WATER, DIFFUSION, AND OSMOSIS
I.
WATER
A. Hydrogen bonds
1. Since oxygen is more electronegative than hydrogen, it hold the shared electrons in
the covalent bond tighter
a) The oxygen has a partial negative charge and the hydrogen atoms partial positive
charges
+
H
-
O
+
H
B. Solvent
1. Water is a solvent for polar and charged molecules
a) Molecules dissolved in water are referred to as solutes (e.g., sugars and salts)
b) Polar molecules have partial charges, charged molecules are ions with full
charges
(1) These molecules are referred to as hydrophilic (water-loving) molecules, since they readily
dissolve in water
c) They partial negative charges of water's oxygen can bond with positive charged
solutes, and the partial positive charges of water's hydrogen atoms can bind with
negative charged solutes
2. Hydrophobic substances
a) Non-polar / non-charged molecules do not form bonds with water
b) These molecules are pushed out of the way as water molecules hydrogen bond to
other water molecules
c) These are referred to as hydrophobic (water fearing) molecules
(1) Hydrophobic molecules tend to aggregate as they are excluded from water -- this aggregation
is referred to as hydrophobic interactions
3. Amphipathic molecules
a) Some compounds (e.g., detergents and phospholipids) contain polar and nonpolar regions
C. Energy
1. It takes energy to break the hydrogen bonds between water molecules
a) Changes boiling point from -80C to 100C
b) Water requires much heat energy to raise its temperature, and releases heat
slowly; this helps maintain a constant body temperature
c) Water loses much heat when changed to vapor; sweating therefore cools animals
rapidly
D. Ionization
1. Dissociation of polar water molecules causes water to ionize to form H + (hydrogen ion
or proton) and OH- (hydroxyl group)
-
H
+
O
O
+
H
H
H
2. In pure water only 0.0000001% (10-7) of the water molecules dissociate
3. pH
a) pH is defined as the negative log of the hydrogen ion concentration
(1) Since we are dealing with logs, each pH unit is a 10-fold difference from the next pH unit
(a) For example, pH 6 has 10X more protons than pH 7, and pH 5 has 100X more protons
than pH 7
b) Pure water with 10-7 hydrogen ions has a pH of 7
(1) When the number of protons equals the number of hydroxyl groups, this is referred to as
neutral
c) If the number of hydrogen ions is more than the number of hydroxyl ions, than the
solution is considered acidic
d) If the number of hydroxyl ions is more than the number of hydrogen ions, than the
solution is considered basic
4. Acids and bases
a) Acids are molecules that release protons, thus causing solutions to become more
acidic
(1) For example, HCl dissociates into H+ and Cl- when added to water
Cl
H
Cl
+
+
H
(2) Another example, carboxylic acid (COOH) dissociates into H+ and COOb) Bases are molecules that either take up protons or release hydroxyl ions
(1) For example, NaOH dissociates into Na+ and OH-
Na O
H
Na+
+
O
H
c) With weak acids and bases, only some of the molecules dissociate
d) With strong acids and bases almost all the molecules dissociate
5. Buffers
a) Buffers are molecules in water which resist changes in pH
(1) Some buffers may take up or release protons or hydroxyl groups
II. DIFFUSION AND OSMOSIS
A. Diffusion
1. A physical process in which molecules move from higher to lower concentrations until
equally distributed
B. Osmosis
1. Movement of water molecules from a region of greater concentration to a region of
lesser concentration across a differentially permeable membrane
C. Tonicity
1. Refers to the strength of a solution in relationship to osmosis
2. Types of solutions
a) Isotonic
(1) Since solute concentration of water is the same on both sides of the membrane, there is no net
gain or loss of water and cells neither swell nor shrink
(2) Animal cells in isotonic solutions are called normal
(3) Plant cells in isotonic solutions are called flaccid
b) Hypotonic
(1) A solution with lower percentage of solute (more water) than in the cell
(2) Animal cells will swell and may burst (lysis)
(3) Plant cells will become turgid
(a) Plant cells do not burst in a hypotonic solution due to the rigid cell wall
c) Hypertonic
(1) Solutions have a higher percentage of solute (less water) outside the cell
(2) This causes the cell to shrink due to net movement of water outward (crenatation)
(3) Plasmolysis refers to the shrinking of the cytoplasm from the cell wall of plant cells due to
osmosis