Water: Properties and Importance in Biology

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Water: Properties and Importance in Biology

Introduction to Water in Biology

Water is a fundamental molecule for life on Earth, covering approximately 75% of the planet's surface. It is the only common substance that naturally exists in all three physical states—solid, liquid, and gas—under Earth's conditions. Life is believed to have originated in water, and all living cells are primarily composed of water, making it essential for biological processes.

Water covers 75% of Earth's surface
Life began in water
Cells are mostly water
Exists as solid (ice), liquid (water), and gas (vapor)

Structure and Polarity of Water Molecules

Water molecules (H2O) are polar, meaning they have partial positive and negative charges due to the unequal sharing of electrons between hydrogen and oxygen atoms. This polarity allows water molecules to form hydrogen bonds with each other, leading to many of water's unique properties.

Polarity: Oxygen is more electronegative than hydrogen, creating a partial negative charge near the oxygen and partial positive charges near the hydrogens.
Hydrogen Bonds: Weak attractions between the partial positive hydrogen of one water molecule and the partial negative oxygen of another.
Dynamic Bonding: At any moment, many water molecules are hydrogen-bonded to their neighbors, but these bonds are constantly forming and breaking.

Four Emergent Properties of Water

Water exhibits four key emergent properties that are critical for supporting life:

Cohesion and Adhesion
Moderation of Temperature
Density: Floating of Ice
Water as a Solvent

Emergent Property 1: Cohesion and Adhesion

Cohesion refers to the attraction between water molecules due to hydrogen bonding, while adhesion is the attraction between water molecules and other substances. These properties are essential for the movement of water in plants and the transport of nutrients.

Cohesion: Water molecules stick to each other, creating surface tension and allowing for the upward movement of water in plant vessels.
Adhesion: Water molecules stick to other surfaces, such as the walls of plant cells, aiding in water transport against gravity.
Example: As water evaporates from leaves (transpiration), cohesion pulls more water upward from the roots, while adhesion helps water cling to cell walls.

Additional info: Surface tension, a result of cohesion, allows small insects to walk on water and droplets to form.

Emergent Property 2: Moderation of Temperature

Water moderates Earth's temperature by absorbing and releasing heat with minimal temperature change, due to its high specific heat capacity. This property helps stabilize environmental and organismal temperatures.

Specific Heat: The amount of heat required to raise the temperature of 1 gram of water by 1°C is higher than most substances.
Hydrogen Bonding: Heat must be absorbed to break hydrogen bonds, so water heats up and cools down slowly.
Example: Coastal areas have milder climates due to the heat-absorbing capacity of oceans.

Equation:

where = heat absorbed or released, = mass, = specific heat, = temperature change.

Emergent Property 3: Density and Floating of Ice

Unlike most substances, water is less dense as a solid than as a liquid. This is due to the hydrogen bonds forming a crystalline structure in ice, keeping molecules further apart.

Ice Floats: Solid water (ice) is less dense than liquid water, so it floats, insulating aquatic life in winter.
Hydrogen Bonds: At temperatures below 4°C, water molecules move too slowly to break hydrogen bonds, resulting in a stable lattice.
Example: Lakes and ponds freeze from the top down, allowing organisms to survive beneath the ice.

Emergent Property 4: Water as a Solvent

Water is known as the "universal solvent" because its polarity allows it to 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 being dissolved (e.g., salt, sugar).
Example: Table salt (NaCl) dissolves in water as water molecules surround and separate the sodium and chloride ions.

Additional info: Hydrophilic substances have an affinity for water, while hydrophobic substances repel water.