Water and Inorganic Ions

• Water is a dipolar molecule:

  • a water molecule consists of 2 hydrogen atoms and an oxygen atom, however the electrons in the covalent bonding are not shared equally. The oxygen has a greater electronegativity (pull on electrons)
  • therefore, water molecules have a slightly positive and slightly negative regions; the oxygen has a partial negative charge, the hydrogen has a partial positive charge
• Water and hydrogen bonding:

  • the positive pole of one molecule is attracted the the negative pole of another
  • the attractive force between opposite charges is a hydrogen bond. A hydrogen bond is relatively weak, however in water there are huge numbers, so they're a significant force
• Ice is less dense than water:

  • water is most dense at 4°C. When water freezes (ice) the hydrogen bonds form a rigid lattice, that holds the molecules further apart than in liquid water (when water freezes it expands making ice less dense - therefore ice floats)
  • it forms an insulating layer and acts as a barrier to the cold. This ensures aquatic organisms can still survive
• Water is a metabolite:

  • water is a metabolite in many metabolic reactions - it is the medium in which enzyme-catalysed reactions take place (it's essential to maintain metabolism). Metabolic reactions occur faster in solution
  • water is used in hydrolysis reactions to break down complex molecules (proteins) and is formed in condensation reactions such as starch and protein synthesis
  • water is one of the raw materials required in photosynthesis and is a product of respiration
• Water is a universal solvent:

  • water dissolves more substances than any other liquid - this is due to it's polar nature. This is beneficial for organisms as metabolic reactions occur faster in solution
  • ionic compounds are made from a positive ion and a negative ion. When dissolved in water the positively charged atom bonds to oxygen and the negatively charged atom bonds to hydrogen. The ions seperate and end up being surrounded by water
  • mineral ions dissolved in water can be taken up by organisms (nitrate ions can be taken up by plants)
• Cohesion and adhesion:

  • cohesion - attraction between molecules of the same type. Water molecules are cohesive as they are polar and have hydrogen bonds
  • adhesion - the attraction between different types of molecules
• Surface tension:

  • surface tension - the measure of how hard it is to break the surface of a liquid. At an interface between air and water, water molecules on the surface form hydrogen bonds with other water molecules but not air (cohesion). The unequal distribution of bonds produces surface tension. This causes the water surface to contract and form a tough film
  • cohesion, adhesion and surface tension are responsible of water transportation through the xylem, the drainage of tears from tear ducts and some insects rely on surface tension to stay afloat on the surface of water
• Water has a high specific heat capacity:

  • water has a high thermal stability due to it's high specific heat capacity. Hydrogen bonds require a lot of energy to break. Therefore, water can gain or lose a lot of heat energy without changing temperature
  • this means there is little variation in temperature in organism's bodies or the water surrounding them. The high specific heat capacity allows water to act as a buffer against sudden temperature changes
• Water has a high latent heat of vaporisation:

  • water's high number of hydrogen bonds means a lot of energy is required to evaporate water. The high specific latent heat explains why sweating is an effective method of cooling
  • when body temperature rises, sweat is secreted. The water molecules in sweat gain energy. When they gain enough, bonds break and water transforms into water vapour. This allows heat to be removed, which cools the body
• Inorganic ions:

  • these occur in the cytoplasm and bodily fluids (some in high and some in low concentrations)
  • hydrogen ions and their role in pH - more H+ ions means it's more acidic
  • iron ions as a component of haemoglobin
  • sodium ions in the co-transport of glucose and amino acids
  • phosphate ions as components of DNA and ATP
• Water is transparent:

  • water allows light to shine through, meaning that aquatic plants underneath the water are able to photosynthesise