Jillian Adler BSCI 105 Notes Chapter 4 water 2 3 14 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 Polar covalent bonds in water molecules result in hydrogen bonding Polar molecule opposite ends have opposite charges Polarity allows water molecules to form hydrogen bonds with each other Four emergent properties of water contribute to Earth s suitability for life Four of water s properties that facilitate an environment for life are o Cohesive behavior o Ability to moderate temperature o Expansion upon freezing o Versatility as a solvent o Cohesion of Water Molecules Cohesion hydrogen bonds hold water molecules together o Cohesion helps the transport of water against gravity in plants Adhesion the attraction between different substances for example between water and plant cell walls Surface tension a measure of how hard it is to break the surface of a liquid o 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 the energy of motion Heat 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 Celsius scale a measure of temperature using Celsius degrees Calorie the amount of heat required to raise the temperature of 1g of water by 1 C Kilocalories kcal 1kcal 1000 cal Waters high specific heat Specific heat a substance is the amount of heat that must be absorbed or lost for 1 g of the substance to change its temperature by 1 C The specific heat of water is 1 cal g C Water resists changing its temperature because of its high specific heat Water s high specific heat can be traced to hydrogen bonding o Heat is absorbed when hydrogen bonds break o 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 the transformation of a substance from liquid gas Heat of vaporization the heat a liquid must absorb for 1 g to be converted to gas Evaporative cooling a liquid evaporates its remaining surface cools o 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 ordered 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 Solution a 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 Hydration shell when an iconic compound is dissolved in water each ion is surrounded by a sphere of water molecules o Water is a versatile solvent due to its polarity which allows it to form hydrogen bonds easily 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 Hydrophilic substance is one that has an affinity for water Hydrophobic substance is one that does not have an affinity for water o Oil molecules are hydrophobic because they have relatively nonpolar bonds Colloid is a stable suspension of fine particles in a liquid Solute Concentration in Aqueous Solutions Most biochemical reactions occur in water Chemical reactions concentration of solutes in an aqueous solution depend on collisions of molecules and therefore on the Molecular mass the sum of all masses of all atoms in a molecule Numbers of molecules are usually measured in moles where 1 mole mol Avogadro s number and the unit dalton were defined such that 6 02 x 1023 6 02 x 1023 molecules 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
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