

• Phosphate (-PO42-): Found in nucleic acids and ATP; involved in energy transfer.

• Methyl (-CH3): Nonpolar; affects gene expression.

• Example: Estradiol and testosterone differ only in the functional groups attached to their carbon skeletons, resulting in different biological activities.

Biological Macromolecules

The Four Major Classes

Living organisms are composed of four major classes of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids.

• Carbohydrates: Sugars and polymers of sugars; serve as energy sources and structural materials.

• Lipids: Hydrophobic molecules including fats, phospholipids, and steroids; important for energy storage and membrane structure.

• Proteins: Polymers of amino acids; perform a wide range of functions including catalysis, transport, and support.

• Nucleic Acids: DNA and RNA; store and transmit genetic information.

Polymers and Monomers

Most biological macromolecules are polymers, long molecules made by linking together smaller units called monomers.

• Polymer: A long molecule consisting of many similar or identical building blocks (monomers) linked by covalent bonds.

• Dehydration Reaction: Monomers are joined by the removal of a water molecule, forming a new bond.

• Hydrolysis: Polymers are broken down into monomers by the addition of water, breaking the bond.

• Example: Starch (a carbohydrate) is a polymer of glucose monomers.

Polymerization Table

Additional info: Lipids are not true polymers, but are included due to their biological importance.