This is useful for humans due to the fact that digestion will only dissolve soluble molecules; and so large, insoluble substances have to be broken down into smaller, soluble molecules with water being the solvent. Plants can only obtain their mineral ions in solution. It is also needed in the processes of diffusion and osmosis. For example, during gaseous exchange, the surfaces of the lungs or in mesophyll cells (in plants) need to be moist as the gaseous exchange takes place in solution. Water can also be used as a medium for transportation in organisms, such as in lymph fluid. It can also be used on a much larger scale in the blood. Plasma in the blood is mostly water and is used to carry hormones, food and waste products (such as urea). Similarly it can be used for translocation in plants in which the water is transported from the roots to the leaves.
Due to the low viscosity of water, it can be used as an effective lubricant, and so reduces friction to allow easy movement in an organism. For example, water is a main component in the synovial fluid in between joints, and it surrounds many internal organs in the body to give protection and lubrication. One example of this is the amniotic fluid to protect the fetus whilst in the womb. Water is also useful in plant cells because when the cell becomes full of water (through osmosis), it will not burst as the cell becomes turgid, and this is important in the cell as it gives it structure and supports the plant. Water can also provide a hydrostatic skeleton, and so the pressure of the liquid inside the organism supports the organism, this can be seen animals such as the earthworm.
The hydrogen bonds hold the water together, making them harder to ‘pull’ apart. This gives strong cohesive forces in the water and so provides a high surface tension. This is useful in habitats; as small animals, such as the pond skater, can run over its surface. These strong cohesive forces are also useful in the transpiration stream in plants, which can allow the water to be pulled up the stem in the xylem tubes without the stream breaking. Water has the highest surface tension of all liquids. At the surface, the molecules inwardly attract each other and therefore occupy the smallest surface area.
Water is unique in the fact that it is less dense in its solid state than as a liquid. It is most dense at around 4 °C, (when it is still a liquid). This is useful because the organisms can live under frozen water as it is liquid and so upholds the habitat, and acts as insulation. The hydrogen bonds hold the water molecules together and a large amount of energy is needed to break these bonds. Therefore, it would be difficult to change the temperature of the water, and this would be useful in an environment, as it would mean that there would be small temperature fluctuations in large bodies of water therefore, enzymes in organisms would not be denatured. In addition, the lower density of water in its solid form allows the conservation of water and grants the circulation of nutrients in large bodies of water (such as the sea).
In conclusion, water is vital for life on Earth. It provides a habitat for many organisms and give structure and support to others. Its properties make it a unique molecule and without it, many organisms would find it extremely difficult to survive.
