Chapter 3: WATER AND THE FITNESS OF THE ENVIRONMENT Overview: The Molecule That Supports All of Life • Water is the biological medium on Earth • All living organisms require water more than any other substance • Most cells are surrounded by water, and cells themselves are about 70–95% water • The abundance of water is the main reason the Earth is habitable Polarity of Water Molecules Results in Hydrogen Bonding • Water is a polar molecule – The two ends have opposite charges – O is partially (-), H is partially (+) • Polarity allows water molecules to form hydrogen bonds with each other Animation: Water Structure – Hydrogen bond + H + O – – + H + – Four emergent properties of water contribute to Earth’s fitness for life 1. Cohesion 2. Moderation of Temperature 3. Insulation of Bodies of Water by Floating Ice (expansion upon freezing) 4. Water as a Solvent 1. Cohesion • Cohesion: the binding together of like molecules by hydrogen bonds – Most often seen in water molecules – helps the transport of water against gravity in plants • Adhesion: an attraction between different kinds of molecules – Ex: between water and plant cell walls Animation: Water Transport Adhesion Water-conducting cells Direction of water movement Cohesion 150 µm • Surface tension: a measure of how hard it is to stretch or break the surface of a liquid – Related to cohesion – Water has a high surface tension because of hydrogen bonding of surface molecules 2. Moderation of Temperature • Water absorbs heat from warmer air and releases stored heat to cooler air • Water can absorb or release a large amount of heat with only a slight change in its own temperature Water’s High Specific Heat • Specific Heat: amount of heat that must be absorbed or lost for 1 g of a substance to change its temperature by 1ºC • The specific heat of water is 1 cal/g/ºC – Water resists changing its temperature because of its high specific heat • Water’s high specific heat linked to hydrogen bonding – Heat is absorbed when hydrogen bonds break – Heat is released when hydrogen bonds form • Water’s high specific heat helps keep temperature steady, within limits that permit life Evaporative Cooling • Evaporation: phase change from liquid to gas at the surface of a liquid • Evaporative Cooling: As a liquid evaporates, its remaining surface cools – Occurs because “hottest” molecules (those with greatest kinetic energy) are most likely to leave as gas – Stabilizes temperatures in organisms and bodies of water • Example: Sweating 3. Insulation of Bodies of Water by Floating Ice • Ice floats in liquid water – Water expands when it freezes – Ice is less dense than liquid water • Occurs because hydrogen bonds in ice are more “ordered” and form a crystalline structure – This keeps large bodies of water from freezing solid and therefore moderate s temperature Hydrogen bond Ice Hydrogen bonds are stable Liquid water Hydrogen bonds break and re-form 4. Water as a Solvent • Solution: a liquid that is a homogeneous mixture of substances • Solvent : the dissolving agent of a solution • Solute: the substance that is dissolved • Aqueous solution: one in which water is the solvent • Water is a versatile solvent due to its polarity, which allows it to form hydrogen bonds easily • When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell _ Na+ + _ _ Na+ + Cl- _ Cl- + _ + _ _ + + _ + _ + _ _ Hydrophilic and Hydrophobic Substances • Hydrophilic substance: has an affinity for water – Polar molecules, ionic compounds • Hydrophobic substance: does not have an affinity for water – Nonpolar molecules – Ex: Oil molecules • A colloid is a stable suspension of fine particles in a liquid – Ex: cotton Acidic and basic conditions affect living organisms • A hydrogen atom in a hydrogen bond between two water molecules can shift from one to the other: – The H atom leaves its electron behind and is transferred as a proton, or hydrogen ion (H+) – The molecule with the extra proton is now a hydronium ion (H3O+), though it is often represented as H+ – The molecule that lost the proton is now a hydroxide ion (OH–) • Water is in a state of dynamic equilibrium in which water molecules dissociate at the same rate at which they are being reformed H O H H O H 2H2O H O H H Hydronium ion (H3O+) O H Hydroxide ion (OH–) Effects of Changes in pH • Concentrations of H+ and OH– are equal in pure water • Adding acids or bases modifies the concentrations of H+ and OH– • Biologists use the pH scale to describe whether a solution is acidic or basic Acids and Bases • Acid: any substance that increases the H+ concentration of a solution • Base: any substance that reduces the H+ concentration of a solution The pH Scale • The pH of a solution is defined by the negative logarithm of H+ concentration, written as pH = –log [H+] • For a neutral aqueous solution [H+] is 10–7 = –(–7) = 7 Buffers • The internal pH of most living cells must remain close to pH 7 • Buffers: substances that minimize changes in concentrations of H+ and OH– in a solution – Accept H+ from solution when they are in excess and donate H+ when they are depleted Threats to Water Quality on Earth • Acid precipitation: rain, snow, or fog with a pH lower than 5.6 – Caused mainly by the mixing of different pollutants with water in the air and can fall at some distance from the source of pollutants – Can damage life in lakes and streams – Effects on soil chemistry are contributing to the decline of some forests • Human activities such as burning fossil fuels threaten water quality • CO2 is released by fossil fuel combustion and contributes to: – A warming of earth called the “greenhouse” effect – Acidification of the oceans • Leads to a decrease in the ability of corals to form calcified reefs Chapter 3 “Need to Knows” 1. The importance of hydrogen bonding to the properties of water. 2. Four unique properties of water, and how each contributes to life on Earth. 3. How to interpret the pH scale. 4. The importance of buffers in biological systems.