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Exam of 2 pages for the course BIOLOGY AND CHEMISTRY at CAM (Explained in detail)
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B io FactsheetSeptember 1998 Number 30
The Biological Importance of Water
Water (H2O) is the most abundant molecule in cells, whole organisms and on earth. This is a consequence of the unique physical and
chemical properties of water. This Factsheet will briefly explain the significance of these physical and chemical properties, then discuss
the primary roles of water in animals and plants.
Water is a polar molecule i.e. it has both positively charged and negatively This layer of water means that the molecules remain dispersed, which
charged areas. Water is made up of two positively charged hydrogen atoms prevents them joining together and settling out. This is known as a colloid
and one negatively charged oxygen atom (Fig 1). or colloidal suspension; the water molecules which surround the large
molecule are weakly bound to the molecule and cannot move away from its
Fig 1. The polarity of water molecules surface. This is important because such molecules provide an osmotic
δ+ effect, helping to draw water into the blood vessels, for example.
H The fact that water is an extremely effective solvent is of great biological
δ- importance. All of the substances which are essential for the functioning of
δ+ cells and whole organisms (glucose, amino acids, fats, vitamins,
O
respiratory gases etc.) are transported around in solution. Similarily, all
H metabolic reactions, catalysed by enzymes, occur in solution.
To summarise, the charged nature of the water molecule (i.e. its polarity),
As a result of this polarity (the uneven distribution of charges), adjacent
gives it stability and allows it to act as a solvent - both essential properties
water molecules are attracted to and become bonded to each other. The
to living organisms.
slight positive charge of a hydrogen atom of one molecule is attracted to the
slight negative charge of an adjacent oxygen atom. A hydrogen bond forms
between them (Fig 2). 2. Thermal properties
The hydrogen bonding between water molecules is also responsible for the
Fig 2. Hydrogen bond between two water molecules unique thermal properties of water. Water has a high specific heat capacity
+ i.e. it takes a lot of energy to raise the temperature of water or conversely,
Hydrogen a lot of energy can be lost before the temperature of water starts to fall.
H
_ bond This is crucial for living organisms which need to maintain particular
+ + temperatures in order to optimise enzyme activity. The high water content
+ O H H of cells and tissues helps them to maintain a constant temperature. In this
H way water acts as a temperature buffer.
second
water Hydrogen bonding is also responsible for the fact that ice is less dense than
molecule water and therefore floats. As the temperature of water decreases, the
_ kinetic energy of the individual molecules decreases and the molecules
first O slow down. This allows each molecule to form the maximum number of
water hydrogen bonds with other water molecules (Fig 3).
molecule
The individual hydrogen bonds are weak but collectively they make water Fig 3. The stable structure formed by hydrogen bonds in ice
very stable i.e. it remains a liquid over a huge range of temperatures (0-
100oC). This is vital in living organisms. H H
O
1. Solvent properties
Polar molecules and ionic compounds such as sodium chloride dissociate in H
H
water. Sodium chloride (NaCl or ‘salt’) is made up of a positively charged
sodium ion (Na+) and a negatively charged chloride ion (Cl -). As a result of O
the opposite charges, the two ions are attracted to each other and a bond H O H
H
forms between them. When the salt is added to water the sodium and O
H
chloride ions split apart (dissociate). They do this because the force of
H
attraction between the negatively charged part of the water molecule and H
O
the positively charged sodium ion is greater than the forces of attraction
between the Na+ and the Cl- ion. The negative part of the water molecule
attracts and pulls away the positive part of the salt molecule. Similarly, the To achieve this structure, the water molecules spread out i.e. expand to
positive part of the water molecule attracts and pulls away the negatively accommodate more bond formation. Because ice floats i.e. water freezes
charged chloride ion. Both the Na + and Cl- become surrounded by water from the top down, many organisms are able to survive below the surface
molecules; in other words they become hydrated. Polar substances which of the ice.
dissociate in water in this way are said to be hydrophilic.
Conversely, when water molecules do escape from the water surface during
Some large molecules have strong intramolecular forces and do not evaporation, a lot of energy is released with them. As a result, evaporation
dissociate or dissolve in water. However, some of these molecules do have (e.g sweating or panting) is an efficient cooling mechanism, allowing living
charged areas on their surface, which attract a layer of water around the organisms to maintain a constant body temperature. The properties and
molecules. functions of water in living systems are summarised overleaf.
1
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