2.1.2 (a)how hydrogen bonding occurs between water molecules, and relate this, and other properties of water, to the roles of water for living organism

To include roles that relate to the properties of water; solvent, transport medium, coolant and as a habitat AND roles illustrated using examples of prokaryotes and eukaryotes. Learners should be able to apply their knowledge and understanding in the context of prokaryotes and eukaryotes.

Importance of water:

• A universal solvent – many substances can dissolve in water

• It is the medium in which all metabolic reactions take place in cells

• It acts as a transport medium

• A relatively high specific heat capacity

• A relatively high latent heat of vaporisation

• Water is less dense when a solid (ice floats on water and can act as a habitat for animals such as polar bears)

• Water has high surface tension and cohesion

• It acts as a reagent

water molecular properties

• 2 hydrogen combine with one oxygen molecule by sharing electrons (covalent bonding)

• dipole moment: due to oxygen having a higher affinity for electrons,

• The oxygen atom attracts the electrons more strongly than the hydrogen atoms resulting in:

  • a weak negatively charged region on the oxygen atom (δ^-)

  • and a weak positively charged region on the hydrogen atoms (δ⁺),

  • this also results in the asymmetrical shape

• is a polar molecule

polarity of water:

• When a molecule has one end that is negatively charged and one end that is positively charged

Hydrogen bonding

• as a result of the polarity of water:

  • hydrogen bonds form between the positive and negatively charged regions of adjacent water molecules

• Hydrogen bonds are weak, when there are few, so they are constantly breaking and reforming. However when there are large numbers present they form a strong structure

dashed lines between o and h

water as a solvent

• As water is a polar molecule many ions (e.g. sodium chloride) and covalently bonded polar substances (e.g. glucose) will dissolve in it

  • because the slightly positive end of water will be attracted to the negative ion (chloride) and the negative end to the positive sodium ion - getting totally surrounded by water molecules

  • This allows chemical reactions to occur within cells

    • (as the dissolved solutes are more chemically reactive when they are free to move about)

  • Metabolites can be transported efficiently (except non-polar molecules which are hydrophobic)

• This process allows prokaryotic cells to exchange substances,

  • such as nutrients and waste products, with their surroundings

  • via the process of diffusion

specific heat capacity

• The specific heat capacity of a substance is the amount of thermal energy required to raise the temperature of 1kg of that substance by 1°C

• Water’s specific heat capacity is 4200 J/kg°C meaning a relatively large amount of energy is required to raise its temperature

  • this is due to the many hydrogen bonds present in water. It takes a lot of thermal energy to break these bonds and a lot of energy to build them, thus the temperature of water does not fluctuate greatly

advantages of specific heat capacity

• Provides suitable stable habitats in aquatic environments such as in lakes and the ocean

• Is able to maintain a constant temperature

  • which is vital in maintaining temperatures that are optimal for enzyme activity in both prokaryotes and eukaryotes

• Water in blood plasma is also vital in transferring heat around the body, helping to maintain a fairly constant temperature

  • As blood passes through more active (‘warmer’) regions of the body, heat energy is absorbed but the temperature remains fairly constant

  • Water in tissue fluid also plays an important regulatory role in maintaining a constant body temperature

high latent heat of vaporisation

• In order to change state (from liquid to gas) a large amount of thermal energy must be absorbed by water to break the hydrogen bonds and evaporate

• This is an advantage for living organisms as only a little water is required to evaporate for the organism to lose a great amount of heat

• This provides a cooling effect for living organisms

  • for example the transpiration from leaves or evaporation of water in sweat on the skin

cohesion and adhesion

• Hydrogen bonds between water molecules allows for strong cohesion between water molecules

  • This allows columns of water to move through the xylem of plants and through blood vessels in animals

  • This also enables surface tension where a body of water meets the air, these hydrogen bonds occur between the top layer of water molecules to create a sort of film on the body of water (this is what allows insects such as pond skaters to float)

• Water is also able to hydrogen bond to other molecules, such as cellulose, which is known as adhesion

  • This also enables water to move up the xylem due to transpiration

Lower density when solid

• when water becomes ice,

  • water molecules are held further apart because each water molecule forms 4 fixed hydrogen bonds to other water molecules making a lattice shape.

• this makes ice less dense then liquid water, which is why ice floats

  • this is useful for living organisms because in cold temperatures ice forms an insulating layer on top of water

  • allowing the organsism below to move around and not freeze