Water

Water is vital to life in animals and plants:

• Good solvent - allowing transport and cell reactions to occur
  • Transports dissolved substances through the body of animals (blood, lymph, tissue fluid) and plants (xylem/phloem)
  • Transports substances into and out of cells through osmosis process e.g. in lungs and digestive system
  • Cell reactions, acting as a medium to bring chemicals into contact and it is involved in some reactions e.g.:
    • hydrolysis: water is used to hydrolyse (split) many substances e.g. proteins into amino acids; fats to fatty acids and glycerol; and polysaccharides to monosaccharides
    • condensation reactions through which smaller molecules combine to form one larger molecule, with the elimination of water (e.g. glu + glu - water -> maltose)
    • photosynthesis (water is broken down to supply hydrogens) and respiration, where water is a necessary product
  • Medium for dispersal e.g. gametes, pollen, spores, fruits and seeds
     
• Good energy absorber - helping to stabilise temperatures
  • A large increase in heat results in a small rise in temperature. Water usually maintains its temperature irrespective of fluctuations to the surrounding environmental temperature. This facilitates enzymes because they have a limited temperature range for activity. Also, aquatic environments show less seasonal and diurnal temperature changes than on land, so growth of aquatic plants is more constant
     

• Lubricant

  • Mucus – used in intestines
  • Synovial fluid – lubricates joints
  • Pleural fluid – friction-free movement of lungs against ribs
  • Pericardial fluid - friction-free movement of heart against ribs
     
• Support
  • Water is not easily compressed, making it useful in supporting organisms
  • Skeleton e.g. earthworms supported by the pressure of water within
  • Water has a high density therefore supporting the weight of aquatic plants and animals e.g. whales
  • Maintains turgidity in plant cells - water is present in vacuoles.
  • Eye - eye shape is due to watery humour in it
  • Foetus – Amniotic fluid supports and protects foetus
     
• Large latent heat of vaporisation - Regulates temperature
  • It absorbs heat to evaporate and so sweating cools animals and transpiration cools plants
     
• High surface tension
  • Resulting from the inward cohesive forces of attraction between molecules causing the surface to have the smallest possible area. Organisms can therefore walk or float on “skin”
     
• Ice floats on freezing water
  • Because water volume increases and therefore density decreases when it freezes it insulates aquatic organisms below the ice from extreme cold
     
• Neutral pH (7)
  • Suitable for most enzymes
     
• High cohesion
  • Water molecules attach to each other and lower adhesion. Water molecules attach to other surfaces, contributing to capillarity in plants (water rising up the stems - against gravity) and soil (causes water to disperse in soil)
     
• Colourless
  • Allows submerged plants to photosynthesise
     
• Hearing and balance in ear
  • Watery liquid in cochlea and semi-circular canals