Sinh học - Chapter 2: Chemical basis of life

Chapter 2The Chemical Basis of Life0Figure 2.0_2Chapter 2: Big IdeasElements, Atoms,and CompoundsChemical BondsWater’s Life-Supporting PropertiesIntroductionChemicals are the stuff that make upour bodies,the bodies of other organisms, andthe physical environment.© 2012 Pearson Education, Inc.IntroductionLife’s chemistry is tied to water.Life first evolved in water.All living organisms require water.The chemical reactions of your body occur in cells consisting of 70–95% water.© 2012 Pearson Education, Inc. ELEMENTS, ATOMS, AND COMPOUNDS © 2012 Pearson Education, Inc.Living organisms are composed of matter, which is anything that occupies space and has mass (weight).Matter is composed of chemical elements.An element is a substance that cannot be broken down to other substances.There are 92 elements in nature—only a few exist in a pure state.2.1 Organisms are composed of elements, in combinations called compounds© 2012 Pearson Education, Inc.Table 2.1A compound is a substance consisting of two or more different elements in a fixed ratio.Compounds are more common than pure elements.Sodium chloride, table salt, is a common compound of equal parts of sodium (Na) and chlorine (Cl).2.1 Organisms are composed of elements, in combinations called compounds© 2012 Pearson Education, Inc.Figure 2.1SodiumChlorineSodium chlorideAbout 25 elements are essential to life.Four elements make up about 96% of the weight of most living organisms. These areoxygen,carbon,hydrogen, andnitrogen.Trace elements are essential but are only needed in minute quantities.2.1 Organisms are composed of elements, in combinations called compounds© 2012 Pearson Education, Inc.2.2 CONNECTION: Trace elements are common additives to food and waterSome trace elements are required to prevent disease.Without iron, your body cannot transport oxygen.An iodine deficiency prevents production of thyroid hormones, resulting in goiter.© 2012 Pearson Education, Inc.Figure 2.2A2.2 CONNECTION: Trace elements are common additives to food and waterFluoride is added to municipal water and dental products to help reduce tooth decay.© 2012 Pearson Education, Inc.Figure 2.2B2.2 CONNECTION: Trace elements are common additives to food and waterSeveral chemicals are added to food tohelp preserve it,make it more nutritious, and/ormake it look better.Check out the “Nutrition Facts” label on foods and drinks you purchase.© 2012 Pearson Education, Inc.Figure 2.2C2.3 Atoms consist of protons, neutrons, and electronsEach element consists of one kind of atom.An atom is the smallest unit of matter that still retains the properties of an element.Three subatomic particles in atoms are relevant to our discussion of the properties of elements.Protons are positively charged.Electrons are negatively charged.Neutrons are electrically neutral.© 2012 Pearson Education, Inc.2.3 Atoms consist of protons, neutrons, and electronsNeutrons and protons are packed into an atom’s nucleus.Electrons orbit the nucleus.The negative charge of electrons and the positive charge of protons keep electrons near the nucleus.© 2012 Pearson Education, Inc.Figure 2.3AHeliumNucleusElectroncloud222ProtonsNeutronsElectronsMass number  42e2.3 Atoms consist of protons, neutrons, and electronsThe number of protons is the atom’s atomic number.An atom’s mass number is the sum of the number of protons and neutrons in the nucleus.The atomic mass is approximately equal to its mass number.© 2012 Pearson Education, Inc.Figure 2.3BCarbonElectroncloudNucleusProtonsNeutronsElectronsMass number  126e6662.3 Atoms consist of protons, neutrons, and electronsAlthough all atoms of an element have the same atomic number, some differ in mass number.Different isotopes of an element havethe same number of protons,but different numbers of neutrons.Different isotopes of an element behave identically in chemical reactions.In radioactive isotopes, the nucleus decays spontaneously, giving off particles and energy.© 2012 Pearson Education, Inc.Table 2.32.4 CONNECTION: Radioactive isotopes can help or harm usLiving cells cannot distinguish between isotopes of the same element.Therefore, radioactive compounds in metabolic processes can act as tracers.This radioactivity can be detected by instruments.Using these instruments, the fate of radioactive tracers can be monitored in living organisms.© 2012 Pearson Education, Inc.2.4 CONNECTION: Radioactive isotopes can help or harm usRadioactive tracers are frequently used in medical diagnosis.Sophisticated imaging instruments are used to detect them.An imaging instrument that uses positron-emission tomography (PET) detects the location of injected radioactive materials.PET is useful for diagnosing heart disorders, cancer, and in brain research.© 2012 Pearson Education, Inc.Figure 2.4AFigure 2.4BHealthy personAlzheimer’s patient2.4 CONNECTION: Radioactive isotopes can help or harm usIn addition to benefits, there are also dangers associated with using radioactive substances.Uncontrolled exposure can cause damage to some molecules in a living cell, especially DNA.Chemical bonds are broken by the emitted energy, which causes abnormal bonds to form.© 2012 Pearson Education, Inc.CHEMICAL BONDS© 2012 Pearson Education, Inc.2.5 The distribution of electrons determines an atom’s chemical propertiesOf the three subatomic particles—protons, neutrons, and electrons—only electrons are directly involved in chemical activity.Electrons occur in energy levels called electron shells.Information about the distribution of electrons is found in the periodic table of the elements.© 2012 Pearson Education, Inc.Figure 2.5HydrogenFirstshellSecondshellThirdshellLithiumSodiumBerylliumMagnesiumBoronAluminumCarbonSiliconNitrogenPhosphorusOxygenSulfurFluorineChlorineHeliumNeonArgon2.5 The distribution of electrons determines an atom’s chemical propertiesAn atom may have one, two, or three electron shells surrounding the nucleus.The number of electrons in the outermost shell determines the chemical properties of the atom.Atoms whose outer shells are not full tend to interact with other atoms, participating in chemical reactions.© 2012 Pearson Education, Inc.2.5 The distribution of electrons determines an atom’s chemical propertiesAtoms with incomplete outer shells tend to react so that both atoms end up with completed outer shells.These atoms may react with each other by sharing, donating, or receiving electrons.These interactions usually result in atoms staying close together, held by attractions called chemical bonds.© 2012 Pearson Education, Inc.2.6 Covalent bonds join atoms into molecules through electron sharingThe strongest kind of chemical bond is a covalent bond in which two atoms share one or more outer-shell electrons.Two or more atoms held together by covalent bonds form a molecule.© 2012 Pearson Education, Inc.2.6 Covalent bonds join atoms into molecules through electron sharingA covalent bond connects two hydrogen atoms in a molecule of the gas H2.There are four alternative ways to represent common molecules.© 2012 Pearson Education, Inc.Table 2.62.6 Covalent bonds join atoms into molecules through electron sharingAtoms in a covalently bonded molecule continually compete for shared electrons.The attraction (pull) for shared electrons is called electronegativity.More electronegative atoms pull harder.© 2012 Pearson Education, Inc.2.6 Covalent bonds join atoms into molecules through electron sharingIn molecules of only one element, the pull toward each atom is equal, because each atom has the same electronegativity.The bonds formed are called nonpolar covalent bonds.© 2012 Pearson Education, Inc.2.6 Covalent bonds join atoms into molecules through electron sharingWater has atoms with different electronegativities.Oxygen attracts the shared electrons more strongly than hydrogen.So, the shared electrons spend more time near oxygen.The oxygen atom has a slightly negative charge and the hydrogen atoms have a slightly positive charge. The result is a polar covalent bond.Because of these polar covalent bonds, water is a polar molecule.© 2012 Pearson Education, Inc.Figure 2.6(slightly )(slightly )(slightly )2.7 Ionic bonds are attractions between ions of opposite chargeAn ion is an atom or molecule with an electrical charge resulting from gain or loss of electrons.When an electron is lost, a positive charge results.When an electron is gained, a negative charge results.Two ions with opposite charges attract each other.When the attraction holds the ions together, it is called an ionic bond.Salt is a synonym for an ionic compound.© 2012 Pearson Education, Inc.Figure 2.7A_s2Transfer of electronNaSodium atomClChlorine atomFigure 2.7A_s2Transfer of electronNaSodium atomClChlorine atomNaSodium ionClChloride ionSodium chloride (NaCl)Figure 2.7BNaCl2.8 Hydrogen bonds are weak bonds important in the chemistry of lifeMost large molecules are held in their three-dimensional functional shape by weak bonds.Hydrogen, as part of a polar covalent bond, has a partial positive charge.The charged regions on molecules are electrically attracted to oppositely charged regions on neighboring molecules.Because the positively charged region is always a hydrogen atom, the bond is called a hydrogen bond.© 2012 Pearson Education, Inc.Figure 2.8Hydrogen bond2.9 Chemical reactions make and break chemical bondsRemember that the structure of atoms and molecules determines the way they behave.Remember that atoms combine to form molecules.Hydrogen and oxygen can react to form water:2H2 + O2 2H2O© 2012 Pearson Education, Inc.2.9 Chemical reactions make and break chemical bondsThe formation of water from hydrogen and oxygen is an example of a chemical reaction.The reactants (H2 and O2) are converted to H2O, the product.Chemical reactions do not create or destroy matter. Chemical reactions only rearrange matter.© 2012 Pearson Education, Inc.Figure 2.9ReactantsProducts2 H2O22 H2O2.9 Chemical reactions make and break chemical bondsPhotosynthesis is a chemical reaction that is essential to life on Earth.Carbon dioxide (from the air) reacts with water.Sunlight powers the conversion to produce the products glucose and oxygen.© 2012 Pearson Education, Inc.WATER’S LIFE-SUPPORTING PROPERTIES© 2012 Pearson Education, Inc.2.10 Hydrogen bonds make liquid water cohesiveThe tendency of molecules of the same kind to stick together is cohesion.Cohesion is much stronger for water than other liquids.Most plants depend upon cohesion to help transport water and nutrients from their roots to their leaves.The tendency of two kinds of molecules to stick together is adhesion.© 2012 Pearson Education, Inc.2.10 Hydrogen bonds make liquid water cohesiveCohesion is related to surface tension—a measure of how difficult it is to break the surface of a liquid.Hydrogen bonds give water high surface tension, making it behave as if it were coated with an invisible film.Water striders stand on water without breaking the water surface.© 2012 Pearson Education, Inc.Figure 2.102.11 Water’s hydrogen bonds moderate temperatureBecause of hydrogen bonding, water has a greater ability to resist temperature change than other liquids.Heat is the energy associated with movement of atoms and molecules in matter.Temperature measures the intensity of heat.Heat is released when hydrogen bonds form.Heat must be absorbed to break hydrogen bonds.© 2012 Pearson Education, Inc.2.11 Water’s hydrogen bonds moderate temperatureWhen a substance evaporates, the surface of the liquid that remains behind cools down, in the process of evaporative cooling.This cooling occurs because the molecules with the greatest energy leave the surface.© 2012 Pearson Education, Inc.2.12 Ice is less dense than liquid waterWater can exist as a gas, liquid, or solid.Water is less dense as a solid than a liquid because of hydrogen bonding.When water freezes, each molecule forms a stable hydrogen bond with its neighbors. As ice crystals form, the molecules are less densely packed than in liquid water. Because ice is less dense than water, it floats.© 2012 Pearson Education, Inc.Figure 2.12HydrogenbondIceHydrogen bonds are stable.Liquid waterHydrogen bonds constantly break and re-form.2.13 Water is the solvent of lifeA solution is a liquid consisting of a uniform mixture of two or more substances.The dissolving agent is the solvent.The substance that is dissolved is the solute.An aqueous solution is one in which water is the solvent.© 2012 Pearson Education, Inc.2.13 Water is the solvent of lifeWater’s versatility as a solvent results from the polarity of its molecules.Polar or charged solutes dissolve when water molecules surround them, forming aqueous solutions.Table salt is an example of a solute that will go into solution in water.© 2012 Pearson Education, Inc.Figure 2.13Ion insolutionSalt crystal2.14 The chemistry of life is sensitive to acidic and basic conditionsIn aqueous solutions, a small percentage of water molecules break apart into ions.Some are hydrogen ions (H+).Some are hydroxide ions (OH–).Both types are very reactive.© 2012 Pearson Education, Inc.2.14 The chemistry of life is sensitive to acidic and basic conditionsA compound that releases H+ to a solution is an acid.A compound that accepts H+ is a base.The pH scale describes how acidic or basic a solution is.The pH scale ranges from 0 to 14, with zero the most acidic and 14 the most basic.Each pH unit represents a tenfold change in the concentration of H+.© 2012 Pearson Education, Inc.2.14 The chemistry of life is sensitive to acidic and basic conditionsA buffer is a substance that minimizes changes in pH. Buffersaccept H+ when it is in excess anddonate H+ when it is depleted.© 2012 Pearson Education, Inc.Figure 2.14Acidic solutionNeutral solutionBasic solutionOven cleanerIncreasingly BASIC(Higher OH concentration)Increasingly ACIDIC(Higher H concentration)NEUTRAL[H][OH]Household bleachHousehold ammoniaMilk of magnesiaRainwaterHuman urineSalivaPure waterHuman blood,tearsSeawaterTomato juicepH scaleBattery acidLemon juice,gastric juiceVinegar, colaFigure 2.14_1Increasingly ACIDIC(Higher H concentration)Tomato juicepH scaleBattery acidLemon juice,gastric juiceVinegar, colaNEUTRAL[H][OH]RainwaterHuman urineSalivaPure waterFigure 2.14_2Oven cleanerIncreasingly BASIC(Higher OH concentration)NEUTRAL[H][OH]Household bleachHousehold ammoniaMilk of magnesiaPure waterHuman blood,tearsSeawaterpH scaleFigure 2.14_3AcidicsolutionNeutralsolutionBasicsolution