Water: The Medium of Life PDF

Water: The medium of life 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...

Water: The medium of life 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 © 2011 Pearson Education, Inc. Figure 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 © 2011 Pearson Education, Inc. Figure 3.2 − Hydrogen + bond Polar covalent bonds − + − + − + Four emergent properties of water contribute to Earth’s suitability for life Four of water’s properties that facilitate an environment for life are – Cohesive behavior – Ability to moderate temperature – Expansion upon freezing – Versatility as a solvent © 2011 Pearson Education, Inc. 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 Adhesion also helps with the transport of water in plants to counter gravity © 2011 Pearson Education, Inc. Figure 3.3 Adhesion Two types of water-conducting cells Cohesion Direction of water movement 300 m  Surface tension is a measure of how hard it is to break the surface of a liquid Surface tension is related to cohesion The ability of water molecules to form hydrogen bonds with each other results in the high surface tension of water © 2011 Pearson Education, Inc. Figure 3.4 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 Water has a high heat of fusion compared to other substances, 80 cal/gram – This is the amount of heat that has to be added or removed to make ice melt or water freeze © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. Water’s High Specific Heat Water resists changing its temperature because of its high specific heat The specific heat of a substance is the amount of heat that must be absorbed or lost for 1 gram of that substance to change its temperature by 1°C A calorie (cal) is the amount of heat required to raise the temperature of 1 g of water by 1°C, so water’s specific heat is 1 cal/g °C © 2011 Pearson Education, Inc.  Water’s 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 © 2011 Pearson Education, Inc. Figure 3.5 Burbank San Bernardino Santa Barbara 73° 90° 100° Los Angeles Riverside 96° (Airport) 75° Santa Ana 84° Palm Springs 70s (°F) 106° 80s Pacific Ocean 68° 90s 100s San Diego 72° 40 miles 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 – For water, 540 cal/gram 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. Figure 3.6 Hydrogen bond Liquid water: Hydrogen bonds break and re-form Ice: Hydrogen bonds are stable 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 © 2011 Pearson Education, Inc.  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 © 2011 Pearson Education, Inc. Figure 3.7 − Na+ + − + − + − − Na+ − + + Cl− Cl− + − − + − + − −  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 © 2011 Pearson Education, Inc. Figure 3.8 + − − + 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc.  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 1023molecules Avogadro’s 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 © 2011 Pearson Education, Inc. 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–) © 2011 Pearson Education, Inc. Figure 3.UN02 + − 2 H2O Hydronium Hydroxide ion (H3O+) ion (OH−)  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 © 2011 Pearson Education, Inc.  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) © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. The pH Scale In any aqueous solution at 25°C the product of H+and OH– is constant and can be written as [H+][OH–] = 10–14 The pH of a solution is defined by the negative logarithm of H+ concentration, written as pH = –log [H+] For a neutral aqueous solution © 2011 Pearson Education, Inc. [H+] is 10–7 = –(–7) = 7  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 © 2011 Pearson Education, Inc. Figure 3.10 pH Scale 0 1 Battery acid 2 Gastric juice, lemon juice Increasingly Acidic H+ [H+] > [OH− ] H+ H+ H+ OH− + 3 Vinegar, wine, OH− H H+ cola H+ H+ Acidic 4 Tomato juice Beer solution 5 Black coffee Rainwater 6 Urine − OH− Saliva OH Neutral 7 Pure water H+ H+ OH− OH− OH− + H [H+] = [OH −] Human blood, tears H+ H+ 8 Seawater Neutral Inside of small intestine solution Increasingly Basic 9 [H+] < [OH− ] 10 Milk of magnesia OH− OH − OH− H+ OH− 11 OH− OH− Household ammonia − H+ OH 12 Basic solution Household 13 bleach Oven cleaner 14 Buffers 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+ © 2011 Pearson Education, Inc.