Introduction
Water is the most abundant liquid on the planet. It covers 2/3rds of
the earth's surface, and at least 60% of living organisms is water.
This value may be as large as 99% - in a jellyfish. The reason for
such a large amount of water making up organisms is the fact that
it is a major cell constituent - more on that later.
Water is a simple molecule yet has many interesting properties
which contribute to its importance.
Structure
The molecule is polar, because the electrons in the bonds between
O and H are pulled slightly toward the O. this means that the O atom
is slightly negative and the two H atoms are slightly positive (shown
by ð+ and ð-). The electrons are shown as small dots; note they are
closer to the O.
These properties are described more in the solvent section. db.
Water is made up of two hydrogen atoms bonded to one oxygen
atom. They are covalently bonded and the intramolecular covalent
bonds are very strong. The structure is non-linear, due to the
electron pair repulsion of the two lone pairs of electrons on the
oxygen atom.
Major cell constituent
Often 2/3rds or more of living cells are water. The water is found mainly in
the cytoplasm, and it plays a vital role in many functions of the cell: in all
organisms - metabolism, and in plants photosynthesis and support. The
actual roles of water in these functions are described more fully later.
Polar solvent / universal solvent
The pulling of the electrons towards the O makes the O electropositive
and the H's electronegative. As opposites attract, not only are the
hydrogen atoms attracted to their O atom electrostatically, but also to the
O atoms in neighbouring molecules. These bonds are called hydrogen
bonds, and are responsible for the great strength and unexpectedly high
melting and boiling points of water.
The diagram above shows the differing electronegativities, and it is for
the polar nature of this molecule that water is such a good solvent.
More substances than any other can dissolve in it, and it has been
named the universal solvent.
In plants, mineral salts can only be obtained when in solution, and water
makes this possible. Likewise in humans, digestion only occurs when the
food is in solution. The reactions of metabolism take place in solution,
and also waste products are removed in solution.
Also, gas exchange in the alveoli requires a moist surface, and gas
exchange takes place in solution.
Both blood and sap are essential for the transportation of food, waste
products and many more substances in animals and plants. These two
mediums consist of mainly water, and it ensures that the substances to be
transported are in a solution (as water is the universal solvent).
Lubricant
Outside of cells, water is also useful by reducing friction and providing
protection and cushion. In joints, water is a major part of synovial fluid,
which prevents the bones from grinding against each other.
Cerebro-spinal fluid, amniotic fluid, mucus and pleural fluid (the latter in
the lungs). All of these are mainly water, and provide either lubrication or
protection and cushion, in different ways.
Support
The turgidity of plants cells is what keeps them upright, and is down to the
fact that the cell exerts a force equal to the force of the water entering the
cell by osmosis. This balance of force makes the cell stiff. This turgidity
helps to support the leaves and stems of many plants.
Reactant
Reactions are constantly occurring in an organism, catalyzed by enzymes.
Water allows many of these reactions to occur, as often the reactants need
to be ionized.
Water also plays a major part in gaining energy from the sun, as it is a key
part in the reaction by which green plants obtain light energy from the
sun and store it as chemical energy:
6H2O + 6CO2 C6H12O6 + 6O2
Monosaccharides (such as alpha glucose) join together to by the process of
condensation to form larger molecules of starch. In this reaction, a
glycogen bond is formed and a water molecule is released when the two
hydroxyl groups join (leaving one O atom). The reverse of this is called
hydrolysis, and in order to break down the larger starch unit into its
glucose sub -units for digestion, a water molecule is needed. This is
another important reaction, where water plays a key part.
Cohesion of molecules
Water molecules are attracted to other water molecules. Water can also
be attracted to other materials, this is known as adhesion. As explained
above, the O atom of one molecule is attracted to the H atoms in the
next. This attractive force is what gives water its cohesive and adhesive
properties.
Surface tension
As the water molecules are cohesive to each other, this results in surface
tension of the water as the molecules along the surface are 'holding' each
other together.
This property allows organisms, such as the pond skater, to settle on or
skate over water. Sexual reproduction
In the process of fertilization, the sperm is transported to the egg in a fluid
medium known as semen - this contains mostly water.
Density
One of the unusual things about water is the fact that the solid form (ice) is
not as dense as the liquid form, and below 4?C the density of water
decreases.
This property means that ice floats on water and insulates it, reducing
the probability of the whole mass of water to freeze. This increases the
chance of life surviving under such harsh conditions.
Temperature control
Water has a very high specific heat capacity (4200J/Kg?C). This means to
raise the temp of water by 1?C, 4200J are needed, so the temperature of
water is not easily changed.
This means that a large body of water like an ocean will heat up in the day
and cool down in the night, but only slightly. This provides an ideal habitat
for marine organisms.
When mammals get hot, they sweat. Sweat contains mostly water, and
therefore a fairly large amount of energy required to evaporate it - this has
a large cooling effect on the body.
Plants lose water via transpiration, which helps the uptake of nutrient rich
water from the soil, and also prevents the temperature of the plant from
rising too high.
Habitat
Water is an ideal medium for life because:
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It provides a protective shield
It prevents the cell from drying out
It provides support and buoyancy to organisms
Its good solvency favours the supply of nutrients to the plants,
and the re moval of waste products from all organisms.
It allows the oxygen required for respiration to be
dissolved in it, and its good mobility means it can get
virtually anywhere in an organism.
Fertilization - a surrounding body of water disperses the
offspring reducing the risk of competition.
Aquatic organisms have very little need for temperature control
mechanisms.
Water filters out harmful UV rays from the sun.
Water is arguably the most important biochemical of all. Water is essential
to life itself, without water life on earth would not exist. Water is a major
component of cells, typically forming between 70 and 95% of the mass of
the cell. This means that we are made from approximately 60% water by
mass. Three-quarters of the planet is covered in water.
Water itself is a simple molecule made up of 2 hydrogen atoms and one
oxygen atom, H20. The hydrogen and oxygen atoms are bonded covalently
as shown in the diagram of waters molecular modal. Water is not a linear
molecule; the two hydrogen atoms form a bond with the oxygen at the
angle of 104.50.
Diagram of molecular modal of water:
Covalent bonds are formed by sharing electrons in the outer orbits of the
quantum shells. In the case of water however the large number of protons
in the oxygen nucleus have a stronger attraction for these shared electrons
than the comparatively tiny hydrogen nuclei. This pulls the electrons
slightly closer to the oxygen nucleus and away from the hydrogen so that
the oxygen develops a slight negative charge and the hydrogen's a slight
positive charge. This makes the molecules slightly polar. Molecules, which
have charged regions, are called polar molecules.
Water has both partial and negative charges and is therefore described as
dipolar. Another thing to mention is that there is an unequal distribution
of charge this is why it is called a dipole.
This slight charge means that when water molecules are close together
the positively charged hydrogen atoms are attracted to the negatively
charged oxygen atoms of another water molecule to form a weak
hydrogen bond (H - bonds). This mutual attraction explains why water is a
liquid at standard temperature and pressure (STP) whereas other, similar
sized, non- polar molecules like methane (CH4), ammonia (NH3) and
hydrogen sulphide (H2S) is gases.
The hydrogen bonds are weak individually but the sheer number of them
means that the total force keeping the molecules together is
considerable. The hydrogen bonds in water are broken when energy is
supplied, e.g. when water is heated. When they break the liquid becomes
a gas (vapour). At 00c and below the H - bonds are at their shortest and
strongest and lined up to form an open regular crystalline structure, which
is ice.
Diagram of water molecules forming hydrogen bonds:
Water is an unusual substance, mostly due to its hydrogen bonds; its
properties allow it to have different abilities.
Water can dissolve polar or ionic substances and can keep them in
solution because of water's own polar properties. Substances that dissolve
in water are known as hydrophilic substances. Ionic substances such as
sodium chloride, NaCl, are made up of positive and negative ions.
Sodium chloride is held in its structure by the strong attraction between
its positive sodium ions and negative chloride ions. Normally these ionic
attractions require a large amount of energy to break but when put into
water the negative oxygen side of the water molecules cluster around the
positive sodium ions Na+ and the positive hydrogen atoms cluster
around the negative chloride ions Cl-. The attraction between the Na +
and Cl- ions is weakened as the ions are separated.
Diagram of sodium chloride dissolving in water:
Water can also separate covalently bonded molecules such as glucose and
sucrose because the polar hydroxyl groups (- OH) in its structure forms
hydrogen bonds with the water separating the molecules from each other.
As you can see the chemical structure of water determines its properties.
The following table lists these properties with examples of their biological
importance.
Water as a solvent: -
It is an excellent solvent for ions and polar molecules because of its
polarity and its hydrogen bonding. This is also because the water
molecules are attracted to them, collect around them and separate them.
This is what happens when a chemical dissolves in water. When a chemical
is in solution it is free to move around and react with other molecules.
Any molecule with a polar region will dissolve, including sugars and
alcohol, and water molecules gather around large protein molecules to
form a special kind of solution called a colloid. Most processes in living cells
take place in this way. Water's properties as a solvent are vital to life as
most biochemical reactions such as respiration occur in solution. This is
why cell cytoplasm contains about 90% water. Water cannot dissolve
hydrophobic substances such as fats and oils; these are used by
organisms as cell membranes to separate cells and also as waterproofing
as they prevent water from entering the organism if it is covered in a
hydrophobic (water-hating) substance.
Hydrophobic interactions in protein structure and in membrane structure
are where it increases the stability of these structures.
Thermal properties: Due to the hydrogen bonding it restricts the movement of water molecules, a relatively
large amount of energy is needed to raise the temperature of water. Water also has
many thermal properties as a temperature stabiliser. Water has a boiling point of lOO oc
and a melting point of Ooc, this is unusual for a molecule of it's size (water's RMM is l8)
because other molecules of a similar size such as CO2 - RMM 44 and ammonia NH4 RMM l8, are all gaseous at room temperature whereas water is a liquid. This is because
of the hydrogen bonds, which hold the water molecules in a liquid state.
Water also has a high specific heat capacity; the result of this is that it takes
4.2 Joules of energy to raise one gram of water by 1 oc. This means that it
takes a lot of heat energy to raise the temperature of water significantly,
but once warm it cools slowly. This is essential to life where internal body
temperature has to be maintained at a constant temperature and
fluctuations can result in a breakdown of essential processes. Large bodies
of water will remain at an almost constant temperature with only very
gradual changes which makes temperature regulations for organisms far
more straightforward.
Because of the large number of bonds holding water molecules together, it
takes 2 kJ per gram of water, which is a considerable amount of energy to
separate the bonds and turn the liquid to vapour.
Water is therefore described as having a high latent heat of evaporation. Animals use this
property of water by using excess body heat to evaporate water from their surfaces,
resulting in them transferring a lot of energy into the environment but only losing a little
water. Sweating and panting are based on
this principle. Also because a large amount of energy must also be transferred from
water to convert from liquid to solid (ice), it is quite difficult for water to freeze, which is
an advantage both to organisms living in water, and to the bodies of living organisms.
Latent heat of evaporation: Water also has a high latent heat of fusion from solid to liquid. It requires 300 J per gram
of ice to melt it to water. This means that water stays liquid. This is vital in the case of
cytoplasm in cells, which is made of a high percentage of water because once frozen the
cell would be irreparably damaged. The freezing point of water is also lowered by
solutes because the soluble molecules disrupt the hydrogen bonds making the water
freeze at a lower temperature and it easier to melt ice. As there are many solutes in
cytoplasm the water will not freeze until well below 0 oc and the cells are protected
until the temperature gets extremely low.
Density and Freezing properties: Water is the only chemical where the solid is less dense than the liquid
form. As water cools it's density increases and the hydrogen bonds
between the water molecules take on a more latticed formation as ice. Yet
ice floats on the surface of water which means its density must be lower
than that of water. Water is at it's most dense at 4oc which is when its
bonds are closest together.
When water freezes the lattice arrangement of its structure move apart
slightly and it floats on the surface. This means that the layer of ice
insulates the water below, which stays at 4 oc, and aquatic life can
continue. This reduces the likelihood of large expanses of water such as
lakes and oceans freezing over, and therefore increases the chances of
organisms living in the water to survive. Changing densities with
temperature, cause currents to form in masses of water, which help to
circulate nutrients within oceans. The fact that water is most dense at 4 Oc
means a layer of water at this temperature will always sink to the
bottom, ensuring a life-supporting band is maintained at the bottom of
even the most heavily frozen lake.
Water as a transport medium: Water is the medium for many reactions, especially those, which occur in
cells. Water is key in condensation reactions where water is removed
from molecules to bond them together, this occurs with many sugars and
carbohydrates. Water is a transport medium in blood, lymphatic,
excretory and digestive systems of animals, and in the vascular systems of
plants. In this role its solvent properties are essential.
Water used for hydrolysis:Water is also used to split up molecules by adding water. This is called
hydrolysis and is essential to animals and plants because it allows them to
utilise stored foods, which are in long chains by breaking off smaller
molecules.
Water used for plants: Respiration produces water as a by-product. A large amount of energy is
produced by oxidising hydrogen, which contains so much energy it is an
explosive gas, into water. Photosynthesis uses water as a source of
hydrogen atoms, which are needed to produce glucose, which is then
stored in the plant cells as starch or used for respiration. Without water
these two essential reactions would not occur and life would not be able to
continue on earth.
Transparent: An important property of water is also it's transparency, it allows sunlight
to pass through it so aquatic plants can photosynthesis and on a larger
scale. This allowed life on earth to begin, since life started in the oceans
with small organisms, which relied on sunlight for the reaction
photosynthesis to take place.
Cohesion and surface tension: Water molecules are highly cohesive because of the hydrogen bonds
between the molecules. Water forms spherical droplets, which have the
maximum inner area and least surface area when in contact with a
hydrophobic material. The cohesive properties of water allows plants to
pull up water through xylem vessels from the roots to the leaves, this is
called the transpiration stream. It also means that the water molecules
where the water meets the air will be tightly held together and the
water molecules below them to form an elastic film known as surface
tension. Small creatures can get stuck in the surface water because they
cannot break the water surface tension, creatures like pond skaters can
move across the surface of the water without sinking as they have
hydrophobic feet which stops them from breaking the surface tension.
Diagram of surface tension:
A pond skater standing on the surface of pond water. This was photographed
through an interferometer, which shows interference patterns made by a
pond skater as it walks on the water's surface. The surface tension of the
water means the pond skater never breaks through the surface.
Incompressibility: Because of water's strong hydrostatic forces water is incompressible. This
provides support for soft- bodied creatures such as worms slugs and
jellyfish, which therefore do not require a supporting skeletal system. Water
allows cells filled with water to become turgid and due to its incompressibility
plants can support themselves.
Roles of water in the environment: Water is very important as an environment for living organisms to live in e.g.
fish, insects. Some organisms spend the whole of their life cycles in water and
are totally dependent on it (e.g. fish), whilst others are only dependent for part
of their life cycles, usually the reproductive phase e.g. amphibians, mosses.
Soft bodied creatures such as jellyfish are made of up to 96 % water. Water also
provides an environment for organisms to live in, 75% of the earth is covered in
water.
Conclusion: In conclusion water's unique properties make it perhaps the most biologically important
substance on the planet. No other substance shares similar properties to water and in the
way that one single molecule can possess such varied and essential characteristics.