The fact that most small biological molecules dissolve in water is very important because all the chemical reactions that take place in cells do so in aqueous solution. Plants cam only obtain mineral salts in solution and human digestion will only dissolve soluble foods, meaning large starch molecules must be broken down into soluble sugars. Also many organisms living in water spend most of their time underwater, yet they require oxygen to respire, and as water is such a good solvent the required oxygen gas is dissolved in water. Water’s solvent properties also allow it to act as a transport medium for polar solutes for example the movement of minerals to lakes and seas and removal of metabolic wastes such as urea and ammonia in urine.
Water’s thermal properties relate to its heat capacity. Water has a high specific heat capacity compared with other liquids. This means that a large increase in energy results in a comparatively small rise in the temperature of the water. This is because much of the heat absorbed is used to break the hydrogen bonds that hold the molecules together. Water is good at maintaining its temperature at a steady level, irrespective of fluctuations in the temperature of the surrounding environment. This is important because the range of temperatures in which biochemical processes can proceed is quite narrow and most organisms cannot tolerate wide variations in temperature. The high thermal capacity of water means that living organisms are very thermostable, and therefore less prone to heat damage by changes in environmental temperatures.
Another of waters thermal properties is its high latent heat of vaporization. This is where hydrogen bonds attract molecules of liquid water to one another and make it hard for these molecules to escape as vapour. Therefore a relatively high-energy input is needed to vaporize water and water has a much higher boiling point than other molecules of the same size. The high latent heat of vaporization of water means that a body can be considerably cooled with a minimal loss of water – this phenomenon is used by mammals in sweating and in reptiles in gaping and may be important in cooling transpiring leaves.
A further important property of water is its surface tension. This is the force that causes the surface of a liquid to contract so that it occupies the least possible area. Hydrogen bonding increases the cohesive forces between water molecules and it is this that causes the molecules which are pulled inwards towards each other to form spherical drops rather than spreading out in a layer. It is this inward pull of the water molecules that creates a skin-like layer at the surface. At ordinary temperatures water has the highest surface tension of any known liquid (except mercury).
This strong cohesive force that exists between water molecules play an important part in the movement of water up the vessels and tracheids in the stems of plants. Were these forces much weaker, trees could not be so tall. Water’s cohesive and adhesive properties also mean that water is viscous, making it a useful lubricant in biological systems. For example, synovial fluid lubricates vertebrate joints and pleural fluid minimizes the friction between lungs and the ribs during breathing. Surface tension also allows the surface film of standing water to support – and provide a habitat for certain aquatic organisms. For example the high surface tension enables small organisms, such as pond skaters, to land on the water’s surface and to move over it.
An additional, but by no means less important, property of water is its density and freezing properties that are so important to aquatic organisms. At temperatures of 0°C and below water forms ice. The arrangement of the water molecules in ice makes it less dense than liquid water. Most liquids increase in density, and so decrease in volume, on solidifying. Water is unusual because the reverse is the case. As a result, ice forms at the surface of bodies of water, such as ponds and lakes, and then continues to float on top of the water. Water has its greatest density at 4°C. This means that in winter, the warmest water is often at the bottom of a lake. As a result, lakes more than more than a metre or two rarely freeze right through.
This is extremely important, especially, to aquatic living organisms, as even in the hardest of winters, organisms including large fish, may survive at the bottom of lakes. If it were not for ice floating on water, the oceans would all be frozen solid except for a thin layer of liquid water near the surface. Even the coldest of oceans are liquid except at certain times of year when they may freeze at their surface. It also allows the water to thaw rapidly when temperatures rise. The changes in the density of the water also allow circulation to be maintained in large bodies of water, which therefore helps nutrient cycling. Floating ice also provides a habitat for organisms such as penguins and polar bears.
A more obvious property of water is its transparency. This feature of water is very important biologically. Were water opaque very few organisms could live in it, because the absence of light would prevent primary producers from photosynthesising. As light penetrates water, aquatic photosynthetic plants can live at some depth. These plants support aquatic food chains, and therefore animal life. It is also important to animals living in water for it allows them to see – their eyes depend on water’s transparency to see.
Water’s colloid formation is also very important although quite unusual. A solid placed in an average liquid may dissolve, and if not, float or sink. However in occasional cases, such as water, an intermediate result is achieved where the solid becomes finely dispersed as particles throughout the liquid. Some molecules have strong intramolecular forces that prevent their solution in water but have charged surfaces and so attract a covering of water molecules. It is this covering that ensures that the molecules remain dispersed throughout the water. Many protein and polysaccharide molecules form colloids. Their most important feature is the large surface area of contact between the particles and the liquid. Cytoplasm, for example, is a colloid, made up largely of protein molecules dispersed in water. Cytoplasm is an example of a hydrophilic colloid. This is where the particles attract water molecules around them and it is this which prevents them aggregating into large particles which would settle out. The attraction of water by hydrophilic colloids causes them to absorb water in a process called imbibition. It is this process that causes dry seeds initially to absorb water.
As we can see water is an extremely complex and diverse molecule with many different properties. I believe that water is extremely important, if not one of the most important molecules, to living organisms. Without it, we, as humans, and all other living organisms could not survive. I think water is important to us as it performs many different functions, it acts as a solvent, it insulates, it provides a habitat for many creatures, it allows aquatic plants to photosynthesise and many other things already mentioned here. Therefore we can say that water is very important to all living organisms.
