general biology unit 3

Water and Life

Introduction

Water is a fundamental molecule for all known forms of life. It exists naturally on Earth in three states: liquid (water), gas (water vapor), and solid (ice). The unique properties of water make it essential for biological processes and the survival of organisms.

Concept 3.1 and Figure 3.2: Structure of Water

Covalent Bonds, Polar Molecules, and Hydrogen Bonds

• Covalent bonds are strong chemical bonds formed when two atoms share one or more pairs of electrons.

• Polar molecules have regions with slight electrical charges due to unequal sharing of electrons. In water (H2O), oxygen is more electronegative than hydrogen, creating a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms.

• Hydrogen bonds are weak attractions between the slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of another. These bonds are responsible for many of water's unique properties.

Example: In a glass of water, each water molecule can form up to four hydrogen bonds with neighboring molecules, creating a dynamic network.

Drawing Hydrogen Bonds

• When illustrating water molecules, show the oxygen atom (O) bonded to two hydrogen atoms (H) at an angle (~104.5°). Use dashed lines to represent hydrogen bonds between the hydrogen of one molecule and the oxygen of another.

Concept 3.2: Emergent Properties of Water

Four Emergent Properties of Water

Water exhibits several properties that are critical for life, arising from its structure and hydrogen bonding:

1. Cohesion and Adhesion

   • Cohesion is the attraction between water molecules due to hydrogen bonding, leading to high surface tension.

   • Adhesion is the attraction between water molecules and other substances, aiding processes like capillary action in plants.

2. Temperature Moderation

   • Water has a high specific heat, meaning it can absorb or release large amounts of heat with little temperature change. This stabilizes climates and helps organisms maintain homeostasis.

   • Evaporative cooling occurs when water evaporates, removing heat and cooling surfaces (e.g., sweating in humans).

3. Expansion Upon Freezing

   • As water freezes, hydrogen bonds stabilize and keep molecules further apart, making ice less dense than liquid water. This allows ice to float, insulating aquatic life in winter.

4. Versatility as a Solvent

   • Water is known as the "universal solvent" because it dissolves many substances, especially ionic and polar compounds, facilitating chemical reactions in cells.

Key Terms: solution, solvent, solute, aqueous solution, hydration shell, hydrophilic (water-loving), hydrophobic (water-fearing), molar mass, mole, molarity.

Why is Water Important for Life?

• Water's ability to dissolve a wide range of substances allows for efficient transport of nutrients and waste in organisms.

• Its high specific heat and evaporative cooling help regulate temperature in living systems.

• The floating of ice protects aquatic ecosystems during cold periods.

Hydrogen Bonding and Water's Properties

• Hydrogen bonds are responsible for water's high cohesion, surface tension, and specific heat.

• These bonds constantly form and break, giving water its fluidity and unique characteristics.

Concept 3.3: Acids, Bases, and pH

Hydrogen Ions, Hydroxide Ions, and Hydronium Ions

• Hydrogen ion (H+): A proton released when a hydrogen atom loses its electron.

• Hydroxide ion (OH-): Formed when water loses a hydrogen ion.

• Hydronium ion (H3O+): Formed when a water molecule gains a hydrogen ion.

Acids, Bases, and the pH Scale

• Acid: A substance that increases the concentration of H+ ions in a solution (e.g., HCl).

• Base: A substance that reduces the concentration of H+ ions, often by increasing OH- (e.g., NaOH).

• pH scale: Measures the concentration of H+ ions in a solution, ranging from 0 (most acidic) to 14 (most basic), with 7 being neutral.

Formula:

$\text{pH} = -\log_{10}[\text{H}^+]$

Examples of Acids and Bases

• Strong acid: Hydrochloric acid (HCl)

• Weak acid: Acetic acid (CH3COOH)

• Strong base: Sodium hydroxide (NaOH)

• Weak base: Ammonia (NH3)

Acidification and Ocean pH

• Increased CO2 in the atmosphere leads to ocean acidification, lowering the pH and affecting marine life, especially organisms with calcium carbonate shells.

pH Scale Table

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

pH Value	Type of Solution	Example
0-2	Strong Acid	Gastric juice, battery acid
3-6	Weak Acid	Vinegar, soda
7	Neutral	Pure water
8-11	Weak Base	Blood, seawater
12-14	Strong Base	Household bleach, oven cleaner

Summary

• Water's structure and hydrogen bonding give rise to its unique properties, which are essential for life.

• Understanding acids, bases, and the pH scale is crucial for studying biological systems and environmental impacts.