The roles water in living organisms and water as a habitat for organisms

Water is the most abundant liquid on the planet. It covers 2/3rds of the earth’s surface, and at least 60% of living organisms is water. This value may be as large as 99% – in a jellyfish. The reason for such a large amount of water making up organisms is the fact that it is a major cell constituent – more on that later.

Water is a simple molecule yet has many interesting properties which contribute to its importance.

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Structure

Water is made up of two hydrogen atoms bonded to one oxygen atom. They are covalently bonded and the intramolecular covalent bonds are very strong. The structure is non-linear, due to the electron pair repulsion of the two lone pairs of electrons on the oxygen atom.

Major cell constituent

Often 2/3rds or more of living cells are water. The water is found mainly in the cytoplasm, and it plays a vital role in many functions of the cell: in all organisms – metabolism, and in plants photosynthesis and support. The actual roles of water in these functions are described more fully later.

Polar solvent / universal solvent

The pulling of the electrons towards the O makes the O electropositive and the H’s electronegative. As opposites attract, not only are the hydrogen atoms attracted to their O atom electrostatically, but also to the O atoms in neighbouring molecules. These bonds are called hydrogen bonds, and are responsible for the great strength and unexpectedly high melting and boiling points of water.

The diagram above shows the differing electronegativities, and it is for the polar nature of this molecule that water is such a good solvent. More substances than any other can dissolve in it, and it has been named the universal solvent.

In plants, mineral salts can only be obtained when in solution, and water makes this possible. Likewise in humans, digestion only occurs when the food is in solution. The reactions of metabolism take place in solution, and also waste products are removed in solution.

Also, gas exchange in the alveoli requires a moist surface, and gas exchange takes place in solution.

Transport

Both blood and sap are essential for the transportation of food, waste products and many more substances in animals and plants. These two mediums consist of mainly water, and it ensures that the substances to be transported are in a solution (as water is the universal solvent).

Lubricant

Outside of cells, water is also useful by reducing friction and providing protection and cushion. In joints, water is a major part of synovial fluid, which prevents the bones from grinding against each other.

Cerebro-spinal fluid, amniotic fluid, mucus and pleural fluid (the latter in the lungs). All of these are mainly water, and provide either lubrication or protection and cushion, in different ways.

Support

The turgidity of plants cells is what keeps them upright, and is down to the fact that the cell exerts a force equal to the force of the water entering the cell by osmosis. This balance of force makes the cell stiff. This turgidity helps to support the leaves and stems of many plants.

Reactant

Reactions are constantly occurring in an organism, catalyzed by enzymes. Water allows many of these reactions to occur, as often the reactants need to be ionized.

Water also plays a major part in gaining energy from the sun, as it is a key part in the reaction by which green plants obtain light energy from the sun and store it as chemical energy:

6H2O + 6CO2 C6H12O6 + 6O2

Monosaccharides (such as alpha glucose) join together to by the process of condensation to form larger molecules of starch. In this reaction, a glycogen bond is formed and a water molecule is released when the two hydroxyl groups join (leaving one O atom). The reverse of this is called hydrolysis, and in order to break down the larger starch unit into its glucose sub-units for digestion, a water molecule is needed. This is another important reaction, where water plays a key part.

Cohesion of molecules

Water molecules are attracted to other water molecules. Water can also be attracted to other materials, this is known as adhesion. As explained above, the O atom of one molecule is attracted to the H atoms in the next. This attractive force is what gives water its cohesive and adhesive properties.

Surface tension

As the water molecules are cohesive to each other, this results in surface tension of the water as the molecules along the surface are ‘holding’ each other together.

This property allows organisms, such as the pond skater, to settle on or skate over water.

Sexual reproduction

In the process of fertilization, the sperm is transported to the egg in a fluid medium known as semen – this contains mostly water.

Density

One of the unusual things about water is the fact that the solid form (ice) is not as dense as the liquid form, and below 4�C the density of water decreases.

This property means that ice floats on water and insulates it, reducing the probability of the whole mass of water to freeze. This increases the chance of life surviving under such harsh conditions.

Temperature control

Water has a very high specific heat capacity (4200J/Kg�C). This means to raise the temp of water by 1�C, 4200J are needed, so the temperature of water is not easily changed.

This means that a large body of water like an ocean will heat up in the day and cool down in the night, but only slightly. This provides an ideal habitat for marine organisms.

When mammals get hot, they sweat. Sweat contains mostly water, and therefore a fairly large amount of energy required to evaporate it – this has a large cooling effect on the body.

Plants lose water via transpiration, which helps the uptake of nutrient rich water from the soil, and also prevents the temperature of the plant from rising too high.

Habitat

Water is an ideal medium for life because:

* It provides a protective shield

* It prevents the cell from drying out

* It provides support and buoyancy to organisms

* Its good solvency favours the supply of nutrients to the plants, and the removal of waste products from all organisms.

* It allows the oxygen required for respiration to be dissolved in it, and its good mobility means it can get virtually anywhere in an organism.

* Fertilization – a surrounding body of water disperses the offspring reducing the risk of competition.

* Aquatic organisms have very little need for temperature control mechanisms.

* Water filters out harmful UV rays from the sun.