Chapter 3: Water and Life – Study Notes for General Biology

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Water: The Molecule That Supports All of Life

Introduction to Water's Role in Biology

Water is essential for all known forms of life and possesses unique properties that make Earth habitable. Its molecular structure and interactions with other molecules underpin many biological processes.

Water as a Universal Medium: Water is the only common substance that exists naturally in all three physical states—solid, liquid, and gas—within Earth's environment.
Emergent Properties: Water's unique properties arise from its molecular structure and interactions, making life possible.
Molecular Structure: The bent shape and polar covalent bonds of water allow it to interact with other molecules through hydrogen bonding.

Structure and Polarity of Water

Polar Covalent Bonds and Hydrogen Bonding

The water molecule (H2O) consists of two hydrogen atoms covalently bonded to one oxygen atom. The unequal sharing of electrons creates a polar molecule, enabling hydrogen bonding.

Polarity: Oxygen is more electronegative than hydrogen, resulting in a partial negative charge near the oxygen and a partial positive charge near the hydrogens.
Hydrogen Bonds: The polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules.
Example: Hydrogen bonds are responsible for water's high surface tension and its ability to dissolve many substances.

Emergent Properties of Water

Four Key Properties Facilitating Life

Water exhibits four emergent properties that contribute to Earth's suitability for life: cohesive behavior, ability to moderate temperature, expansion upon freezing, and versatility as a solvent.

Cohesion: Hydrogen bonds hold water molecules together, aiding the transport of water in plants against gravity.
Adhesion: Water molecules can also adhere to other substances, such as plant cell walls, facilitating movement through capillaries.
Surface Tension: Water has a high surface tension due to hydrogen bonding, making it difficult to break the surface.
Temperature Moderation: Water absorbs and releases heat with minimal temperature change, stabilizing environments.
Expansion Upon Freezing: Ice is less dense than liquid water because hydrogen bonds form a crystalline structure, allowing ice to float.
Versatility as a Solvent: Water's polarity enables it to dissolve ionic and polar substances, making it the solvent of life.

Cohesion, Adhesion, and Surface Tension

Transport and Stability in Biological Systems

Cohesion and adhesion are critical for the movement of water in plants and the stability of biological systems.

Cohesion: Water molecules stick together, enabling the upward transport of water in plant vessels.
Adhesion: Water molecules adhere to other materials, such as cell walls, aiding capillary action.
Surface Tension: The measure of how difficult it is to stretch or break the surface of a liquid; water's surface tension is unusually high.
Example: Water droplets form beads on surfaces due to surface tension.

Moderation of Temperature by Water

Specific Heat and Heat Transfer

Water moderates temperature by absorbing and releasing heat, helping organisms and environments maintain stable conditions.

Kinetic Energy: The energy of motion; thermal energy is the kinetic energy associated with random movement of atoms or molecules.
Heat: Thermal energy transferred from one body to another.
Calorie (cal): The amount of heat required to raise the temperature of 1 g of water by 1°C.
Specific Heat: The amount of heat that must be absorbed or lost for 1 g of a substance to change its temperature by 1°C.
Water's Specific Heat:
High Specific Heat: Water resists temperature changes due to hydrogen bonding; heat is absorbed when bonds break and released when bonds form.
Example: Coastal climates are milder due to water's temperature moderation.

Evaporative Cooling

Heat of Vaporization and Cooling Effects

Evaporation allows organisms and environments to cool as water changes from liquid to gas.

Heat of Vaporization: The amount of heat a liquid must absorb for 1 g to be converted to gas.
Evaporative Cooling: As water evaporates, the surface cools, stabilizing temperatures in organisms and bodies of water.
Example: Sweating in humans helps regulate body temperature.

Floating of Ice on Liquid Water

Density and Environmental Impact

Ice floats because it is less dense than liquid water, which is crucial for aquatic life and Earth's climate.

Hydrogen Bonding: In ice, hydrogen bonds are more ordered, creating a lattice that spaces molecules further apart.
Density: Water reaches its greatest density at 4°C; ice is less dense and floats.
Environmental Impact: If ice sank, bodies of water would freeze solid, threatening life.
Example: Polar ecosystems depend on floating ice for survival.

Water: The Solvent of Life

Solutions, Solvents, and Solutes

Water's polarity makes it an excellent solvent, capable of dissolving a wide range of substances.

Solution: A homogeneous mixture of substances.
Solvent: The dissolving agent (water in aqueous solutions).
Solute: The substance dissolved.
Aqueous Solution: Water is the solvent.
Hydration Shell: Water molecules surround ions or polar molecules, facilitating dissolution.
Hydrophilic vs. Hydrophobic: Hydrophilic substances have an affinity for water; hydrophobic substances do not and are typically nonpolar.
Example: Salt (NaCl) dissolves in water due to hydration shells.

Concentration of Solutes in Aqueous Solutions

Molecular Mass, Moles, and Molarity

Understanding solute concentration is essential for chemical reactions in biological systems.

Molecular Mass: The sum of all atomic masses in a molecule.
Mole (mol): molecules (Avogadro's number).
Molarity (M): Number of moles of solute per liter of solution.
Example: Preparing a 1 M NaCl solution involves dissolving 1 mole of NaCl in 1 liter of water.

Acidic and Basic Conditions Affect Living Organisms

Acids, Bases, and pH Scale

The balance of hydrogen ions (H+) and hydroxide ions (OH-) in water affects cellular processes and organismal health.

Acid: Substance that increases H+ concentration in solution.
Base: Substance that reduces H+ concentration, often by donating OH- or accepting H+.
pH Scale: Measures H+ concentration;
Neutral Solution: M, so pH = 7.
Acidic Solution: pH < 7; Basic Solution: pH > 7.
Buffer: Substance that minimizes changes in H+ and OH- concentrations.
Example: Human blood is buffered to maintain a pH close to 7.4.

Ocean Acidification: A Threat to Marine Life

Impact of Human Activities on Water Chemistry

Burning fossil fuels increases atmospheric CO2, which dissolves in oceans, forming carbonic acid and lowering pH. This process, called ocean acidification, threatens marine organisms that rely on carbonate ions for shell and reef formation.

Carbonic Acid Formation: CO2 + H2O → H2CO3
Reduction of Carbonate Ions: H+ ions combine with carbonate to form bicarbonate, reducing availability for calcification.
Example: Coral reefs are at risk due to decreased carbonate ion concentration.

Table: Comparison of Acidic, Neutral, and Basic Solutions

Type of Solution	[H+]	[OH-]	pH Value	Characteristic
Acidic	> 10-7 M	< 10-7 M	< 7	Donates H+ in solution
Neutral	10-7 M	10-7 M	7	Equal H+ and OH-
Basic	< 10-7 M	> 10-7 M	> 7	Donates OH- or accepts H+

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