Topic 1: Water

Water is an essential molecule for life as we know it. Water's unique properties allow life to exist and thrive within the conditions found on Earth. On this page, we will explore these properties and how they support life and the biological processes necessary for life.

Polarity

Perhaps the most important property of water, that leads to its other properties and abilities, is its structure which leads to the polarity of its molecules.

Cohesion

Due to its polarity and the number of hydrogen bonds formed between water molecules, water displays the property of cohesion. This is when water essentially is attracted to or sticks to water. This property explains why water falls from clouds as raindrops. The individual molecules of water are attracted to each other and therefore come together in the form of drops. One of the biological systems that relies upon cohesion, at least in part, is the ability of plants to draw water up to their leaves from the soil. As water is cohesive, water molecules help pull other water molecules upwards in a continuous stream. 

Adhesion

The polarity of water and its tendency to form hydrogen bonds also leads to the formation of these bonds between water molecules and other polar substances. Water tends to stick to these substances. This also aids in the movement of water up a plant, as the water molecules are attracted to and bond with the walls of the vessels it is being drawn through (xylem). You will have observed adhesion in action if you've ever viewed water in a test tube or measuring/graduated cylinder. The forming of the meniscus is due to water being pulled up the sides of the test tube due to adhesion. The narrower the vessel, the higher the water is drawn. As xylem are microscopic tubes, this leads to a significant amount of movement upwards.

​Surface Tension

At the surface of a body of water, the water molecules are held more strongly together due to their cohesive forces. As there are only other water molecules to the sides and below, and not above, the water molecules at the surface are more strongly held to each other. This creates surface tension that allows for things like some insects to appear to walk on water. It is also why water is pulled into droplets. 

Water as an Effective Solvent

Both polar molecules and ionic compounds will dissociate in water and form a solution. This is extremely important to biological systems as most substances that are essential to the functioning of cells (glucose, amino acids, vitamins, oxygen, carbon dioxide) must be transported in solution. Also, all metabolic, biochemical reactions catalyzed by enzymes, must occur in solution.

Thermal Properties of Water

The polarity of water and the resultant hydrogen bonds that form between water molecules also leads to the unique thermal properties of water. The number of hydrogen bonds between water molecules means that water is able to absorb a relatively large amount of energy before it increases in temperature, compared to other similar substances. This is known as the specific heat capacity. Its significance to life is that water acts as a temperature buffer, maintaining the inter and extracellular environments at fairly stable temperatures, allowing biochemical reactions to continue. 

Water also has a high latent heat of vaporization, which means that water carries a lot of energy with it when it evaporates, making it a very effective coolant. 

Finally, as water cools and becomes solid, the maximum of four hydrogen bonds forms between all water molecules. In order for this to occur, the water molecules need to spread out to take on the correct orientations to form the bonds. This causes ice to be less dense than liquid water, meaning ice floats. This is important to life as it means living organisms are able to survive under frozen lakes and other bodies of water as they freeze from the top down and rarely freeze completely.