The structure of water
Water molecules are 'charged' meaning they are polar molecules, with the oxygen atom being slightly negative (delta -) (δ-) and the hydrogen atoms being slightly positive (delta +) (δ+). It is these opposite charges on water molecules that attract each other, forming weak hydrogen bonds. In-fact it is mostly due to hydrogen bonds in water that give this essential molecule its many important properties essential for life.
Life on Earth evolved in the water and all life depends on water for survival.
Just think, around 80% of the mass of living organisms is water - and almost all biochemical reactions that take place do so in aqueous solution (e.g. the cytosol of cells).
Do you recall the other chemicals that make up living things, the organic macromolecules - belonging to the four groups?
Proteins
Nucleic acids
Carbohydrates and
Lipids.
Between these four groups 93% of the dry mass of living organisms is made up, the remaining 7% is made up from small organic molecules such as inorganic ions and vitamins.
The Important Properties of water
Describe several biologically important properties of water
The biological significance of water: Water has Several key biologically important properties.
Water is the universal solvent: a major constituent of cells (e.g. cytosol) and has many substances dissolved in it (i.e. forming the cytoplasm). Dissolved substances, such as Sodium and Chloride ions are components of the extracellular fluid in multicellular organisms. Consequently, water has many biologically significant roles in a cell being the medium in which metabolic reactions take place. Not only is water the medium for biochemical reactions to occur, but is also directly involved in the breakdown of larger molecules. Water is also an important by-product of many biochemical reactions. Beyond the biochemically important properties of water, it is also essential for structural support and turgor. Making up around two thirds of any organism and three fourths of the Earth, water is extremely abundant – and super important for life as we know it. In fact, life evolved in water for approximately 2 billion years before migrating to land!
Water is liquid at room temperature, and a lot of energy is required for it to change state. It is this property that makes water thermally stable: and as a result water benefits life as it stabilises the temperature of an organism and the environment. So, aquatic environments are thermally stable, for example ice is less dense than water, which is why ice floats. This benefits the environment as the ice insulates the water beneath it – maintaining a relatively stable and valuable habitat. Terrestrial organisms on the other hand, take advantage of evaporative cooling – sweating and transpiration for example, to maintain core temperatures.
Water molecules are Cohesive. Hydrogen bonds hold water molecules together, for example this is important in transpiration, as water is ‘pulled’ upwards in a plant from the root system. The cohesiveness of water molecules is also responsible for providing high surface tension – (again, due to the hydrogen bonding between water molecules). It is this property that allows small insects, such as the water strider, to literally walk on water! Additionally, due to the polarity of water, water is adhesive. This can be demonstrated with capillary action – which causes water molecules to rise against the force of gravity within narrow tubes (again think about transpiration and the way water moves up the xylem of plants). Here links should also be made to the importance of surface area: as the narrower the tube – the greater the surface area for the adhesion of water molecules – resulting in water rising higher in the “tube”.
Finally water is a universal solvent – thus many molecules move freely in cells allowing for a many biochemical reactions to take place. Water is essential to life and of course understanding water and its properties is essential to the study of biology.
To summarise: water has many biologically important properties and in this overview several have been discussed. Cohesion (hydrogen bonds holding water molecules together) and subsequent properties such as adhesion and surface tension.
High specific heat capacity - hydrogen bonds absorb heat when they break but release heat when they are formed – resulting in the Thermal stability of water.
Lower density of Ice – resulting in stable aquatic environments, i.e. lakes don’t freeze completely solid, therefore fish and other aquatic life are better able to survive winter.