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Without water, life could not exist on this planet. It is doubly important to living organisms because it is both a vital chemical constituent of living cells and for many a habitat too. It covers three quarters of the earth's surface and is the only compound known to man that exists naturally in the three states of matter- solid ice is found at both poles culminating in a formidable climate for living beings; liquid water is found in even the driest region; water vapour is found in hugely variable amounts as part of the earth's atmosphere, which shields its inhabitants from the deadly radiation emitted from the sun.
Water is such a fundamental part of human life that the first temperature scale to ever have been constructed, the Celsius scale, has its 'bench marks', as it were, set around water's freezing point (0oC) and boiling point (100oC).
Water makes up 65- 70% of our total body mass, and this mass remains relatively constant throughout the day.
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This means that the 2-3 dm3 of water lost daily from the body must be replaced by fluids or food consumed by us each day. The importance of water to life becomes clear when it is considered that a human being deprived of food may live for up to 60 days, but for only a few days if deprived of water. Therefore, judging by the impressive introduction, it would be worthwhile to have a look at some of water's properties.
Water's physical properties are rather unusual and are due mostly to its small size, its polarity and to hydrogen bonding between its molecules.
Water is polar- this means there is an uneven charge distribution over the molecule. In water, one end of the molecule has a slightly negative charge, the oxygen end, and one end has a slightly positive charge, the hydrogen end. This is known as a dipole:
The more electronegative oxygen atom tends to attract the more electropositive hydrogen atoms. Water molecules therefore have a slight electrostatic attraction for each other, opposite charges coming together via forces of attraction, behaving as if they were 'sticky'. These attractions are not as strong as normal ionic bonds and are known as hydrogen bonds. These created hydrogen bonds give water a slight structure. This explains the unusually high boiling point of water. Although the individual hydrogen bonds are not very strong, due to the shear number of them found in, for example a pan of water, a large amount of heat energy is needed in order to break all the hydrogen bonds and vaporise the liquid water, thus giving water a high boiling temperature. Here is an example of the hydrogen bonds:
Having observed the structure of the molecules of water, some of the biologically significant properties of water can be examined: Solvent Properties: Water is an excellent solvent for polar molecules. These include ionic substances such as salt, whose charged particles (ions) disassociate when dissolved in water, and some non-ionic substances such as sugar and simple alcohols that contain polar groups within the molecules such as the hydroxyl groups of the sugars and alcohols.
When ionic substances are dissolved into water, the electrostatic attraction between the polar water molecules and the ions of the substance exceed the attraction between the cation and anion of the ionic substance. This causes the water to break up the ionic lattice of the substance and culminates in the formation of hydration shells; molecules of water arranged around the individual cations and anions of the substance. Here is a diagram to represent this:
Once a substance is in solution, its molecules or ions are free to move about, thus making is more chemically reactive than if it were a solid. Therefore, the majority of the chemical reactions that occur in the cell take place in aqueous solutions. Non-polar substances, such as lipids, are immiscible with water (does not mix with water), and so provide 'compartments' within the cell, such as membrane within the cell does.
Non-polar parts of the molecules are repelled by the water molecules and group together in its presence, such as when oil forms large oil droplets when subjected to water. This property of non-polar molecules means that they are hydrophobic, where as ionic substances are hydrophilic (are not water hating). Such hydrophobic interactions are important in maintaining the stability of membranes and many protein molecules. Water's solvent properties also mean that it acts as a transport medium in the blood. Here, soluble proteins and ions are carried dissolved in the plasma.
Specific Heat Capacity:
Water has a high specific heat capacity. Specific heat capacity is the amount of heat energy in joules needed to raise the temperature of 1kg of water by 1oC. As water has a high specific heat capacity (4200j/kg/oC), a large increase of heat energy is accompanied by a relatively small increase of the temperature of the water. This is because (as discussed earlier), much of the energy is needed to break the magnitude of hydrogen bonds prevalent in the water, which restrict the mobility of the water molecules. This means that temperature variances are minimised due to the high specific heat capacity. This creates a more stable environment for aquatic organisms. It also means that biochemical processes will occur over a smaller temperature range, proceeding at more constant rates and are less likely to be inhibited by extreme temperatures.
Latent Heat of Evaporation:
Water has a high latent heat of evaporation. Latent heat of evaporation is the amount of heat energy required by 1kg of water to be vaporised into gas, without a change of temperature. Conversely, it can also be taken as an indicator of the attractive forces between the molecules in the liquid, as it is these that must be overcome in order for the liquid water to be vaporised into a gas. Again, the reason for water's high latent heat of vaporisation, is due to the hydrogen bonding between the molecules, which need to be broken in order to vaporise it. The energy imparted to the water molecules to vaporise the substance results in a loss of temperature from the surroundings, rather like an endothermic reaction.
This produces a cooling effect which is exploited by many mammals when they pant and sweat. The high latent heat of vaporisation of the water means that mammals can lose large amounts of heat energy this way, with minimal losses of water from the body. Some mammals have special sweat glands on the surface of their skin, which produce a salty, aqueous solution. The heat produced by the vaso-dialated capillaries below the skin, evaporates off this solution, cooling the organism.
Density of Ice:
The density of water decreases below 4oC and so ice (which is at 0oC) floats on water, being less dense than it. Water is the only substance whose solid form is less dense than its liquid form. The reason this unique property exists is, surprise surprise, again down to its hydrogen bonds. As water cools below 4oC and then freezes, the molecules lose increasing amounts of energy. This causes them to vibrate less and contract together. As this occurs, more hydrogen bonds form between the molecules (as water does not have 100% hydrogen bonding). When these hydrogen bonds form, they push the water molecules between which they have formed apart, decreasing the density of the water.
Therefore, at 0oC, there is 100% hydrogen bonding, causing ice to be less dense than water. Since ice floats on water, it forms at the surface first and at the bottom last. If ponds froze from the bottom upwards, aquatic life would not be able to survive in arctic climates. Once the ice has formed at the surface of ponds, it insulates the rest of the pond, increasing the chance of survival of the organisms in the water. The fact that water below 4oC will rise to the top due to it's low density, allows for circulation in large bodies of water, which results in nutrient cycling, providing a chance for colonisation of water at greater depths.
Incompressibility:
Due to the fact that water is incompressible, it is invaluable to many soft-bodied organisms as a hydro skeleton, such as earthworms. Here fluid is secreted within the body and enclosed by the muscles in the body wall. The fluid presses out against the muscles, which are in turn able to contract against the fluid. The combined effect of the pressure of the fluid within and the contracting of the muscles helps to maintain the shape and form of the animal. Usually there are two types of muscles, radial and longitudinal. They are an antagonistic muscle pair and when they work in tandem exerting pressure on the fluid in different directions, the organism is able to move its body.
However, the pressure is localised in organisms that are segmented, which means only certain segments will move or change shape. The incompressibility of water is also used in the functioning of the male sex organ, the penis. Within this rod-like feature, is erectile tissue. During arousal, blood flows into the penis through the network of capillaries in it. This causes a pressure to be exerted outwards against the tissue, allowing an erection to be achieved.
High Surface Tension and Cohesion:
Cohesion is the force whereby the individual water molecules seem to 'stick' together. This is because as water molecules are polar, they are electrically attracted to each other and are held by hydrogen bonds. This is known as the 'cohesion tension theory'. This theory is prevalent during the uptake of water through the xylem of all plants. Here water is transported up the plant in the transpiration stream- at the leaves water is evaporated away, almost 'sucking' up water through the roots. This 'sucking' action pulls the water up the xylem as one long chain, as the molecules stick to each other due to cohesion.
This can be performed as since water has high cohesion strength, it means that a relatively large tension is needed to break a column of water. At the surface of water, a force called surface tension exists between the molecules due to the force of cohesion between the water molecules acting inwardly. This causes the surface of the water to take up the least possible surface area, which is ideally a sphere, as the surface molecules are pulled tightly into the body of the liquid (this is because there are no molecules of water above the surface molecules). This is why water droplets form on the surface of materials. Water has a higher surface tension than any other liquid, which allows many organisms to move over its surface, almost skating over it. An example of an organism like this is the pond-skater. Here is an example of the cohesion present in a droplet of water:
The Importance of Water to Living Organisms. (2020, Jun 02). Retrieved from https://studymoose.com/importance-water-living-organisms-4686-new-essay
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