Water

• The tissues of mammals are 60-70% water. The tissues of plants are 90% water.
• Water is a very small, dipolar molecule made up of two hydrogen atoms covalently bonded to one oxygen atom.
• The electrons is water are not shared evenly between the oxygen and hydrogen atoms. The larger oxygen atoms pulls the electrons closer towards it, so is slightly negatively charged. The smaller hydrogen atoms are slightly positively charged. These slight charges attract one another to form a weak interaction called a hydrogen bond. These are continually broken and reformed.
• Metabolite- many metabolic reactions require water as a reactant or produce it as a product (eg photosynthesis, respiration, hydrolysis and condensation reactions).
• High latent heat of vaporisation- the evaporation of water requires a lot of energy to break the hydrogen bonds. When water evaporates, it removes heat and has a cooling effect, ed sweating and transpiration.
• Liquid with relatively high boiling point- water remains a liquid over a wide range of temperatures making it a good transport medium. The continual making and breaking of hydrogen bonds makes it difficult for water molecules to escape the liquid state.
• Low density of ice- as water is cooled and the molecules slow down, more hydrogen bonds form holding the water molecules in a crystalline structure which is less dense than liquid water. Ice therefore floats on water and insulates the organisms that live beneath it.
• High specific heat capacity- Thermostability- the hydrogen bonds in water prevent the movement of the water molecules so relatively large amounts of energy are required to raise the temperature of water. This means large bodies of water are thermostable even when there are large external fluctuations of temperature.
• Cohesion- hydrogen bonds cause water molecules to stick together. This results in surface tension at the surface of water. Water can be transported up the xylem in a transpiration stream as cohesion between water molecules makes long thin columns of water difficult to break.
• Adhesion- water molecules stick to other surfaces, eg the walls of xylem.
• Solvent- The polarity of water allows it to be a good solvent. Any polar molecules like ions will dissolve well in water (negative ions would be attracted to the positive end of the water molecule and vise versa). Water molecules tend to cluster around any charged parts of solute molecules. This allows for metabolic reactions like photosynthesis to take place in solution in the cytoplasm of plant cells.
• Transport medium- since water is a good solvent and remains liquid over a wide range of temperatures, it is a good transport medium.
• Transparent- aquatic organisms such as algae and seaweed are able to photosynthesise as light passes through water.
• Water provides the hydrostatic skeleton seen in animals such as jellyfish.
• Water provides turgidity in plant cells and in the eye.