Water is a simple molecule, yet it is fundamental to life.
In active living cells, two-thirds, or often more, of the area is occupied by water,
and two-thirds of the globe is covered in water.
Water is therefore extremely abundant, and in biological terms it has great importance
both inside cells, and externally, for example as a habitat.
simple molecule of H2O has many interesting physical and chemical properties
which contribute to its importance. It is the only substance which can be found naturally
in all three states - solid (ice), liquid and gas (water vapor).
Water molecules bond to each other by means of hydrogen bonds, and this raises its
melting and boiling points substantially. If these bonds did not exist, its boiling point
would be in the region of -120C, rather than the actual value of 100C.
Water is also very good at ionizing substances due to its structure, and is therefore
a good solvent.
These properties, and many others, will be considered by looking at the main areas
in which water is of biological significance.
Water as a Major Cell Constituent
is the most abundant component of any organism.
Humans are 60% water, and most organisms are 60-90% water. The lowest water
content can be found in plant seeds (20%), and the highest in jellyfish (99%),
and this is the cause of their transparency. The water is found mainly in the protoplasm,
and here it plays vital roles in many functions, for example in metabolism in all
organisms, and photosynthesis and support in plants.
The roles of water in each of these functions, and many more are described below.
Water as a Solvent
molecules have the well-known formula H2O, each molecule
containing two hydrogen atoms and one oxygen atom.
These atoms are covalently bonded together, sharing electrons to hold them together.
However, these electrons are not evenly distributed within the molecule.
Oxygen has an affinity towards electrons, and therefore draws them away from the
hydrogen atoms in the molecule.
This attraction makes the oxygen part of the molecule very slightly negatively charged,
and it is said to be electronegative. Consequently, the electrons are slightly further
away from the hydrogen atoms in the molecule that they would usually be,
and this part of the molecule is electropositive, possessing a small positive charge.
The molecule is said to be polar, and as opposite charges attract one another, hydrogen
atoms of one water molecule are weakly bonded to the oxygen atoms in adjacent
water molecules. These weak bonds are called hydrogen bonds. This is the reason behind
the unexpectedly high melting and boiling points of water, when compared to similar
compounds such as H2S (hydrogen sulphide - gaseous at room temperature),
sulfur being in the same group as oxygen in the Periodic Table.
diagram above, illustrates the polar nature of the water molecule.
The H and O represent hydrogen and oxygen atoms respectively,
and the black circles represent the electrons which are shared
(one from each hydrogen atom, and two from the oxygen atom, one for each bond),
and they are shown nearer to the oxygen atom, as it attracts them.
The d+ represents the small electropositive charge on each of the hydrogen atoms,
and the d- represents the electronegative charge on the hydrogen atom.
The polar nature of the water molecule is the reason that water is such a good solvent,
as it can easily ionize substances. Water has been called the 'universal solvent',
as more substances can dissolve it in that in any other solvent.
Often in living organisms, substances must be solution, and water is the solvent
which makes this a possibility. For example, plants can only obtain mineral salts in
solution, and human digestion will only dissolve soluble foods, so large starch
molecules must be broken down in soluble sugars, such as glucose.
The crucial reactions of metabolism take place in the protoplasm with the materials
in solution. Waste products can also be removed in solution, for example in urine.
exchange requires a moist surface, since gas exchange takes place
In mammals, the alveoli in the lungs are moist with water, to allow gas exchange,
and many plants have moist surfaces in their leaves (the mesophyll cells)
for gas exchange.
organisms living in water spend much or all of their time under the surface.
They require oxygen gas to respire, and as water is such a good solvent, the
required oxygen gas is dissolved in the water. Four parts of oxygen will dissolve
in 100 parts of water, and this is sufficient oxygen for most marine life.
Water in the process of Transport
produced in organisms often need to be transported to other parts of the
organism. In humans, blood is used to transport food, hormones, oxygen, waste
products and so on, and similarly in plants, sap is used to transport food
and other substances. Both of these mediums for transports (blood and sap)are
mainly water, as this is the solvent which can dissolve the products to be transported.
Blood allows oxygen to be transported to tissues, and waste products to be
quickly removed. It also facilitates the transport of hormones, to control various organs.
Water as a Reactant
any one time, many reactions are occurring in an organism,
catalyzed by enzymes. Water, being an excellent solvent, allows many of these
reactions to occur, as often the substances need to be ionized.
However, water does not just allow reactions to take part, it can also take part
as a reactant in reactions. The most basic example of this is photosynthesis.
The energy from the sun must be harnessed and stored in organic molecules,
such as glucose. Apart from some bacteria and algae, green plants are the only
organisms which can do this, by the process of photosynthesis, and all other
organisms rely on plants to trap the energy, which is then used by them when
the plants are consumed. Without photosynthesis, there would be no way for
organisms to obtain energy, so life would be impossible.
The equation for photosynthesis shows that water is a vital reactant in the reaction.
mentioned earlier, substances can only be absorbed into the gut, when in
However, large molecules such as starch have been made by combining smaller
units such as glucose. In this process known as condensation, water molecules are released.
The reverse of this reaction, hydrolysis, is carried out in digestion, and as water molecules
are released in condensation, they must be supplied in hydrolysis.
This is another example of an important reaction where water is a reactant.
Water enables metabolism, by allowing the required substances to ionize,
but it also provides the hydrogen necessary for the processes of metabolism.
Water as a mechanism of Support
cells have a cell wall in addition to the usual cell
membrane, so the
cells are not likely to burst (lysis), as can happen in animal cells when too
much water enters them by osmosis.
Plant cells can therefore become turgid - this occurs when water diffuses in
the cell by osmosis up to the point where the cell wall prevents further water intake,
by exerting a force equal to the osmotic force, by which the water was diffusing in.
This makes the cell stiff, or turgid. The turgidity of plant cells in important in support,
as it helps to support leaves, and the stems of herbaceous plants.
there is a weaker solution outside, water enters the cell by osmosis, moving
down its concentration gradient
The cell does not resist the entry of water, as there is little or no 'wall-pressure'
|As the cell becomes turgid the cell wall exists a 'wall-pressure' (W.P.) which resists further water entry by osmosis. The turgidity of the cell gives support to non-woody organs of a plant.|
Water as a Lubricant
meet at joints, and at these joints lubrication must be provided
make sure the bones do not scrape against each other causing damage,
and enabling free easy movement by reducing friction.
A synovial membrane at joints encloses a fluid called synovial fluid,
which acts at the lubricant. Water is a major part of this fluid.
internal organs have watery fluids surrounding them to
give protection, lubrication, and possibly other functions.
For example, in humans the brain contains cerebro-spinal fluid, and the lungs have
pleural fluid in pleural membranes. Eyes contain aqueous and vitreous humours to
maintain the shape of the eye, help focusing, and to allow nutrients, oxygen and
wastes to diffuse in and out of the eye. When a fetus is developing, it is protected
by a watery fluid, known as amniotic fluid. Finally, another important lubricant,
mucus, in the gut allows food to pass more easily through the gut.
Water in Sexual Reproduction
fertilization, a male sex cell (sperm) must
reach a female sex cell (ovum)
in order to fuse producing a zygote, which will develop into a new individual.
The sperm is often transported in a fluid medium known as semen,
which contains mostly water.
Water in the process of Temperature Control
has a high specific heat capacity - the approximate value is 4200J/kg°C.
This means that to raise the temperature of 1kg of water by 1°C, 4200J of
energy are required, so the temperature of water is not easily changed.
A large mass of water, such as an ocean, will heat up slowly during the day, and
cool slowly at night, so its temperature does not change much.
This provides an ideal habitat for marine organisms, with only small variations
sweat when their body temperature is too high.
Sweat consists of mainly water, and the large amount of energy required to
evaporate the sweat from the surface of the organisms is taken from the organism,
thus giving a large cooling effect. Plants also lose water in a process called
transpiration, and this helps water uptake by the roots, and has an effect on the
temperature of the plant, and helps to keep this from rising too high.
high water content of cells gives them insulation, and protects them from
temperature changes, thus helping to keep cells at a fairly constant optimum temperature.
Water as a Habitat
originally evolved in an aqueous habitat, and although life now exists
it still provides a good habitat for a wide variety of life.
Water is still an ideal medium for life, and there are many reasons behind this:
A large mass of water can surround the organisms, providing a protective
which can prevent the cells drying out, as they could on land.
It also makes it easier for life by providing support and buoyancy to organisms
(see Water as a mechanism of Support), so rigid structures, such as woody tissues
(plants) and bones (animals), are not required in such quantities (or even at all)
Water's good solvency and mobility favor the supply of nutrients to organisms,
and the removal of waste products, due their concentration gradients and diffusion.
It also allows the oxygen required for respiration to be dissolved in the water,
from where many organisms can obtain it. For example, fish take in water via
their mouths, and it passes over the gills, allowing oxygen to diffuse into the
blood capillaries, and carbon dioxide to diffuse into the water, to be expelled.
Fertilization is made much easier by a surrounding mass of water, and when
offspring are produced, water tends to disperse then, reducing the risk of competition.
A large mass also has a fairly constant temperature, due to the high specific
capacity of water (see Water in the process of Temperature Control).
This is extremely important, as at the surface of oceans, the annual temperature variation
is on average 10°C, but at a depth of 20m, this is reduced to only 1 or 2°C. Thus,
aquatic organisms have very little need for the temperature control mechanisms which
are required by land-based organisms.
Water filters out harmful ultra-violet (UV) rays from the sun
ecosystem can be divided into various zones, as shown in
on the below.
intertidal zone is zone near the shore between high and low-water
where there are few organisms. The coastal zone extends from low-water mark to the end
of the continental shelf, and this is the main area of the coastal ecosystem, containing 98%
of marine life. Seaweed, and many fish shoals are in this area, so it is used for fishing.
oceanic zone is the zone most remote from land-masses, and
is therefore mainly
It can be divided into three vertical zones. The euphotic zone extends from the surface to a
depth of 100-200m depth, and this is the area in which there is enough light available for
photosynthesis. Most life forms are in this zone, rather than in the lower zone.
The next zone, the bathyal goes down to a depth of 2000m, corresponding to where the
continental slope ends. The bottom, cold, dark layer is the abyssal zone, and contains many
species, but these are mainly bacteria, causing decomposition.
ecosystems can vary widely depending on the surrounding geology,
land-use and levels of pollution. Freshwater ecosystems tend to be eutrophic,
allowing nutrients and organic matter to accumulate. If this process is too severe,
eutrophication occurs resulting in an oxygen shortage.
However, some freshwater ecosystems are obligotrophic - the water flows off rocks,
and therefore contains few nutrients, and so there are few life forms.
The freshwater ecosystem can be divided into two zones - the limnetic zone is the
uppermost zone, where light can penetrate, and the region below is the profundal zone.
Most fish live in this zone, whilst the vegetation is near the surface.
Survival of Marine Life during Winter
has an unusual property - the solid form (ice) has a lower density that
whereas other compounds are more dense in the solid form.
Water actually has its' highest density at a temperature of 4°C.
This means that in a mass of water, such as a pond, the water at the bottom will always
have a temperature of 4°C. In the Winter, ponds often freeze over, but the temperature
at the base is likely to stay near 4°C. Normally, the cooler water would fall, forcing the
warmer water to rise, but in the case of water, as the water at 4°C is most dense,
this stays at the bottom, and the colder water above does not fall, so the temperature at
the bottom does not change much. This allows life, especially fish, to survive during
Letts Revise Guide - GCSE Biology (Julian Ford-Robertson)
Biological Science 1 - Organisms, Energy and Environment (Green, Stout, Taylor, Soper)
Basic Facts Biology (T.A. McCahill)
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