Why is water important for life ?

1.

Water is an excellent solvent

Water is an excellent solvent. That means that many different types of materials can dissolve in water - forming solutions. Water is the solvent that transports many essential molecules and other particles around the body. These include nutrients and waste products from the body's metabolic processes.

2.

Ease of movement of water molecules through biological membranes

Particles such as some ions and molecules need to be able to move around biological organisms. One way in which this happens is in solutions (mentioned above) e.g. transport of oxygen in blood around the vascular system. Movement of solutions within defined channels such as blood vessels and lymphatic vessels is easily explained by comparison with e.g. the movement of the fluids along pipes.
Some ions and molecules in biological organisms also need to be able to move through tissues, and membranes e.g. cell membranes. They move by the processes of diffusion, osmosis and active transport - of which osmosis is the diffusion of water, an important process in living organisms.

3.

Water takes part in many chemical reactions

Chemical reactions only happen when the reactants make contact with each other (sometimes via intermediary steps e.g. involving catalysts). Solutions are often good "mediums" for chemical reactions because the solvent, e.g. water, encloses solutes - which could potentially be "reactants" if there is a possibility of them reacting with each other if and when they collide - in a common volume of space, be that a test tube in a laboratory or an organ or tissue in the body. When two or more potential reactants are in the same solution they may collide and react with each other. The probability of this happening depends on several factors including the concentration of the solutes, the temperature of the solution and, in some cases, the presence (or not) of an appropriate catalyst for the reaction.

Water molecules participate in decomposition reactions whereby certain macromolecules are broken-down into smaller parts. Examples include the breakdown of carbohydrates and proteins during the digestive process.

Water is also produced by chemical reactions occurring within the body in which relatively small organic compounds (called "monomers") join together in "synthesis reactions" to form larger and more complex molecules called "macromolecules" required by the body for specific functions e.g. nucleic acids and hormones.

4.

Water can act as a lubricant, i.e. to reduce friction between moving surfaces.

Water (incl. solutions of which water is the solvent) serves an important lubrication function.

This is essential in many parts of the body, especially:

• in the thoracic and abdominal cavities where internal organs (e.g. the heart and lungs, and the organs of the digestive system) are located next to each other and slide over one another as the body moves around).
• at joints e.g. synovial joints where structures such as bones, ligaments and tendons need to move smoothly relative to each other without being impeded by friction between the different structures/surfaces.

5.

The thermal properties of water are well-suited to support life.

Water has a high specific heat.
The specific heat of a substance is the quantity of heat per unit mass needed to increase the temperature of the substance by one degree Celsius.

More energy is needed to increase the temperature of water compared with that of other solvents because hydrogen bonds hold the water molecules together.

The definition of specific heat can be summarized as:

However, the relationship expressed in the above equation does not apply if a phase change (e.g. from a liquid to a liquid) happens because heat added or removed during a phase change does not change the temperature.

The specific heat of water is 4.18 J/g° C.
This is much higher than for many other substances e.g. NaCl 0.864 J/g°C, Fe 0.450 J/g°C and Cu 0.385 J/g°C.

The thermal properties of water that affect human and animal biology include:

• Compared with other materials water can absorb or release a relatively large amount of heat energy while only adjusting its own temperature by a relatively small amount. Therefore the fact that water accounts for a significant proportion of body mass helps the body to cope with environmental temperature variations and maintain the body's temperature within a safe and comfortable range.
• Similarly, compared with other materials water also needs a relatively large amount of heat energy in order to evaporate (i.e. change state from liquid to gas). This is called the "latent heat of evaporation" and the value for water is approx. 2270 kJ/kg at 1 atm pressure.
  Therefore the evaporation of sweat from the surface of the skin is very efficient in helping to cool the body because it removes relatively large amounts of heat from the body as the sweat evaporates.
  - This also explains the limitation of fur on an animal's ability to cool itself in high temperatures, hence the use of cooling water pools by animals with dense fur living in hot climates.

6.

Other biologically useful properties of water include its cohesion, adhesion and surface tension

The structure of water molecules includes "polar covalent bonds" which means that electrons (hence electric charge) are unevenly distributed around the molecule, hence some parts of the molecule are relatively more "positive" and others relatively more "negative" - compared with each other.

This results in some water having some useful properties:

• Cohesion - means that water molecules are attracted to other water molecules (e.g. forming droplets)
• Adhesion - means that water molecules are often attracted to other materials (though not all other materials)
• Surface Tension - means that water molecules pull towards each other at interfaces with other matter e.g. air. The result is the smallest possible surface area of water.
• Capillary Action is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to external forces like gravity. Water is capable of capillary action due to its properties of adhesion (i.e. some water molecules are attracted to molecules of another adjacent material) and cohesion (i.e. other water molecules stay attached to and so "follow" neighbouring water molecules moving along a channel or surface due to attractions to successive non-water molecules).

An example of capillary action in human biology is the drainage of constantly produced tear fluid from the eye.