B2.1.2

• Hydrogen bonds form between water molecules. The shared negative electrons from hydrogen and oxygen are pulled towards the oxygen atom, leaving the opposite side of the hydrogen atoms slightly positively charged (δ+)
• In a hydrogen bond oxygen has a greater share of the electrons in the covalent bond as it is more electronegative than hydrogen. This gives it a slight negative charge (δ-)
• A polar molecule is a molecule with positive and negative regions. For example water.
• A hydrogen bond is formed when the slightly negatively charged oxygen atoms are attracted to the slightly positively charged hydrogen atoms of other water molecules, with a lone pair from the oxygen atom in between.
• Roles of water include…

  • solvent
  • transport medium
  • coolant
  • habitat
• Water‘s polarity allows it to act as a solvent...

  • water molecules are attracted to other polar substances
  • water molecules cluster around charged solute molecules or ions
  • The water attracts them, helping them to separate and stay apart, dissolving them and creating a solution
• Only polar molecules and ions will dissolve in water, non-polar substances will not
• Most biological reaction take place in solutions, for example the cytosol is mainly water.
• Due to the combined efforts of cohesion and adhesion water exhibits capillary action, making it ideal for transport in processes such as transpiration.
• Cohesion- the attraction between molecules of the same substance due to polarity. (Allows water to move as one mass)
• Adhesion- the attraction between two different polar surfaces.
• Capillary action - the process by which water can rise up a narrow tube against the force of gravity
• Capillary action allows water to be transported in the xylem
• Solvent properties also enable the transport of dissolved compounds and ions into and out of the cell and around the body via the water in the blood.
• What properties allow water to act as a solvent?

  polarity
• What properties of water allow it to act as a transport medium?

  • Cohesion
  • adhesion
  • capillary action
  • solvent (polarity)
• What properties of water enable it to act as a coolant?

  High specific heat capacity and high latent heat of vaporisation.
• Waters high specific heat capacity means water can absorb and release large amounts of energy with little temperature change. This is due to the large amount of energy needed to overcome hydrogen bonds. This makes water a good coolant.
• Water’s high latent heat of vaporisation means that a lot of energy is used up when water evaporates (to break the hydrogen bonds), removing a lot of heat with it. For example when sweat evaporates it cools down the skins surface.
• Water’s high specific heat capasity prevents temperature fluctuations that are outside the range suitable for life.
• As ice is less dense than water it forms on the surface creating an insulating layer for the water bellow stopping it from freezing. This also prevents water freezing from the bottom up.
• Ice is less dense than water as the frozen hydrogen bonds form a solid crystal-like lattice where molecules are set at fixed distances. This is why ice expands. Liquid hydrogen bonds are constantly being broken and reformed.
• The surface tension created by cohesion is also strong enough to support small insects such as pond skaters.
• What properties of water allow it to act as a habitat?

  • high specific heat capasity
  • density of water
  • high surface tension (cohesion)
• Monomers are the small units from which larger molecules are made
• Polymers are molecules made from a large number of monomers joined together in a chain
• Condensation is a reaction in which monomers combine covalently to form polymers (polymerisation) or macromolecules (eg lipids) and water is removed.
• Condensation equation…


  monomer + monomer + monomer → polymer + 2H₂O
• Condensation picture equation…


  HO-🟥-H + HO-🟥-H → HO-🟥-🟥-H + H₂O
• Hydrolysis is a reaction in which covalent bonds are broken when water is added
• Hydrolysis equation…


  polymer + 2H₂O → monomer + monomer + monomer
• Hydrolysis picture equation…


  HO-🟥-🟥-H + H₂O → HO-🟥-H + HO-🟥-H
• Macromolecules are large molecules. Polymers can be macromolecule, however not all macromolecules are polymers as the subunits of polymers have to be the same repeating units
• What elements are present in carbohydrates?


  Carbon , hydrogen , and oxygen
• what is the general formula of carbohydrates?


  Cₓ(H₂O)ₐ
• What elements are present in lipids?


  Carbon , hydrogen and oxygen
• what elements are present in proteins?


  carbon , hydrogen , oxygen , nitrogen and sulfur
• What elements are present in nucleic acids?


  Carbon , hydrogen , oxygen , nitrogen and phosphorus
• Monosaccharides are single sugar monomers , all of which are reducing sugars.
• Glucose is a hexose monosaccharide as it is composed of 6 carbon atoms