Water Supports All of Life

Importance of Water for Life

Water is essential for all known forms of life and plays a critical role in biological systems. Its unique physical and chemical properties make it indispensable for cellular processes and the maintenance of life on Earth.

• Abundance: Approximately 70% of Earth's surface is covered by water.

• Cellular Composition: Most cells are surrounded by water, and cells themselves are composed of 70-95% water.

• Human Body: Humans are composed of about 70% water.

• States of Matter: Water is the only common substance that exists naturally in all three physical states—solid, liquid, and gas—under Earth's environmental conditions.

• Molecular Shape and Charge: The bent shape and polar nature of the water molecule are crucial to its biological functions.

Water – Hydrogen Bonding

Structure and Bonding in Water Molecules

The molecular structure of water and its ability to form hydrogen bonds are fundamental to its properties.

• Polarity: Oxygen is highly electronegative, while hydrogen is less so, resulting in polar covalent bonds within the water molecule.

• Hydrogen Bonds: Water molecules are attracted to each other via hydrogen bonds, which are relatively strong intermolecular forces.

• Energy: Hydrogen bonds require significant energy to break and release energy when formed.

• Partial Charges: The oxygen atom carries a partial negative charge, and the hydrogen atoms carry partial positive charges.

Emergent Properties of Water

Key Properties Facilitating Life

Water exhibits several emergent properties that are vital for sustaining life on Earth.

• Cohesive & Adhesive Behavior: Water molecules stick to each other and to other substances.

• Ability to Moderate Temperature: Water can absorb and release heat with minimal temperature change.

• Expansion Upon Freezing: Water becomes less dense as it freezes, allowing ice to float.

• Versatility as a Solvent: Water can dissolve a wide variety of substances, making it an excellent medium for chemical reactions.

Cohesion & Adhesion

Interactions Between Water Molecules and Other Substances

Cohesion and adhesion are two important properties resulting from hydrogen bonding in water.

• Cohesion: The tendency of water molecules to stick together due to hydrogen bonding.

• Adhesion: The attraction between water molecules and different substances, such as plant cell walls.

• Example: Cohesion allows for surface tension, while adhesion helps water move upward in plants through capillary action.

Surface Tension

Definition and Biological Significance

Surface tension is a measure of how difficult it is to break the surface of a liquid. Water has an unusually high surface tension due to hydrogen bonding.

• Surface Tension: Results from cohesive forces among water molecules at the air-water interface.

• Biological Example: Enables small insects to walk on water and supports the formation of droplets.

Moderation of Temperature

Water's Role in Thermal Regulation

Water moderates temperature by absorbing and releasing heat with minimal change in its own temperature.

• Heat Absorption and Release: Water absorbs heat when hydrogen bonds break and releases heat when they form.

• Specific Heat: The specific heat of water is high (1 cal/g·°C), allowing it to buffer temperature changes.

• Evaporative Cooling: As water evaporates, the surface cools, helping regulate temperature in organisms and environments.

Units of Heat

• Calorie (cal): Amount of heat required to raise the temperature of 1 g of water by 1°C.

• Kilocalorie (kcal): 1 kcal = 1,000 cal; commonly used in food energy.

• Joule (J): SI unit of energy; 1 cal = 4.184 J.

Expansion Upon Freezing

Density Changes and Ecological Impact

Water expands upon freezing, making ice less dense than liquid water.

• Density: Density is mass per unit volume (g/mL). Ice is less dense than liquid water due to the ordered arrangement of hydrogen bonds.

• Ecological Importance: Floating ice insulates water below, protecting aquatic life in cold climates.

• Maximum Density: Water reaches its maximum density at 4°C.

Versatility as a Solvent

Water as the Universal Solvent

Water's polarity allows it to dissolve many substances, facilitating chemical reactions in biological systems.

• Solution: A homogeneous mixture of substances.

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

• Solute: The substance dissolved.

• Rule of Solubility: "Like dissolves like"—polar solutes dissolve in polar solvents, non-polar solutes in non-polar solvents.

• Hydration Shell: When ionic compounds dissolve, water molecules surround each ion, forming a hydration shell.

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

Hydrophilic and Hydrophobic Substances

Affinity for Water

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

• Hydrophobic: Substances that repel water, typically non-polar (e.g., oils, lipids).

• Biological Relevance: Cell membranes are composed of hydrophobic lipid bilayers, which separate internal and external environments.

Solute Concentration in Aqueous Solutions

Calculating Concentrations

• Mole (mol): 1 mol = molecules (Avogadro's number).

• Molar Mass: Mass (g) of 1 mol of molecules; sum of atomic weights.

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

Acidity and Basicity

pH and Ionization of Water

Water can dissociate into ions, affecting the pH of solutions.

• Dissociation: A hydrogen atom in a hydrogen bond may shift, leaving its electron behind and becoming a hydrogen ion (H+).

• Hydronium Ion (H3O+): Water molecule with an extra proton.

• Hydroxide Ion (OH-): Water molecule that lost a proton.

• Concentration in Pure Water: At 25°C, [H+] and [OH-] are both M.

• Acid: Substance that increases H+ concentration.

• Base: Substance that reduces H+ concentration.

pH Scale

• Definition:

• Neutral Solution: pH = 7

• Acidic Solution: pH < 7

• Basic Solution: pH > 7

• Biological Fluids: Most have pH values between 6 and 8; human blood is about 7.4.

Buffers

Maintaining pH Stability

• Buffer: Substance that minimizes changes in concentrations of H+ and OH- in solution.

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

• Example: Bicarbonate buffer system in blood:

Acidification: A Threat to Our Oceans

Human Impact on Water Chemistry

• CO2 Emissions: Burning fossil fuels releases CO2, which dissolves in oceans and forms carbonic acid.

• Ocean Acidification: Increased H+ ions reduce carbonate ions, affecting organisms that build calcium carbonate shells (e.g., corals).

• Equation:

Key Terms and Concepts

Summary Table: Water Properties and Definitions

Practice and Application

• Be able to convert between calories and joules:

• Calculate molarity:

• Distinguish between solvent and solute, hydrophilic and hydrophobic substances, acids and bases.

Additional info: Some explanations and equations were expanded for clarity and completeness.