Chapter 2 The Chemical Basis of Life

Transcription

1 Chapter 2 The Chemical Basis of Life PowerPoint Lectures Campbell Biology: Concepts & Connections, Eighth Edition REECE TAYLOR SIMON DICKEY HOGAN Lecture by Edward J. Zalisko

2 Figure 2.0-1

3 Figure Chapter 2: Big Ideas H O H Elements, Atoms, and Compounds Chemical Bonds Water s Life- Supporting Properties

4 Introduction Chemicals are the raw materials that make up our bodies, the bodies of other organisms, and the physical environment.

5 Introduction Life s chemistry is tied to water. Life first evolved in water. All living organisms require water. Cells consist of about 75% water.

6 ELEMENTS, ATOMS, AND COMPOUNDS

7 2.1 Organisms are composed of elements, in combinations called compounds 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 by ordinary chemical means. There are 92 elements in nature only a few exist in a pure state.

8 Table 2.1

9 2.1 Organisms are composed of elements, in combinations called compounds A 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).

10 Figure Sodium (Na) Chlorine (Cl) Sodium chloride (NaCl)

11 Figure Sodium (Na)

12 Figure Chlorine (Cl)

13 Figure Sodium chloride (NaCl)

14 2.1 Organisms are composed of elements, in combinations called compounds About 25 elements are essential for human life. Four elements make up about 96% of the weight of most living organisms. These are oxygen, carbon, hydrogen, and nitrogen. Trace elements are essential but are only needed in minute quantities.

15 2.2 CONNECTION: Trace elements are common additives to food and water Some 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.

16 Figure 2.2a

17 2.2 CONNECTION: Trace elements are common additives to food and water Fluoride is usually added to municipal water and dental products to help reduce tooth decay.

18 Figure 2.2b

19 2.2 CONNECTION: Trace elements are common additives to food and water Several chemicals are added to food to help preserve it, make it more nutritious, and/or make it look better. Check out the nutrition facts label on foods and drinks you purchase.

20 Figure 2.2c

21 2.3 Atoms consist of protons, neutrons, and electrons Each 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.

22 2.3 Atoms consist of protons, neutrons, and electrons Neutrons 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.

23 Figure 2.3 Nucleus 2e Electron cloud Protons Neutrons Nucleus 2 Electrons

24 2.3 Atoms consist of protons, neutrons, and electrons The 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.

25 Figure 2.3 Nucleus 2e Electron cloud Protons Neutrons Nucleus 2 Electrons

26 2.3 Atoms consist of protons, neutrons, and electrons Although all atoms of an element have the same atomic number, some differ in mass number. Different isotopes of an element have the 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.

27 Table 2.3

28 2.4 CONNECTION: Radioactive isotopes can help or harm us Living 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. By using these instruments, the fate of radioactive tracers can be monitored in living organisms.

29 2.4 CONNECTION: Radioactive isotopes can help or harm us Radioactive 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 and cancer and in brain research.

30 Figure 2.4a

31 Figure 2.4b Healthy person Alzheimer s patient

32 2.4 CONNECTION: Radioactive isotopes can help or harm us In 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.

33 CHEMICAL BONDS

34 2.5 The distribution of electrons determines an atom s chemical properties Of the three subatomic particles protons, neutrons, and electrons only electrons are directly involved in the chemical activity of an atom. Electrons can be located in different electron shells, each with a characteristic distance from the nucleus. An atom may have one, two, or more electron shells. Information about the distribution of electrons is found in the periodic table of the elements.

35 Figure 2.5a

36 Figure 2.5b-0 First shell Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Second shell Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Argon Third shell

37 Figure 2.5b-1 Hydrogen Lithium Beryllium Boron Carbon Sodium Magnesium Aluminum Silicon

38 Figure 2.5b-2 Helium Nitrogen Oxygen Fluorine Neon Phosphorus Sulfur Chlorine Argon

39 2.5 The distribution of electrons determines an atom s chemical properties The number of electrons in the outermost shell, called the valence shell, determines the chemical properties of the atom. Atoms whose outer shells are not full tend to interact with other atoms in ways that enable them to complete or fill their valence shells.

40 2.5 The distribution of electrons determines an atom s chemical properties When two atoms with incomplete outer shells react, each atom will share, donate, or receive electrons, so that both partners end up with completed outer shells. These interactions usually result in atoms staying close together, held by attractions called chemical bonds.

41 2.6 Covalent bonds join atoms into molecules through electron sharing In a covalent bond, two atoms, each with an unpaired electron in its outer shell, actually share a pair of electrons. Two or more atoms held together by covalent bonds form a molecule. A covalent bond connects two hydrogen atoms in a molecule of the gas H 2.

42 Animation: Covalent Bonds

43 2.6 Covalent bonds join atoms into molecules through electron sharing There are four alternative ways to represent common molecules. The hydrogen atoms in H 2 are held together by a pair of shared electrons. In an oxygen molecule (O 2 ), the two oxygen atoms share two pairs of electrons, forming a double bond, indicated in a structural formula by a pair of lines.

44 Figure Molecular Formula Electron Distribution Diagram Structural Formula Space-Filling Model H 2 Hydrogen H H Single bond O 2 Oxygen O O Double bond H CH 4 Methane H C H H Nonpolar covalent bonds H 2 O Water H O H Polar covalent bonds (slightly ) Polar covalent bonds in a water molecule H O H (slightly +) (slightly +)

45 Figure Molecular Formula Electron Distribution Diagram Structural Formula Space-Filling Model H 2 Hydrogen H H Single bond O 2 Oxygen O O Double bond

46 Figure Molecular Formula Electron Distribution Diagram Structural Formula Space-Filling Model H CH 4 Methane H C H H Nonpolar covalent bonds H 2 O Water H O H Polar covalent bonds

47 Figure (slightly ) O H H (slightly +) (slightly +) Polar covalent bonds in a water molecule

48 2.6 Covalent bonds join atoms into molecules through electron sharing H 2 and O 2 are molecules composed of only one element. Methane (CH 4 ) and water (H 2 O) are compounds, substances composed of two or more different elements.

49 2.6 Covalent bonds join atoms into molecules through electron sharing Atoms in a covalently bonded molecule continually compete for shared electrons. The attraction (pull) for shared electrons is called electronegativity. More electronegative atoms pull harder.

50 2.6 Covalent bonds join atoms into molecules through electron sharing In 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.

51 2.6 Covalent bonds join atoms into molecules through electron sharing Water 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.

52 2.7 Ionic bonds are attractions between ions of opposite charge An ion is an atom or molecule with an electrical charge resulting from gain or loss of one or more 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.

53 Animation: Ionic Bonds

54 Figure 2.7a-1 Na Cl Na Sodium atom Cl Chlorine atom

55 Figure 2.7a-2 + Na Cl Na + Cl Na Sodium atom Cl Chlorine atom Na + Sodium ion Cl Chloride ion Sodium chloride (NaCl)

56 Figure 2.7b-0 Cl Na +

57 Figure 2.7b-1

58 2.8 Hydrogen bonds are weak bonds important in the chemistry of life In living organisms, most of the strong chemical bonds are covalent, linking atoms to form a cell s molecules. Crucial to the functioning of a cell are weaker bonds within and between molecules. One of the most important types of weak bonds is the hydrogen bond, which is best illustrated with water molecules.

59 2.8 Hydrogen bonds are weak bonds important in the chemistry of life The hydrogen atoms of a water molecule are attached to oxygen by polar covalent bonds. Because of these polar bonds and the wide V shape of the molecule, water is a polar molecule that is, it has an unequal distribution of charges. This partial positive charge allows each hydrogen to be attracted to a nearby atom that has a partial negative charge.

60 2.8 Hydrogen bonds are weak bonds important in the chemistry of life Weak hydrogen bonds form between water molecules. Each hydrogen atom of a water molecule can form a hydrogen bond with a nearby partially negative oxygen atom of another water molecule. The negative (oxygen) pole of a water molecule can form hydrogen bonds to two hydrogen atoms. Thus, each H 2 O molecule can hydrogen-bond to as many as four partners.

61 Animation: Water Structure

62 Figure 2.8 ( ) (+) Hydrogen bond (+) ( ) ( ) (+) (+) Polar covalent bonds ( )

63 2.9 Chemical reactions make and break chemical bonds Remember that the structure of atoms and molecules determines the way they behave. Atoms combine to form molecules. Hydrogen and oxygen can react to form water: 2 H 2 + O 2 2 H 2 O

64 2.9 Chemical reactions make and break chemical bonds The formation of water from hydrogen and oxygen is an example of a chemical reaction. The reactants (H 2 and O 2 ) are converted to H 2 O, the product. Chemical reactions do not create or destroy matter. Chemical reactions only rearrange matter.

65 Figure H 2 + O 2 2 H 2 O Reactants Products

66 2.9 Chemical reactions make and break chemical bonds Photosynthesis is a chemical reaction that is essential to life on Earth. Carbon dioxide (from the air) reacts with water. Sunlight powers the conversion of these reactants to produce the products glucose and oxygen.

67 WATER S LIFE-SUPPORTING PROPERTIES

68 2.10 Hydrogen bonds make liquid water cohesive The tendency of molecules of the same kind to stick together is cohesion. Cohesion is much stronger for water than for 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.

69 2.10 Hydrogen bonds make liquid water cohesive Cohesion 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.

70 Animation: Water Transport

71 Figure 2.10

72 2.11 Water s hydrogen bonds moderate temperature Thermal energy is the energy associated with the random movement of atoms and molecules. Thermal energy in transfer from a warmer to a cooler body of matter is defined as heat. Temperature measures the intensity of heat that is, the average speed of molecules in a body of matter.

73 2.11 Water s hydrogen bonds moderate temperature Heat must be absorbed to break hydrogen bonds. Heat is released when hydrogen bonds form. To raise the temperature of water, hydrogen bonds between water molecules must be broken before the molecules can move faster. Thus, when warming up, water absorbs a large amount of heat and when water cools, water molecules slow down, more hydrogen bonds form, and a considerable amount of heat is released.

74 2.11 Water s hydrogen bonds moderate temperature Earth s giant water supply moderates temperatures, helping to keep temperatures within limits that permit life. Water s resistance to temperature change also stabilizes ocean temperatures, creating a favorable environment for marine life.

75 2.11 Water s hydrogen bonds moderate temperature When a substance evaporates, the surface of the liquid that remains behind cools down; this is the process of evaporative cooling. This cooling occurs because the molecules with the greatest energy leave the surface.

76 Figure 2.11

77 2.12 Ice floats because it is less dense than liquid water Water 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.

78 Figure Ice Hydrogen bonds are stable. Hydrogen bond Liquid water Hydrogen bonds constantly break and re-form.

79 Figure

80 2.13 Water is the solvent of life A 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.

81 2.13 Water is the solvent of life Water 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.

82 Figure 2.13 Positive hydrogen ends of water molecules attracted to negative chloride ion Negative oxygen ends of water molecules attracted to positive sodium ion Cl Na Na Cl + + Salt crystal

83 2.14 The chemistry of life is sensitive to acidic and basic conditions In liquid water, 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.

84 2.14 The chemistry of life is sensitive to acidic and basic conditions A substance that donates hydrogen ions to solutions is called an acid. A base is a substance that reduces the hydrogen ion concentration of a solution. The ph scale describes how acidic or basic a solution is. The ph scale ranges from 0 to 14, with 0 the most acidic and 14 the most basic. Each ph unit represents a 10-fold change in the concentration of H + in a solution.

85 2.14 The chemistry of life is sensitive to acidic and basic conditions A buffer is a substance that minimizes changes in ph. Buffers accept H + when it is in excess and donate H + when it is depleted.

86 Increasingly BASIC (Higher OH concentration) Increasingly ACIDIC (Higher H + concentration) Figure ph scale Battery acid Lemon juice, gastric juice Vinegar, cola Tomato juice H + H + H + OH OH H + H + H + H + H + Acidic solution NEUTRAL [H + ] = [OH ] Rainwater Human urine Saliva Pure water Human blood, tears Seawater H + H + OH OH H + OH OH OH H + H + Neutral solution Milk of magnesia Household ammonia Household bleach Oven cleaner OH OH OH OH H + OH OH OH H + Basic solution

87 Increasingly ACIDIC (Higher H + concentration) Figure ph scale 0 Battery acid 1 NEUTRAL [H + ] = [OH ] Lemon juice, gastric juice Vinegar, cola Tomato juice Rainwater Human urine Saliva Pure water

88 Increasingly BASIC (Higher OH concentration) Figure NEUTRAL [H + ] = [OH ] ph scale 7 8 Pure water Human blood, tears Seawater Milk of magnesia Household ammonia Household bleach Oven cleaner

89 Figure H + H + H + OH OH H + H + H + H + H + Acidic solution OH OH H + OH OH OH H + H + H + H + Neutral solution OH OH OH OH H + OH OH OH H + Basic solution

90 2.15 SCIENTIFIC THINKING: Scientists study the effects of rising atmospheric CO 2 on coral reef ecosystems Carbon dioxide is the main product of fossil fuel combustion, increasing in the atmosphere, and linked to global climate change.

91 2.15 SCIENTIFIC THINKING: Scientists study the effects of rising atmospheric CO 2 on coral reef ecosystems About 25% of this human-generated CO 2 is absorbed by the vast oceans. CO 2 dissolved in seawater lowers the ph of the ocean in a process known as ocean acidification.

92 2.15 SCIENTIFIC THINKING: Scientists study the effects of rising atmospheric CO 2 on coral reef ecosystems As seawater acidifies, the extra hydrogen ions (H + ) combine with carbonate ions (CO 3 2 ) to form bicarbonate ions (HCO 3 ). This reaction reduces the carbonate ion concentration available to corals and other shellbuilding animals.

93 2.15 SCIENTIFIC THINKING: Scientists study the effects of rising atmospheric CO 2 on coral reef ecosystems In a controlled experiment, scientists looked at the effect of decreasing carbonate ion concentration on the rate of calcium deposition by reef organisms. The lower the concentration of carbonate ions, the lower the rate of calcification, and thus the slower the growth of coral animals. The results from experimental and observational field studies of sites where ph naturally varies have dire implications for the health of coral reefs and the diversity of organisms they support.

94 Figure 2.15a Calcification rate (mmol CaCO 3 /m 2 day) [CO 2 3 ] (μmol/kg of seawater) Source: Adaption of figure 5 from Effect of Calcium Carbonate Saturation State on the Calcification Rate of an Experimental Coral Reef by C. Langdon, et al., from Global Biogeochemical Cycles, June 2000, Volume 14(2). Copyright 2000 by American Geophysical Union. Reprinted with permission of Wiley Inc.

95 Figure 2.15b Rising CO 2 bubbles lower the ph of the water

96 2.16 EVOLUTION CONNECTION: The search for extraterrestrial life centers on the search for water The emergent properties of water support life on Earth. When astrobiologists search for signs of extraterrestrial life on distant planets, they look for evidence of water. The National Aeronautics and Space Administration (NASA) has found evidence that water was once abundant on Mars.

97 You should now be able to 1. Describe the importance of chemical elements to living organisms. 2. Explain the formation of compounds. 3. Describe the structure of an atom. 4. Distinguish between ionic, hydrogen, and covalent bonds. 5. Define a chemical reaction and explain how it changes the composition of matter. 6. List and define the life-supporting properties of water. 7. Explain the ph scale and the formation of acid and base solutions.

98 Figure 2.UN01 Protons (+ charge) determine element Neutrons (no charge) determine isotope + Atom Nucleus Electrons ( charge) + form negative cloud and determine chemical behavior

99 Figure 2.UN02 Liquid water: Hydrogen bonds constantly break and re-form Ice: Stable hydrogen bonds hold molecules apart

100 Figure 2.UN03-0 Atoms have positively charged have neutral have negatively charged (a) number present equals atomic number of each element (b) (d) number may differ in (c) number in outer shell determines formation of H Chemical Bonds electron transfer between atoms creates electron sharing between atoms creates Na Cl H H ions (e) attraction between ions creates unequal equal sharing creates sharing creates (f) (g) nonpolar covalent bonds H ( ) O H (+) (+) water example is has important qualities due to polarity and can lead to (h) H (+) ( ) O

101 Figure 2.UN03-1 have positively charged Atoms have neutral have negatively charged (a) (b) (c) number present equals atomic number of each element (d) number may differ in number in outer shell determines formation of H Chemical Bonds

102 Figure 2.UN03-2 Chemical Bonds electron transfer between atoms creates electron sharing between atoms creates Na Cl H H ions (e) attraction between ions creates unequal sharing creates equal sharing creates (f) (g) nonpolar covalent bonds H ( ) O H (+) (+) water example is has important qualities due to polarity and can lead to (h) H (+) ( ) O

103 Figure 2.UN04 F K Fluorine atom Potassium atom