Chapter 3 Water and Life: Properties, Functions, and Biological Importance

Water and Life

The Molecule That Supports All of Life

Water is essential for all known forms of life. Its unique chemical and physical properties make Earth habitable and support biological processes.

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

• Its molecular structure and ability to form hydrogen bonds give rise to emergent properties critical for life.

• Water is a polar molecule with a bent shape, allowing it to interact with other molecules via hydrogen bonding.

Structure and Bonding in Water

• Polar covalent bonds hold the hydrogen and oxygen atoms together within a water molecule, resulting in a partial negative charge near the oxygen and partial positive charges near the hydrogens.

• Hydrogen bonds form between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of another, leading to water's unique properties.

Example: The partial charges allow water molecules to form networks of hydrogen bonds, which are responsible for cohesion, adhesion, and high surface tension.

Emergent Properties of Water

Four Key Properties

Water's emergent properties that facilitate life on Earth include:

• Cohesive behavior

• Ability to moderate temperature

• Expansion upon freezing

• Versatility as a solvent

Cohesion and Adhesion

• Cohesion: The attraction between water molecules due to hydrogen bonding. This property helps water move upward against gravity in plants (transpiration).

• Adhesion: The attraction between water molecules and other substances, such as plant cell walls, aiding water transport in biological systems.

• Surface tension: A measure of how difficult it is to break the surface of a liquid. Water's high surface tension allows small organisms to walk on its surface.

Example: Water moves from roots to leaves in plants due to cohesion and adhesion, enabling nutrient transport.

Moderation of Temperature by Water

Water stabilizes temperatures in organisms and environments by absorbing and releasing heat with minimal temperature change.

• Kinetic energy: The energy of motion.

• Thermal energy: The total kinetic energy of molecules in a body of matter.

• Temperature: The average kinetic energy of molecules.

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

Specific heat is the amount of heat required to change the temperature of 1 g of a substance by 1°C. Water has a high specific heat due to hydrogen bonding, which minimizes temperature fluctuations and helps maintain stable environments for life.

Example: The evaporation of sweat cools the body as water molecules with the highest kinetic energy leave as vapor.

Expansion Upon Freezing

Unlike most substances, water expands as it freezes due to the formation of stable hydrogen bonds, making ice less dense than liquid water.

• Ice floats on liquid water, insulating aquatic life in cold climates.

• Water reaches its maximum density at 4°C.

Example: If ice sank, lakes and oceans would freeze solid from the bottom up, threatening aquatic ecosystems.

Water: The Solvent of Life

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

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

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

• Solute: The substance that is dissolved.

• Aqueous solution: A solution in which water is the solvent.

Example: Table salt (NaCl) dissolves in water as the polar water molecules surround and separate the Na+ and Cl- ions.

Hydrophilic and Hydrophobic Substances

• Hydrophilic substances have an affinity for water (e.g., salts, sugars, proteins).

• Hydrophobic substances do not interact well with water due to nonpolar bonds (e.g., oils, fats).

• Hydrophobic molecules are major components of cell membranes, contributing to their structure and function.

Acids, Bases, and pH

Acids and Bases

• Acid: A substance that increases the H+ concentration of a solution.

• Base: A substance that reduces the H+ concentration, often by accepting H+ or releasing OH-.

• Examples of acids: citric acid, salicylic acid, sulfuric acid ($H_2SO_4 \rightarrow 2H^+ + SO_4^{2-}$), hydrochloric acid ($HCl \rightarrow H^+ + Cl^-$).

• Examples of bases: sodium hydroxide ($NaOH \rightarrow Na^+ + OH^-$), bleach.

pH Scale

• pH measures the concentration of H+ ions in a solution.

• The pH scale ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

• Each pH unit represents a tenfold change in H+ concentration.

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

Equation:

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

pH Value	Solution Type	Examples
0-6	Acidic	Battery acid, lemon juice, gastric juice
7	Neutral	Pure water, human blood
8-14	Basic	Milk of magnesia, bleach, oven cleaner

Acid-Base Chemistry in Water

• A hydrogen atom in a hydrogen bond between two water molecules can shift, forming ions:

$H_2O + H_2O \rightleftharpoons H_3O^+ + OH^-$

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

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

Environmental Impact: Ocean Acidification

Acidification: A Threat to Our Oceans

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

• CO2 dissolves in seawater, forming carbonic acid ($CO_2 + H_2O \rightarrow H_2CO_3$).

• Carbonic acid dissociates, increasing H+ concentration and lowering ocean pH.

• This process threatens marine life, especially organisms that build shells from calcium carbonate.

Example: Ocean acidification can reduce the availability of carbonate ions needed by corals and shellfish to form their skeletons and shells.

Process	Chemical Equation
CO2 dissolves in water	$CO_2 + H_2O \rightarrow H_2CO_3$
Carbonic acid dissociates	$H_2CO_3 \rightarrow H^+ + HCO_3^-$
Bicarbonate dissociates	$HCO_3^- \rightarrow H^+ + CO_3^{2-}$
Calcium carbonate formation	$CO_3^{2-} + Ca^{2+} \rightarrow CaCO_3$

Additional info: Ocean acidification is a growing concern for global biodiversity and food security, as it can disrupt marine ecosystems and fisheries.