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

Concept 3.1: Polar covalent bonds in water molecules result 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

Concept 3.2: Four emergent properties of water contribute to EarthÕs suitability for life

¥          Four of waterÕs properties that facilitate an environment for life are

         Cohesive behavior

         Ability to moderate temperature

         Expansion upon freezing

         Versatility as a solvent

Cohesion of Water Molecules

¥           Collectively, hydrogen bonds hold water molecules together, a phenomenon called cohesion

¥           Cohesion helps the transport of water against gravity in plants

¥           Adhesion is an attraction between different substances, for example, between water and plant cell walls

¥           Surface tension is a measure of how hard it is to break the surface of a liquid

¥           Surface tension is related to cohesion

Moderation of Temperature by Water

¥                    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

Heat and 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

¥           The Celsius scale is a measure of temperature using Celsius degrees (¡C)

¥           A calorie (cal) is the amount of heat required to raise the temperature of 1 g of water by 1¡C

¥           The ÒcaloriesÓ on food packages are actually kilocalories (kcal), where 1 kcal = 1,000 cal

¥           The joule (J) is another unit of energy where
1 J = 0.239 cal, or 1 cal = 4.184 J

WaterÕs High Specific Heat

¥           The specific heat of a substance is the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1¼C

¥           The specific heat of water is 1 cal/g/¼C

¥           Water resists changing its temperature because of its high specific heat

¥           WaterÕs high specific heat can be traced to hydrogen bonding

         Heat is absorbed when hydrogen bonds break

         Heat is released when hydrogen bonds form

¥           The high specific heat of water minimizes temperature fluctuations to within limits that permit life

Evaporative Cooling

¥           Evaporation is transformation of a substance from liquid to gas

¥           Heat of vaporization is the heat a liquid must absorb for 1 g to be converted to gas

¥           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

Floating of Ice on Liquid Water

¥           Ice floats in liquid water because hydrogen bonds in ice are more Òordered,Ó making ice less dense

¥           Water reaches its greatest density at 4¡C

¥           If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth

Water: The Solvent of Life

¥           A solution is a liquid that is a homogeneous mixture of substances

¥           A solvent is the dissolving agent of a solution

¥           The solute is the substance that is dissolved

¥           An aqueous solution is 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

¥           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

Hydrophilic and Hydrophobic Substances

¥           A hydrophilic substance is one that has an affinity for water

¥           A hydrophobic substance is one that does not have an affinity for water

¥           Oil molecules are hydrophobic because they have relatively nonpolar bonds

¥           A colloid is a stable suspension of fine particles in a liquid

Solute Concentration in Aqueous Solutions

¥           Most biochemical reactions occur in water

¥           Chemical reactions depend on collisions of molecules and therefore on the concentration of solutes in an aqueous solution


¥           Molecular mass is the sum of all masses of all atoms in a molecule

¥           Numbers of molecules are usually measured in moles, where 1 mole (mol) = 6.02 x 1023 molecules

¥           AvogadroÕs number and the unit dalton were defined such that 6.02 x 1023 daltons = 1 g

¥           Molarity (M) is the number of moles of solute per liter of solution

Possible Evolution of Life on Other Planets with Water

¥           The remarkable properties of water support life on Earth in many ways

¥           Astrobiologists seeking life on other planets are concentrating their search on planets with water

¥           To date, more than 200 planets have been found outside our solar system; one or two of them contain water

¥           In our solar system, Mars has been found to have water

Concept 3.3: 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 hydrogen 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

¥           Though statistically rare, the dissociation of water molecules has a great effect on organisms

¥           Changes in concentrations of H+ and OH can drastically affect the chemistry of a cell

¥           Concentrations of H+ and OH are equal in pure water

¥           Adding certain solutes, called acids and bases, modifies the concentrations of H+ and OH

¥           Biologists use something called the pH scale to describe whether a solution is acidic or basic (the opposite of acidic)

Acids and Bases

¥           An acid is any substance that increases the H+ concentration of a solution

¥           A base is any substance that reduces the H+ concentration of a solution

The pH Scale

¥           In any aqueous solution at 25¡C the product of H+ and OH is constant and can be written as

¥           The pH of a solution is defined by the negative logarithm of H+ concentration, written as

¥           For a neutral aqueous solution, [H+] is 10–7, so

¥           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


¥           The internal pH of most living cells must remain close to pH 7

¥           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+

Acidification: A Threat to Water Quality

¥           Human activities such as burning fossil fuels threaten water quality

¥           CO2 is the main product of fossil fuel combustion

¥           About 25% of human-generated CO2 is absorbed by the oceans

¥           CO2 dissolved in sea water forms carbonic acid; this process is called ocean acidification

¥           As seawater acidifies, H+ ions combine with carbonate ions to produce bicarbonate

¥           Carbonate is required for calcification (production of calcium carbonate) by many marine organisms, including reef-building corals

¥           Calcium carbonate disassociates in acidic seawater, resulting in shell deterioration of such marine organisms, as well as of established reef

¥           The burning of fossil fuels is also a major source of sulfur oxides and nitrogen oxides

¥           These compounds react with water in the air to form strong acids that fall in rain or snow

¥           Acid precipitation is rain, fog, or snow with a pH lower than 5.2

¥           Acid precipitation damages life in lakes and streams and changes soil chemistry on land