HideShow resource information

Structure of H20

Water is a polar molecule.  This means it has an uneven distribution of charge.

Water has the symbol H2O as one molecule contains two hydrogen atoms and one oxygen atom.The oxygen atom has a slight negative charge.  The two hydrogen atoms have a slight positive charge. 


The opposite charges of water molecule are attracted to each other and form hydrogen bonds – this is called cohesion.

Molecular morbility- the weakness of individual hydrogen bonds means that individual water molecules continually jostle one another when in the liquid phase.

1 of 7

Water and its biological significance

Water as a solvent- Water is slightly ionised, so are other molecules such as salts, sugars and amino acids. •This means they will dissolve in water.

Biological significance-Once a substance is dissolved in water it can be easily transported.E.g. excretory products, hormones, digestive juices.Once a substance is in solution its molecules or ions are free to move about and are therefore more chemically reactive.

Surface tension- exists due to the water molecules being more attracted to each other than to the air molecules above it.
The attraction between molecules of similar substances  is called cohesion. 
This surface is not easily broken and water has a greater surface tension than most other liquids.

Biological significance-Tall trees depend on cohesion of water molecules in order to transport water up from the roots to the leaves.Many animals rely on surface tension to support their bodies so they can ‘walk on water’.

2 of 7

Water and its biological significance

High specific heat capacitity-This is the amount of heat energy (joules) required to raise the temperature of 1kg of water by 1°C. Water has a high specific heat capacity – i.e. it needs to gain a lot of heat energy to raise its temperature.

Biological significance-Fluctuations in temperature are minimised in cells.Creates a constant sea temperature for aquatic organisms.

High latent heat of vaporation-This means a lot of heat energy is needed in order to evaporate it. When water evaporates it draws a lot of heat energy out of the material underneath it

Biological significance- Creates a cooling effect. E.g. sweating, panting and cooling in plants – a lot of heat is lost without the loss of too much water.

High latent heat of fusion-This means water must lose a lot of heat energy before it forms ice crystals, i.e. it is difficult to freeze

Biological significance-Ice crystals are less likely to form inside cells.Water in ice form cannot transport substances or act as a solvent. Liquid water is essential for life.

3 of 7

Water and its biological significance

Density-Water is most dense at 4°C and then its density decreases as it freezes so ice floats.

Biological significance-A floating section of ice will insulate the water below and allow aquatic life to live in sub zero conditions.

Transparent-It is transparent to sunlight. 

Biological significance-This means aquatic plants can still photosynthesise beneath the water surface as light can penetrate below.

Metabolite- Metabolism = the sum of all the reactions that occur within a cell/organism.  (2 types of reaction: anabolic & catabolic) Important reactions depend on water.

Biological significance-Hydrolysis (addition of water to break covalent bonds) is important in digestion. Glucose could not be formed if photosynthesis stopped. Many reactions would not occur if water was not present. 

4 of 7


Osmosis- Diffusion of water from an area of high water potential to an area of low water potential through a partially permeable membrane.


5 of 7

Water potential

Measured in KPa- Unit of pressure


6 of 7


  • Hypertonic- lower/more negative water potential
  • Hypotonic- higher/less nagative water potential ( closer to 0KPa)
  • Isotonic- same water potential


7 of 7


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Biological molecules resources »