Aspects of Biochemistry
Water Structure & Properties pg 79-81
tetrahedrai
Net negative charge
-
arrangem ent
aroundO atomm
Attraction of
bonding
electrons to the
.
oxygen creates
local negative
and positive
1 1 0p m
176 pm
partial charges
...
105
H
H-
hydrogen
Net positive charge
bond
31% Sinapv
In water, each-hydrogen nucleus is
fesciats, n
bound
atom
to the central oxygen
between them; this shared electron
In H20,
pair is called covalent chemicalbond.
leaving four
are used for this purpose,
shared
electrons of oxygen
two of the six outer-shell
.only
'erisn
are
electrons which
are
organized into
two
that
by a pair of electrons
four
non-bonding pairs. The
electron
pairs surrounding
minimize
possible in order to
the oxygen tend
a tetrahedral
This would ordinarly result in
chargé.
negative
is
repulsionis between these clouds.of electron
therefore the H-O-H bond angle)
(and
pairs
between
these
geometry in which the angle
remain closer to the oxygen atom,
pairs
non-bonding
two
the
because
109.5°. However,
effectively pushing the two
the two covalent bonding pairs,
to
exert a
each
arrange themselves as far from
other
as
stronger repulsion against
arrangement in which
result is a distorted tetrahedral
atoms closer together. The
hydrogen
the H-O-H angle is 104.5.
Hydrogen Bonding
Nare 2tnftin
T
h
gen e n
not
and negative charges are
The H20 molecule is electrically neutral, but the positive
schematic diagram
in intensity in the
distributed uniformly. This is illustrated by the gradation
Colnt
at the
(negative) charge is concentrated
end of the molecule, owing partly to the nonbonding
here. The electronic
oxygen
electrons
(solid circles), and to oxygen's high
nuclear
H
HO
charge which
exerts stronger attractions on the electrons. This charge
displacement constitutes
an
electric dipole,
+
represented by the
arrow at the bottom; you can think of this dipole as the electrical
"image" of a water molecule.
ydrogen bond.
O
Opposite
charges attract, so the partially-positive
hydrogen ätom on one water molecule electrostatically
jattracted to the partially-negative oxygen on a
neighboring molecule. This process is called hydrogen
7pm
is
bonding. Notice that the hydrogen bond (shown by the
OHbond
99pm
jdashed line) is somewhat longer than the covalent 0-H
Mio bond. This means that it is considerably weaker; it is so
Jweak, in fact, that a given hydrogen bond cannot survive
for too long.
Water Properties
cthcariun
feret
2ebjen mliedis
Sktnts
Water hasunusugf properties due to its small size, polarity and hydrogen bonding between
molecules. Somé of these include: solvent properties, thermal properties, density and freezing
properties, adhesion and cohesion and water acting as a reagent.
Artion
1. Solvent properties
bihu
etilas
A solvent is a substance in which.a solute dissolves by dispersing as individual molecules or ions.
The polarity of water makes it an excellent solvent for other polar molecules, as such this
means that non-polar substances do not dissolve in water.
properties of water are vital in biology, because many biochemical reactions take
place only within aqueous solutions (e.g., reactions in the cytoplasm and blood). In addition,
water is used to transport biological molecules. When an ionic or
polar compound enters water,
it is surrounded by water molecules. The
relatively small size of water molecules typically allows
many water molecules to surround one molecule of solute. The partially negative dipoles of the
The solvent
attracted
positive dipoles.
water
in
are
general,
ionic and
to
positively charged components of the solute,
and vice
versa
for the
polar substances such
as acids, alcohols, and salts are
easily soluble in
water, and non-polar substances such as fats and oils are not. Non-polar molecules
stay
together in water because it is energetically more favorable for the water molecules to
hydrogen bond to each other than to engage in van der Waals interactions with non-polar
Suhk
m
it
Mi
ArC
hyedphyle (kik
iiv
hydrphk(atr h
.
molecules.
An
example of
Na cations and CI
an
anions,
salt, the sodium chloride, NaCi, separates into
water molecules. The ions are then easily
surrounded
ionic solute is table
each
being
by
transported away from their crystalline iattice
into solution.
tension
2. Cohesion and surface
water a
bonds
surface tension
high cohesiveness and, consequently,
give
The strong hydrogen
and the water
onto a nonsoluble surface
This is evident when small quantities of water are put
when water is carried through xylem up
stays together as drops. This feature is important
column together, and
intermolecular attractions hold the water
the
in
stems
strong
plants;
tension would
Other liquids with iower surface
prevent tension caused by transpiration pull.
the xylem vesse
air pockets and rendering
have a higher tendency to "rip", forming vacuum or
or skate over its
allows small insects to settle on
inoperative. The high surface tension of water
surface
3. Transport medium
Substances
are
dissolved in
water
and transported to
various
parts of the
body. It
is
thhe
canal and
and excretory systems, aiimentary
transport medium in the blood, lymphatic
in
xylem
and phloem.
4. Thermal properties pg 81.
fusion
heat of vaporization, high heat of
Thermal properties include: high heat capacity, high
hence m o r e energy is required
movement of water molecules;
Hydrogen bonding restricts the
bodies of water are slow to
Because of this property large
to raise the temperature of water.
for aquatic organisms.
a stable environment/habitat
hence
change
provides
temperature,
5. Reactivity
Chemically, water
acid ar base Occasionaily the term hydroxic
is amphoteric: able to act as an
of 7 (neutral), the
acid in a chemical reaction. At a pH
acid is used when water acts as an
(H,0') o r hydrogen i o n s
is equal to that of the hydronium
concentration of hydroxide ions (OH)
concentration of
the solution becomes acidic (higher
ions. If the equilibrium is disturbed,
(H)
hydronium ions)
or
basic (higher
Water can act as either
an
acid
concentration
or a
base in
of hydroxide ions).
reactions.
According
to the
Brønsted-Lowry systern,
in a reaction, and a base as
which donates a proton (an H+ ion)
a
as
defined
species
is
acid
an
water acts as a base; when
When reacting with a stronger acid,
o n e which receives a proton.
in the
it receives an H+ ion from HCI
it acts as an acid. For instance,
weaker
acid,
a
with
reacting
equilibrium:
HCI+H20-> HO' +CI
Here water is
acting
also do this, such
as
as a
base,
by receiving an
H+ ion. An acid donates
NH3:
in the reaction with ammonia,
NH3+H,0 ---> NHaOH
an
H+
ion, and
water can
6. Density and Freezing properties
The solid form of water; ice, is less dense than its liquid. Below 4°C the density of water starts
and hence insulate below it, this adds to the
of water with
reason why large bodies of water never freeze completely. Changes of density
in the ocean.
temperature cause currents which help to maintain the circulation of nutrients
to decrease. Ice therefore
floats
on
liquid
water
Summary of some of water properties
In the liquid sate, the positive
ends of some molecules are
sightly at racted to the
WATER
he gative ends of others.
These sight attractions gwe
As water freezes, it
crystalizes. (Most solids are
Crystakine, meaning that their
molecules are arrange d in a
regular internal order.j In Ice,
that order irwoves lining the
water a cohesion that most
iquids do not have. Because
molecules ip with negatwe
the eects of this are most
and positive ends adjoinng
easiy seen at the sunface of a |
each other. Because the
O-2 charge)
volume of water, e cal the
es are not symmetic
Cohesion "surface tension".
This efect is what allows you
getting them into this
orientation means that they
cannot be packed as closely
stightly, or toflaa a needle on
a glass of water.
together. Since xacty the
to overl a glass of water,
sarne molecules occUpy mare
space when frozen, ice is less
H(1)
H(+1)
The surface tension or
cohesion of water has mary
important erwronmental
consequences.
For example,
it is what allcws water to mowe
upaard in the pore spaces of
SOll supplying water to
shallow roots long after a rain.
Similarty, most of the water
fransport through planis, from
root io eaves, Is
accomplished by tensional
pulling of the water through
HO molecule
Note theteven though the molecule ie neutral
that ie, the-2 ohage af the aygen is kslanced
by the twot1chargee aftha hydrgene,tha
f o e s a tha moecules have slght chages
bacausethe molecule
not completey
The hycdrcgen end s sighthy pcs ithe
sYmmatC.
and the Cgen end s lighdy nagative. This is
caled a 'pole" molaoula.
dense than water, and s0
floats. Virtualky all other
materials are denser in the
solid state, and so WDuld sink
in their liquid equval ents.
If ice sank in water, aquatic life
0 uld be much re stricted or
impossible. As a body of
water frOZe n wirter, it wOuld
fre eze from tha boftom u
The next warm season would
Vwamn the upper p art, but the
ice at the bottom Would not
melt. Ater several years, all
the water would be fraz en solid
the xylem tubea
throughout the cold season.
Biological importance of wateer
High water content of cells (makes up 70-95%)
Good solvent
and medium
for diffusion
Reagent in hydrolysis
Support for aquatic organism
Fertilization for swimming gametes.
THE CHEMICALS OF LIFE
biologic/ siginiicance of the properties of wate
able 1 The
Slgnificance for lving organisms
Property of water
Water is
a
liquid at
room
temperature
Water is the universal solvent: it
dissolves more substances than any
Provides
a
liquid environment inslde cells
and
happen in
The chemical reactions inside cals
environments for
aquatic
aqueous
organisms
to tive in
solution;
water
is aisa the main
allows
some
aquatic organisms
transport medium in organisms
other common solvent
Water forms a surface film at
Water forms a skin at its surface: it has
a high aurface tension
3uch
as
pond skaters
to
land
air-water Intarface; this
an
on
surface of
the
the surface of a body of
allowing aquatic life to survive
ice forms on
lce floats on water: ice is laess dense
than liquid water
water
pond
a
insulates
and
move over
and
the
so
water
it
below,
that, for
exampl
allows capillarity
Along with low viscosity, this property
channels in soil, against gravity
narrow
through
water can move upwards
Water is wet. adhesion makes it stick
to polar surfaces
The environment inside
Water has a very high spec+ic
heat capacity
environments
changes; aquatic
organisms resists temperature
have relatively stable temperaturas
a cooling
from it; this is used as
when water evaporates
Heat is lost from a surface
transpiration in plants
and
mammais
in
mechanisn, for example sweating
Weter has a high latent heat of
vaporisation
aquatic
habitate
sunllght
enables
Cell contents and
Water has a high iatent heat
are
weather
slow to freeze in cold
of fusion
Transmission
Water is coiourless and transparent
of
Water supports organisms
Water is denser than air
structuressuch as
chemicel reactions
Most
Water combines with many organic
form hydrated molecules
is
v e a s reproductive
whales; it also supports and disperses
large fruits
such
as coconuts
as a
skeleton
(called
a
hydrostatic skeletan)
an
organlc molecules
their physical and
Water
Water has a lcw visity (it flows
w a t e r moiecules can
as
to photosynthesise
in many digestive
for photosynthesis; it also takes part
Water is a major raw material
food molecules by hydrolysis
reactions, breaking down
Water takes part in many
freety:
larvae and
large
important structural agent, acting
in worms and turgid plants
Water
Water is difficult to compress
moiscules to
as
aquatic plants
slide easily
can
in
occur
a
hydrated
chemical properties
are
form in
cells; if the
water
is removed,
affected
watery solutions can act
fiow freely through narrow vessels;
move easily down the oesophagus
to
food
allows
mucus
as a lubricant,
for example
over eack other
to
can be pulled all the way up
Continuous columns of water
Water has e high tensile strength:
water c0iuns do not hrazk or pull
In xylem vessals durlng transpiration
aparr e2siiy
******"*****
QuICK cHECK
Which property vf
a
water
makes
hydrostatic skeleton?
b
2
Why is waters transparen y
Which
biological process uses
it suitable fir
use as
.
essential for most, of life on Earth?
"
the latent heat.of vaporisation ofwater?
Food for thought
if
pond became
Suggest what would happen to pond skaters their
polluted with detergent.
the top of
a
tree