Inorganic Molecules

Inorganic molecules do not contain carbon and were not synthesised from a biological origin (oxides of carbon and carbonates are exceptions) 

Certain inorganic molecules play important roles in maintaing living organisms

Oxygen and Carbon Dioxide

  • Oxygen is needed in most organisms to release energy from organic molecules (via aerobic respiration)
  • Organic molecules are synthesised by plants from an inorganic stock of atmospheric carbon dioxide (via photosynthesis)
  • These two processes are in many ways inter-related, with O2 being released by photosynthesis and CO2 being released by respiration

The Role of Oxygen and Carbon Dioxide in Living Organisms


  • Mineral nutrients are the chemical elements required by living things other than the four main elements of organic molecules (C, H, O, N)
    • Phosphorus is a part of nucleic acids, cell membranes and ATP
    • Sulphur is found in certain amino acids and can play an important role in the structure and function of proteins
    • Calcium is important in the growth and development of bones and teeth
    • Sodium and potassium are involved in neuronal signalling (nervous communication)
    • Iron is found in red blood cells and is necessary for the transport of oxygen in blood
    • Magnesium is found in chlorophyll, which is required for photosynthesis


  • Living organisms usually consist of approximately 70 - 90% water as it functions as the fluid medium that bathes the cells and tissues
  • Water has a specific structure with associated properties that are critical for the maintenance of living organisms

Structure of a Water Molecule:

  • Water (H2O) is made up of two hydrogen atoms covalently bound to an oxygen atom
  • While this bonding involves the sharing of electrons, they are not shared equally 
  • The oxygen atom, having more protons (+ve), attract the electrons (-ve) more strongly (i.e. the oxygen has a higher electronegativity)
  • Thus the oxygen atom becomes slightly negative and the hydrogen atoms become slightly positive

Hydrogen Bonding between Water Molecules:

  • Covalently bonded molecules that have a slight potential charge are said to be polar
  • The slightly charged regions of the water molecule can attract other polar or charged compounds
  • Water molecules can associate via weak hydrogen bonds (F/O/N bonding with H)

Structure and Bonding of Water Molecules

Properties of Water:

  • Water has a high specific heat capacity
    • It can absorb a lot of energy with little change in form due to the extensive hydrogen bonding, making water a good medium for metabolic reactions
    • It also has a high heat of vaporisation - this allows sweat to be an efficient form of evaporative cooling
  • Water is very cohesive
    • Because water molecules are polar, they will form intermolecular associations with each other (cohesion) and other polar molecules (adhesion)
    • This allows water to travel up the stems of plants (against gravity) without requiring high levels of energy for transport
    • It also means water has a high surface tension, allowing small insects to walk along the surface of water without disturbing its integrity
  • Water is the 'universal' solvent
    • Water, due to its polarity, can dissolve other polar substances as well as ionic compounds (but not non-polar substances)
    • While individual water molecules cannot sufficiently weaken and break the intramolecular attraction between ions, large enough quantities can 
    • This makes water a very efficient transport medium for hydrophilic ('water-loving') substances, but not hydrophobic substances
  • Water is less dense as a solid
    • Unlike most substances, water expands when frozen to ice (the arrangement of water molecules in an ice crystal creates empty spaces)
    • This is important as it means ice will float on water - this prevents the oceans from freezing over when surface temperatures are sub-zero