Water and the Fitness of the Environment

Water: The Molecule That Supports All of Life Water is the biological medium here on Earth All living organisms require water more than any other substance

Three-quarters of the Earth’s surface is submerged in water The abundance of water is the main reason the Earth is habitable Figure 3.1

Water is a compound in which: The electrons in its covalent bonds are shared unequally. Unequality is due to electronegativity of oxygen This causes water to be a polar molecule Polarity means water molecules carry opposite charges on opposite ends.

The polarity of water results in: Weak electrical attractions between neighboring water molecules. These interactions are called hydrogen bonds.

Figure 2.10

Life-Supporting Properties of Water Most of water’s life-supporting properties are explained by: The polarity of water molecules, and The resulting hydrogen bonding

These properties include: Water Properties These properties include: Water’s cohesive nature Water’s ability to moderate temperature Freezing of water into floating ice Versatility of water as a solvent

Water molecules stick together as a result of hydrogen bonding. The Cohesion of Water Water molecules stick together as a result of hydrogen bonding. This is called cohesion. Cohesion is vital for water transport in plants. Adhesion is the sticking of water to the cell wall It resists the downward pulling of gravity

Figure 2.12

Hydrogen bonds give water: Surface tension is: The measure of how difficult to stretch or break the surface of a liquid. Hydrogen bonds give water: An unusually high surface tension.

Figure 2.13

Heat and Temperature The Celsius scale: A calorie (cal): A measure of temperature using Celsius degrees (°C) A calorie (cal): The amount of heat required to raise the temperature of 1 g of water by 1°C kilocalories (kcal): 1 kcal = 1,000 calories The “calories” on food packages are Kcal The joule (J): Is another unit of energy 1 J = 0.239 cal, or 1 cal = 4.184 J Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Temperature (number): Heat and Temperature Heat and temperature: Are related, but two different concepts. Heat (Energy): A measure of the total amount of kinetic energy due to molecular motion in a body of matter Temperature (number): A measure of the intensity of heat due to the average kinetic energy of molecules

Water’s High Specific Heat The specific heat of a substance: Is the amount of heat that must be absorbed or lost for 1 gram of that substance to change its temperature by 1ºC The specific heat of water is: 1 cal/g/ºC

Water’s high specific heat is due to hydrogen bonding: Heat is absorbed when hydrogen bonds break Heat is released when hydrogen bonds form. The high specific heat of water allows it to minimize temperature fluctuations to within limits that permit life

Heat of vaporization Is the quantity of heat a liquid must absorb for 1 gram of it to be converted from a liquid to a gas

Water Moderation to Temperature Water can absorb and store large amounts of heat while only changing a few degrees in temperature. Water has a strong resistance to temperature change because of hydrogen bonding Earth’s giant water supply causes temperatures to stay within limits that permit life. Evaporative cooling removes heat from the Earth and from organisms.

The Biological Significance of Ice Floating When water molecules get cold, they move apart, forming ice. A chunk of ice has fewer molecules than an equal volume of liquid water. Hydrogen bonds more stable in ice than in water The density of ice is lower than liquid water. Ice floats preventing ponds, lakes, and even oceans from freezing solid. Marine life could not survive if bodies of water froze solid.

Figure 2.15

Water as the Solvent of Life A solution is a liquid consisting of two or more substances evenly mixed. The dissolving agent is called the solvent. The dissolved substance is called the solute. When water is the solvent, the result is an aqueous solution. Example: NaCl in water

Figure 2.16

Water as the Solvent of Life Compound that dissolve in water are: Ionic compounds e.g. NaCl Nonionic polar compounds e.g. Sugars Many different polar compounds and ions are dissolved in the water of biological fluids such as: Blood Sap of plants Liquid within cells (intracellular fluid) Liquid between cells (extracellular fluid)

Affinity to Water: Hydrophilic Substances (ionic or polar): Have affinity to water Hydrophobic substances (nonionic & nonpolar): Rebel water e.g oils Colloid: A stable suspension of fine particles in a liquid Hydrophilic but doesn’t dissolve in water

Solute Concentration in Aqueous Solutions Is the number of solute molecules in a volume of solution Molecular mass: The sum of the masses of all atom in a molecule For sucrose (C12H22O11) is 342 daltons Mass of C is 12; (12 x 12 = 144) Mass of H is 1; (22 x 1 = 22) Mass of O is 16; (11 x 16 = 176)

Solute Concentration continued Weighing small number of molecules is impractical Substances are usually measured in “mole” units A mole represents exactly 6.02 x 1023 molecules That number is also called Avogadro’s number 1 mole of a substance has exactly the same number of molecules as a mole of any other substance Example (ethanol vs sucrose ) Molarity is the number of moles of solute per liter of solution

Water dissociates into hydrogen and hydroxide ions: Water Dissociation Water dissociates into hydrogen and hydroxide ions: H2O H+ + OH- Dissociation is reversible Changes in the concentration of these ions can have a great affect on living organisms At equilibrium water molecules concentration greatly exceeds ion concentration In pure water, concentration of each H+ & OH- is equal (10-7 M at 25 oC)

To describe the acidity of a solution, we use: Acids, Bases, and pH Acid A chemical compound that donates H+ ions to solutions; e.g. HCl H+ + Cl- Base A substance that accepts H+ ions and removes them from solution, either Directly e.g. NH3, or Indirectly e.g. NaOH Na+ + OH- To describe the acidity of a solution, we use: The pH scale.

The pH Scale The pH of a solution Is determined by the relative concentration of hydrogen ions (–ve log of H+ concentration) Is low in an acidic solutions (high H+ concentration) Is high in a basic solutions (low H+ concentration) At a value of 7 the solution is neutral (equal H+ & OH- concentrations)

Figure 2.17

pH and Living Cells How this is achieved? Cellular processes are very sensitive to: Slight changes in H+ and OH- concentration s The internal pH of most living cells must: Remain close to pH 7 How this is achieved?

Buffers Buffers are substances that resist pH change. They accept H+ ions when in excess. They donate H+ ions when depleted. Buffering is not foolproof. Example: acid precipitation

The Threat of Acid Precipitation Can damage life in lakes and streams On land, washes away certain minerals (Mg & Ca ions) that: Help buffer soil solution Are essential nutrients to plant growth Increases solubility of toxic minerals (Aluminum) in the soil Adversely damages forests through changing soil chemistry

The Threat of Acid Precipitation The pH of clean uncontaminated rain water is 5.6 Acid precipitation refers to: Rain, snow, or fog with a pH lower than pH 5.6 Is caused primarily by the mixing of different pollutants with water in the air

Can damage life in Earth’s ecosystems Acid precipitation Can damage life in Earth’s ecosystems 1 2 3 4 5 6 7 8 9 10 11 12 13 14 More acidic Acid rain Normal rain More basic Figure 3.9