Water and the Fitness of The Environment Chapter 3

Goals Explain how the polarity of water results in hydrogen bonding. Discuss the properties of water that make life possible and provide examples. Define buffers, explain how they work, and give an example.

Water as a Polar Molecule Oxygen is very electronegative and attempts to pull the electrons from hydrogen making it a polar molecule Produces a slightly negative charge for oxygen and a slightly positive charge for the hydrogens Hydrogen bonding occurs as an attraction between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of another water molecule

Properties of Water that Allow Life to Exist Cohesion/Adhesion High specific heat Evaporative cooling Expansion upon freezing A versatile solvent Ability to repel hydrophobic substances and attract hydrophilic substances

1. Cohesion/Adhesion Cohesion The attraction of like molecules due to hydrogen bonding Gives water “structure” compared to other liquids Responsible for: Insects walking on water Transport of water and dissolved nutrients against gravity in plants (water to water) Surface tension

1. Cohesion/Adhesion Adhesion The attraction of different molecules due to hydrogen bonding Responsible for: Transport of water and dissolved nutrients against gravity in plants as well (water to walls of plant cells in the xylem) The meniscus that forms in a graduated cylinder

2. High Specific Heat The specific heat of a substance is defined as the amount of heat that must be absorbed or lost for 1g of that substance to change its temperature by 1˚C Having a high specific heat means that A LOT of heat must be absorbed or lost in order to change the temperature of the water by even 1˚C This property allows large bodies of water to moderate air temps and allows ocean temps to stay fairly constant all year round for living organisms Consider why coastal regions have mild temperatures…

2. High Specific Heat

3. Evaporative Cooling As a liquid evaporates, its remaining surface cools The hot water molecules evaporate leaving behind slower moving (cooler) water molecules Contributes to the stability of temps in lakes and ponds Provides a mechanism (sweating) to prevent overheating for terrestrial organisms

4. Expansion Upon Freezing Water is less dense as a solid than it is as a liquid…hence, ice floats When hydrogen bonds form as water cools, they cause water molecules to stay a specific distance apart, resulting in a crystalline lattice, which causes ice to expand Floating ice insulates the water below, preventing it from freezing, thus allowing life to exist in the liquid water below

5. A Versatile Solvent Made possible due to hydrogen bonding Able to dissociate salts and other water-soluble substances How does it work? The oxygen of a water molecule with a slight negative charge is attracted to positively charged ions The hydrogens of a water molecule with a slight positive charge are attracted to negatively charged ions Slowly they pick away at a salt and surround the ions forming a hydration shell

5. A Versatile Solvent Allows polar compounds and ions to be transported throughout living organisms in biological fluids such as blood, the sap of plants, and the liquid within cells

6. Ability to Repel Hydrophobic Substances and Attract Hydrophilic Substances Nonpolar and nonionic substances cannot form hydrogen bonds with water and therefore do not dissolve in the presence of water These substances are called hydrophobic substances Ex) oil is nonpolar Cell membranes are composed of molecules that are hydrophobic and for good reason…consider what would happen to cells if their membranes were hydrophilic!

Acids & Bases An acid increases the H+ concentration of a solution A base decreases the H+ concentration of a solution Some bases accept H+ Other bases dissociate into a cation and OH- which combines with H+ to form water Recap: [H+] > [OH-] = Acidic solution [H+] < [OH-] = Basic solution [H+] = [OH-] = Neutral solution

pH Scale pH scale goes from 0-14 7 = neutral 0-6 = acids 8-14 = bases Most biological fluids have pH values in the range of 6 to 8 Each pH unit represents a tenfold difference in H+ and OH- concentrations A solution of pH 3 is 10x as acidic as a pH4 A solution of pH 3 is 100x as acidic as a pH5 A solution of pH 3 is 1000x as acidic as a pH6

Buffers The internal pH of most living cells must remain close to pH7 Buffers are substances that minimize changes in concentrations of H+ and OH– in a solution A buffer is a solution of a weak acid and its corresponding base

Buffers Ex) Carbonic acid (H2CO3) and bicarbonate ions (HCO3-) If an acid is added, some of the H+ combines with bicarbonate ions to form carbonic acid If a base is added, the reverse happens, carbonic acid ionizes into H+ and bicarbonate ions This buffering system is present in the blood, where it is important for preventing significant changes in pH that could disrupt the ability of the blood to carry vital oxygen to tissues

Buffers We also use buffers to relieve indigestion. The lining of the stomach constantly secretes hydrochloric acid but excessive amounts inhibit digestion and cause discomfort. Antacids are composed of a salt such CaCO3 (calcium carbonate) which acts as a buffer in your stomach