The role of water in the lives of organisms
The role of water in the lives of organisms
Water (Hydrogen Oxide - H2O), is a compound that is found everywhere. It can occur as solid, liquid and gas phases. It forms as much as 2/3 of the Earth's surface and is vital to all varieties of life. It occurs in all living organisms and has a remarkable combination or properties which make it a very useful substance. These properties include high solvent capacity, which enables water to dissolve a wide range or polar substances, high chemical stability, and excellent thermal properties, which make it ideal as a medium for transporting energy in the form of heat in a cooling system in a living organism. Water in fact has the highest specific heat capacity of all known substances, which means it is the best suited liquid for this job.
Diagram of a water molecule
One of the main roles of water in the lives of organisms is to act as a transport medium. Both inside the organisms themselves, and also outside transporting the materials vital for the survival of the organism. Some small animals do not have a circulatory system, but all larger animals do. The circulatory fluids of these animals are made up mostly of water. This water acts as a solvent for a wide variety of polar compounds. It is these complex organic compounds that allow useful substances such as oxygen, which is transported by haemoglobin, and glucose to be carried around the body.
The second major group of living organisms is plants. Although very different to animals, they still require water to survive. The xylem in a plants stem is there to carry water around the plant. Water is absorbed through the roots, and then transported to different parts of the plant where it is made use of. A key use for water in a plant is in the chemical process photosynthesis. In this reaction, water and carbon dioxide are used to produce glucose, which the plant then respires to make chemical energy, or ATP. However, photosynthesis takes place in small ...
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The second major group of living organisms is plants. Although very different to animals, they still require water to survive. The xylem in a plants stem is there to carry water around the plant. Water is absorbed through the roots, and then transported to different parts of the plant where it is made use of. A key use for water in a plant is in the chemical process photosynthesis. In this reaction, water and carbon dioxide are used to produce glucose, which the plant then respires to make chemical energy, or ATP. However, photosynthesis takes place in small organelles called chloroplasts, and these are found in the palisade layer, a layer near the surface of the leaf. In order for the water to reach these areas of the plant it must travel through the xylem. This has other uses for the plant as well, such as turgidity. Plants require water in order to remain turgid and upright. If they loose a lot of water they loose the ability to remain standing, and this makes it difficult for the plant to recover. A similar set of vessels called phloem are used to transport glucose around a plant. The glucose cannot however be carried in phloem on its own, so it must first be dissolved in water. Water molecules are attracted to the polar hydroxyl groups in glucose, so this is possible. Not only does water help to carry around useful substances in the plant, it also provides the plant with minerals from the soil. These minerals are dissolved in the water, and absorbed through the plant's roots into the stem where they are distributed to all parts of the plant. Water is sometimes known as a 'universal solvent' because it dissolves more substances than any other solvent. Because of the way atoms are arranged in a water molecule, water is able to form weak hydrogen bonds with other water molecules, and also different substances which contain polar molecules.
Diagram showing the weak hydrogen bonds between water and a dissolved substance
A plant's main purpose in life is to reproduce. All its energy goes into making seeds, which are then dispersed naturally to allow new plants to grow. The efficiency of this however, depends on the method the plant is designed to use. One plant which uses water as its method of transporting seeds is the palm tree. This tree often grows along side the sea and rivers, and its seeds (the coconut) are often found hundreds of miles from its parent tree. The coconut is one of many plants that use water to transport its seeds.
Although we have established that water is transported around plants and animals for various different reasons, we have not discussed what happens at the ultra-cellular level. Two very important processes occur involving water at this stage: osmosis and diffusion. Osmosis is the process in which water moves across a semi-permeable membrane down a concentration gradient. In plants this enables the plant to control the amount of water in an area, and therefore control its turgidity. An increase in the concentration of an impurity, such as potassium ions causes water to move in. This process takes place in the guard cells and surrounding cells on the underside of a leaf. The increase in volume of the guard cells controls the size of the stomata on the leaf. Diffusion is very similar to osmosis, however instead of the water moving it is the substance dissolved in the water. In this case water is used as the solvent to carry substances such as glucose which are then used in the plant.
Not only does water provide substances necessary for organisms to live, it also gives some living things a place to live. Amphibious creatures, fish, and some mammals can all live in water. In normal conditions water contains dissolved oxygen and carbon dioxide. Both plants and animals can live in these conditions keeping the oxygen and carbon dioxide levels in equilibrium. Water plants absorb nutrients from the water in a similar way to land plants, and the fact that water is naturally colourless allows sunlight to penetrate and allow green plants to photosynthesise. By providing a habitat for small organisms and plants, water is also providing a food source for larger animals.
Water has one unique property which means it is less dense in its solid state than liquid, therefore ice floats on water. This provides an insulating layer which keeps the liquid water below at a fairly constant temperature, allowing life to survive even with very cold outside temperatures. Even when there is no ice on the surface, water is very good at holding its heat which helps organisms to survive all year round.
There are a group of small animals which use water as their habitat, but do not live in it. They are very small and incredibly light in weight. This combined with the large surface area of their feet allows them to stand on the surface of the water. Water has a layer of stronger bonds across its surface called 'surface tension'. This provides enough strength to support the weight of small animals.
Water is important because without it life could not be supported. Water is needed for photosynthesis, so in an atmosphere with no water plants would die out completely. This would reduce the oxygen supply in the air dramatically, causing animals to die. There would also be mass droughts planet wide. On a smaller level, a cell without water would simply cease to function. Normal transport of materials in and out of the cell would not happen, and it would die.
Richard Strofton
