BIO 201 Lecture Notes - Lecture 2: Equilibrium Constant, Chemical Polarity, Hydrolysis

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Lecture Two 2-1-17
The Chemistry of Life
I. Everything that happens in your cells is a function of energy and chemistry
II. Bonds and Interactions
a. Covalent
i. Connect atoms to form molecules
ii. A function of electron cell configuration
1. How many covalent bonds an atom likes to form depends on how many
free electrons there are in the outer (valence) shell
2. Octet Rule pefeee fo 8 eletos i the ato’s oute shell
iii. The force that holds two atoms together through a pair of shared electrons
iv. Carbon is the most important
1. Has four free electrons, so forms 4 covalent bonds
b. Ionic
i. A very strong force, but not quite as strong as a covalent bond
ii. Referred to as bonds, but are much easier to pull apart
1. Interactions
2. Like two opposite pulls of magnets
iii. Often large difference in electronegativity
iv. Between two oppositely charged atoms
v. NaCl is likely the strongest ionic bond in nature
c. Hydrogen
i. Referred to as bonds, but are much easier to pull apart
1. Interactions
2. Like two opposite pulls of magnets
ii. Formed by polar molecules
iii. Oppositely-charged dipoles attract
1. The opposite charges are slightly less strong than in ionic bonds, so the
interaction is slightly less strong
iv. In water, Hydrogen bonds to the oxygen of another molecule
1. One of the reasons that it is very important to life on earth
v. Slightly less strong than ionic bonds
d. Van der Waals
i. Truly not bonds
1. Mysterious interactions
ii. Weakest of the interactions
iii. Occur between hydrophobic, nonpolar molecules
iv. Very random transient dipoles
v. If a dipole is created in one of them, it will create a
domino effect where the entire group begins stabilizing
dipoles (become slightly negative to counteract a
positive charge, vice versa) in an otherwise nonpolar
molecule
vi. These are VERY tiny dipoles
vii. These molecules do not mix with water
1. Also called the Hydrophobic Effect
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Lecture Two 2-1-17
The Chemistry of Life
a. Non-polar, hydrophobic molecules do not dissipate in water
b. It is energetically unfavorable
viii. These are weak interactions, however, combined they can be very strong
1. Reason that geckos can walk up vertical glass
a. There are millions or billions of Van der Waals interactions that
are occurring when a gecko puts its foot up on glass
2. Taken together, Van der Waals forces make a difference; individual they
do not
III. Electronegativity
a. Regardless of how many bonds an atom like to form, some atoms like electrons more
than other atoms
b. This desire for electrons is called electronegativity
c. Oxygen Nitrogen Carbon Hydrogen
i. Most to least electronegative
d. Na and Cl are at opposite extremes
i. Na and Cl are not held together by covalent bonds
ii. It looks instead like Cl steals an electron completely from Na, leading a charge
iii. Cl is now an anion, and Na is a cation
iv. This is an ionic bond
1. Negatively-charge chloride is attracted to positively-charged sodium
IV. Dipoles
a. Small difference in electronegativity
b. H2O
i. Oxygen does not steal the electron completely away from water
ii. Share of electrons
1. Covalent bond
iii. In water, oxygen steals some electron density from hydrogen
iv. L-shaped molecule (i.e. bent)
c. Creates charge asymmetry a dipole
i. A tiny magnet
ii. Not a full positive and negative charge, but a net shift from neutral
d. Any molecule with a dipole, or any part of a dipole, is called polar
V. Solubility
a. An important feature of biological molecules
i. How cells form and the organization and function of cells is determined in part
by the solubility of the molecules involved
b. Usually in water
i. Cells are essentially bathed in water and
made of water
ii. Most of the mass of cells is water
c. When you put a solute into a solvent, the forces of
entropy (disorder) favor dissolution
d. However, the solute molecules may be held
together by strong bonds/interactions
http://www.holland-hills.com/images/saltwater2.jpg
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