Water is life

ZIMSEC O Level Biology Notes:Chemical building blocks of Life: Bio-molecules


Structure of a water molecule

  • Water is essential for life.
  • Approximately 75% earth’s surface is water.
  • Life evolved in water.
  • An average adult body is 50 to 65 percent water
  • This rises to about 80% in a child
  • A water molecule is made up of 2 hydrogen atoms and one oxygen atom.
  • It has a bent shape and an angle of 104.5°.
  • Creates a slightly negative oxygen and a slightly positive hydrogen allowing formation of hydrogen bonds.

Hydrogen bonding

  • A type of polar/intermolecular interaction.
  • Critical for:
  • Maintaining protein structure, that is primary, secondary and tertiary.
  • Takes part in enzymatic reactions.
  • Movement of water in plant stems.
  • Ionization of water: H2O -> H+ + OH
  • Generally it breaks down to form a hydrogen ion (H+), and hydroxide ion (OH-).
  • At 25°C, 1 liter of water contains 10-7 moles of H+ ions: 10-7 moles/liter.


  • Is a convenient way of indicating H (Hydrogen ion) concentration
  • It is used to measure alkalinity or acidity of a solution
  • pH = – log [H+]
  • For water, pH = – log[10-7] = 7
  • Since for each H+ in pure water, there is one OH-, pH of 7 indicates that it is neutral.
  • Water also acts as a buffer due to a reservoir for H+ .
  • This means it maintains relatively constant pH over buffering range.

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