2. Water is central to chemistry
1. Nearly all biological molecules assume their
shapes/functions in response to the physical and
chemical properties of the surrounding water
2. The medium for the majority of biochemical
reactions is water. Reactants & products of
metabolic reactions, nutrients as well as waste
products, depend on water for transport within
and between cells
3. Water itself actively participates in many chemical
reactions that support life. Frequently, the ionic
components of water, the H+ and OH- ions, are
the true reactants. In fact, the reactivity of many
functional groups on biological molecules depends
on the relative concentrations of H+ and OH- in the
surrounding medium.
3. Physical Properties of Water
1) Water is a polar molecule
2) Hydrophilic substances dissolve in
water
3) The hydrophobic effect causes
nonpolar substances to aggregate
water
4) Water moves by osmosis and solutes
move by diffusion
4. 1) Water is a Polar Molecule
Hydrogen atoms occupy
2 corners of the
tetrahedron, and the
nonbonding electron
pairs of the oxygen atom
occupy the other two
corners
Oxygen with unshared
electrons carries partial
negative charge, and
hydrogen atoms each
carry a partial positive
charge
5. Polar and ionic molecules have
positive and negative charges and
are therefore attracted to water
molecules because water molecules
are also polar. They are said to be
hydrophilic because they interact
with (dissolve in) water by forming
hydrogen bonds.
Nonpolar molecules are
hydrophobic (means "water
fearing"). They do not
dissolve in water.
6.
7. Hydrogen Bond
Electrostatic attractions
between dipoles of water
molecules solvent
The strength of interaction
is maximal when the O-H
covalent bond of one
molecule points directly
toward the lone pair
electron cloud of the other
8. Ice is a crystal of hydrogen-
bonded water molecules
Each water molecule is
tetrahedrally surrounded by 4
nearest neighbors to which it is
hydrogen bonded
In ice, each water molecule
forms four hydrogen bonds, the
maximum possible for a water
molecule, creating a regular
crystal lattice. By contrast, in
liquid water at room temperature
and atmospheric pressure, each
water molecule hydrogen-bonds
with an average of 3.4 other
water molecules. This crystal
lattice structure makes ice less
dense than liquid water, and
thus ice floats on liquid water.
9. The structure of liquid water is
irregular
Consists of 3-7 membered rings, in
contrast to 6-membered rings of ice
Liquid water consists of a rapidly
fluctuating, three-dimensional network of
hydrogen-bonded H2O molecules
10. Hydrogen bonds and other
weak interactions influence
biological molecules
Weak electrostatic
forces:
ionic bond
hydrogen bonds
van der Waals
forces
11. 2) Hydrophilic substances
dissolve water
The polar character of water makes it an
excellent solvent for polar and ionic
materials (hydrophilic)
Nonpolar substances are insoluble in
water (hydrophobic)
Polar solvents (like water) weaken the
attractive forces between oppositely
charged ions (NaCl)
12.
13. Solubilities of polar and
ionic substances..
Are enhanced when they carry
functional groups:
Hydroxyl (OH)
Carbonyl (C=O)
Carboxylate (COO-)
Ammonium (NH3+)
14. 3) Hydrophobic effect
causes nonpolar
substances to aggregate in
water
Tendency of water to minimize its
contacts with hydrophobic molecules
= hydrophobic effect
Many large molecules (proteins,
nucleic acids & cellular membranes)
assume their shapes at least partially
in response to the hydrophobic effect
In order to maximize their hydrogen-
bonding ability, these water
molecules orient themselves to form
a hydrogen-bonded network
enclosing the cavity
15.
16. Amphiphiles form micelles
and bilayers
Most biological
molecules have both
polar (or charged) and
nonpolar segments are
therefore
simultaneously
hydrophilic and
hydrophobic
Ex. Palmitate and
oleate are amphiphilic
and amphipathic
20. 4) Water moves by osmosis
and solutes by diffusion
Water has colligative properties (physical
properties that depend on solute concentration)
Ex. freezing point, BP, osmotic pressure
Osmosis – movement of solvent across the
membrane from a region of high concentration (ex.
Water) to a region of low concentration (ex. Water
with solute)
Osmotic pressure – pressure that must be applied
to solution to prevent the inward flow of water;
proportional to concentration of solute
21. ..Cont..Water moves by
osmosis and solutes by
diffusion
Diffusion – molecules moving
randomly until the concentration of
solute is the same on both sides of
membranes
Basis for dialysis – useful for
separating large molecules (proteins/
nucleic acids) from smaller molecules
28. Chemical Properties of
Water
Water ionizes to form H+ and OH-
Acids and bases alter the pH
Buffers resist changes in pH
29. 1) Water ionizes to form H+
and OH-
Water is a neutral molecule with very slight
tendency to ionize
Ionization/dissociation: H2O H+ + OH-
H+ from hydronium ion, H3O+ proton
jumping from one water molecule to
another
Proton jumping – also responsible for acid-
base reactions are fast reactions that take
place in aqueous solution
30. pH
Amount of hydrogen ions in a solution
Most physiological solutions have hydrogen ion
concentrations near neutrality (ex. Human blood is
slightly basic)
Devised by Danish Biochemist Soren Sorenson to
compare the value of H+, to test the acidity of the
beer he was making
The higher the pH, the lower is H+ concentration;
vice-versa
pH water = 7.0
Acidic = pH < 7.0
Basic = pH > 7.0
31.
32. 2) Acids and bases alter the
pH
Biological molecules (ex. Proteins,
nucleic acids) have numerous functional
groups that act as acids/bases (ex.
Carboxyl, amino groups) – influence the
pH of surrounding aqueous medium
Acid – donates proton
Base – accepts proton
33. 3) Buffers resist changes in
pH
Buffers – solutions which resist change
in pH by the addition of small amounts
of acids or bases
Proteins, organic acids and
bicarbonates – buffers in body
