3.1 Chemical Elements and Water


3.1.1  State that the most frequently occurring chemical elements in living things are carbon, hydrogen, oxygen and nitrogen

The approximate proportions of the four main elements in living things are:

• Carbon (19%)                        • Hydrogen (10%)                        • Oxygen (65%)                        • Nitrogen (3%)

3.1.2  State that a variety of other elements are needed by living organisms, including sulphur, calcium, phosphorus, iron and sodium

Outside of the four main elements, living things may contain trace amounts of 20 or so other elements, including: 

• Sulphur (0.25%)                        • Calcium (1.5%)                        • Phosphorus (1%)                        • Iron (0.7%)                        • Sodium (0.15%)

3.1.3  State one role for each of the elements mentioned in 3.1.2

Sulphur (S):  Found in certain amino acids (cysteine and methionine), allowing proteins to form disulphide bonds

Calcium (Ca):  Found in bones and teeth, also involved in neurotransmitter release in synapses

Phosphorus (P):  Component of nucleic acids and cell membranes

Iron (Fe):  Found in haemoglobin (animals), allowing for oxygen transport

Sodium (Na):  Involved in the generation of nerve impulses in neurons

3.1.4  Draw and label a diagram showing the structure of water molecules to show their polarity and hydrogen bond formation

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

3.1.5  Outline the thermal, cohesive and solvent properties of water

Thermal Properties

  • Water has a high specific heat capacity (the measure of energy required to raise the temperature of 1 g of substance by 1°C)
  • Water has a high heat of vaporisation (amount of energy absorbed per gram as it changes from a liquid to a gas / vapour)
  • Water has a high heat of fusion (amount of energy required to be lost to change 1 g of liquid to 1 g of solid at 0°C)
  • These properties occur as a result of the extensive hydrogen bonding between water molecules - this allows water to absorb considerable amounts of energy with little change in form (H-bonds need to be broken first)

Cohesive Properties

  • Water molecules are strongly cohesive (they tend to stick to one another)
  • Water molecules will also tend to stick to other molecules that are charged or polar (adhesion) 
  • These properties occur as a result of the polarity of a water molecule and its ability to form hydrogen bonds with appropriate molecules

Solvent Properties

  • Water can dissolve many organic and inorganic substances that contain electronegative atoms  (such as fluorine, oxygen and nitrogen)
  • This occurs because the polar attraction of large quantities of water molecules can sufficiently weaken intramolecular forces (such as ionic bonds) and result in the dissociation of the atoms

Other Properties

  • Water is transparent, allowing light to pass through it (important for photosynthesis)
  • Water expands when frozen, becoming less dense / lighter (important for life on earth - oceans don't freeze)

3.1.6  Explain the relationship between the properties of water and its use in living organisms as a coolant, medium for metabolic reactions and transport medium


  • Both plants and animals use the evaporation of water from the surfaces of their bodies to facilitate cooling (sweating and panting in animals, transpiration from leaves in plants)
  • Water can be used to carry heat to cooler places in our bodies (countercurrent exchange of thermal energy)

Medium for Metabolic Reactions

  • Water can dissolve many organic and inorganic substances to facilitate chemical reactions
  • Water can also absorb thermal energy released as a by-product of many chemical reactions

Transport Medium

  • The forces of attraction between water molecules help facilitate the transport of water up the xylem of plants
  • Water is an effective transport medium for dissolved substances (in plants, minerals from the soil and sugars from the leaves can be transported in water through the xylem and phloem respectively; while in animals, water in the blood is used to transport oxygen, glucose and urea)

Surface Tension

  • The force of attraction between water molecules makes water sufficiently dense for some smaller organisms to move along its surface