Water

• what is water?

  its a medium for metabolic reactions and an important constituent of cells, being 65-95% of the mass of many plants and animals
• how much of the human body is made of water?

  about 70%
• what is a water molecule?

  its a dipole, which means it has a positively charged end (hydrogen) and a negatively charged end (oxygen) but no overall charge
• what does dipole mean?

  a polar molecule, with a positive and a negative charge, separated by a very small distance
• what do you call a molecule with separated charges?

  polar
• what can form between the positive charge on a hydrogen atom of one molecule and a negative charge on an oxygen atom of another molecule?

  hydrogen bonds
• what is the strength of a hydrogen bond?

  very weak but when in abundance they can become stronger
• what does the very large number of hydrogen bonds in water do?

  they make the molecules difficult to separate and gives water a wide range of physical properties vital to life
• what is a hydrogen bond?

  the weak attractive force between the partial positive charge of a hydrogen atom of one molecule and the partial negative charge on another atom - usually oxygen or nitrogen
• what is this an image of?

  water molecules showing hydrogen bonding

  
• how is water as a solvent important?

  • living organisms obtain their key elements from aqueous solutions
  • water is such a good solvent that its been called the ‘universal solvent’
  • because water molecules are dipoles, they attract charged particles (e.g. ions and other charged molecules like glucose) - these then dissolve in water so chemical reactions take place in solution
  • it acts as a transport medium
  • non-polar molecules, such as lipids, don’t dissolve in water
• what are examples of water acting as a transport medium?

  • in animals, plasma transports dissolved substances
  • in plants, water transport minerals in the xylem, and sucrose and amino acids in the phloem
• how is water being a metabolite important?

  • water is used in many biochemical reactions as a reactant (e.g. with carbon dioxide to produce glucose in photosynthesis)
  • many reactions in the body involve hydrolysis, where water splits a molecule
  • in condensation reactions, water is a product
• what is hydrolysis?


  the breaking down of large molecules into smaller molecules, by the addition of a molecule of water
• what is a condensation reaction?

  chemical process in which 2 molecules combine to form a more complex molecule, with the elimination of a molecule of water
• why is water having a high specific heat capacity important?

  • this means a large amount of heat energy is needed to raise its temperature
  • this is because the hydrogen bonds between water molecules restrict their movement, resisting an increase in kinetic energy and therefore resisting an increase in temperature
  • this prevents large fluctuations in water temperature, which is important in keeping aquatic habitats stable, so that organisms don’t have to adapt to the extremes of temperature
  • it also allows enzymes within cells to work efficiently
• why is water having a high latent heat of vaporisation important?

  • this means a lot of energy is needed to change it from a liquid to vapour
  • this is important, for example, in temperature control, where heat is used to vaporise water from sweat on the skin or from a leaf’ surface - as the water evaporates, the body cools
• what is specific heat capacity?

  the energy required to raise the temperature of 1g of a substance through 1$^.$c
• what is the latent heat of vaporisation?

  the energy required to convert 1g of a liquid into vapour at the same temperature
• how is water having cohesion important?

  • water molecules attract each other forming hydrogen bonds
  • individually these are weak but, because there are so many of them, the molecules stick together in a lattice
  • this sticking together is called cohesion
  • it allows columns of water to be drawn up xylem vessels in plants
• how is water having high surface tension important?

  • cohesion between water molecules at the surface produces surface tension
  • at ordinary temperatures water has the highest surface tension of any liquid except mercury
  • in a pond, cohesion between water molecules at the surface produces surface tension so that the body of an insect, such as the pond skater, is supported
• how is water having high density important?

  • water is denser than air and, as a habitat for aquatic organisms, provides support and buoyancy
  • water has a maximum density at 4$^.$c
  • ice is less dense than liquid water as the hydrogen molecules hold the molecules further apart than they are in the liquid - so ice floats on water
  • ice is a good insulator and prevents large bodies of water losing heat and freezing completely, so organisms beneath it survive
• why is water being transparent important?

  • it allows light to pass through
  • this lets aquatic plants photosynthesise effectively
• what does cohesion mean?

  the attraction of water molecules for each other because of the dipole structure of water, producing hydrogen bonds between them