Water and Life

The Molecule That Supports All of Life

Water is fundamental to life on Earth, serving as the medium in which most biological processes occur. Its unique chemical and physical properties make it essential for the existence and maintenance of living organisms.

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

• Emergent properties of water make Earth suitable for life, including its ability to moderate temperature, dissolve a wide range of substances, and support biological structures.

• The structure of the water molecule allows it to interact with other molecules through hydrogen bonding.

Structure and Bonding in Water

Polar Covalent Bonds and Hydrogen Bonding

The chemical structure of water gives rise to its remarkable properties. Each water molecule (H2O) consists of two hydrogen atoms covalently bonded to one oxygen atom.

• Polar covalent bonds: The electrons in the O–H bonds are shared unequally, spending more time near the oxygen atom, which is more electronegative. This creates a partial negative charge (δ-) on the oxygen and a partial positive charge (δ+) on the hydrogens.

• Polarity: The unequal sharing of electrons makes water a polar molecule, with an uneven distribution of charge.

• Hydrogen bonds: The polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules. These are weak bonds individually, but collectively they have significant effects on water's properties.

Emergent Properties of Water

Overview of Four Key Properties

Water's structure leads to four emergent properties that are crucial for life:

1. Cohesive behavior

2. High specific heat

3. Expansion upon freezing

4. Versatility as a solvent

1. Cohesion of Water Molecules

Cohesion refers to the attraction between molecules of the same substance. In water, this is primarily due to hydrogen bonding.

• Cohesion: Hydrogen bonds hold water molecules together, allowing for the transport of water against gravity in plants (e.g., from roots to leaves).

• Adhesion: Attraction between water molecules and other substances, such as plant cell walls, which also aids in water transport.

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

• Example: Water droplets forming beads on a leaf due to cohesion and surface tension.

2. Moderation of Temperature by Water

Water helps stabilize 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 associated with the random movement of atoms or molecules.

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

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

• Example: Large bodies of water, like oceans, moderate coastal climates by absorbing heat during the day and releasing it at night.

3. Water's High Specific Heat

Specific heat is the amount of heat required to change the temperature of 1 gram of a substance by 1°C. Water has a high specific heat, which allows it to resist temperature changes.

• Definition: The specific heat of water is 1 cal/(g·°C).

• Mechanism: Much of the heat absorbed by water is used to break hydrogen bonds; conversely, heat is released when hydrogen bonds form.

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

• Food Calories: The "Calories" listed on food packages are actually kilocalories (kcal), where 1 kcal = 1,000 cal.

• Example: Water's high specific heat helps maintain stable temperatures in organisms and environments.

Formula:

4. Expansion Upon Freezing

Unlike most substances, water expands as it freezes, making ice less dense than liquid water.

• Hydrogen bonds in ice are more "ordered," causing molecules to be spaced further apart.

• Ice floats on liquid water, insulating the water below and providing a habitat for certain organisms.

• Example: If ice sank, lakes and oceans would freeze solid from the bottom up, making life in water impossible.

5. Versatility as a Solvent

Water is known as the "universal solvent" because it can dissolve a wide variety of substances, especially ionic and polar compounds.

• 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.

• Hydration shell: When an ionic compound dissolves in water, each ion is surrounded by water molecules.

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

• Example: Table salt (NaCl) dissolving in water as Na+ and Cl- ions become surrounded by hydration shells.

Hydrophilic and Hydrophobic Substances

Substances can be classified based on their affinity for water.

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

• Hydrophobic: Substances that do not have an affinity for water, often because they are nonpolar (e.g., oils, fats).

• Hydrophobic molecules are major components of cell membranes, contributing to the formation of biological barriers.

Acids, Bases, and pH

Acidic and Basic Conditions Affect Living Organisms

Water can dissociate into hydrogen ions (H+) and hydroxide ions (OH-), and the balance of these ions is critical for biological systems.

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

• Base: A substance that reduces the H+ concentration of a solution.

• Strong acids and bases: Dissociate completely in water.

• Weak acids and bases: Reversibly release and accept hydrogen ions, helping to buffer changes in pH.

The pH Scale

The pH scale measures the concentration of hydrogen ions in a solution, ranging from 0 (most acidic) to 14 (most basic).

• pH is defined as:

• Neutral solution: pH = 7 (e.g., pure water)

• Acidic solution: pH < 7 (e.g., gastric juice, vinegar)

• Basic solution: pH > 7 (e.g., seawater, household bleach)

Buffers

Buffers are substances that minimize changes in the concentrations of H+ and OH- in a solution, helping to maintain a stable pH in biological systems.

• Most buffers consist of a weak acid and its corresponding base, which combine reversibly with H+ ions.

• The internal pH of most living cells is close to 7, and buffers are essential for maintaining this stability.

Acidification: A Threat to Our Oceans

Human activities, such as 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 (H2CO3), which lowers ocean pH.

• This process disrupts the chemical balance of marine environments and can lead to coral bleaching and harm to marine life.

Relevant chemical equation:

Additional info: Ocean acidification reduces the availability of carbonate ions (CO32-), which are necessary for marine organisms to build shells and skeletons.