Generally, the main biological importance of water is that life cannot exist without it. In fact on a percentage basis, the majority of any organism is comprised of water. Additionally, it is believed that life first originated in the bodies of water on the earth. The importance of water is seen in such third world countries as Kenya. Where the inhabitants are forced to drink water so dirty and contaminated that they know it might kill them. However, they still drink it because it’s essential for life.
The solvent properties of water are essential for living organisms to survive. The solubility of the substances needed by the organism depends on the ability of water to interact with them, and the polarity of water plays a critical role.
Water is an excellent solvent for ionic compounds such as sodium chloride. This is because cations like sodium ions become surrounded by a shell of water molecules each attracted to the positive charge by the slight negative charge on the water-oxygen atoms. Anions are hydrated through the attraction of the slight positive charge of water-hydrogen atoms to their negative charge. Substances, in particular polar substances, which dissociate in water, are known as hydrophilic.
Covalent compounds are harder for water to act as the solvent. Some molecules have strong intramolecular forces, which prevent their solution in water but have charged surfaces, which attract a covering of water molecules. This covering ensures that the molecules remain dispersed throughout the water, rather than forming large aggregates, which could settle out. The dispersed particles and liquid around them collectively form a colloid. Such substances are sugar and alcohols. This is important because such molecules provide an osmotic effect, which helps to draw water into the blood vessels of living organisms.
The fact that water is a very effective solvent allows all of the substances essential for the functioning of cells and organisms (glucose, amino acids, vitamins, fats, respiratory gases) are transported around in solution in the blood. This means that water acts as a transport medium for the polar solutes. It carries things needed by cells to cells and products from cells to other cells and waste products to be excreted. In mammals, this happens in the blood and in plants, it happens in the xylem and phloem.
Similarly, all metabolic reactions, catalysed by enzymes occur in solution. Chemicals can only react with each other if the molecules and ions are free to move and collide. If they were in solid form it wouldn’t happen. Therefore all metabolic reactions and all of life are dependent on the solvent properties of water.
The thermal properties of water are also of great biological importance. Water has a high specific heat capacity i.e. it takes a lot of energy to raise the temperature of the water or conversely a lot of energy can be lost before the temperature of water starts to fall. This is crucial for living organisms, which need to maintain a particular temperature to optimise enzyme activity. The high water content of cells and tissues helps them to maintain a constant temperature. In this way, water acts as a temperature buffer. This is vital in endothermic organisms.
The hydrogen bonding between water molecules is responsible for the unique thermal properties of water. The individual hydrogen bonds are weak but collectively they make water very stable i.e. it remains a liquid over a huge range of temperatures.
Hydrogen bonding is responsible for the fact that ice is less dense than water and therefore floats. As the temperature decreases so does the kinetic energy of the molecules and they slow down. This allows each molecule to form the maximum number of hydrogen bonds with other water molecules. When this happens the water molecules spread out i.e. expand to accommodate more bond formations and because ice floats i.e. water freezes from the top to the bottom, aquatic organisms are able to survive below the surface of the ice.
This is because the maximum density of water occurs at 4. As the surface water gets colder it becomes denser and sinks to the bottom and warmer water from below rising to the surface and begins to cool. If this continued, however, ponds would freeze from the bottom to the top killing many aquatic organisms. Instead what happens is that when the surface water gets colder than 4 it’s actually less dense so therefore stays closest to the surface. Therefore that layer of water gets colder and colder until it freezes, leaving aquatic organisms to live below the ice, which acts as an insulator.
Conversely, when water molecules escape from the water surface during evaporation a lot of energy is released. As a result, evaporation (sweating) is an efficient cooling mechanism, allowing living organisms to maintain a constant body temperature.
Water plays a very important role in other areas of life as well, such as, In-plant cells water confers turgidity. This is essential in maintaining maximum leaf surface area. Therefore maximum light absorption takes place and consequently the rate of photosynthesis increases. If water is lost during hot conditions then wilting occurs, decreasing leaf surface area and photosynthesis, which highlights the significance of water to plants.
Water is the means by which living organisms are able to survive. It is also the means by which organisms reproduce. Organisms, which employ sexual reproduction, use water to bring the male and female gametes together in the process of fertilisation.
Therefore without water living organism would perish within minutes. Merely the fact that about 80% of the earth is covered with water shows its importance to living organisms. Water is essential in order for living organisms to function, in a lot of cases it acts as a home to aquatic animals, it allows organisms to reproduce and has certain solvent properties which allow processes such as respiration to occur in living organisms. The change of state of water allows aquatic animals to survive and other organisms in very hot conditions. Without water, life would simply cease to exist and for that reason, the biological properties of water are the most important thing to living organisms.
Biofact sheet by Catherine Brown
Water notebook by Allan Lowe
The biological importance of water from A level biology handbook
Advanced Biology by Mary and Geoff Jones
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