Water

• Key water information

  • Most common molecule
  • Photosynthesis reactant
  • Important habitat for many (covers most of Earth’s surface)
  • ~70% of body mass for most organisms (mammals ~65%)
  • Aqueous media: blood plasma, tears, urine, and cell sap
  • Small
  • 2 covalent bonds
  • Formula is H_2O
  • Oxygen is electronegative, slight negative charge (δ-) and hydrogen is slightly positively charged (δ+)
  • Uneven charge distribution makes water molecule polar (dipolar; slight positive, other slight negative)
  • Unequal electron sharing: Oxygen has more protons, so a greater attraction for electrons than hydrogen
• Hydrogen Bonds


  • Water is polar and attracts itself (positive end attracted to negative end of other)
  • Form a hydrogen bond ( H-bond) with weak intermolecular forces
  • Gives water many special properties
• Water as Universal Solvent

  • Many substances dissolve in water due to their polarity or dissociation to give polar ions
  • Essential transport medium for polar molecules and ions; main part of blood, tissue fluid, urine, and phloem sap
  • Facilitates metabolic reactions; soluble reactants move freely in aqueous cell cytoplasm
  • Aquatic organisms and plants absorb soluble O_2 for aerobic respiration and CO_2 for photosynthesis
  • Also absorb soluble mineral ions (and in non-aquatic plants through soil water via roots)
• High Latent Heat of Vaporisation

  • “Energy required to change a given quantity of liquid into a gas”
  • Water has a high latent heat of vaporisation since extra energy is required to break hydrogen bonds
  • Efficient cooling technique in mammals (temperature regulation), sweat absorbs skin heat and turns it to water vapor (evaporation)
  • Cool leaves in transpiration, water in mesophyll cell walls evaporates into leaf air spaces; state change needs thermal energy, taken from leaf
• High Specific Heat Capacity

  • “Amount of substance required to increase temperature of 1g of substance by 1°C”
  • Substance’s temperature is related to kinetic energy; faster molecules = higher temperature
  • Extra energy needed to break hydrogen bonds for free movement
  • Use to aquatic organisms; thermostable, temperature changes little compared to surrounding air, environment (less energy on temperature control)
  • Gas solubility doesn’t change very much either
  • Internal temperature in organisms cell changes slowly because cytoplasm is thermostable medium for enzymes and metabolic reactions
• Density and Freezing Properties

  • High density due to H-bonds; most organisms have high water %; so similar density and float
  • Density change cause currents; circulate nutrients
  • Density lowers as temperatures fall below 4°C (max density); pond bottom is safe non-frozen habitat, ice formed at 0°C has lower density; floats
  • Water expands when freezing; slow molecules allow max H-bonds to form, pushing molecules apart; makes a crystal lattice
  • Floating ice is good as it thermally insulates water (also a habitat itself)
  • Large energy loss required to change liquid water to ice; less likely
• Cohesion and Surface Tension

  • Water attraction to itself is cohesion; strong cohesive forces allow water to move up xylem vessels as a continuous column in transpiration stream
  • Cohesion of water molecules on pond surface makes it behave as a type of skin; surface tension
• Other Properties

  Transparency:

  • Allows light to penetrate (key for photosynthesis)
  • Not all light wavelengths of light can penetrate
  • Blue light penetrates further
  • Deeper water has a lower light intensity

  Reactivity:

  • Important reactant in many chemical reactions
  • Condensation (two molecules are joined by removing water)
  • Hydrolysis (two molecules are split by adding water)

  High Boiling Point (100°C):

  • Lots of energy is required to break hydrogen bonds
  • It remains liquid at room temperatures
  • Providing an aquatic habitat for many organisms and liquid cytoplasm
• Other Roles

  • Lubricant e.g. synovial fluid in joints, saliva in mouth and mucus in gut
  • Hydrostatic Skeleton for plant turgidity, stomatal opening, penis erection and fluid in body cavities
  • Protective Cushion e.g. amniotic fluid around foetus and cerebrospinal fluid
  • Reproduction, aquatic organisms release gametes into water for external fertilisation