CH. 3 Water and the Fitness of the Environment

The Effects of Water’s Polarity The polarity of water molecules results in hydrogen bonding Organisms depend on the cohesion of water molecules Water moderates temperatures on Earth Oceans and lakes don’t freeze solid because ice floats Water is the solvent of life

The polarity of water molecules results in hydrogen bonding The bonds that hold together the atoms in a water molecule are polar covalent bonds, with the oxygen region of the molecule having a partial negative charge and the hydrogens having a partial positive charge. Polar molecule-opposite ends of the molecule have opposite charges. The extraordinary qualities of water are emergent properties resulting from the hydrogen bonding that orders molecules into a higher level of structural organization.

Organisms depend on the cohesion of water molecules Water molecules stick to each other as a result of hydrogen bonding. In liquid form, water is constantly forming, breaking, and reforming, so that at any instant a substantial percentage of molecules are bonded to their neighbor, making water more structured that most liquids. Cohesion-the binding together of like molecules, often by hydrogen bonds Cohesion due to hydrogen bonding contributes to the transport of water against gravity in plants. Adhesion- the clinging of one substance to another Adhesion of water to the walls of the vessels helps counter the downward pull of gravity. Surface Tension- a measure of how difficult it is to stretch or break the surface of a liquid Water has a greater surface tension than most other liquids.

Water moderates temperatures on Earth Heat and Temperature Kinetic energy- the energy of motion Heat- the measure of total quantity of kinetic energy due to molecular motion in a body of matter Temperature- measures the intensity of heat due to the average kinetic energy of the molecules Calorie (cal)- the amount of heat energy it takes to raise the temperature of 1 g of water by 1 degree celsius

Water moderates temperatures on Earth Water’s High Specific Heat The ability of water to stabilize temperature stems from its relatively high specific heat. Specific heat- the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1 degree celsius Because of its high specific heat relative to other substances, water will change its temperature less when it absorbs or loses a given amount of heat. A calorie of heat causes a relatively small change in the temperature of water because much of the heat energy is used to disrupt hydrogen bonds before the water molecules begin moving faster. Because of its high specific heat, the water that covers most of the Earth keeps temperature fluctuations on land and in water within limits that permit life. Because organisms are made primarily of water, they are more able to resist changes in their own temperatures than if they were made of a liquid with a lower specific heat.

Water moderates temperatures on Earth Evaporative Cooling 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 Water’s high heat of vaporization helps moderate Earth’s climate. Evaporative cooling- as a liquid evaporates, the surface of the liquid left behind cools down Provides a mechanism that prevents terrestrial organisms from overheating.

Oceans and lakes don’t freeze solid because ice floats Water is one of the few substances that are less dense as a solid than a liquid. While other materials contract when solidified, water expands. If ice sank, then eventually all ponds, lakes, and even oceans would freeze solid, making life as we know it impossible on Earth.

Water is the solvent of life Solution- a liquid that is a completely homogeneous mixture of two or more substances Solvent- the dissolving agent Solute- the substance being dissolved Aqueous solution- solution where water is the solvent Hydration cell- the sphere of water around each dissolved ion

Water is the solvent of life Hydrophilic and Hydrophobic Substances Hydrophilic- substance that has an affinity for water Hydrophilic is used even if the substance can’t be dissolved– because the molecules are too large, for instance. Cellulose is hydrophilic but has molecules that are too large to be dissolved. Cellulose is also present in the walls of water-conducting vessels in a plant. Hydrophobic- substances that repel water Usually because they are nonpolar or non-ionic Hydrophobic molecules are major ingredients of cell membranes

Water is the solvent of life Solute Concentration in Aqueous Solutions Most of the chemical reactions that occur in organisms involve solutes dissolved in water. Mole (mol)- a practical way to measure molecules; equal in number to the molecular weight of a substance Molecular weight- the sum of the weights of all the atoms in a molecule A mole of one substance has the exact same number of molecules as a mole of any other substance Avogadro’s number- 6.02x10^23 Measuring in moles makes it convenient for scientists working in the laboratory to combine substances in fixed ratios of molecules. Molarity- the number of moles of solute per liter of solution; the unit of concentration most often used by biologists for aqueous solutions.

The Dissociation of Water Molecules Organisms are sensitive to changes in pH –Hydroxide ion (OH-)– the water molecule that has lost a proton; charge of -1 –Hydrogen ion (H+)– a single proton with a charge of +1 Acid precipitation threatens the fitness of the environment

Organisms are sensitive to changes in pH Acids and Bases Acid- a substance that increases the hydrogen ion concentration of a solution Base- substance that reduces the hydrogen ion concentration of a solution Some bases reduce H+ concentration directly by accepting hydrogen ions. Other bases reduce the H+ concentration indirectly by dissociating to form hydroxide ions, which then combine with hydrogen ions in the solution to form water. A solution in which the H+ and OH- are equal is said to be neutral.

Organisms are sensitive to changes in pH The pH Scale In any aqueous solution the product of H+ and OH- concentrations is constant at 10^(-14) An acid not only adds hydrogen ions to a solution, but also removes hydroxide ions because of the tendency of for H+ to combine with OH- and form water. A base has the opposite effect. The pH of a solution is defined as the negative logarithm (base 10) of the hydrogen ion concentration The pH of a neutral solution is 7, the midpoint of the scale. A pH calue less that 7 denotes an acidic solution (the lower the more acidic).The pH for basic solutions is above 7. Buffers- substances that minimize changes in the concentrations of H+ and OH- in a solution

Acid precipitation threatens the fitness of the environment One of the most serious assaults on water quality is acid precipitation. Acid precipitation- rain, snow, or fog that is more acidic than pH 5.6 It is caused primarily by the presence in the atmosphere of sulfur oxides and nitrogen oxides, gaseous compounds that react with water to form strong acids, which fall to Earth with rain or snow. A major source of the oxides come from the burning of fossil fuels. Strong acidity can alter the structure of biological molecules and prevent them from carrying out the essential chemical processes of life. Acid rain and snow can bring about profound changes in soils by affecting the solubility of soil minerals.