3.1.7 Water

• Water is vital to

  living organisims
• Water makes up about 80% of


  a cells contents
• Water has many important functions inside and outside a cell

  ● is a metabolite● is a solvent● helps with temperature control● its molecules are very cohesive
• Cohestion


  is the attraction between water molecules
• Water is a metabolite in many


  important metabolic reactions— e.g condensation / hydrolysis reactions
• Water is a solvent, which means


  some substances dissolve in it● Most metabolic reactions take place in solution —e.g. in the cytoplasm of eukaryotic and prokaryotic cells — so water’s pretty essential
• Water helps with temperature control because

  it has a high :● latent heat of vaporisation ● specific heat capacity
• Water molecules are very cohesive (they stick together), this helps


  ● water transport in plants● transport in other organisms
• A molecule of water (H2O) is


  one atom of oxygen (O) joined to two atoms of hydrogen (H2) by shared electrons

  
• Water is a polar molecule because

  ● the shared negative hydrogen electrons are pulled towards the oxygen atom— the other side of each hydrogen atom is left with a slight positive charge (d+). ● The unshared negative electrons on the oxygen atom give it a slight negative charge (d-)

  
• Water is a polar molecule as it has


  ● a slight (partial) negative charge on one side ● a slight (partial) positive charge on the other

  
• Hydrogen bonds


  are weak bonds between ● a slightly positively charged hydrogen atom in one molecule● a slightly negatively charged atom in another molecule
• Hydrogen bonds form between water molecules because


  the slightly negatively charged oxygen atoms of water attract the slightly positively charged hydrogen atoms of other water molecules

  
• Hydrogen bonding gives water some of its useful properties such as

  ● being an excellent solvent — many substances can dissolve in water● relatively high specific heat capacity● relatively high latent heat of vaporisation● is less dense when a solid● has high surface tension and cohesion● acts as a reagent​
• Partial Charges

  
• Metabolic reaction


  is a chemical reaction that happens in a living organism to keep the organism alive
• Metabolite


  a substance involved in a metabolic reaction
• Most biological reactions take place

  in solution
• A molecule is polar if it has

  ● a slightly negatively charged sideand ● a slightly positively charged side
• Water is an important metabolite

  ● Many metabolic reactions involve a condensation or hydrolysis reaction● A hydrolysis reaction requires a molecule of water to break a bond● A condensation reaction releases a molecule of water as a new bond is formed
• Some reactions where water is a metabolite are when

  ● amino acids are joined together to make polypeptides (proteins)— by condensation reactions● Energy from ATP is released — through a hydrolysis reaction
• A lot of important substances in biological reactions are


  ionic
• Ionic


  ● they’re made from one positively charged atom or molecule and one negatively charged atom or molecule— e.g a salt is made from a positive sodium ion and a negative chloride ion
• Because water is polar

  ● the slightly positively charged end of a water molecule will be attracted to the negative ion● the slightly negatively charged end of a water molecule will be attracted to the positive ion— means the ions will get totally surrounded by water molecules (they’ll dissolve)

  
• Water's polarity makes it useful as a solvent, this means


  ● living organisms can take up useful substances dissolved in water — (e.g. mineral ions) ● these dissolved substances can be transported around the organisims body
• Solvent


  a substance capable of dissolving another substance
• Polar molecules, such as glucose, dissolve in water because


  hydrogen bonds form between them and the water molecules
• Latent heat


  is the heat energy that's needed to change a substance from one state to another, — e.g. from a liquid to a gas
• Water has a high latent heat of vaporisation because

  ● water evaporates (vaporises) when the hydrogen bonds holding water molecules together are broken● allows the water molecules on the surface of the water to escape into the air as a gas— It takes a lot of energy (heat) to break the hydrogen bonds between water molecules● so a lot of energy is used up when water evaporates
• Water has a high latent heat of vaporisation as


  lots of heat is used to change it from a liquid to a gas
• High latent heat of vaporisation is useful for living organisms because

  ● means they can use water loss through evaporation to cool down without losing too much water— When water evaporates, it carries away heat energy from a surface, which cools the surface and helps to lower the temperature— e.g. when humans sweat to cool down
• Hydrogen bonds give water a high


  specific heat capacity
• Specific heat capacity


  this is the energy needed to raise the temperature of 1 gram of a substance by 1 °C
• Water has a high specific heat capacity because

  ● When water is heated, a lot of the heat energy is used to break the hydrogen bonds between the water molecules— This means there is less heat energy available to increase the temperature of the water
• Water has a high specific heat capacity as


  it takes a lot of energy to heat it up
• High specific heat capacity is useful for living organisms because


  ● it means that water doesn’t experience rapid temperature changes— makes water a good habitat because the temperature under water is likely to be more stable than on land● The water inside organisms also remains at a fairly stable temperature— helping them to maintain a constant internal body temperature
• Enzyme activity is affected by temperature


  ● Some important biological processes need enzymes to work— e.g. digestion and respiration● these may not work properly if the organism’s temperature is not kept fairly stable
• Cohesion


  attraction between molecules of the same type — e.g. two water molecules
• Cohesive


  they tend to stick together
• Water molecules are very cohesive because

  they're polar