Water & Life

Introduction

Water is a fundamental molecule that supports all known forms of life. Its unique physical and chemical properties make it indispensable for biological processes and the maintenance of life on Earth.

• Water covers about 70% of Earth's surface and is present in all living cells, which themselves are composed of 70–95% water.

• Humans are approximately 70% water by body mass.

• Water is the only common substance that exists naturally in all three physical states: solid, liquid, and gas.

• The shape and charge of the water molecule are crucial to its biological importance.

Structure of Water and Hydrogen Bonding

Polar Covalent Bonds and Molecular Polarity

The water molecule (H2O) is composed of two hydrogen atoms covalently bonded to one oxygen atom. The oxygen atom is highly electronegative, resulting in polar covalent bonds and a bent molecular shape.

• Electronegativity: Oxygen attracts electrons more strongly than hydrogen, creating a partial negative charge near the oxygen and a partial positive charge near the hydrogens.

• Polarity: This unequal sharing of electrons makes water a polar molecule.

Hydrogen Bonding

Water molecules form hydrogen bonds with each other due to their polarity.

• Hydrogen bonds are weak individually but strong collectively, requiring significant energy to break.

• Hydrogen bonds are responsible for many of water's unique properties.

Emergent Properties of Water

Overview

Water exhibits four key emergent properties that are essential for life:

• Cohesive and adhesive behavior

• Ability to moderate temperature

• Expansion upon freezing

• Versatility as a solvent

Cohesion and Adhesion

Definitions and Biological Importance

Cohesion and adhesion are two related but distinct properties of water that arise from hydrogen bonding.

• Cohesion: The attraction between water molecules due to hydrogen bonding. This property allows water to form droplets and contributes to surface tension.

• Adhesion: The attraction between water molecules and other substances (e.g., cell walls of plants). Adhesion helps water move against gravity in plant vessels.

• Surface tension: A measure of how difficult it is to stretch or break the surface of a liquid. Water has a high surface tension due to cohesive forces.

Example: Cohesion and adhesion enable the upward movement of water through plant stems (capillary action), which is essential for transporting nutrients.

Ability to Moderate Temperature

Thermal Properties of Water

Water can absorb or release large amounts of heat with only a slight change in its own temperature, due to its high specific heat.

• Kinetic energy: The energy of motion; in molecules, this is called thermal energy.

• Temperature: Represents the average kinetic energy of molecules in a substance.

• Heat: The transfer of thermal energy from one body to another.

• Specific heat: The amount of heat required to raise the temperature of 1 gram of a substance by 1°C. For water, this is 1 cal/(g·°C).

• Water's high specific heat helps stabilize temperatures in organisms and environments.

• Evaporative cooling: As water evaporates, the surface cools, helping regulate temperature in living organisms.

Formulas:

• 1 calorie (cal) = amount of heat to raise 1 g of water by 1°C

• 1 kilocalorie (kcal) = 1,000 cal

• 1 joule (J) = 0.239 cal; 1 cal = 4.184 J

Expansion Upon Freezing

Density and Ice Formation

Water is less dense as a solid (ice) than as a liquid, which is unusual among substances.

• Density: Mass per unit volume (g/mL).

• Hydrogen bonds in ice are more "ordered," causing molecules to be spaced further apart, making ice less dense than liquid water.

• Ice floats, insulating the water below and allowing aquatic life to survive in cold climates.

• Water reaches its greatest density at 4°C.

Example: If ice sank, bodies of water would freeze solid from the bottom up, making life impossible in aquatic environments.

Versatility as a Solvent

Solubility and Solutions

Water is known as the "universal solvent" due to its ability to dissolve a wide variety of substances.

• Solution: A homogeneous mixture of two or more substances.

• Solvent: The dissolving agent (water in aqueous solutions).

• Solute: The substance that is dissolved.

• "Like dissolves like": Polar and ionic solutes dissolve in polar solvents like water; non-polar solutes dissolve in non-polar solvents.

• When ionic compounds dissolve, each ion is surrounded by a hydration shell of water molecules.

• Water can also dissolve large polar molecules, such as proteins, if they have ionic and polar regions.

Hydrophilic and Hydrophobic Substances

• Hydrophilic: Substances with an affinity for water (e.g., salts, sugars).

• Hydrophobic: Substances that do not interact well with water (e.g., oils, lipids) due to non-polar bonds.

Concentration of Solutes in Aqueous Solutions

Moles, Molar Mass, and Molarity

Most chemical reactions in organisms occur in aqueous solutions. Concentrations are often measured in moles and molarity.

• Mole (mol): 6.022 × 1023 molecules (Avogadro's number).

• Molar mass: The mass (in grams) of 1 mole of a substance, equal to the sum of atomic masses.

• Molarity (M): Number of moles of solute per liter of solution.

Formula:

Acids, Bases, and pH

Water Dissociation and pH Scale

Water can dissociate into hydrogen ions (H+) and hydroxide ions (OH-), affecting the pH of solutions.

• Acid: Increases H+ concentration in solution.

• Base: Reduces H+ concentration (often by increasing OH-).

• In pure water at 25°C:

• pH: Defined as the negative logarithm of the H+ concentration:

• Neutral solution: , so pH = 7

• Acidic solutions: pH < 7; Basic solutions: pH > 7

• Most biological fluids have pH values between 6 and 8.

Buffers

Buffers are substances that minimize changes in pH by accepting or donating H+ ions. They are crucial for maintaining stable internal environments in organisms.

• Most buffers consist of a weak acid and its corresponding base.

• Bicarbonate buffer system is important in human blood:

Acidification: A Threat to Our Oceans

Human Impact and Ocean Acidification

Human activities, such as burning fossil fuels, increase atmospheric CO2, a portion of which is absorbed by oceans, leading to ocean acidification.

• CO2 dissolves in seawater, forming carbonic acid, which lowers ocean pH.

• Increased H+ ions combine with carbonate ions, reducing carbonate availability for marine organisms that build calcium carbonate shells and skeletons.

• This threatens coral reefs and other marine life dependent on calcification.

Key Terms and Concepts

• Cohesion, adhesion, surface tension, kinetic energy, temperature, heat, specific heat, pH

• Be able to convert between calories and joules

• Know the difference between solvent and solute

• Know the difference between hydrophilic and hydrophobic

• Be able to calculate molarity

• Know the difference between acid and base

Summary Table: Properties of Water