Biochemical
Properties of Water
Nikki Rose N. Labiano, RChE
Lecturer
Water: The Medium of Life
● Water plays a central role in the
chemistry of all life
● Proteins, polysaccharides,
nucleic acids, and membranes all
assume their characteristic
shapes in response to water
● The chemical properties of water
are related to the functions of
biomolecules, entire cells, and
organisms
Water: The Medium of Life
● The geometry of the water
molecule and its properties as a
solvent play a major role in
determining the properties of
living systems
● When two hydrogens and one
oxygen share electrons via
covalent bond, a water molecule
is formed
Water molecule is made of covalent bonds, while water molecules bind to
each other with hydrogen bonds.
Electronegativity
- The tendency of an atom to
attract electrons to itself in a
chemical bond (i.e. to become
negative)
- Oxygen and nitrogen are both
highly electronegative, much
more so than carbon and
hydrogen
- Molecules, or even different regions of the same molecule, fall
into two general classes, depending on how they interact with
water.
● Hydrophilic ● Hydrophobic
- water loving - water fearing
Five Critical Properties of Water
1. Water as a solvent
- Ionic substances tend to readily dissolve in water: Hydrophilic
- A molecule or ion surrounded by solvent molecules is solvated
- When the solvent is water, the molecules of ions are hydrated
Five Critical Properties of Water
1. Water as a solvent
- The solubility in water depends upon the ratio of polar to nonpolar groups
- The larger the portion of nonpolar groups, the less soluble the molecule is in
water
- The larger portion of polar groups (e.g. hydroxyl groups), the more soluble the
molecule is in water
Five Critical Properties of Water
1. Water as a solvent
- Molecules that are nonpolar (e.g. hydrocarbons such as oils) tend to be not
soluble in water: they are hydrophobic
Five Critical Properties of Water
1. Water as a solvent
- There are also amphipathic
molecules that contain both
hydrophilic parts and
hydrophobic parts
- Amphipathic molecules
refers to the molecule that has
one end with a polar, water-
soluble group and another
end with a nonpolar,
hydrocarbon group that is
insoluble in water
- In the presence of water, they
tend to form structures called
micelle
Five Critical Properties of Water
2. Water has high specific heat capacity, conduction of heat, and
heat of vaporization
- Because of water’s extensive hydrogen bonding, it is hard to raise its
temperature (compared to other liquids)
- You have to add enough heat energy to break the hydrogen bonds
between water molecules before they are free to start vibrating faster.
(Increased vibration = increased kinetic energy = increased temperature)
- Because cells are mostly made of water, this makes it easier for living
creatures to maintain a constant body temperature
Specific heat capacity – amount of energy needed to raise the temperature of
1 g of a substance by 1°C
- For water, Cp = 4.184 J/g-°C
Five Critical Properties of Water
2. Water has high specific heat capacity, conduction of heat, and
heat of vaporization
- Because of water’s extensive hydrogen bonding, it takes a lot of
energy for water to evaporate. Before it can change from liquid to gas,
it has to break all those hydrogen bonds holding the water molecule
together .
- Then the individual water molecules start vibrating with higher kinetic
energy, eventually breaking free and leaving as a gas (water vapor)
- Living organisms can take advantage of this in the form of
“evaporative cooling” – when water evaporates, it carries away heat
energy, leaving a cooler animal behind (think how a dog pants to cool
off)
Five Critical Properties of Water
3. Adhesion and Cohesion
- Water molecules are very cohesive
(attracting to each other) because of the
molecule’s polarity.
- This is why you can fill a glass of water
just barely above the rim without spilling
or a drop of water can flow over a glass
window without breaking.
- Cohesive forces are responsible for
surface tension, a phenomenon that
results in the tendency of a liquid’s surface
to resist rupture when placed under tension
or stress
Five Critical Properties of Water
3. Adhesion and Cohesion
- The cohesive force are also related to the
water’s property of adhesion, or the
attraction between water molecules and
other molecules.
- For instance, adhesion enables water to
“climb” upwards through thin glass tubes
(called capillary tubes) placed in a beaker of
water
- This upward motion against gravity, known
as capillary action, depends on the
attraction between water molecules and the
glass walls of the tube (adhesion), as well as
on interactions between water molecules
(cohesion)
Five Critical Properties of Water
3. Adhesion and Cohesion
Five Critical Properties of Water
4. Water is the only substance occurring naturally in all three phases
- Water is the only substance which in solid phase is less dense than in liquid
phase. Water has a maximum density at around 4°C
- Thus, ice floats over liquid water. This allows fish to survive even if the top of a
lake, river, or ocean is frozen because it also insulates the liquid water
underneath
Five Critical Properties of Water
5. Dissociation of Water Molecules: Acids, Bases, and pH
- Pure water consists of low concentration of hydronium ions and an
equal concentration of hydroxide ions
- Acids are proton-donors and bases are proton acceptors
Water and Biomolecules
- The oxygen atom of water carries a partial negative charge
and thus water could act as a nucleophile (donates an electron
to an electrophile) in hydrolysis reactions
- Water is central to the assembly and three-dimensional shape
of proteins and nucleic acids
- As well as providing a medium in which enzyme structure and
activity is maintained, water is a reactant in many reactions.
- Nearly 35% of all identified enzymes (namely hydrolases and
some lyases) involve water as a substrate or product.
Water and Biomolecules
- Water molecules locked up in the active site can modulate
interactions with substrates and contribute to catalysis.
- The DNA helix expanding and contracting depends on its
hydration status. Water hydrates DNA by forming links with the
polar atoms at the edges of the pairs.
- Most x-ray crystal structures of proteins show water molecules
in fixed positions around the exterior of the protein molecules,
forming hydrogen-bonding networks that include hydrophilic
groups in the protein main and side chains.
