Water: The Molecule of Life

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.

The polarity of water molecules results in hydrogen bonding. The water molecule is a polar molecule: The opposite ends have opposite charges Polarity allows water molecules to form hydrogen bonds with each other. Animation: Water Structure

Question #1: Based on your knowledge of the polarity of water, this solute molecule is most likely positively charged. negatively charged. hydrophobic. nonpolar. Answer: b Source: Barstow - Test Bank for Biology, Sixth Edition, Question #22 Discussion Notes for the Instructor The key to this question is the orientation of the water molecules that are interacting with the solute molecule. Choices C, D, and E are eliminated because the water molecules are hydrogen bonding with the solute. This implies the solute is charged. Since it is the hydrogens of the water molecules that are interacting with the solute the solute must be negatively charged. There are several questions which can be asked to guide the discussion of this question including: Are the water molecules interacting with the solute? What is/are the orientation/s of the water molecules relative to the solute molecule? What are the charges on the various components of a water molecule? Using these questions as an outline, discussion of the choices might look like this: Choice A, the hydrogens of the water molecule, which have a partial positive charge, are interacting with the solute. Like charges repel Choice B, correct Choice C, if the solute was neutrally charged the water molecules, being polar, would try to avoid interacting with the solute Choice D, similar to Choice C Choice E, similar to Choice C

Four properties of water that facilitate an environment for life: Cohesive behavior Ability to moderate temperature Expansion upon freezing Versatility as a solvent

Animation: Water Transport Cohesion Hydrogen bonds hold water molecules together. Cohesion and adhesion help in the transport of water against gravity in plants. Animation: Water Transport

Water’s High Surface Tension A measure of how hard it is to break the surface of a liquid

Surface Tension

Moderation of Temperature Heat vs. Temperature Kinetic energy is the energy of motion Heat is a 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 Water can absorb or release a large amount of heat with only a slight change in its own temperature. Water can moderate air temperatures because it absorbs heat from warmer air and releases stored heat to cooler air.

Water’s High Specific Heat the amount of heat that must be absorbed or lost for 1 gram of a substance to change its temperature by 1ºC Water’s high specific heat minimizes temperature fluctuations to within limits that permit life. Heat is absorbed when hydrogen bonds break. Heat is released when hydrogen bonds form.

Water’s High Heat of Vaporization Amount of heat a liquid must absorb for 1 gram to evaporate Hydrogen bonds must be broken for water molecules to evaporate. As a liquid evaporates, its remaining surface cools, a process called evaporative cooling. Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water.

Insulation of Bodies of Water by Floating Ice Ice floats in liquid water because hydrogen bonds in ice are more “ordered,” making ice less dense. If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth.

Hydrophilic vs. Hydrophobic Hydrophilic: substance with an affinity for water Hydrophobic: substance without an affinity for water

The Solvent of Life Solution, solvent, solute Water is a versatile solvent due to its polarity and because it readily forms hydrogen bonds. When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules, a hydration shell.

The Solvent of Life Water can also dissolve compounds made of nonionic polar molecules. Even large polar molecules such as proteins can dissolve in water if they have ionic and polar regions. Lysozyme molecule in a nonaqueous environment. Lysozyme molecule in an aqueous environment.

Biological Chemistry is “Wet” Chemistry Most biochemical reactions occur in water. Chemical reactions depend on collisions of molecules and therefore on the concentration of solutes in an aqueous solution.

Determining Solute Concentration Molecular Mass sum of all masses of all atoms in a molecule measured in daltons Example: sucrose (C12H22O11) (C=12, H=1, O=16) 12(12) + 22(1) + 11(16) = 342 daltons Numbers of molecules are measured in moles. 1 mole = 6.02 x 1023 molecules 1 gram = 6.02 x 1023 daltons 1 mole of substance = molecular mass in grams Molarity is the number of moles of solute per liter of solution.

Question #2: How many grams of the molecule in this figure would be required to make 1 L of a 0.2 M solution of the molecule? (Carbon=12, Oxygen=16, Hydrogen=1) 8 12 24 60 Answer: b Source: Barstow - Test Bank for Biology, Sixth Edition, Question #30 2(12) + 4(1) + 2(16) = 60 g/mol 60 g/mol x 0.2 mol/L = 12 g/L C2H4O2

Dissociation of water molecules leads to acidic and basic conditions that affect living organisms A hydrogen atom in a hydrogen bond between two water molecules can shift from one to the other The hydrogen atom leaves its electron behind and is transferred as a proton, or hydrogen ion (H+) Though statistically rare, the dissociation of water molecules has a great effect on organisms because changes in concentrations of H+ and OH- can drastically affect the chemistry of a cell. Hydronium ion (H3O+) Hydroxide ion (OH–)

Effects of changes in pH Acids and Bases Acid: any substance that increases the H+ concentration of a solution Base: is any substance that reduces the H+ concentration of a solution Biologists use the pH scale to describe how acidic or basic a solution is. Pure water [H+] = [OH-]

pH Scale 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Oven cleaner Household bleach Household ammonia Milk of magnesia Seawater Pure water Human blood Urine Rainwater Black coffee Tomato juice Vinegar, beer, wine, cola Digestive (stomach) juice, lemon juice Battery acid Neutral [H+] = [OH–] Increasingly Acidic [H+] > [OH–] Increasingly Basic [H+] < [OH–] The pH Scale The pH of a solution is determined by the relative concentration of hydrogen ions. Acidic solutions have pH values less than 7. Basic solutions have pH values greater than 7. Most biological fluids have pH values in the range of 6 to 8.

[10−7][10−6] = 10−14 [10−8][10−7] = 10−14 [10−9][10−5] = 10−14 [10−10][10−4] = 10−14 [10−11][10−3] = 10−14 [10−12][10−2] = 10−14 [10−13][10−1] = 10−14 [10−14][10−0] = 10−14 [10−6][10−8] = 10−14 [10−5][10−9] = 10−14 [10−4][10−10] = 10−14 [10−3][10−11] = 10−14 [10−2][10−12] = 10−14 [10−1][10−13] = 10−14 [10−0][10−14] = 10−14 The pH Scale Compresses the range of H+ and OH− concentrations by employing logarithms. The pH of a solution is defined as the negative logarithm (base 10) of the hydrogen ion concentration pH = −log[H+] [H+][OH−] = 10−14

Question 3: The following are pH values: cola-2; orange juice-3; coffee-5; human blood-7.4. Which of these liquids has the highest concentration of OH-? cola orange juice coffee human blood Answer: e Source: Taylor - Student Study Guide for Biology, Sixth Edition, Test Your Knowledge Question #18

Question 4: If the pH of a solution is increased from pH 8 to pH 9, it means that the concentration of H+ is 10 times greater than what it was at pH 8. concentration of H+ is 100 times less than what it was at pH 8. concentration of OH- is 10 times greater than what it was at pH 8. concentration of OH- is 100 times less than what it was at pH 8. Answer: c Source: Barstow - Test Bank for Biology, Sixth Edition, Question #43

Question 5: Acid precipitation has lowered the pH of a particular lake to 4.0. What is the hydroxide ion concentration of the lake? [10−7][10−6] = 10−14 [10−8][10−7] = 10−14 [10−9][10−5] = 10−14 [10−10][10−4] = 10−14 [10−11][10−3] = 10−14 [10−12][10−2] = 10−14 [10−13][10−1] = 10−14 [10−14][10−0] = 10−14 [10−6][10−8] = 10−14 [10−5][10−9] = 10−14 [10−4][10−10] = 10−14 [10−3][10−11] = 10−14 [10−2][10−12] = 10−14 [10−1][10−13] = 10−14 [10−0][10−14] = 10−14 10-7 M 10-4 M 10-10 M 10-14 M Answer: c Source: Campbell/Reece - Biology, Sixth Edition, EOC Self-Quiz Question #10 Discussion Notes for the Instructor To answer this question students need to know the relationship between pH and hydroxide concentration.

Buffers Buffers are substances that minimize changes in concentrations of H+ and OH- in a solution Most buffers consist of an acid-base pair that reversibly combines with H+