Water is a solvent for numerous biochemical moleculesm giving solutions and enabling:
- Transport of nutrients, e.g. glucose and amino acids in blood, and sucrose in phloem
- Removal of excretory products, e.g. ammonia, urea
- Secretion of substances, e.g. hormones, digestive juices.
Role of water in metabolic reactions
The majority of essential metabolic reactions take place in solution in water. Water is a raw material or a product of many metabolic reactions.
- Hydrolysis involves the addition of water (hydro) in the breakdown (lysis) of large biological molecules into their monomers/subunits, e.g. proteins into amino acids. (Water is release during condensation reactions).
- Water produced as a metabolic product of respiration is essential for organisms, especially those living in dry habitats. It is the final electron acceptor in the electron transport chain.
- Water is needed for photosynthesis as water is split in the photolysis of water, in order to replace the electrons lost from chlorophyll, and to provide protons for the NADPH.
Role of water in support
Water is not easily compressed and has an important role in support in plants and animals
- The uptake of water by plant cells creates a pressure against the rigid cell wall.
- This turgor pressure helps non-woody plants to remain upright.
- Water provides buoyancy for aquatic organisms, e.g whales.
- Water has a high specific tension and water molecules have cohesive forces holding them together, due to hydrogen bonding between water molecules.
- These properties allow aquatic insects to walk on the surface of water, and water to be pulled through xylem in plants.
- Transpiration of water from the stomata allows water to be pulled up the stem. This brings essential nutrients with it.
Role of water in temperature regulation
Water has a high specific heat capacity which means it adsorbs a lot of heat energy for its temperature rise, and loses a lot to cool. This help to:
- Reduce temperature fluctations in organisms - especially large ones.
- Minimise increases in temperature in cells as a result of biochemical reactions.
- Reduce fluctations in temperature in aquatic habitats.
A lot of heat is needed to turn water into vapour (it has a high latent heat of vaporisation)
- This helps some animals to maintain a constant body temperature as a high amount of heat energy is removed from the body to evaporate sweat or during planting.
- In plants evaporation of water from leaves has a cooling effect.
At 4*C water is at its maximum density and becomes less dense as it freezes.
- Water is denser as a liquid than as a solid
- Thus cold water forms ice on the upper surface, insulating the aquatic organisms below.
- Water must lose a relatively large amount of heat energy to freeze, making the formation of ice crystals in cells less likely.
These notes are aimed at people studying for A2 Biology, but are suitable for other exams and levels too.
Originally written by froggymills on TSR Forums.
Article by TSR User on Thursday 15 February 2018
The Biological Importance of Water
- Length: 885 words (2.5 double-spaced pages)
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The Biological Importance of Water
Water is made up of 2 basic components, these being Oxygen and
Hydrogen. Water has the molecular formula H2O.
We are about two-thirds water and require water to live. Life now,
could not have evolved without liquid water and would die without it.
For example, droughts cause famines and floods cause death and disease
still to this day. Because of its importance, water is the most
studied material on Earth.
Water in human biology
Water is very important as part of the diet of all humans and other
living organisms. For humans, 60% of the water we take in comes from
drinks, 30% comes from food and the remaining 10% is metabolic water,
a product of respiration. Most substances found in the body, dissolve
in water except fats and other large polymers such as fibrous
proteins. Water is the medium in which substances are transported
around the body. This shows how two thirds of the body is made up of
water. It is the most important, yet basic substance ever known.
This is due to the hydrogen bonds that hold water molecules together.
It is very important especially to plants which use the cohesiveness
to take water in through the xylem. The cohesiveness gives the water
the "stickiness" or surface tension and basically holds the droplet or
droplets, together. This is due to the cohesive forces between the
molecules. Cohesion is when water molecules are attracted to other
water molecules. Water can also be attracted to other materials, which
is called adhesion.
The oxygen in water has a negative charge and the hydrogen has a
positive charge. The hydrogen in one water molecule is attracted to
the oxygen from other water molecules. This attractive force is what
gives water its cohesive and adhesive properties.
An example of cohesion is when a drop of water lies on a flat surface,
it does not break up, it remains whole due to the stickiness of the
individual water particles.
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Biological Importance Molecules Living Organisms Fats Metabolic Substances Drinks Polymers
Water is a polar molecule. This helps many substances to dissolve in
Molecules that have ends with partial negative and positive charges
are known as polar molecules. It is this polar property that allows
water to separate polar solute molecules and explains why water can
dissolve so many substances.
Water molecules have a weak, partial negative charge at one region of
the molecule (the oxygen atom) and a partial positive charge elsewhere
(the hydrogen atoms).
Therefore, when water molecules are close together, their positive and
negative regions are attracted to the oppositely-charged regions of
nearby molecules. The force of attraction is called a hydrogen bond.
Each water molecule is hydrogen bonded to four others.
The hydrogen bonds that form between water molecules account for some
of the essential properties of water.
· The attraction created by hydrogen bonds keeps water liquid over a
wider range of temperature than is found for any other molecule its
· The energy required to break multiple hydrogen bonds causes water to
have a high heat of vaporization, which, means a large amount of
energy is needed to convert liquid water, where the molecules are
· Attracted through their hydrogen bonds, to water vapour, where they
The 3 States of water
Water exists in 3 different states, which can be changed using the
processes boiling, freezing, melting and condensing.
Probably the most important state of water is as a liquid. It is
mostly found as a liquid on earth due to the earth's temperature.
Water is a liquid between the temperatures 0oC - 100oC.
At below 0oC, water as a liquid becomes solid or "ice". This is
because the arrangement of hydrogen bonds between water molecules is
more regular than in liquid water.
Above 100oC, the liquid water becomes a gas or "steam". The atoms
become further away and spread out.
Water is also essential for life, since it provides the correct medium
for the transport of nutrients into, and waste products out of, cells.
Water acts as the environment between and around cells through which
other molecules can move.
One obvious example is blood, which is largely water, and which acts
as the transportation system for almost all of the body's essential
Water and Heat
Water absorbs a lot of heat before it temperature rises. This is due
to the inter-molecular forces. Because the hydrogen bonding in water
has stronger inter-molecular forces, the heat capacity is higher.
This table shows the comparison of waters specific heat capacity
against other materials.
Specific Heat (cal/g-C)
When water is heated to 100oC, additional heat must be applied in
order to cause the water to evaporate (form steam). This added heat is
called the latent heat of vaporization. This latent heat must also be
removed when water vapor condenses.
When water is cooled to 0°C, an additional amount of heat must be
removed in order to form ice. This additional heat is called the
latent heat of fusion. This latent heat must also be added when ice
Not only does water have a high specific heat capacity, it is also a
good conductor of heat in comparison to air which is useful for many
animals in different ways.
Water is the medium in which all chemical reactions in the body take
place. Blood, which contains 80% water, is vital in transporting
oxygen to the tissues and carbon dioxide from the tissues as well as
being the life support system for the body.
In its major functions, water acts as:
· an ideal lubricant to transport feed;
· an aid in excretion;
· a regulator of body temperature; and
· A buffering agent to regulate pH of body fluids.
Water's properties make it an important factor in the transfer of heat
and the regulation of temperature in the body. Because water has a
high specific heat it is ideally suited as a temperature buffering
system for the body.
Animals need a continuous supply of water for maximum efficiency.
Because water functions as a lubricant in the transport of feed and
aids in the excretion of waste products from the body, the intake must
equal the output lost through urine, feces and evaporation otherwise
known as sweat.
Water also plays a key part in digestion.
Water is also essential in plants, mainly in the process
photosynthesis which keeps the plant alive and healthy.
Water is the most essential of all substances, upon which all life