Properties of Water, Chemical Bonds, and pH in Biology

Properties of Water and Chemical Foundations of LifeProperties of Water and Chemical Foundations of Life

Objectives

• Understand the five properties of water essential for life.

• Explain the relationship between kinetic energy, heat, and temperature.

• Define pH and calculate the pH of a solution.

• Describe the importance of carbon in biological molecules.

• Understand isomers and their biological significance.

• Recognize various functional groups in organic chemistry.

Review of Chemical Bonds and Molecules

Covalent Bonds and Molecular Structure

Atoms combine to form molecules through chemical bonds. The most important type of bond in biological molecules is the covalent bond, which involves the sharing of electron pairs between atoms.

• Covalent bonds can result in polar or non-polar molecules depending on the electronegativity difference between the atoms.

• Polar molecules have regions of partial positive and negative charge (e.g., H2O), while non-polar molecules have an even distribution of charge (e.g., O2, CH4).

Example: Water (H2O) is a polar molecule due to the difference in electronegativity between hydrogen and oxygen.

Water: Structure and Unique Properties

Structure of Water

A single water molecule consists of two hydrogen atoms covalently bonded to one oxygen atom, forming a bent or "V" shape. This structure imparts unique properties to water.

• Chemical equation: $2H_2 + O_2 \rightarrow 2H_2O$

• Water molecules are polar, with a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms.

Properties of Water Due to Polarity

The polarity of water molecules leads to several emergent properties essential for life:

• Hydrogen bonding: Water molecules form hydrogen bonds with each other and with other polar substances.

• Cohesion: Water molecules stick together, aiding in processes like water transport in plants.

• Temperature moderation: Water absorbs and releases heat slowly, stabilizing temperatures on Earth.

• Ice floats: Solid water (ice) is less dense than liquid water, preventing bodies of water from freezing solid.

• Solvent of life: Water dissolves a wide variety of substances, facilitating biochemical reactions.

Hydrogen Bonding in Water

The "V" shape and polarity of water molecules result in hydrogen bond formation, which organizes water into a higher level of structural organization. Each water molecule can form up to four hydrogen bonds, leading to unique physical properties.

• Hydrogen bonds are weak individually but strong collectively, giving water high cohesion and surface tension.

• Hydrogen bonding is responsible for water's high specific heat and heat of vaporization.

Biological Importance of Water's Properties

Cohesion, Adhesion, and Surface Tension

• Cohesion: The attraction between water molecules due to hydrogen bonding. This allows for the transport of water against gravity in plants (e.g., capillary action).

• Adhesion: The clinging of water molecules to other substances, aiding in processes like water transport in plant vessels.

• Surface tension: The measure of how difficult it is to stretch or break the surface of a liquid. Water has a high surface tension, allowing small organisms (e.g., water striders) to walk on its surface.

Temperature Moderation

• Kinetic energy: The energy of motion; all atoms and molecules possess kinetic energy.

• Heat: The total kinetic energy of all molecules in a system.

• Temperature: The average kinetic energy of molecules, measured in degrees Celsius (°C).

• Specific heat: The amount of heat required to raise the temperature of 1 gram of a substance by 1°C. Water has a high specific heat, allowing it to buffer temperature changes.

• Heat of vaporization: The amount of heat required to convert 1 gram of a liquid to gas. Water's high heat of vaporization enables evaporative cooling (e.g., sweating).

Density and Ice Formation

• Density: Defined as mass per unit volume ($\text{Density} = \frac{\text{mass}}{\text{volume}}$).

• Water is most dense at 4°C. Below this temperature, water expands as it cools, and ice forms a crystalline lattice that is less dense than liquid water.

• Because ice floats, aquatic life can survive under the ice during winter.

Water as the Solvent of Life

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

• Solvent: The dissolving agent (e.g., water).

• Solute: The substance being dissolved.

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

• Molarity (M): The number of moles of solute per liter of solution.

• Molecular mass: The sum of the masses of all atoms in a molecule, measured in Daltons (Da).

• Avogadro's number: $6.02 \times 10^{23}$ molecules per mole.

Example: Calculating the molecular mass of glucose ($C_6H_{12}O_6$): $(6 \times 12) + (12 \times 1) + (6 \times 16) = 180$ Daltons

Hydrophilic vs. Hydrophobic Substances

• Hydrophilic: "Water-loving"; substances that are polar or ionic and dissolve easily in water.

• Hydrophobic: "Water-fearing"; substances that are non-polar and do not dissolve in water.

Acids, Bases, and pH

Dissociation of Water

• Water can dissociate into hydrogen ions ($H^+$) and hydroxide ions ($OH^-$): $H_2O \rightleftharpoons H^+ + OH^-$

• In pure water at 25°C: $[H^+] = 1 \times 10^{-7} \text{M}$ and $[OH^-] = 1 \times 10^{-7} \text{M}$

Acids and Bases

• Acid: A substance that increases the concentration of $H^+$ ions in solution (e.g., $HCl \rightarrow H^+ + Cl^-$).

• Base: A substance that decreases the concentration of $H^+$ ions, either by accepting $H^+$ directly (e.g., $NH_3 + H^+ \rightarrow NH_4^+$) or by producing $OH^-$ ions (e.g., $NaOH \rightarrow Na^+ + OH^-$).

pH Scale

• pH: Defined as the negative logarithm of the hydrogen ion concentration: $pH = -\log [H^+]$

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

• Example: If $[H^+] = 1 \times 10^{-7} \text{M}$, then $pH = -\log(1 \times 10^{-7}) = 7$.

Buffers

• Buffer: A substance that minimizes changes in pH by accepting or donating $H^+$ ions as needed.

Ocean Acidification

• Increased atmospheric $CO_2$ dissolves in seawater, forming carbonic acid and lowering ocean pH.

• Lower pH reduces carbonate ion ($CO_3^{2-}$) availability, affecting marine organisms that rely on calcification (e.g., corals).

Summary Table: Properties of Water

Property	Description	Biological Importance
Cohesion	Attraction between water molecules via hydrogen bonds	Enables water transport in plants
Adhesion	Attraction between water and other substances	Assists capillary action
High Specific Heat	Resists temperature change	Stabilizes climate and body temperature
Ice Floats	Solid water is less dense than liquid	Insulates aquatic life in winter
Solvent of Life	Dissolves many substances	Facilitates biochemical reactions

Additional info: The notes reference isomers, carbon, and functional groups as objectives, but detailed content on these topics is not present in the provided materials. For completeness, students should review these topics in their textbook or lecture notes.