Life Chemistry and Energy

1. 2 Life Chemistry and Energy

2. Chapter 2 Life Chemistry and Energy • Key Concepts • 2.1 Atomic Structure Is the Basis for Life’s Chemistry • 2.2 Atoms Interact and Form Molecules • 2.3 Carbohydrates Consist of Sugar Molecules • 2.4 Lipids Are Hydrophobic Molecules • 2.5 Biochemical Changes Involve Energy

3. Chapter 2 Opening Question Why is the search for water important in the search for life?

4. Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry Living and nonliving matter is composed of atoms.

5. Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry Like charges repel; different charges attract. Most atoms are neutral because the number of electrons equals the number of protons. Dalton—mass of one proton or neutron (1.7 × 10–24 grams) Mass of electrons is so tiny, it is usually ignored.

6. Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry Element—pure substance that contains only one kind of atom Living things are mostly composed of 6 elements: Carbon (C) Hydrogen (H) Nitrogen (N) Oxygen (O) Phosphorus (P) Sulfur (S)

7. Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry The number of protons identifies an element. Number of protons = atomic number For electrical neutrality, # protons = # electrons. Mass number—total number of protons and neutrons

8. Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry A Bohr model for atomic structure—the atom is largely empty space, and the electrons occur in orbits, or electron shells.

9. Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry Behavior of electrons determines whether a chemical bond will form and what shape the bond will have.

10. Figure 2.1 Electron Shells

11. Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry Atoms with unfilled outer shells tend to undergo chemical reactions to fill their outer shells. They can attain stability by sharing electrons with other atoms or by losing or gaining electrons. The atoms are then bonded together into molecules.

12. Concept 2.1 Atomic Structure Is the Basis for Life’s Chemistry Octet rule—atoms with at least two electron shells form stable molecules so they have eight electrons in their outermost shells.

13. Concept 2.2 Atoms Interact and Form Molecules Chemical bond is an attractive force that links atoms together to form molecules. There are several kinds of chemical bonds.

14. Table 2.1 Chemical Bonds and Interactions

15. Concept 2.2 Atoms Interact and Form Molecules Ionic bonds Ions are charged particle that form when an atom gains or loses one or more electrons. Cations—positively charged ions Anions—negatively charged ions Ionic bonds result from the electrical attraction between ions with opposite charges. The resulting molecules are called salts.

16. Figure 2.2 Ionic Bond between Sodium and Chlorine

17. Concept 2.2 Atoms Interact and Form Molecules Ionic attractions are weak, so salts dissolve easily in water.

18. Concept 2.2 Atoms Interact and Form Molecules Covalent bonds Covalent bonds form when two atoms share pairs of electrons. The atoms attain stability by having full outer shells. Each atom contributes one member of the electron pair.

19. Figure 2.3 Electrons Are Shared in Covalent Bonds

20. Concept 2.2 Atoms Interact and Form Molecules Carbon atoms have four electrons in the outer shell—they can form covalent bonds with four other atoms.

21. Figure 2.4 Covalent Bonding (Part 1)

22. Figure 2.4 Covalent Bonding (Part 2)

23. Concept 2.2 Atoms Interact and Form Molecules Properties of molecules are influenced by characteristics of the covalent bonds: Orientation—length, angle, and direction of bonds between any two elements are always the same. Example: Methane always forms a tetrahedron.

24. Concept 2.2 Atoms Interact and Form Molecules Strength and stability—covalent bonds are very strong; it takes a lot of energy to break them. Multiple bonds Single—sharing 1 pair of electrons Double—sharing 2 pairs of electrons Triple—sharing 3 pairs of electrons N N C H C C

25. Concept 2.2 Atoms Interact and Form Molecules Degree of sharing electrons is not always equal. Electronegativity—the attractive force that an atomic nucleus exerts on electrons It depends on the number of protons and the distance between the nucleus and electrons.

26. Table 2.2 Some Electronegativities

27. Concept 2.2 Atoms Interact and Form Molecules If two atoms have similar electronegativities, they share electrons equally, in what is called a nonpolar covalent bond. If atoms have different electronegativities, electrons tend to be near the most attractive atom, in what is called a polar covalent bond

28. Concept 2.2 Atoms Interact and Form Molecules Hydrogen bonds Attraction between the δ– end of one molecule and the δ+ hydrogen end of another molecule forms hydrogen bonds. They form between water molecules. They are important in the structure of DNA and proteins.

29. Figure 2.5 Hydrogen Bonds Can Form between or within Molecules

30. Concept 2.2 Atoms Interact and Form Molecules Water molecules form multiple hydrogen bonds with each other—this contributes to high heat capacity.

31. Concept 2.2 Atoms Interact and Form Molecules A lot of heat is required to raise the temperature of water—the heat energy breaks the hydrogen bonds. In organisms, presence of water shields them from fluctuations in environmental temperature.

32. Concept 2.2 Atoms Interact and Form Molecules Water has a high heat of vaporization—a lot of heat is required to change water from liquid to gaseous state. Thus, evaporation has a cooling effect on the environment. Sweating cools the body—as sweat evaporates from the skin, it transforms some of the adjacent body heat.

33. Concept 2.2 Atoms Interact and Form Molecules Hydrogen bonds also give water cohesive strength, or cohesion—water molecules resist coming apart when placed under tension. This permits narrow columns of water to move from roots to leaves of plants.

34. Concept 2.2 Atoms Interact and Form Molecules Any polar molecule can interact with any other polar molecule through hydrogen bonds. Hydrophilic (“water-loving”)—in aqueous solutions, polar molecules become separated and surrounded by water molecules Nonpolar molecules are called hydrophobic (“water-hating”); the interactions between them are hydrophobic interactions.

35. Figure 2.6 Hydrophilic and Hydrophobic

36. Concept 2.2 Atoms Interact and Form Molecules Functional groups—small groups of atoms with specific chemical properties Functional groups confer these properties to larger molecules, e.g., polarity. One biological molecule may contain many functional groups.

37. Figure 2.7 Functional Groups Important to Living Systems (Part 1)

38. Figure 2.7 Functional Groups Important to Living Systems (Part 2)

39. Concept 2.2 Atoms Interact and Form Molecules Macromolecules Most biological molecules are polymers (poly, “many”; mer, “unit”), made by covalent bonding of smaller molecules called monomers.

40. Concept 2.2 Atoms Interact and Form Molecules Proteins: Formed from different combinations of 20 amino acids • Carbohydrates—formed by linking similar sugar monomers (monosaccharides) to form polysaccharides • Nucleic acids—formed from four kinds of nucleotide monomers • Lipids—noncovalent forces maintain the interactions between the lipid monomers

41. Concept 2.2 Atoms Interact and Form Molecules Polymers are formed and broken apart in reactions involving water. • Condensation—removal of water links monomers together • Hydrolysis—addition of water breaks a polymer into monomers

42. Figure 2.8 Condensation and Hydrolysis of Polymers (Part 1)

43. Figure 2.8 Condensation and Hydrolysis of Polymers (Part 2)

44. Concept 2.3 Carbohydrates Consist of Sugar Molecules Carbohydrates Source of stored energy • Transport stored energy within complex organisms • Structural molecules that give many organisms their shapes • Recognition or signaling molecules that can trigger specific biological responses

45. Concept 2.3 Carbohydrates Consist of Sugar Molecules Monosaccharides are simple sugars. Pentoses are 5-carbon sugars Ribose and deoxyribose are the backbones of RNA and DNA. Hexoses (C6H12O6) include glucose, fructose, mannose, and galactose.

46. Figure 2.9 Monosaccharides (Part 1)

47. Figure 2.9 Monosaccharides (Part 2)

48. Concept 2.3 Carbohydrates Consist of Sugar Molecules Monosaccharides are covalently bonded by condensation reactions that form glycosidic linkages. Sucrose is a disaccharide.

49. Concept 2.3 Carbohydrates Consist of Sugar Molecules Oligosaccharides contain several monosaccharides. Many have additional functional groups. They are often bonded to proteins and lipids on cell surfaces, where they serve as recognition signals.

50. Concept 2.3 Carbohydrates Consist of Sugar Molecules Polysaccharides are large polymers of monosaccharides; the chains can be branching. Starches—a family of polysaccharides of glucose Glycogen—highly branched polymer of glucose; main energy storage molecule in mammals Cellulose—the most abundant carbon-containing (organic) biological compound on Earth; stable; good structural material