Atoms: The Building Blocks of Life

1. Chapter 2 The Chemical Context of Life

2. Chapter 2 The Chemical Context of Life • What is an atom? • Smallest unit of matter that retains the physical & chemical properties of its element • Element – a substance that cannot be broken down into other substances by chemical rxn • What is an atom made of? • Proton – (+1) charge, found in nucleus, 1 amu/1 dalton • Neutron – no charge, found in nucleus, 1 amu/1 dalton • Electron – (-1) charge, orbit nucleus, mass negligible

3. Nucleus (a) (b) Figure 2.4 Simplified models of a helium (He) atom Cloud of negative charge (2 electrons) Electrons

4. Chapter 2 The Chemical Context of Life • What is an atom? • What is an atom made of? • What do these numbers mean? • 23Na 12C • 11Na 6C • What is an isotope? Radioisotope? • Atoms of an element that have the same atomic # but different atomic mass (due to different #s of neutrons) • Unstable isotope where the nucleus spontaneously decays emitting subatomic particles &/or energy as radioactivity. • 5. What are radioisotopes used for? Atomic mass = protons + neutrons Atomic # = # of protons

5. Chapter 2 The Chemical Context of Life • What is an atom? • What is an atom made of? • What do these numbers mean? • What is an isotope? Radioisotope? • What are radioisotopes used for? •  Radiation cancer therapy (“nuclear medicine”) • - http://www.rsc.org/chemistryworld/News/2011/May/11051102.asp •  Cell research (trace path of protein from ribosome-to-final destination) • How are atoms held together? •  Chemical bonds!!! • 7. What are the different types of bonds? • Covalent – sharing of electrons • Polar covalent – UNequal sharing • Non-polar covalent – equal sharing • Ionic – complete transfer of electrons

6. Hydrogen atoms (2 H) In each hydrogen atom, the single electron is held in its orbital by its attraction to the proton in the nucleus. + + 1 2 3 When two hydrogen atoms approach each other, the electron of each atom is also attracted to the proton in the other nucleus. + + The two electrons become shared in a covalent bond, forming an H2 molecule. + + Hydrogen molecule (H2) • Formation of a covalent bond Figure 2.10

7. Name (molecular formula) Electron- shell diagram Space- filling model Structural formula Hydrogen (H2). Two hydrogen atoms can form a single bond. H H Oxygen (O2). Two oxygen atoms share two pairs of electrons to form a double bond. O O Figure 2.11 A, B • Single and double covalent bonds (a) (b)

8. Name (molecular formula) Electron- shell diagram Space- filling model Structural formula (c) Water (H2O). Two hydrogen atoms and one oxygen atom are joined by covalent bonds to produce a molecule of water. H O H (d) Methane (CH4). Four hydrogen atoms can satisfy the valence of one carbon atom, forming methane. H H H C H Figure 2.11 C, D • Covalent bonding in compounds

9. The lone valence electron of a sodium atom is transferred to join the 7 valence electrons of a chlorine atom. Each resulting ion has a completed valence shell. An ionic bond can form between the oppositely charged ions. – + 1 2 Cl Na Na Cl Cl– Chloride ion (an anion) Na+ Sodium on (a cation) Na Sodium atom (an uncharged atom) Cl Chlorine atom (an uncharged atom) Sodium chloride (NaCl) • An ionic bond • Is an attraction between anions and cations Figure 2.13

10. Chapter 2 The Chemical Context of Life • What is an atom? • What is an atom made of? • What do these numbers mean? • What is an isotope? Radioisotope? • What are radioisotopes used for? • How are atoms held together? • What are the different types of bonds? • What is electronegativity? • Atom’s ability to attract & hold electrons • In Biology – most concerned with N & O

11. Because oxygen (O) is more electronegative than hydrogen (H), shared electrons are pulled more toward oxygen. d– This results in a partial negative charge on the oxygen and a partial positive charge on the hydrogens. O H H d+ d+ H2O • In a polar covalent bond • The atoms have differing electronegativities • Share the electrons unequally d– Figure 2.12

12. H Water (H2O) O A hydrogen bond results from the attraction between the partial positive charge on the hydrogen atom of water and the partial negative charge on the nitrogen atom of ammonia. H  +  – Ammonia (NH3) N H H d+ + H Figure 2.15 Hydrogen Bonds • A hydrogen bond • Forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom  –  +  +

13. Molecular shape • Determines how biological molecules recognize and respond to one another with specificity

14. Nitrogen Carbon Hydrogen Sulfur Oxygen Natural endorphin Morphine (a) Structures of endorphin and morphine. The boxed portion of the endorphin molecule (left) binds toreceptor molecules on target cells in the brain. The boxed portion of the morphine molecule is a close match. Natural endorphin Morphine Endorphin receptors Brain cell (b) Binding to endorphin receptors. Endorphin receptors on the surface of a brain cell recognize and can bind to both endorphin and morphine. Figure 2.17

15. Chapter 2 The Chemical Context of Life • What is an atom? • What is an atom made of? • What do these numbers mean? • What is an isotope? Radioisotope? • What are radioisotopes used for? • How are atoms held together? • What are the different types of bonds? • What is electronegativity? • How are bonds created between atoms?

16. + 2 H2 + O2 2 H2O Reactants Reaction Product • Chemical reactions • Convert reactants to products