BackWater and Life: Properties, Structure, and Biological Importance
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Chapter 3: Water and Life
The Molecule That Supports All of Life
Water is essential for all known forms of life and is unique in its ability to exist in all three physical states—solid, liquid, and gas—under natural Earth conditions. Its molecular structure and interactions give rise to properties that are critical for sustaining life.
Concept 3.1: Polar Covalent Bonds in Water Molecules Result in Hydrogen Bonding
The water molecule (H2O) is polar because oxygen is more electronegative than hydrogen, causing electrons to spend more time near the oxygen atom. This polarity allows water molecules to form hydrogen bonds with each other, which are weak individually but strong collectively.
Polar Covalent Bond: A type of covalent bond where electrons are shared unequally, resulting in partial charges.
Hydrogen Bond: A weak bond between the slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of another.
Concept 3.2: Four Emergent Properties of Water Contribute to Earth’s Suitability for Life
Water’s structure leads to four key properties that support life: cohesion, moderation of temperature, expansion upon freezing, and versatility as a solvent.
Cohesion and Adhesion of Water Molecules
Cohesion refers to the attraction between water molecules due to hydrogen bonding, which helps transport water against gravity in plants. Adhesion is the attraction between water molecules and other substances, such as plant cell walls.
Surface Tension: The measure of how difficult it is to break the surface of a liquid; water has a high surface tension due to hydrogen bonding.
Ability to Moderate Temperature
Water moderates temperature by absorbing heat from warmer air and releasing it to cooler air. It can absorb or release large amounts of heat with only slight changes in its own temperature due to its high specific heat.
Specific Heat: The amount of heat required to change 1 g of a substance by 1°C. For water, it is 1 cal/(g·°C).
Heat of Vaporization: The heat a liquid must absorb for 1 g to be converted to gas.
Evaporative Cooling: As water evaporates, its surface cools, helping organisms and environments regulate temperature.
Expansion Upon Freezing
Water is less dense as a solid than as a liquid because hydrogen bonds in ice are more ordered, creating a crystalline structure. This property allows ice to float, insulating bodies of water and protecting aquatic life.
Impact of Climate Change on Ice
Global warming is reducing the extent of glaciers and sea ice, threatening species that depend on ice for survival and altering ecosystems.
Versatility as a Solvent
Water’s polarity makes it an excellent solvent, capable of dissolving a wide range of substances. When ionic compounds dissolve, water molecules surround each ion, forming hydration shells. Water can also dissolve large polar molecules, such as proteins, if they have ionic and polar regions.
Solution: A homogeneous mixture of two or more substances.
Solvent: The dissolving agent (water in aqueous solutions).
Solute: The substance dissolved.
Hydrophilic and Hydrophobic Substances
Hydrophilic substances have an affinity for water, while hydrophobic substances do not. Hydrophobic molecules, such as oils, are major components of cell membranes.
Solute Concentration in Aqueous Solutions
Chemical reactions in organisms often occur in aqueous solutions. Concentrations are measured in moles (using Avogadro’s number) and molarity (moles per liter).
Possible Evolution of Life on Other Planets
Scientists search for extraterrestrial life by looking for water on other planets. Mars, for example, has evidence of water in the form of dark streaks on its surface.
Concept 3.3: Acidic and Basic Conditions Affect Living Organisms
Water molecules can dissociate into hydronium ions (H3O+) and hydroxide ions (OH-). The balance of these ions is crucial for cellular processes.
Acid: Increases the concentration of H+ in solution.
Base: Reduces the concentration of H+, often by increasing OH-.
The pH Scale
The pH scale measures the concentration of hydrogen ions in a solution. It is defined as the negative logarithm of the H+ concentration:
Neutral solutions have a pH of 7.
Acidic solutions have pH < 7.
Basic solutions have pH > 7.
Buffers
Buffers are substances that minimize changes in pH by accepting or donating hydrogen ions. Most buffers consist of a weak acid and its corresponding base, maintaining the internal pH of cells near neutrality.
Acidification: A Threat to Our Oceans
Human activities, such as burning fossil fuels, increase atmospheric CO2, which dissolves in oceans and forms carbonic acid. This process, called ocean acidification, reduces carbonate ion concentration, threatening organisms that rely on calcium carbonate for their skeletons and shells.
Example: Coral reefs are particularly vulnerable to ocean acidification, as reduced carbonate availability impairs their ability to build skeletons.
Additional info: The chemical equations for ocean acidification are:
