Chemistry of Life Part I

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Chemistry of Life Part I Common Constituents and
Bonds
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Covalent Bonds

• the sharing of a pair of valence electrons by two
  atoms

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LE 2-10
e-
e-
Hydrogen molecule (H2)
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• A single covalent bond, or single bond, is the
  sharing of one pair of valence electrons
• A double covalent bond, or double bond, is the
  sharing of two pairs of valence electrons
• Covalent bonds can form between atoms of the same
  element or atoms of different elements
• A molecule is two or more covalently bonded atoms
• Strong bond

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LE 2-11b
Name (molecular formula)
Electron- shell diagram
Structural formula
Space- filling model
Oxygen (O2)
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LE 2-11c
Name (molecular formula)
Electron- shell diagram
Structural formula
Space- filling model
Water (H2O)
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• Electronegativity
• an atoms attraction for the electrons in a
  covalent bond
• Examples Oxygen and nitrogen are highly
  electronegative
• The more electronegative an atom, the more
  strongly it pulls shared electrons toward itself

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• Nonpolar covalent bond
• the atoms share the electron equally
• -molecule has no charge (neither positive nor
  negative)

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LE 2-11b
Molecular Oxygen
Name (molecular formula)
Electron- shell diagram
Structural formula
Space- filling model
Oxygen (O2)
Nonpolar covalent molecule Neutral
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• Polar covalent bond
• one atom is more electronegative
• the atoms do not share the electron equally
• Partial negative and positive charges

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LE 2-12
H2O Polar covalent molecule
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Ionic Bonds

• Formed by the transfer of electrons from one atom
  to another
• After transfer, both atoms charged
• A charged atom (or molecule) is called an ion
• Weaker than covalent bond

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• Anion
• negatively charged ion
• Cation
• positively charged ion
• Ionic bond
• attraction between an anion and a cation

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LE 2-13
Na
Cl
Na
Cl
Sodium atom (an uncharged atom)
Chlorine atom (an uncharged atom)
Sodium ion (a cation)
Chlorine ion (an anion)
Sodium chloride (NaCl)
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• Ionically bonded atoms
• ionic compounds, or salts e.g. NaCl
• often crystals

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LE 2-14
Na
Cl
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Hydrogen Bonds

• when a hydrogen atom covalently bonded to one
  electronegative atom is attracted to another
  electronegative atom
• Example water (H2O)
• Weak, but many together are strong

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LE 2-15
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Water (H2O)
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Hydrogen bond
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Ammonia (NH3)
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Van der Waals Interactions

• Attraction between adjacent atoms by fleeting
  charge differences
• Very weak
• Collectively, can be strong, as between molecules
  of a geckos toe hairs and a wall surface

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Autumn K, Sitti M, Liang YA, Peattie AM,
Hansen WR, Sponberg S, Kenny TW, Fearing R,
Israelachvili JN, Full RJ. 2002. Evidence for van
der Waals adhesion in gecko setae. Proc Natl Acad
Sci USA 99(19)12252-6. Department of
Biology, Lewis Clark College, Portland, OR
97219, USA. autumn_at_lclarke.edu
Kellar Autumn
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Order of Relative Bond Strength
Covalent gtionicgt hydrogengt Van der Waals
In biological systems, often many weak bonds
collectively are strong and help stabilize
structures. Example DNA double helix held
together through H-bonds
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Molecular Structure of Molecules is Crucial For
example Between hormone and hormone receptor
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LE 2-17a
Carbon
Nitrogen
Hydrogen
Sulfur
Natural endorphin
Oxygen
Morphine
Structures of endorphin and morphine
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LE 2-17b
Natural endorphin
Morphine
Endorphin receptors
Brain cell
Binding to endorphin receptors