1. Introduction
Life on earth is described as a carbon based phenomenon and yet a water based phenomenon. Life probably originated in water about 4 billion years ago. All living cells depend on water for their existence. In most living cells, water is the most abundant molecule, accounting for 60% to 90% of the mass of the cell. An understanding of water and its properties is important in biochemistry. The macromolecular components of the cell assume their shapes in response to water. Some molecules interact extensively with water, other molecules or parts of molecules that do not dissolve easily in water tend to associate with each other. In addition, much of the metabolic machinery of the cell operates in an aqueous environment. Water is an essential solvent as well as a subatrate for many cellular reactions.
It is the polarity of water that allows water to separate polar molecules and explains why water can dissolve so many substances.
2. Structure of Water
2.1 Polarity
Water can exist in all 3 states of matter on earth. The general formula is H2O. The water molecule is V-shaped and the angle between he 2 covalent O-H bonds is 104.5o. The unusual properties of water arise from its angled shape and the intermolecular bonds that it can form. Water is polar and covalently bonded. A covalent bond involves atoms becoming more stable when their outer electron shells become full. In the case of water, a covalent bond is formed when 2 hydrogen atoms combine with and oxygen atom by sharing electrons resulting in a stable molecule which is unreactive. The shape of the water molecule is triangular.
Overall, the water molecule is electrically neutral but in both the oxygen-hydrogen bonds, the oxygen nucleus draws electrons away from the hydrogen nucleus thus there is a net negative charge on the oxygen atom and a net positive charge on the hydrogen atom. Such a molecule that carries unequal distribution of electrical charge is termed as a polar molecule.
2.2 Hydrogen Bonding
This involves the formation of a hydrogen bond between the hydrogen side of one water molecule and the oxygen side of another water molecule. Hydrogen bonds are weak and they are constantly being formed, broken and reformed in water. Although each hydrogen bond is weak, collectively, they form important forces. Each water molecule can form 4 hydrogen bonds to a maximum of 4 other water molecules or 4 other different molecules. A substantial percentage of the water molecules are bonded to their neighbours. These large number of hydrogen bonds accounts for strong internal cohesion as a liquid which explains its higher melting point, boiling point and heat of vaporization than most liquids. The advantage of weak bonds is hat the contact between molecules can be brief. Come together, respond to one another in some way and then separate when required to.
3. Physical Properties of Water & their Significance.
3.1 Solvent Properties
Water is an excellent solvent for polar substances. These include ionic substances like salts, which contains ions and some non-ionic substances like sugars which contain polar groups within the molecules. Charged regions of polar water are attracted to positive and negative molecules. This attraction is sufficient to overcome the attractions between the ions themselves. Polar functional groups form hydrogen bonds readily with water molecules. Non-polar molecules are repelled by water.
Water’s solvent properties help it to function as a transport medium. The aqueous solution of life contains a diversity of dissolved substances.
3.2 Thermal Properties
3.2.1 High Heat Capacity
Specific heat capacity of water is the amount of heat to be absorbed or loss to change the temperature of 1g of water by 1oC. High heat capaciry means a large increase in heat gain or loss results in a relatively small rise and drop in temperature respectively.
Having high heat capacity means water is relatively slow to heat up or cool down therefore temperature changes within water are minimized. This is important in the maintenance of a constant external and internal environment for organisms and cells, as biological processes occur over a small temperature range.
3.2.2 High Heat of Vaporization
Latent heat of vaporization is the measure of the amount of heat to be absorbed to vaporize 1g of a liquid i.e. to overcome the attractive forces that it can escape as a gas. Water having a high heat of vaporization means that a large amount of energy is needed to break the hydrogen bonds between water molecules and to move molecules far apart to allow vaporization of water.
There is a cooling effect. Energy used by water molecules to vaporize them results in loss of energy from the surrounding leading to he cooling effect which is made use of in sweating and panting of animals and in transpiration in plants to prevent overheating. Large amount of heat loss upon evaporation of water from surfaces of organisms. It contributes to the stability of temperature of lakes and ponds.
3.2.3 High Heat of Fusion
Latent heat of fusion is a measure of heat energy required to convert a substance from solid to liquid without a change in temperature. High heat of fusion means ice requires relatively a large amount of heat energy to melt it. Conversely, water must lose relatively large amount of energy of heat to freeze. The contents of cells and their environment are less likely to freeze.
3.2.4 Density and Freezing Properties
Most liquids contract on cooling, reaching their maximum density at their freezing point. Water is unusual in reaching its maximum density at 4oC. At 0oC, the freezing point of water, the molecules are arranged in a hexagonal crystalline network in which each water molecule is hydrogen bonded to 4 others. The hydrogen bonds keep the molecules far apart enough to make ice less dense than water at 4oC Below this temperature, water density decreases and its volume increases hence ice floats.
Changes in density of water are important in the survival of many aquatic organisms. The temperature of open water is affected by air temperature above the water. Very cold air causes the body of water to cool from the surface downwards. When the temperature of water falls to 4oC, the dense water sinks, bringing the warmer water to the surface. Convention currents of this sort delay the freezing of a body of water. Ultimately water at the surface freezes. As water freezes, it releases heat to the water below and insulates it, making it less likeiy to freeze.
These properties of water prevent water organisms from being freezed to death in winter. Ice floating on water helps to insulate water below to increase survival chance of aquatic organisms. The convention current helps in nutrient cycling.
3.3 Intermolecular Attractions
Cohesion is the tendency of molecules of the same type to be attracted to one another. Cohesion forces in liquids are much stronger than in gases. Unlike gases, liquids cannot be expanded or compressed to any degree. Hydrogen bonding increases cohesive forces between water molecules. Cohesion helps in the upward transport of water in the xylem when evaporation occurs at the leaves. Water that evaporated from the leaves “pulls” water from the xylem vessels and water molecules leaving the xylem vessels “pulls” water from further down in the vessels all the way down to the roots.
Adhesion is the tendency of molecules of different type to be attracted to one another. It exists between walls of xylem vessels and the water within them and aid to counter the downward pull of gravity in the upward transport of water in plants.
Surface tension is related to cohesion. It is a measure of how difficult it is to stretch or break the surface of a liquid. At the surface of water, the molecules are arranged so that they are hydrogen bonded to one another and to the water molecules below the surface. The effect of hydrogen bonding creates a tight skin-like layer at the surface which is termed surface tension. This allows small insects to walk on the surface of the water without breaking the surface.
3.4 Low Viscosity
Viscosity is a measure of a substance’s resistance to motion. Having low viscosity, water is able to flow fast. Water is often used as a lubricant.
3.5 Light Penetration
Light rays penetrate water with relative ease because water is transparent. In clear water, red and yellow light can reach a depth of 50m and blue and violet light can go down to 200m.. This property enables photosynthetic plants to inhabit in water.
3.6 Incompressibility
As water is not compressible, it plays a role as a structural agent. It acts as a hydrostatic skeleton in some animals for maintaining shape and locomotion.
