Structure of water
- dipolar molecule (“di” = 2 & “polar” = charges)
- uneven distribution of charge - O2 slightly negative, H2 slightly positive
- delta symbol (+-): slight positive/negative
- due to charges: hydrogen bonds form between an O2 & H2 of (different) neighbouring molecules
Key properties of water and why?
Why? formation of H bonds & dipolar molecule
1. Metabolite (e.g. condensation & hydrolysis reactions)
2. Important solvent in reactions
3. High specific heat capacity - it buffers temperature
4. Large latent heat of vaporisation - providing a cooling effect - w loss of water through evaporation
5. Strong cohesion between water molecules - supports water columns & provides surface tension
- transport medium due to solvent & metabolite
Metabolite?
- water involved in many metabolic reactions (metabolism) e.g. photosynthesis, hydrolysis & condensation
- reason why essential that approx 90% of plasma is water & cytoplasm in cells largely composed of water
Solvent?
- good/universal solvent so - many substances dissolve in it
- so many essential polar substances dissolve in water - enables them to be transported easily around animals & plants - blood/xylem to cells
Why?
- due to dipolar: polar/charged molecules dissolve readily in it
- slight + charge on H2 atoms attract any - ions in solutes & slight - on O2 atoms attract any + ions in solutes
- polar molecules = hydrophilic (aka attracted to water)
BUT:
- non-polar molecules e.g. lipid can’t dissolve in water = hydrophobic (aka repelled by water)
Why high specific heat capacity?
- H bonds between molecules need to be broken to raise temp of water - requires a lot of heat energy…
- temp of water remains relatively stable - even if surround temp fluctuates significantly
- so internal temp of plants & animals remain relatively constant despite outside temps (because large portion of organism is water)
- important for enzyme controlled reactions so: don’t denature/reduce in activity w temp fluctuations
- buffers changes in temp so - thermostable - stable environment for aquatic organisms
Large latent heat of vaporisation?
- a lot of energy required to evaporate water (convert liquid to gas)
- due to H bonds - heat energy needed to break between the molecules to turn into a gas
- so: provides cooling effect e.g. sweat releases water onto skin -> large amounts of heat energy from skin transferred to water -> evaporate - removing a lot of heat & cooling organism w little loss of H2O
- e.g. transpiration in plants
Strong cohesion?
- H2O molecules sticky tgt by H bonds
- due to this - water moves up xylem in plants (due to transpiration) as continuous column (rather than individual molecules) & due to adhesion (the ability of a substance to stick to a different substance/ the attraction between unlike molecules)
- also: provides surface tension - enables small invertebrates to move & live on surface - habitat away from predators within
Electronegativity?
the measure of an atom’s pull on a bonded pair of electrons
Condensation reaction?
a chemical reaction which joins 2 molecules tgt w the formation of a chemical bond and involves the elimination of a molecule of water
Hydrolysis reaction?
a chemical reaction which breaks a chemical bond between two molecules and involves the use of a water molecule
Importance of density & freezing
- as water freezes it expands (water molecules are now held further apart by hydrogen bonds) so - frozen water is less dense than liquid water so - floats
-frozen ice layer insulates the water below it so - the water below doesn’t freeze (allowing sustained life below)
Transparent?
light travels through water, allowing aquatic plants/algae to photosynthesise
