Water and Life: Properties of Water, Acids, Bases, and Environmental Impact

Water and Life

Importance of Water in Biological Systems

Water is essential for life on Earth, serving as the medium in which most biological processes occur. It is the only common substance that exists naturally in all three physical states: solid, liquid, and gas.

• Major component of organisms: Most cells are 70-95% water.

• Earth's surface: Three-quarters is covered by water.

• Role in chemical reactions: Water acts as a reactant in many biochemical reactions.

• Origin of life: Life began in water and most cells are surrounded by water.

Properties of Water

Structure and Polarity

The unique properties of water arise from its molecular structure and hydrogen bonding.

• Polar covalent bonds: Water molecules have a bent shape, with two hydrogen atoms bonded to an oxygen atom. Oxygen is more electronegative, making water a polar molecule with partial charges (δ- on oxygen, δ+ on hydrogen).

• Hydrogen bonding: The polarity allows water molecules to form hydrogen bonds with each other and with other polar substances.

Key Properties of Water

• Polarity: Enables water to dissolve many substances, making it a universal solvent.

• Cohesion: Water molecules stick together due to hydrogen bonding, aiding in the transport of water in plants.

• Adhesion: Water molecules cling to other surfaces, such as plant vessel walls.

• High heat capacity: Water absorbs or releases large amounts of heat with little temperature change.

• High surface tension: Water resists breaking at its surface, allowing small organisms to walk on it.

• Density anomaly: Solid water (ice) is less dense than liquid water, causing ice to float.

Water as a Solvent

Water's polarity makes it an excellent solvent for ionic and polar substances.

• Solution: Composed of a solvent (dissolving agent) and a solute (substance dissolved).

• Hydration shell: Each dissolved ion is surrounded by a sphere of water molecules.

• Biological chemistry: Most reactions occur in aqueous solutions.

Hydrophilic vs. Hydrophobic Substances

• Hydrophilic: Substances with an affinity for water (e.g., ionic and polar molecules) that dissolve easily.

• Hydrophobic: Substances that repel water (e.g., oils), typically nonpolar and nonionic.

Cohesion and Adhesion

• Cohesion: Hydrogen bonds hold water molecules together, crucial for water transport in plants.

• Adhesion: Water molecules adhere to other surfaces, aiding movement against gravity in plant vessels.

Thermal Properties of Water

• High heat capacity: The amount of heat needed to raise 1 g of water by 1°C. Water stabilizes temperature in organisms and environments.

• Hydrogen bonds: Increase melting and boiling points. Without them, water would melt at -100°C and boil at -91°C.

• High specific heat: Water resists temperature changes, stabilizing climates and internal conditions in organisms.

• High heat of vaporization: 580$calrequiredtoevaporate$1 g of water at room temperature. Evaporative cooling (e.g., sweating) helps regulate temperature.

Surface Tension

• Definition: The force required to stretch or break the surface of a liquid.

• Cause: Collective strength of hydrogen bonds.

• Example: Raft spiders can walk on water due to high surface tension.

Density of Ice vs. Liquid Water

• Ice floats: Ice is about 10% less dense than liquid water at 4°C due to its lattice structure.

• Ecological importance: Floating ice insulates water below, allowing aquatic life to survive in cold climates.

• Habitat: Ice provides habitat for Arctic animals.

Effect of Climate Change on the Arctic

• Warming: Reduces ice cover, affecting habitats and food sources for polar species.

• Acidification: Increased CO2 leads to ocean acidification, threatening marine ecosystems.

Acids, Bases, and pH

Acidic and Basic Conditions

Water can dissociate into hydrogen ions (H+) and hydroxide ions (OH-), affecting cellular chemistry.

• Dissociation equation: $H_2O>H^{+}+OH^{-}$

• Acids: Increase H+ concentration; donate protons or accept electrons. Example: $HCl>H^{+}+Cl^{-}$

• Bases: Reduce H+ concentration; accept protons or donate electrons. Example: $NaOH>Na^{+}+OH^{-}$; $OH^{-}+H^{+}>H_2O$

pH Scale

• Definition: Measures H+ concentration.

• Formula: $pH=-l{og}[H^{+}]$

• Range: 1 (acidic) to 14 (basic); 7 is neutral.

• Biological relevance: Most cells function near pH 7; human blood pH is about 7.4.

Buffers

• Function: Minimize changes in pH by accepting or donating H+ ions.

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

• Example: Carbonic acid ($H_2CO_3$) in blood acts as a buffer: $CO_2+H2O>H_2CO_{3>}HCO_3^{-}+H^{+}$

Environmental Impact: Acidification

Acidification of Water Bodies

• Human activities: Burning fossil fuels releases CO2, sulfur oxides, and nitrous oxides.

• Ocean acidification: $CO_2+H_2O>H_2CO_3$; carbonic acid lowers ocean pH, harming marine life.

• Acid precipitation: Sulfur and nitrous oxides form strong acids in rain, affecting soil and aquatic ecosystems.

• Regulation: Clean Air Act (1990) in the U.S. aimed to reduce harmful emissions.

Summary Table: Properties of Water

Property	Description	Biological Importance
Polarity	Unequal sharing of electrons	Universal solvent
Cohesion	Hydrogen bonds between molecules	Transport in plants
Adhesion	Clinging to other surfaces	Movement in vessels
High Heat Capacity	Absorbs/releases heat slowly	Temperature stability
High Surface Tension	Resists surface breaking	Habitat for small organisms
Density of Ice	Ice less dense than liquid	Insulates aquatic life

Summary Table: pH Scale

pH Range	Type	Examples
1-6	Acidic	Stomach acid, lemon juice
7	Neutral	Pure water, blood
8-14	Basic	Bleach, baking soda

Additional info: Academic context and expanded explanations were added to ensure completeness and clarity for college-level biology students.