Chapter 3: Water and Life: Properties, Functions, and Importance in Biology

Ch. 3 - Water & Life

Properties of Water

Water is a unique molecule essential for life, exhibiting several properties that make it vital for biological systems. These properties arise from its molecular structure and ability to form hydrogen bonds.

• Polar Covalently Bonded: Water (H2O) is a polar molecule, meaning it has a partial positive charge on each hydrogen atom and a partial negative charge on the oxygen atom. This polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules.

• Hydrogen Bonding: The attraction between the positive region of one water molecule and the negative region of another leads to hydrogen bonding, which is responsible for many of water's unique properties.

• Attraction to Other Molecules: Water can attract and dissolve other polar molecules (such as sugars) and ions, making it an excellent solvent.

Temperature Stabilization

Water plays a crucial role in moderating temperature changes in organisms and their environments due to its high specific heat and ability to form and break hydrogen bonds rapidly.

• Hydrogen Bonds and Temperature: Hydrogen bonds can be created and disrupted quickly, allowing water to absorb or release heat with minimal temperature change.

• Biological Importance: This property helps maintain stable temperatures within living organisms and ecosystems, supporting life.

Temperature and Heat

Definitions and Units

Understanding heat and temperature is essential for studying water's role in biology.

• Calorie (cal): The amount of heat required to raise the temperature of 1 gram of water by 1°C. It is also the amount of heat released when 1 gram of water cools by 1°C.

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

Water's High Specific Heat

Specific heat is the amount of heat that must be absorbed or lost for 1 gram of a substance to change its temperature by 1°C. Water has a high specific heat compared to most other substances.

• Specific Heat of Water: $\text{Specific heat of water} = 1 \text{ cal/(g °C)}$

• Resistance to Temperature Change: Water resists changing its temperature because of its high specific heat, which is due to hydrogen bonding.

• Moderation of Climate: Large bodies of water can moderate air temperature, leading to milder climates in coastal areas.

Evaporative Cooling

Heat of Vaporization and Cooling Effect

Evaporative cooling is a process where the surface of a liquid becomes cooler during evaporation, as molecules with the highest energy leave the liquid phase.

• Heat of Vaporization: The amount of heat a liquid must absorb for 1 gram to be converted from liquid to gas.

• Evaporative Cooling: As water evaporates, the remaining surface cools, helping to stabilize temperatures in organisms (e.g., sweating in humans, transpiration in plants) and bodies of water.

Floating of Ice on Liquid Water

Density and Structure

Water is less dense as a solid (ice) than as a liquid, which is unusual among substances and has important ecological consequences.

• Crystalline Lattice: At 0°C, water molecules form a crystalline lattice held together by hydrogen bonds, keeping the molecules farther apart than in liquid water.

• Density: Ice is less dense than liquid water, which is why it floats. Water reaches its greatest density at 4°C.

• Ecological Importance: Floating ice insulates the water below, allowing aquatic life to survive in cold climates.

Water: The Solvent of Life

Solutions and Solubility

Water's polarity makes it an excellent solvent, capable of dissolving a wide variety of substances necessary for life.

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

• Solvent: The dissolving agent of a solution (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 a sphere of water molecules called a hydration shell.

• Versatility: Water can dissolve ionic compounds, polar molecules, and even large polar molecules like proteins.

Hydrophilic and Hydrophobic Substances

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

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

Buffers, pH, Acids, and Bases

pH and Its Measurement

The pH of a solution measures its acidity or alkalinity, which is crucial for maintaining proper cellular function.

• pH Scale: Ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

• Definition: $\text{pH} = -\log [\text{H}^+]$

• Neutral Solution: In pure water at 25°C, $[\text{H}^+] = 10^{-7}$ M, so pH = 7.

• Litmus Paper: A common tool for testing pH in laboratories and pools.

Acids and Bases

• Acids: Substances that donate hydrogen ions (H+), lowering the pH of a solution. Strong acids dissociate completely in water (e.g., hydrochloric acid).

• Bases: Substances that donate hydroxide ions (OH-), raising the pH of a solution. Strong bases dissociate completely in water (e.g., sodium hydroxide).

Buffer Systems

Buffers help maintain stable pH in biological systems by absorbing excess H+ or OH-.

• Optimal pH Range: Most cellular functions occur between pH 7.2 and 7.6. Deviations can cause organ failure or death.

• Example Buffer System: The carbonic acid (H2CO3) and bicarbonate ion (HCO3-) system in blood.

• Reactions:

$\mathrm{H_2CO_3} \rightleftharpoons \mathrm{H^+} + \mathrm{HCO_3^-}$

• If excess H+ is present, bicarbonate binds with H+ to form carbonic acid.

• If excess OH- is present, carbonic acid dissociates to release H+, which combines with OH- to form water.

Acidification: A Threat to Our Oceans

Ocean Acidification and Its Effects

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

• CO2 Dissolution: CO2 dissolves in seawater, forming carbonic acid, which dissociates to release H+ ions.

• Impact on Marine Life: Increased H+ ions combine with carbonate ions (CO32-), reducing the availability of carbonate needed for calcification (formation of calcium carbonate, CaCO3) by marine organisms such as corals and shellfish.

• Key Reactions:

$\mathrm{CO_2} + \mathrm{H_2O} \rightarrow \mathrm{H_2CO_3}$ $\mathrm{H_2CO_3} \rightarrow \mathrm{H^+} + \mathrm{HCO_3^-}$ $\mathrm{H^+} + \mathrm{CO_3^{2-}} \rightarrow \mathrm{HCO_3^-}$ $\mathrm{Ca^{2+}} + \mathrm{CO_3^{2-}} \rightarrow \mathrm{CaCO_3}$

• Ecological Consequences: Ocean acidification threatens coral reefs and marine ecosystems by impairing the ability of organisms to build shells and skeletons.

Summary Table: Water's Properties and Biological Importance

Property	Description	Biological Importance
Polarity	Unequal sharing of electrons creates partial charges	Allows formation of hydrogen bonds; makes water a good solvent
High Specific Heat	Requires large amount of energy to change temperature	Stabilizes climate and internal body temperature
High Heat of Vaporization	Requires large amount of energy to evaporate	Enables evaporative cooling (e.g., sweating)
Lower Density as Solid	Ice floats on liquid water	Insulates aquatic environments in cold climates
Versatile Solvent	Dissolves many substances	Facilitates chemical reactions in cells