Transcript Chapter 2: Chemical Foundations

By: Debbie Schwagerman
January 31, 2005
Atomic Bonds and Molecular Interactions
Each atom has a defined
number and geometry of
covalent bonds.
Atomic Bonds and Molecular Interactions
Electrons are shared unequally in
polar covalent bonds. Atoms
with higher electronegativity
values have a greater attraction
for electrons.
Atomic Bonds and Molecular Interactions
Covalent bonds are much
stronger and more stable
than noncovalent bonds.
Atomic Bonds and Molecular Interactions
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Ionic bonds result from the attraction of a positively charged ion (cation)
for a negatively charged ion (anion).
The atoms that form the bond have very different electronegativity values
and the electron is completely transferred to the more electronegative
atom.
Ions in aqueous solutions are surrounded by water molecules, which
interact via the end of the water dipole carrying the opposite charge of the
ion.
Atomic Bonds and Molecular Interactions
Van der waals interactions are caused by transient dipoles.
Atomic Bonds and Molecular Interactions
The hydrophobic effect causes
nonpolar molecules to adhere to
one another.
Atomic Bonds and Molecular Interactions
Molecular complementarity
permits tight, highly specific
binding of biomolecules.
Chemical Building Blocks of Cells
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Proteins
 Amino Acids
Nucleic Acids
 Nucleotides
Polysaccharides
 Monosaccharides
Chemical Building Blocks of Cells
Common structure of amino
acids.
Chemical Building Blocks of Cells
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20 amino acids.
All amino acids in nature are L form.
Structure consists of Ca, to which an amino group, a carboxyl
group, a hydrogen atom, and a variable group.
Amino acids are classed according to their R group.
Chemical Building Blocks of Cells
Common structure
of
nucleotides.
Chemical Building Blocks of Cells
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Common structure: phosphate group,
base, and a five-carbon sugar.
Sugar is either DNA or RNA.
Bases are adenine, guanine, cytosine,
thymine (DNA), and uracil (RNA).
Nucleotides link together to build
nucleic acids.
Chemical Building Blocks of Cells
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Monosaccharides are carbohydrates of
combinations of carbon and water in a
one-to-one ratio.
Except for fructose, all sugars are in
nature are D form.
D-Glucose (C6H12O6) is primary energy
source.
Chemical Building Blocks of Cells
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Polysaccharides:
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Disaccharides are simplest
polysaccharides.
Anomeric carbon of one sugar molecule is
linked to hydroxyl oxygen of another
sugar molecule.
Polysaccharides can contain dozens to
hundreds of monosaccharides.
Chemical Equilibrium
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The extent to which a reaction can proceed and the rate at which the
reaction takes place determines which reactions occur in a cell.
Reactions in which the rates of the forward and backward reactions are
equal, so that the concentrations of reactants and products stop changing,
are said to be in chemical equilibrium.
At equilibrium, the ratio of products to reactants is a fixed value termed
the equilibrium constant (Keq) and is independent of reaction rate.
Chemical Equilibrium
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Keq depends on the nature of the reactants and products, the temperature,
and the pressure.
The Keq is always the same for a reaction, whether a catalyst is present or
not.
Keq equals the ratio of the forward and reverse rate constants (Keq = kf/kr).
The concentrations of complexes can be estimated from equilibrium
constants for binding reactions.
Biochemical Energetics
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The change in free energy ∆G is the most useful measure for predicting the direction
of chemical reactions in biological systems. Chemical reactions tend to proceed in the
direction for which ∆G is negative.
A chemical reaction having a positive ∆G can proceed if it is coupled with a reaction
having a negative ∆G of larger magnitude.
The chemical free energy change ∆G equals -2.3RT log keq. Thus the value of ∆G
can be calculated from the experimentally determined concentrations of reactants and
products at equilibrium.