1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 2 The Chemical Context of Life

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Elements and Compounds An element is a substance that cannot be broken down to other substances by chemical reactions A compound is a substance consisting of two or more elements in a fixed ratio

3. LE 2-2 SodiumChlorineSodium chloride

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some applications of radioactive isotopes in biological research: – Dating fossils (can date based on how much left – its half life) – Tracing atoms through metabolic processes – Diagnosing medical disorders – can “see” problems

5. LE 2-7b Third energy level (shell) Second energy level (shell) First energy level (shell) Atomic nucleus Energy absorbed Energy lost

6. LE 2-8 First shell Hydrogen 1 H Lithium 3 Li Second shell Third shell Sodium 11 Na Beryllium 4 Be Magnesium 12 Mg Boron 5 B Aluminum 12 Al Silicon 14 Si Carbon 6 C Nitrogen 7 N Phosphorus 15 P Oxygen 8 O Sulfur 16 S Chlorine 17 Cl Fluorine 9 F Neon 10 Ne Argon 18 Ar Helium 2 He Atomic number Element symbol Electron-shell diagram Atomic mass 2 He 4.00

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Valence electrons are those in the outermost shell, or valence shell The chemical behavior of an atom is mostly determined by the valence electrons (it bonds to fill these!)

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 2.3: The formation and function of molecules depend on chemical bonding between atoms Atoms with incomplete valence shells can share (covalent) or transfer (ionic) valence electrons with certain other atoms These interactions usually result in atoms staying close together, held by attractions called chemical bonds

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Covalent Bonds A covalent bond is the sharing of a pair of valence electrons by two atoms In a covalent bond, the shared electrons count as part of each atom’s valence shell – see how these atoms both have 8 now!!

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Electronegativity is an atom’s attraction for the electrons in a covalent bond The more electronegative an atom, the more strongly it pulls shared electrons toward itself

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In a nonpolar covalent bond, the atoms share the electron equally In a polar covalent bond, one atom is more electronegative, and the atoms do not share the electron equally

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Weak Chemical Bonds Most of the strongest bonds in organisms are covalent bonds that form a cell’s molecules Weak chemical bonds, such as ionic bonds and hydrogen bonds, are also important Weak chemical bonds reinforce shapes of large molecules and help molecules adhere to each other

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Hydrogen Bonds A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom In living cells, the electronegative partners are usually oxygen or nitrogen atoms

14. LE 2-17a Natural endorphin Morphine Carbon Hydrogen Nitrogen Sulfur Oxygen Structures of endorphin and morphine

15. LE 2-17b Natural endorphin Morphine Brain cell Endorphin receptors Binding to endorphin receptors

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chemical Reactions: Involve making and breaking of bonds to form new chemicals. Reactants – go into the reaction Products are formed from the reaction A reversible arrow means that reaction happens both ways continuously :