General Biology 121- Chapter 3: Water and Life
General Biology 121- Chapter 3: Water and Life Bio 121
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This 6 page Class Notes was uploaded by Audrey Notetaker on Sunday September 27, 2015. The Class Notes belongs to Bio 121 at Syracuse University taught by Wiles in Fall 2015. Since its upload, it has received 16 views. For similar materials see Intro to biological sciences in Biological Sciences at Syracuse University.
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Date Created: 09/27/15
Chapter 3 Water and Life 31 Polar covalent bonds in water molecules result in hydrogen bonding Oxygen is more electronegative than hydrogen so the electrons shared by the atoms spend more time closer to oxygen than to hydrogen These are a particular type of covalent bonds called This sharing of electrons and water s Vlike shape make it a polar molecule meaning that its overall charge is unevenly distributed In water the oxygen region of the molecule has a partial negative charge and each hydrogen has a partial positive charge The positive hydrogen of one molecule is attracted to the negative oxygen of a nearby molecule The 2 molecules are thus held together by a hydrogen bond When water is in its liquid form its hydrogen bonds are very fragile The hydrogen bonds form break and reform with great frequency A molecule may be polar either as a result of polar bonds due to differences in electronegativity or as a result of an asymmetric arrangement of nonpolar covalent bonds and nonbonding pairs of electrons Water is a polar molecule 32 4 Emergent properties of water contribute to Earth s suitability for life K I a i a 7 a i a a r E a A a a 77 a 7 a We mm mu 2M m g a va y x L y y L 4 i s H y v Q g ga L3 Water molecules stay close to each other as a result of hydrogen bonding Many of the molecules are linked by multiple hydrogen bonds These linkages make water more structured than most other liquids Collectively the hydrogen bonds hold the substance together This is called Cohesion due to hydrogen bonding contributes to the transport of water and dissolved nutrients against gravity in plants Adhesion is the clinging of one substance to another Related to cohesion is It s the measure of how difficult it is to stretch or break the surface of a liquid At the interface between water and air is an ordered arrangement of water molecules hydrogen lhm faHCWl fTEW p WEEMW EIW bonded to one another and to water below This gives water an unusually high surface tension making it behave as though it were coated with an invisible lm whater Water moderated air temperature by absorbing heat from air that s warmer and releasing the stored heat to air that is cooler Water is effective as a heat bank because it can absorb or release a relatively large amount of heat with only a slight change in its own temperature Temperature and Heat Anything that moves has the energy of motion Atoms and molecules have kinetic energy because they are always moving but not in any particular direction The faster a molecule moves the greater its kinetic energy The kinetic energy associated with the random movement of atoms or molecules is called Thermal energy is related to temperature but they re not the same thing is a measure of energy that represents the average kinetic energy of the molecules in a body of matter But the total thermal energy depends in part on the matter s volume Whenever 2 objects of different temperature are brought together thermal energy passes from the warmer to the cooler object until the 2 are the same temperature Molecules in the cooler object speed up at the expense of the thermal energy of the warmer object An ice cube cools a drink not by adding coldness to the liquid but by absorbing thermal energy from the liquid as the ice itself melts Thermal energy in transfer from one body of matter to another is de ned as Water s High Speci c Heat The ability of water to stabilize temperature stems from its relatively high speci c heat The of a substance is de ned as the amount of heat that must be absorbed or lost for 1 gram of that substance to change its temperature by 1 C Because of the high speci c heat of water relative to other materials water will change its temperature less than other liquids when it absorbs or loses a given amount of heat While heat must be absorbed in order to break hydrogen bonds heat must be released when hydrogen bonds form Hydrogen bonds must break to raise water temperature Speci c heat of water helps organisms maintain relatively constant internal temperature The high speci c heat of water also tends to stabilize ocean temperatures creating a favorable environment for marine life Eva porative Cooling Molecules of any liquid stay close together because they are attracted to one another Molecules moving fast enough to overcome these attractions can depart the liquid and enter the air as a gas This transformation from a liquid to a gas is called vaporization or Some evaporation occurs at any temperature is the quantity of heat a liquid must absorb for 1 gram of it to be converted from the liquid to the gaseous state For the same reason that water has a high speci c heat it also has a high heat of vaporization relative to most other liquids Water s high heat of vaporization is another emergent property resulting from the strength of its hydrogen bonds which must be broken before the molecules can exit from the liquid in the form of water vapor The high amount of energy required to vaporize water has a wide range of effects On a global scale for example it helps moderate Earth s climate On an organism level water s high heat of vaporization accounts for the severity of steam burns These burns are caused by the heat energy released when steam condenses into liquid on the skin As a liquid evaporates the surface of the liquid that remains behind cools down its temperature decreases This occurs because the hottest molecules those with the greatest kinetic energy are the most likely to leave as gas Evaporative cooling of water contributes to the stability of temperature in lakes and ponds and also provides a mechanism that prevents terrestrial organisms from overheating For example evaporation of water from the leaves of a plant helps keep the tissues in the leaves from becoming too warm in the sunlight Evaporation of sweat from human skin dissipates body heat and helps prevent overheating on a hot day or when excess heat is generated by strenuous activity 41 xig y D ci if4 H i4 o g 9 a 9 lrf I ll 3 Water is one of the few substances that are less dense as a solid than as a liquid In other words ice oats on liquid water When water solidi es it expands Hydrogen bonding is the cause of this Even in liquid water many of the molecules are connected by hydrogen bonds but only transiently The hydrogen bonds are constantly breaking and reforming The ability of ice to oat due its lower density is a crucial factor in the suitability of the environment for life If ice sank then all ponds lakes and even oceans would freeze solid making life on Earth impossible Global warming which is caused by carbon dioxide and other greenhouse gases in the atmosphere is having an effect on icy environments around the world For example in the Arctic sea the temperature increase has affected the seasonal balance between ice and liquid water causing ice to form later in the year to melt earlier and to cover a smaller area The rate at which glaciers and Arctic sea ice are disappearing is a challenge to animals that depend on ice for survival Q U L i ii l L i A sugar cube placed in a glass of water will dissolve with a little stirring The glass will then contain a uniform mixture of sugar and water the concentration of dissolved sugar will be the same everywhere in the mixture A liquid that s a completely homogeneous mixture of 2 or more substances is called a The dissolving agent of a solution is the and the substance that s dissolved is the In the previous example water is the solvent and sugar is the solute An is one in which the solute is dissolved in water water is thus the solvent Water is a versatile solvent a quality we can trace to the polarity of the water molecule For example let s say we put a spoonful of table salt sodium chloride in water Then the surface of each grain of salt is exposed to the solvent These ions and regions of the water molecules are attracted to each other due to their opposite charges The oxygen regions of the water molecules are negatively charges and are attracted to sodium cations The hydrogen regions are positively charges and are attracted to chloride anions As a result water molecules surround the individual sodium and chloride ions separating and shielding them from one another The sphere of water molecules around each dissolved ion is called a Water then dissolves all the ions The result is a solution of 2 solutes sodium cations and chloride anions homogeneously mixed with water the solvent A compound doesn t need to be ionic to dissolve in water Many compounds are dissolved in the water like sap from plants blood and the liquid within all cells Hydrophilic and Hydrophobic Substances means to have a tendency to mix with dissolve in or to be wetted by water But in some cases substances can be hydrophilic without actually dissolving Some molecules in cells are so big that they don t dissolve like cotton Substances that are nonionic and nonpolar actually seem to repel water these substances are An example would be vegetable oil It doesn t mix stably with waterbased substances like vinegar 32 Acidic and basic conditions affect living organisms Most of the chemical reactions in organisms involve solutes dissolved in water Sometimes a hydrogen atom participating in a hydrogen bond between 2 water molecules shifts from 1 molecule to the other When this happens the hydrogen atom leaves its electron behind and what s transferred is a a single proton with a charge of 1 The water molecule that lost a proton is now a which has a charge of 1 The proton binds to the other water molecule making that molecule a ion lt71 D lt7 33 7 lt7 23 r13 5 Fae H 5 le l tj 2l Luci An is a substance that increases the hydrogen ion concentration of a solution It s also a proton donor and dissociates in solutions to yield H and an anion For example when hydrochloric acid HCI is added to water hydrogen ions dissociate from chloride ions HCH Cl This source of H results in an acidic solution one having more H than OH A substance that reduces the hydrogen ion concentration of a solution is called a Some bases reduce the H concentration directly by accepting hydrogen ions It s a proton acceptor and dissociates in solutions to yield hydroxide ions OH Other bases reduce the H concentration indirectly by dissociating to form hydroxide ions which combine with hydrogen ions and form water Solutions with a higher concentration of OH than H are known as basic solutions A solution in which the H and OH concentrations are equal is said to be neutral I is a numeric scale used to specify the acidity or alkalinity of an aqueous solution Solutions with a pH less than 7 are acidic the lower the number the more acidic the solution Solutions with a pH greater than 7 are basic Thus the higher the number the more basic the solution Lastly solutions with a pH of 7 are neutral The presence of substances called buffers allows biological uids to maintain a relatively constant pH despite the addition of acids and bases A is a substance that minimizes changes in the concentrations of H and OH in a solution It does so by accepting hydrogen ions from the solution when they re in excess and donating hydrogen ions to the solution when they ve been depleted Several buffers contribute to pH stability in human blood and many other biological solutions
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