Water also has a high latent heat of fusion from solid to liquid, and so if possible water says liquid. This is vital to cells, which are made up of a very high percentage of water, because frozen cells cannot be repaired. Water containing solutes lowers the melting and boiling points of water, therefore unless the temperature gets to well below 0°C, cells are protected against freezing due to the large number of solutes in the cytoplasm.
Due to the polarity of water it can easily ionise substances, many reactions take place in water as solutions, because ionic, polar and covalently bonded molecules can all dissolve in water easily. The water molecules are attracted to them, and separate all the molecules out therefore allowing them all to react with each other. This is how most processes work in solutions. Plants can only get minerals in a solution and human digestion only dissolves soluble food. Starch has to be broken down to glucose, which is soluble.
Substances that dissolve in water are called ‘hydrophilic’ and those that don’t are called ‘hydrophobic’. These include fats and oils. Hydrophobic molecules are important because they can increase stability and create a waterproof layer. They are pushed together instead of pulled apart when met with water, and therefore useful in parts of the cell membrane and protein structure where water is needed to be kept out.
Although pure water has low conductivity, when ions are dissolved in it the solution can be conductive. This can happen in the cytoplasm and is important for some cells such as nerve cells where electrical impulses are used.
Water plays a vital role inside cells, especially in photosynthesis and respiration. In plant cells it is used as a raw material for photosynthesis to make glucose, and in the plant mesophyll cells where moisture is needed for gas exchange. Water is also used in animals in the lungs, where oxygen is dissolved into it to transfer it through the alveoli into the bloodstream, and vice versa for carbon dioxide.
Water is also used in hydrolysis and condensation reactions, which are used to turn polysaccharides into monosaccharides, and forming glycosidic bonds. Hydrolysis is essential to both animals and plants as it lets them make the most of stored food in long chains by making the chains smaller.
Osmosis, diffusion of molecules through a semipermeable membrane from a place of higher concentration to a place of lower concentration, is used in all cells to transport substances. It is used in root hairs to take up water for the plant in the transpiration stream, and in blood to let molecules into cells where needed.
Plants have cell walls as well as membranes, and when the plants become full of water, turgid, due to osmosis they do not burst. This is because the cell wall exerts an equal force to the osmotic force. This provides support in the stems and leaves of herbaceous plants. Because of their polarity, water molecules are attracted to many surfaces making it able to enter very narrow spaces such as between cells due against the force of gravity. This is called ‘Capillary attraction’ or more simply ‘capillarity’ and is involved in transporting water in the stem of a plant.
Water is incompressible due to the strong hydrostatic pressures, and this means that some animals do not need a skeleton to support them, for example worms, slugs and jellyfish, who rely on the pressure of the fluid inside them to keep their shape. In the same way it is the pressure of the water in the blood in the penis that makes in erect.
Water can also be used as transport inside organisms. It is used in the lymphatic, excretory and digestive systems as well as the all-important blood. Blood is mainly water, and it transports oxygen, food, hormones, waste products such as urea and other substances. Water can be used for this because substances can be dissolved in it, and then transported quickly to wherever, tissues or organs. Water is also used in fertilization as the sperm travel in semen – mainly water – to reach the ovum. In plants sap is used to transport food amongst other things.
Molecules of water are highly cohesive because of their hydrogen bonds. Water forms spherical droplets, which have maximum inner area and least surface area. These properties allow plants to pull up water through their xylem vessels, in the transpiration stream.
The molecules are held tightly together when in contact with air, and the inner ones form an elastic film called ‘surface tension’. Water skaters are animals that use this to their advantage and can move across water easily with sinking because they don’t break the surface tension. This is because they have hydrophobic feet.
Water is also viscous and therefore useful as a lubricant. It is used in the synovial fluid in joints to aid the manoeuvrability of them.
It can also be used as protection for organs inside organisms such as the eyes, lungs, heart and brain. The brain contains cerebro-spinal fluid, and the heart has pericardial fluid which both help the organs. The lungs are surrounded by pleural fluid in between pleural membranes, which help the lungs expand and contract easily, giving lubrication and protection. The vitreous and aqueous humours in the eye help keep the shape of it as well as providing a suitable medium for light to travel through. Inside the intestines is mucus that helps the food pass easily along to get digested by villi. Amniotic fluid is a serous liquid that is very important to foetuses whilst growing inside the womb, providing support and protection from the outside world.
Water also provides a habitat. Aquatic animals can breathe due to oxygen dissolved in the water. Due to its solvency, nutrients can also be easily supplied and waste easily taken away, because of changes in the density of water with temperature, currents are formed which take away or bring in substances for organisms. Water is transparent; therefore photosynthesis can still take place, meaning that plants can live at the very bottom of the sea. Water filters out harmful UV rays, provides buoyancy and maintains roughly a constant temperature. Water is most dense at 4°C, and when it freezes the ice formed floats on the surface due to the lattice arrangement of its structure. Staying at 4°C the ice insulates the water and lets aquatic life continue without a huge temperature change causing devastation to animals that don’t have temperature control.
In conclusion, water’s distinctive properties has made it undoubtedly one of the most important molecules on Earth. Water is useful in many ways, and even helped in the very beginning of life. Although some animals have now moved onto land or sky, all still rely on water in some way or another, such as food, protection or as a universal solvent.
