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:
Adhesion to Vessel Walls:
As the molecules of water are polar, not only do they tend to stick together but they also have a slight attraction to the walls of the vessels within which they are contained. This causes them to stick to the walls, a force known as adhesion. An example of this is capillary action, which is defined as the elevation of depression of the surface of a liquid when it is in contact with the sides of a solid. Capillary action depends on the forces created by surface tension and the forces of adhesion of the water molecules to the solid. If the forces of adhesion of the liquid to the solid are greater than the forces of cohesion within the liquid (surface tension), the surface of the liquid will dome upwards, and the liquid will rise up the tube. This action is present by water in clean glass tubes. If the forces of cohesion are greater than the forces of adhesion, the surface of the liquid will dome inwards, and the liquid will begin to fall (if there is a way for it to escape). This action is present by water in greasy glass tubes (in which the force of adhesion is small).
Lubrication:
Water is used very commonly, both biologically and artificially as a lubricant. A lubricant is a substance which may be fluid to semi-fluid, and is used to reduce the friction and abrasion between moving parts. This friction, especially in biological terms, may lead to serious damage of the parts involved. All tissue within the human body is fragile, and therefore, to a certain extent, must be protected against the everyday ‘wear-and-tear’ to which they are subjected. A common biological lubricant is mucus. It is composed of carbohydrate fibres with water trapped within them- this explains why mucus becomes hard and flaky once it has dried out and all of the water has escaped. Due to its lubrication property, mucus is found lining passages, which experience the friction of moving materials. For example, the oesophagus is lined with mucus in order to reduce the abrasion as the bolus of food swallowed travels down the food pipe, preventing damage to any epithelium tissue. Water is also found as a constituent of synovial fluid, secreted by membranous sacs, known as bursae, found between the bones. The thick synovial fluid reduces the friction as the bones move over each other at the joints, decreasing the amount of damage through use.
Transparency:
Water’s transparency is a huge benefit to many organisms. Transparency is the ability of a substance to allow light to pass through it. This is what allows the depths of the ocean to be colonised by large numbers of aquatic plants, which form the basis of every aquatic food chain. As the light is able to penetrate through the water, it enables the plants to photosynthesise. This not only keeps them and the millions of other organisms dependent on them for food alive, but it also allows oxygen to be produced and eventually released into the atmosphere from the water. Another example of how water’s transparency benefits living organisms is the ability for mammals to see, as the aqueous and vitreous humour found in mammalian eyes (which helps keep the pressure within the eyes high) largely consists of water.
No doubt, reading through these properties of water conjures up images of the shear essentialness of water within the mind. Put simply, without this modest compound, earth would be a barren place, devoid of all live. It is our very essence and yet it is still a slave to us, such is the parasitic nature of man. We use it care free for cooking and cleaning, for bathing and carrying away wastes. Our factories use more water than any other material and our demand for it is ever increasing, as the population of this fourth rock from the sun soars, almost exponentially. However, mockingly, it is also our master. Great civilisations have risen where water was plentiful- they have fallen where supplies failed. Men have killed one another over muddy holes and have been forced to worship rain Gods. Often droughts have been partnered by famine, disease and war.
As demand for water grows, man will need to make better use of his supplies. The more he learns about this fourth element of the earth, the better he will be able to face his challenge.
