Chapter 3: Water and Life

Concept 3.1: Polar Covalent Bonds in Water Molecules Result in Hydrogen Bonding

  • Polar Covalent Bond: A covalent bond between atoms that differ in electronegativity. Shared electrons are pulled closer to the more electronegative atom (oxygen), making it slightly negative (δ−\delta-) and the other atom (hydrogen) slightly positive (δ+\delta+).

  • Polar Molecule: A molecule with an uneven distribution of charges in different regions. Water is a polar molecule.

  • Hydrogen Bond: A weak chemical bond formed when the slightly positive hydrogen atom of a polar covalent bond in one molecule is attracted to the slightly negative atom in another molecule.

  • Bond Capacity: A single water molecule can form up to four hydrogen bonds with surrounding molecules.

  • Binding Energies Comparison:

    • Covalent bond energy: ≈80 kcal/mol\approx 80\text{ kcal/mol}


    • Hydrogen bond energy: ≈5.0 kcal/mol\approx 5.0\text{ kcal/mol}


    • Van der Waals energy: <1 kcal/mol< 1\text{ kcal/mol}


Concept 3.2: Four Emergent Properties of Water Contribute to Earth's Suitability for Life

  • Cohesive Behavior

    • Cohesion: The linking together of like molecules, often by hydrogen bonds.

    • Adhesion: The clinging of one substance to another, such as water clinging to plant cell walls by means of hydrogen bonds.

    • Biological Role: Cohesion and adhesion work together to transport water against gravity from roots to leaves in plants.

    • Surface Tension: A measure of how difficult it is to stretch or break the surface of a liquid. Water has high surface tension due to the collective strength of surface hydrogen bonds.

  • Ability to Moderate Temperature

    • Heat: The total amount of kinetic energy due to the random motion of atoms or molecules in a body of matter (thermal energy). Energy in its most random form

    • Kinetic Energy: The energy associated with the relative motion of objects. Moving matter can perform work by imparting motion to other matter.

    • Temperature: A measure of the intensity of heat, reflecting the average kinetic energy of the molecules.

      • Units & Constants Based on Water:

        • Celsius Scale: A temperature scale (°C) equal to 5/9(°F - 32) that measures the freezing point of water at 0°C and the boiling point of water at 100°C.

        • Calorie (cal): The amount of heat energy required to raise the temperature of 1 g of water by 1°C; also the amount of heat energy that 1 g of water releases when it cools by 1°C. The Calorie (with a capital C), usually used to indicate the energy content of food, is a kilocalorie.

        • Kilocalorie (kcal / Calorie): A thousand calories; the amount of heat energy required to raise the temperature of 1 kg of water by 1°C.

        • Joule (J): A unit of energy: 1 J = 0.239 cal; 1 cal = 4.184 J.

        • Specific Heat: The amount of heat that must be absorbed or lost for 1 g of a substance to change its temperature by 1°C.

        • Heat of Vaporization: The quantity of heat a liquid must absorb for 1 g of it to be converted from the liquid to the gaseous state.

  • Expansion Upon Freezing

    • Ice floats because hydrogen bonds lock water molecules into a crystalline lattice at freezing temperatures, making ice less dense than liquid water.

  • Versatility as a Solvent

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

    • Solvent: The dissolving agent of a solution. Water is the most versatile solvent known.

  • Hydration Shell: The sphere of water molecules surrounding a dissolved ion.

  • Key Quantitative Concepts:

    • Molecular Mass: The sum of the masses of all atoms in a molecule.

    • Mole (mol): An amount equal to a substance's molecular weight in grams, containing Avogadro's number (6.02×10236.02 \times 10^{23}) of molecules.

    • Molarity (M): Solute concentration measured as the number of moles of solute per liter of solution (moles/L\text{moles/L}).


Concept 3.3: Acidic and Basic Conditions Affect Living Organisms

  • Why do we need to understand pH to understand biology?

    • It’s directly used for:

      • Energy processing, e.g. in metabolism and photosynthesis

      • Digestion

      • Regulating a variety of chemical reactions

      • Controlling charge and organization of biological molecules

    • It’s a standard example of concentration, and concentrations drive all chemical reactions.

    • It’s a standard example of an exponential scale, and understanding these are crucial to understanding reproduction, length scales, and any scientific measurement that spans multiple powers of 10.

  • Dissociation of Water: Water molecules dissociate into a hydroxide ion (OH−\text{OH}^-) and a hydronium ion (H3O+\text{H}_3\text{O}^+, commonly represented as H+\text{H}^+).

  • pH Definition: A measure of hydrogen ion concentration, defined mathematically as:

    pH=−log⁡[H+]or[H+]=10−pH M\text{pH} = -\log[\text{H}^+] \quad \text{or} \quad [\text{H}^+] = 10^{-\text{pH}}\text{ M}


  • pH Scale (0 to 14):

    • Acidic: pH<7\text{pH} < 7 where [H+]>[OH−][\text{H}^+] > [\text{OH}^-].

    • Neutral: pH=7\text{pH} = 7 where [H+]=[OH−][\text{H}^+] = [\text{OH}^-].

    • Basic (Alkaline): pH>7\text{pH} > 7 where [H+]<[OH−][\text{H}^+] < [\text{OH}^-].

  • Biological Importance of pH:

    • Used directly in energy processing (metabolism and photosynthesis).

    • Essential for digestion and regulating chemical reactions.

    • Controls charges and overall organization of biological molecules.

  • Buffers: Solutions containing a weak acid and its conjugate base that minimize changes in pH when acids or bases are added.

  • Ocean Acidification: Decreasing pH of ocean waters caused by the absorption of excess atmospheric CO2\text{CO}_2, which reacts with water to form carbonic acid (H2CO3\text{H}_2\text{CO}_3).