2.2 - Water

• Covalent bonding in water:

  • While this bonding involves the sharing of electrons, they are not shared equally
  • The number of protons in each atom  is different; oxygen atoms have 8 whilst hydrogen atoms have just 1
  • Having more protons the oxygen atoms attract the electrons more strongly•
  • Thus the oxygen atom becomes slightly negative and the hydrogen atoms become slightly positive• (i.e. the oxygen has a higher electronegativity)
• Polarity:

  • Covalently bonded molecules that have a slight potential charge are said to be polar
  • The slightly charged regions of the water molecule can attract other polar or charged compounds
  • Water molecules can associate via weak hydrogen bonds
  • Hydrogen bonds are transitory in nature – they constantly form, break and re-form
• Cohesion:

  • This property occurs as a result of the polarity of a water molecule and its ability to form hydrogen bonds
  • Although hydrogen bonds are weak the large number of bonds present (each water molecule bonds to four others in a tetrahedral arrangement) gives cohesive forces great strength
  • Water molecules are strongly cohesive (they tend to stick to one another)
• Water droplets form because the cohesive forces are trying to pull the water into the smallest possible volume, a sphere.
• Surface tension is caused by the cohesive hydrogen bonding resisting an object trying to penetrate the surface.
• Adhesion:

  • This property occurs as a result of the polarity of a water molecule and its ability to form hydrogen bonds
  • Water molecules tend to stick to other molecules that are charged or polar for similar reasons that they stick to each other
  • Again similarly individual hydrogen bonds are weak, but large number of bonds gives adhesive forces great strength
• Water droplets stick to surface and seem to defy gravity because of form because the adhesive forces that bond them to the surface of the grass blade.
• • Capillary action is caused by the combination of adhesive forces causing water to bond to a surface, e.g. the sides of a xylem vessel and the cohesive forces bonding water molecules together.
  • Capillary action is helpful in the movement of water during transpiration and also when you drink using a straw.
• Solvent:

  • Water can dissolve many organic and inorganic substances that have charged or polar regions.
  • The polar attraction of large quantities of water molecules can interrupt intramolecular forces (such as ionic bonds) and resulting in the dissociation of the atoms
  • Positive atoms, e.g. Na+ end up being surrounded by the negative oxygen regions of water molecules and the Cl- being surrounded by the positive hydrogen region of water molecules
  • Because of this water is often (wrongly) referred to as being the ‘universal solvent’, it is however a very good solvent for many substances.
• Metabolic reactions happen most readily in solutions of water – water in cells dissolves the reactants /substrates
• Cells are mostly water therefore diffusion into and out of them happens most easily if the substance concerned is in solution, e.g. before oxygen diffuses from the alveoli to the blood it  dissolves into the moist layer lining the alveoli.
• • Soluble substances such as sucrose can be easily transported around the plant in the phloem.
  • Once dissolved in the water of the phloem the sucrose can be moved to where it is needed by mass flow.
• Hydrophilic:

  • substances that are chemically attracted to water.
• • All substances that dissolve in water are hydrophilic, including polar molecules such as glucose, and particles with positive or negative charges such as sodium and chloride ions.
  • Substances that water adheres to, cellulose for example, are also hydrophilic.
• Hydrophobic:

  • substances that are insoluble in water
• • Molecules are hydrophobic if they do not have negative or positive charges and are nonpolar
  • All lipids are hydrophobic, including fats and oils
  • Hydrophobic molecules dissolve in other solvents such as propanone (acetone)
• Sodium chloride:

  • An ionic compound that is freely soluble in water, dissolving to form sodium ions (Na+) and chloride ions (Cl ), which are carried in blood plasma.
• Amino acids:

  • Have both negative and positive charges.
  • Thus, they are soluble in water but their solubility varies depending on the R group, some of which are hydrophilic while others are hydrophobic.
  • All amino acids are soluble enough to be carried dissolved in blood plasma.
• Glucose:

  • A polar molecule.
  • It is freely soluble in water and is carried dissolved in blood plasma.
• Oxygen:

  • A nonpolar molecule.
  • Because of the small size of the molecule it dissolves in water but only sparingly and water becomes saturated with oxygen at relatively low concentrations.
  • The amount of oxygen that blood plasma can transport around the body is far too little to provide for aerobic cell respiration.
  • This problem is overcome by the use of haemoglobin in red blood cells.
• Fats molecules:

  • Entirely nonpolar, are larger than oxygen and are insoluble in water.
  • They are carried in blood inside lipoprotein complexes.
  • These are groups of molecules with a single layer of phospholipid on the outside and fats inside.
• Cholesterol molecules:

  • Are hydrophobic, apart from a small hydrophilic region at one end.
  • This is not enough to make cholesterol dissolve in water and instead it is transported with fats in lipoprotein complexes.
• Water thermal properties:

  • Water has a high specific heat capacity (4.2 Joules is required to raise the temperature of 1 g of water by 1°C)
  • Water has a high heat of vaporisation (amount of energy needed to change from a liquid to a gas or vapour)
  • Water has a high heat of fusion (amount of energy needed to be lost to change liquid water to ice)
  • These properties are due to many hydrogen bonds that need to be formed or broken to change the temperature or state of water
  • Therefore the temperature of water remains relatively stable
• • Water is used by Leaves as a coolant.
  • The heat lost from leaves for evaporation  prevents them over-heating.
  • If the leaves get too hot enzymes in their cells will start to denature.
• Water as a coolant:

  • High temperatures damage tissues and denature proteins
  • It takes a lot of energy for water to change temperature
  • This means that it will cool and heat more slowly than air or land
  • Thus it's useful to animals in hot climates - who use water or mud to cool off in the hot day
• • When water evaporates it removes a lot of energy from the system
  • This is felt as a cooling sensation - excess heat energy is removed from the body (latent heat of evaporation)
  • The skin and their blood vessels are cooled
  • This also helps aquatic habitats remain at fairly constant temperatures in hot summers
• • Water makes up 70% of the body, including the blood
  • Because it is resistant to temperature change, cooler blood from some parts of the body can be circulated to other parts, cooling them down
• Methane

  • Waste product of anaerobic respiration in certain prokaryotes living in anaerobic conditions
  • Methane can be used as a fuel
  • If present in the atmosphere it contributes to the greenhouse effect.
• Polarity is the key chemical property that causes the major differences seen in the physical properties of water and methane.
• Methanogenic prokaryotes:

  • can be found in swamps, wetlands, the guts of animals (including cattle and sheep)
  • can also be found in waste dumps