Water and Fitness in the Environment Chapter 3: Water and Fitness in the Environment AP Biology Mrs. Madalon

Overview: The Molecule That Supports All of Life Water = biological medium on Earth All living organisms require water more than any other substance Cells: Most cells surrounded by water Cells are about 70–95% water Abundance of water = main reason Earth is habitable Why? Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Concept 3.1: The polarity of water molecules results in hydrogen bonding Water is a polar molecule: The opposite ends have opposite charges Polarity  water molecules form hydrogen bonds with each other Four of water’s properties that facilitate an environment for life are: Cohesive behavior Strong adhesion Ability to moderate temperature Expansion upon freezing Versatility as a solvent Concept 3.2: Four emergent properties of water contribute to Earth’s fitness for life Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 3-UN7

Concept 2.2 Atoms Interact and Form Molecules Hydrogen bonds: Attraction between the δ– end of one molecule and the δ+ hydrogen end of another molecule. They form between water molecules and within larger molecules. Although much weaker than covalent bonds, they are important in the structure of DNA and proteins.

Figure 2.4 Hydrogen Bonds Can Form between or within Molecules Figure 2.4 Hydrogen Bonds Can Form between or within Molecules (A) A hydrogen bond forms between two molecules because of the attraction between an atom with a partial negative charge on one molecule and a hydrogen with a partial positive charge on a second molecule. (B) Hydrogen bonds can form between different parts of the same large molecule.

Water has strong cohesion & high surface tension Cohesion: Hydrogen bonds hold water molecules together helps the transport of water against gravity in plants Surface tension: measure of how hard it is to break the surface of a liquid; related to cohesion cohesion—water molecules resist coming apart when placed under tension. Hydrogen bonding between liquid water molecules and solid surfaces allows for adhesion between the water and the solid surface. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Water has strong adhesion Adhesion: attraction between unlike substances water and plant cell walls Capillary Action: this adhesive property allows H2O to rise against the force of gravity. moves H2O in plants

Moderation of Temperature Water absorbs heat from warmer air; releases stored heat to cooler air Kinetic energy: the energy of motion Heat: measure of the total amount of kinetic energy due to molecular motion Temperature: measures the intensity of heat due to the average kinetic energy of molecules Celsius scale: measure of temperature using Celsius degrees (°C) Calorie (cal): amount of heat required to raise the temperature of 1 g of water by 1°C “Calories” on food packages are actually kilocalories (kcal) 1 kcal = 1,000 cal Joule (J): 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

Moderation of Temperature A lot of heat energy is required to raise the temperature of water—the heat energy breaks the hydrogen bonds. In organisms, presence of water shields them from fluctuations in environmental temperature.

Water’s High Specific Heat Specific heat: the degree to which a substance changes temperature in response to a gain or loss of heat. Water’s high specific heat  resists changing temperature  better for LIFE Hydrogen bonding  water’s high specific heat Heat absorbed when hydrogen bonds break Heat is released when hydrogen bonds form Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Evaporative Cooling Evaporation: liquid  gas Water has a high heat of vaporization: a lot of heat energy is required to change water from the liquid to gaseous state (to break the hydrogen bonds). Thus, evaporation has a cooling effect on the environment. [Evaporative Cooling] Sweating cools the body—as sweat evaporates from the skin, a large amount of heat is taken with it and you are cooled. helps stabilize temperatures in organisms and bodies of water Evaporative cooling: as a liquid evaporates, its remaining surface cools

Evaporative Cooling Heat of Fusion: energy required to change water from a solid to liquid Heat of vaporization: energy required to change water from a liquid to a gas The horizontal lines between the different physical states of water indicate the absorption of energy without a rise in temperature. The energy associated with each of these transition has a special name… Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Insulation of Bodies of Water by Floating Ice Hydrogen bonds in ice are more “ordered” = ice less dense  Ice floats Water reaches its greatest density at 4°C If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth

The Solvent of Life!! Solution: liquid that is a homogeneous mixture of substances Solvent: dissolving agent of a solution Solute: substance that is dissolved Aqueous solution: one in which water is the solvent Water = versatile solvent due to its polarity Hydration shell: When an ionic compounds dissolve in water, each ion is surrounded by a sphere of water molecules Even large polar molecules like proteins can dissolve in water if they have ionic and polar regions UNIVERSAL SOLVENT

Hydrophilic and Hydrophobic Substances Solute Concentration in Aqueous Solutions Most biochemical reactions occur in water Chemical reactions depend on collisions of molecules thus on the concentration of solutes in an aqueous solution Molarity (M): number of moles of solute per liter of solution Hydrophilic: has an affinity for water; “water-loving” Hydrophobic: does not have an affinity for water; “water-fearing” How can you tell if a molecule is hydrophobic? Colloid: stable suspension of fine particles in a liquid Hydrophilic and Hydrophobic Substances Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Hydrophilic and Hydrophobic Substances POLAR MOLECULES LIKE EACH OTHER! Any polar molecule can interact with any other polar molecule through hydrogen bonds. Hydrophilic (“water-loving”): in aqueous solutions, polar molecules become separated and surrounded by water molecules. Nonpolar molecules are called hydrophobic (“water-hating”); the interactions between them are hydrophobic interactions.

Hydrophilic and Hydrophobic Hydrophilic and Hydrophobic (A) Molecules with polar covalent bonds are attracted to polar water (they are hydrophilic). (B) Molecules with nonpolar covalent bonds show greater attraction to one another than to water (they are hydrophobic). The color convention in the models shown here (gray, H; red, O; black, C; green, F) is often used.

Atoms Interact and Form Molecules When one atom is much more electronegative than the other, a complete transfer of electrons may occur. This makes both atoms more stable because their outer shells are full. The result is two ions—electrically charged particles that form when atoms gain or lose one or more electrons. 20

Ionic Attraction between Sodium and Chlorine Figure 2.6 Ionic Attraction between Sodium and Chlorine When a sodium atom reacts with a chlorine atom, the chlorine fills its outermost shell by “stealing” an electron from the sodium. In so doing, the chlorine atom becomes a negatively charged chloride ion (Cl–). With one less electron, the sodium atom becomes a positively charged sodium ion (Na+).

Atoms Interact and Form Molecules Cations—positively charged ions Anions—negatively charged ions Ionic attractions result from the electrical attraction between ions with opposite charges. The resulting molecules are called salts or ionic compounds. 22

Atoms Interact and Form Molecules Ionic attractions are weak, so salts dissolve easily in water. place text art pg 25 here

Concept 3.3: Acidic and basic conditions affect living organisms Hydrogen atom in hydrogen bond between two water molecules can shift from one to the other: Hydrogen atom leaves electron behind transferred as a proton or hydrogen ion (H+) Molecule with the extra proton is = hydronium ion (H3O+) 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 Water in state of dynamic equilibrium Rate of water molecules dissociate = rate of water molecules reformed RARE but changes in concentrations of H+ and OH– can drastically affect the chemistry of a cell How? Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Acids and Bases pH Scale: Scientists us to determine whether solution is acidic or basic Most biological processes within pH range 6-8 Acid: any substance that increases the H+ concentration of a solution pH less than 7 Base: any substance that reduces the H+ concentration of a solution pH more than 7 [ H+ ] = [ OH– ] in pure water Any aqueous solution at 25°C the product of H+ and OH– is constant [H+][OH–] = 10–14 pH of a solution is defined by the negative logarithm of H+ concentration pH = –log [H+] Neutral aqueous solution [H+] is 10–7 = –(–7) = 7

Acidic [H+] > [OH–] Neutral [H+] = [OH–] 7 Basic [H+] < [OH–] 14 Fig. 3-UN5 Acidic [H+] > [OH–] Acids donate H+ in aqueous solutions Neutral [H+] = [OH–] 7 Bases donate OH– or accept H+ in aqueous solutions Basic [H+] < [OH–] 14

Buffers Internal pH of most living cells MUST remain close to pH 7 Buffer: substances that minimize changes in concentrations of H+ and OH– in a solution Most buffers consist of acid-base pair Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Threats to Water Quality on Earth Acid precipitation: rain, snow, or fog with a pH lower than 5.6 can fall far from pollutant source can damage life in lakes and streams Effects on soil chemistry are contribute to decline of some forests Pollutants + water in air = Acid Rain Burning fossil fuels  Poor water quality CO2 is released by fossil fuel combustion: “Greenhouse effect” Acidification of the oceans this decrease in the ability of corals to form calcified reefs Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

WHAT DID WE COVER? Water is necessary for LIFE! Water is polar Water has strong cohesion & high surface tension Water has strong adhesion Water has ability to moderate temperature Water has a high specific heat capacity Evaporative cooling Ice floats- unique density properties Water is the universal solvent Hydrophilic vs. Hydrophobic pH

You should now be able to: List and explain the four properties of water that emerge as a result of its ability to form hydrogen bonds Distinguish between the following sets of terms: hydrophobic and hydrophilic substances; a solute, a solvent, and a solution Define acid, base, and pH Explain how buffers work Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings