1. Lecture Outlines by Gregory Ahearn, University of North Florida Copyright © 2011 Pearson Education Inc. Chapter 2 Atoms, Molecules, and Life

2. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Chapter 2 At a Glance  2.1 What Are Atoms?  2.2 How Do Atoms Interact to Form Molecules?  2.3 Why is Water So Important to Life?

3. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.1 What Are Atoms?  2.1 What Are Atoms? –Atoms are basic structural units composed of still smaller particles –Electrons orbit the nucleus at fixed distances –Life depends on the ability of electrons to capture and release energy

4. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.1 What Are Atoms?  Atoms are the fundamental structural units of matter and are composed of three types of particles –In the central atomic nucleus, there are positively charged protons and uncharged neutrons –In orbit around the nucleus are negatively charged particles called electrons

5. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.1 What Are Atoms?  Atoms are electrically neutral because their number of positive protons and negative electrons is equal

6. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Atomic Models (a) Hydrogen (H) electron shell atomic nucleus ee pp (b) Helium (He) n n ee pp pp ee Fig. 2-1

7. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.1 What Are Atoms?  The number of protons in the nucleus of an atom is known as its atomic number  The atomic mass of an element is the total mass of its protons, neutrons, and electrons

8. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.1 What Are Atoms?  Elements and isotopes –An element is a substance that cannot be broken down by ordinary chemical reactions –All atoms belong to one of 92 types of naturally occurring elements

9. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.1 What Are Atoms?  Elements and isotopes (continued) –An atomic number (number of protons) is the defining value for an element –All atoms of an element have the same atomic number –For example, carbon has six protons, nitrogen has seven

10. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Table 2-1

11. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.1 What Are Atoms?  Elements and isotopes (continued) –However, atoms of an element may vary in the number of neutrons they have in the nucleus –Variant atomic forms of an element are called isotopes –Some isotopes are radioactive (meaning that they spontaneously break apart, forming different atoms and releasing energy) and are used in research –At room temperature, elements may occur as solids, liquids, or gases

12. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Atomic Structure

13. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.1 What Are Atoms?  Electron shells –Larger atoms can accommodate more electrons than smaller ones can –Electrons are distributed around the nucleus of an atom in electron shells –The first shell, or energy level, holds two electrons –The second shell holds up to eight

14. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Electron Shells in Atoms Fig. 2-2 Carbon (C) Oxygen (O) Phosphorus (P) Calcium (Ca) Ca O C P 4e4e 6e6e 5e5e 2e2e 8e8e 8e8e 8e8e 2e2e 2e2e 2e2e 2e2e 6p6p 8p8p 15p  20p  6n6n8n8n 16n20n

15. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.1 What Are Atoms?  Electron shells (continued) –The roles of the nucleus and the electrons are: –The nucleus provides stability –The electrons interact with other atoms (for example, to form chemical bonds) –Chemical bonds are the force of attraction between neighboring atoms that holds them together within a molecule

16. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.1 What Are Atoms?  Energy capture and release –Life depends on electrons capturing and releasing energy –Electron shells correspond to energy levels –When energy excites an atom, it causes an electron to jump from a lower- to a higher- energy shell –Later, the electron falls back into its original shell, releasing the energy

17. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Energy Capture and Release Fig. 2-3 heat energy light   An electron absorbs energy The energy boosts the electron to a higher-energy shell The electron drops back into lower-energy shell, releasing energy as light     1 2 3

18. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Atoms interact when there are vacancies in the outermost electron shells  Highly reactive free radicals can damage cells  Chemical bonds hold atoms together in molecules

19. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Ionic bonds form among charged atoms that are called ions  Covalent bonds form between uncharged atoms that share electrons  Hydrogen bonds are attractive forces between polar molecules

20. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Interaction between atoms –Molecules consist of two or more atoms, from the same or different elements, that are held together by interactions among their outermost electron shells –A substance made of atoms of different elements is a compound –Reactions between atoms depend upon the configuration of electrons in the outermost electron shell

21. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Interaction between atoms (continued) –Atoms will not react with other atoms if the outermost shell is completely empty or full (such atoms are considered inert) –Example: Neon, with eight electrons in its outermost shell is full, and therefore is inert

22. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Interaction between atoms (continued) –Atoms will react with other atoms if the outermost shell is partially full (such atoms are considered reactive) –Example: Oxygen, with six electrons in its outermost shell, can hold two more electrons, and so is susceptible to reacting

23. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Interaction between atoms (continued) –Reactive atoms gain stability through electron interactions (chemical reactions) –To empty the outermost shell, electrons can be lost –To fill the outermost shell, electrons can be gained –Electrons can be shared with another atom when both atoms have full outermost shells

24. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Interaction between atoms (continued) –Hydrogen and oxygen atoms gain stability by interacting with each other –Single electrons from each of two hydrogen molecules fill the outer shell of an oxygen atom

25. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Free radicals –Some cellular reactions produce free radicals –A free radical is a molecule in which atoms have one or more unpaired electrons in their outer shells

26. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Free radicals (continued) –Free radicals are highly unstable and reactive –Free radicals steal electrons, destroying other molecules –Cell death can occur from free radical attack

27. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Free radicals (continued) –Free radicals are implicated in heart disease, Alzheimer’s, cancer, and aging –Antioxidants like vitamins C and E render free radicals harmless

28. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Chemical bonds hold atoms together within molecules –A chemical reaction is a process by which new chemical bonds are formed or existing bonds are broken, converting one substance into another –There are three major types of chemical bonds: ionic bonds, covalent bonds, and hydrogen bonds

29. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Table 2-2

30. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Ions and ionic bonds –Atoms that have lost or gained electrons, thereby altering the balance between protons and electrons, are charged, and are called ions –Atoms that have lost electrons become positively charged ions (e.g., sodium: Na + ) –Atoms that have gained electrons become negatively charged ions (e.g., chlorine: Cl – )

31. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Ions and ionic bonds (continued) –Oppositely charged ions that are attracted to each other are bound into a molecule by ionic bonds

32. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Ionic Bonds

33. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e The Formation of Ions and Ionic Bonds Fig. 2-5 Electron transferred Na  Cl  Na  Cl  Na  Cl  (b) Ions (c) An ionic compound: NaCl Attraction between opposite charges – – – – – – – –– – 11p  11n – – – –– – – – – – – – – – –– – – 17p  18n Sodium ion (  ) Chloride ion (–) (a) Neutral atoms Sodium atom (neutral)Chlorine atom (neutral) – – – –– – – – – – – 11p  11n –– – – –– – – – – – – – –– – – 17p  18n

34. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Ions and ionic bonds (continued) –Salt crystals are formed by a repeated, orderly arrangement of sodium and chloride ions

35. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Covalent bonds form between uncharged atoms that share electrons –An atom with a partially full outermost electron shell can become stable by sharing electrons with another atom, forming a covalent bond –Two electrons (one from each atom), when shared, form a covalent bond

36. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Covalent bonds form between uncharged atoms that share electrons (continued) –Covalent bonds are found in H 2 (single bond), O 2 (double bond), N 2 (triple bond), and H 2 O –Covalent bonds are stronger than ionic bonds but vary in their stability

37. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Covalent bonds form between uncharged atoms that share electrons (continued) –Because biological molecules must function in a watery environment where ionic bonds rapidly dissociate (break down), the atoms in most biological molecules, such as those found in proteins, sugars, and fats, are joined by covalent bonds

38. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Table 2-3

39. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  How electrons are shared determines whether a covalent bond is nonpolar or polar –Atoms within a molecule may have different nuclear charges –Those atoms with a greater positive nuclear charge pull more strongly on electrons in a covalent bond

40. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  How electrons are shared determines whether a covalent bond is nonpolar or polar (continued) –In molecules like H 2, both atoms exert the same pulling force on bond electrons and the bond is called a nonpolar covalent bond

41. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  How electrons are shared determines whether a covalent bond is nonpolar or polar (continued) –In molecules where atoms of different elements are involved (H 2 O), the electrons are not always equally shared and these covalent bonds are called polar covalent bonds

42. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  How electrons are shared determines whether a covalent bond is nonpolar or polar (continued) –A molecule with polar bonds may be polar overall –H 2 O is a polar molecule –The (slightly) positively charged pole is around each hydrogen –The (slightly) negatively charged pole is around the oxygen

43. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Covalent Bonds

44. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Covalent Bonds Involve Shared Electrons Fig. 2-6 (b) Polar covalent bonding in water (H 2 O) + (+) (oxygen: slightly negative) (–) (hydrogens: slightly positive) 8p8n8p8n + _ _ _ _ _ _ _ _ _ _ _ _ (a) Nonpolar covalent bonding in hydrogen gas (H 2 ) (hydrogens: uncharged) + + Electrons spend equal time near each nucleus Same charge on both nuclei Larger positive charge Electrons spend more time near the larger nucleus Smaller positive charge

45. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Hydrogen bonds are attractive forces between polar molecules –Polar molecules—such as those in water—have partially charged atoms at their ends –Hydrogen bonds form when partial opposite charges in different molecules attract each other –The partially positive hydrogen atoms of one water molecule are attracted to the partially negative oxygen on another

46. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.2 How Do Atoms Interact to Form Molecules?  Hydrogen bonds are attractive forces between polar molecules (continued) –Polar biological molecules can form hydrogen bonds with water, each other, or even within the same molecule –Hydrogen bonds are comparatively weak but, collectively, can be quite strong

47. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Hydrogen Bonds in Water Fig. 2-7

48. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water molecules attract one another –Cohesion is the tendency of the molecules of a substance to stick together –Hydrogen bonding between water molecules produces high cohesion –Water cohesion explains how water molecules can form a chain in delivering moisture to the top of a tree

49. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water molecules attract one another (continued) –Cohesion of water molecules along a surface produces surface tension –Spiders and water striders rely on surface tension to move across the surface of ponds

50. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Cohesion Among Water Molecules Creates Surface Tension Fig. 2-8

51. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water molecules attract one another (continued) –Water molecules stick to polar or charged surfaces according to the property called adhesion –Adhesion helps water climb up the thin tubes of plants to the leaves

52. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water interacts with many other molecules –Water is an excellent solvent –A wide range of substances dissolve (are completely surrounded and dispersed) in water to form solutions

53. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Water is Polar

54. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Water as a Solvent Fig. 2-9

55. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water interacts with many other molecules (continued) –Water-soluble molecules are hydrophilic –Water molecules are attracted to and can surround (and thereby dissolve) ions or polar molecules, such as sugars and amino acids

56. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water interacts with many other molecules (continued) –Water-insoluble molecules that repel and drive together uncharged and nonpolar molecules like fats and oils are hydrophobic –The “clumping” of nonpolar molecules is called hydrophobic interaction

57. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Oil and Water Don’t Mix Fig. 2-10

58. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water-based solutions can be acidic, basic, or neutral –A small fraction of water molecules break apart into ions: H 2 O  OH – + H +

59. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Some Water Is Ionized Fig. 2-11 hydrogen ion (H  ) hydroxide ion (OH  ) water (H 2 O)  (  )(  ) O HH O H H

60. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water-based solutions can be acidic, basic, or neutral (continued) –Solutions where H + > OH – are acidic –For example, hydrochloric acid ionizes in water: HCl  H + + Cl – –Lemon juice and vinegar are naturally occurring acids

61. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water-based solutions can be acidic, basic, or neutral (continued) –Solutions where OH – > H + are basic –For example, sodium hydroxide ionizes in water: NaOH  Na + + OH – –Baking soda, chlorine bleach, and ammonia are basic

62. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water-based solutions can be acidic, basic, or neutral (continued) –The degree of acidity of a solution is measured using the pH scale –pH 0–6 is acidic (H + > OH – ) –pH 7 is neutral (H + = OH – ) –pH 8–14 is basic (OH – > H + )

63. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e The pH Scale Fig. 2-12 1 molar hydrochloric acid (HCI) stomach acid (2)lemon juice (2.3) "acid rain" (2.5–5.5) beer (4.1)tomatoes (4.5)black coffee (5.0)normal rain (5.6) milk (6.4) pure water (7.0) seawater (7.8–8.3)baking soda (8.4) antacid (10) household ammonia (11.9)washing soda (12) oven cleaner (13.0) 1 molar sodium hydroxide (NaOH) 01234567891011121314 pH value H  concentration in moles/liter 10 0 10 –1 10 –2 10 –3 10 –4 10 –5 10 –6 10 –7 10 –8 10 –9 10 –10 10 –11 10 –12 10 –13 10 –14 neutral (H   OH  ) (H   OH  ) (H  < OH  ) vinegar, cola (3.0) urine (5.7) blood, sweat (7.4) chlorine bleach (12.6) drain cleaner (14.0) orange (3.5) increasingly acidic increasingly basic

64. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  A buffer helps maintain a relatively constant pH in a solution –A buffer is a compound that accepts or releases H + in response to a pH change –The bicarbonate buffer found in our bloodstream prevents pH changes

65. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  A buffer helps maintain a relatively constant pH in a solution (continued) –If the blood becomes too acidic, bicarbonate accepts (and absorbs) H + to make carbonic acid HCO 3 – + H +  H 2 CO 3 bicarbonate hydrogen ion carbonic acid

66. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  A buffer helps maintain a relatively constant pH in a solution (continued) –If the blood becomes too basic, carbonic acid liberates hydrogen ions to combine with OH – to form water H 2 CO 3 + OH –  HCO 3 – + H 2 O carbonic acid hydroxide ion bicarbonate water

67. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water moderates the effects of temperature change –Very low or very high temperatures may damage enzymes or slow down important chemical reactions

68. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water moderates the effects of temperature change (continued) –The energy required to heat 1 gram of a substance by 1°C is called its specific heat –It takes a lot of energy to heat water –Temperature reflects the speed of molecular motion –It requires 1 calorie of energy to raise the temperature of 1g of water 1°C (the specific heat of water), which is a very slow process

69. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water moderates the effects of temperature change (continued) –The heat of vaporization is the amount of heat needed to cause a substance such as water to evaporate (to change from a liquid to a vapor) –Evaporating water uses up heat from its surroundings, cooling the nearby environment (as occurs during sweating)

70. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water moderates the effects of temperature change (continued) –It takes a lot of energy to cause water to evaporate –Because the human body is mostly water, a sunbather can absorb a lot of heat energy without sending her/his body temperature soaring

71. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Hydrogen Bonds and Specific Heat

72. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Water’s High Specific Heat and High Heat of Vaporization Fig. 2-13

73. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water forms an unusual solid: ice –Most substances become denser when they solidify from a liquid –Ice is unusual because it is less dense than liquid water –Water molecules spread apart slightly during the freezing process

74. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Author Animation: Hydrogen Bonds in Water

75. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Water and Ice Fig. 2-14

76. Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 2.3 Why Is Water So Important to Life?  Water forms an unusual solid: ice (continued) –Ice floats in liquid water –Ponds and lakes freeze from the top down and never freeze completely to the bottom –Many plants and fish are therefore saved from freezing