A1.1

• Water


  The medium of life
• Water


  • Liquid water is thought to have existed on earth for more than 4 billion years
  • Water makes up more than 70% of the earth's surface
• Hydrogen bonds


  Consequence of the polar covalent bonds within water molecules
• Formation of hydrogen bonds in water

  1. Water molecules can associate via weak hydrogen bonds

  2. Hydrogen bonds are transitory in nature – they constantly form, break and re-form
• Cohesion of water molecules

  • Due to hydrogen bonding
  • Gives cohesive forces great strength due to the large number of bonds present
• Adhesion of water


  • Water molecules tend to stick to other molecules that are charged or polar
  • Adhesive forces are strong due to the large number of hydrogen bonds
• Capillary action


  Caused by the combination of adhesive forces causing water to bond to a surface and the cohesive forces bonding water molecules together
• Solvent properties of water

  • Water can dissolve many organic and inorganic substances that have charged or polar regions
  • Allows for metabolic reactions and transport in plants and animals
• Hydrophilic


  Substances that are chemically attracted to water
• Hydrophilic substances


  • Glucose
  • Amino acids
• Blood plasma consists mainly of water (95%) plus dissolved substances which it transports
• Hydrophilic


  Substances that water adheres to, e.g. cellulose
• A space filling molecular diagram of glucose showing the positive and negative charges
• Solvent properties of water

  Linked to its role as a medium for metabolism and for transport in plants and animals
• Blood plasma


  Consists mainly of water (95%) plus dissolved substances which it transports
• Glucose


  Polar molecule hence freely soluble, carried by the blood plasma
• Amino acids


  Positive and negative charges (due to the amine and acid groups) therefore soluble in water, R group varies and determines the degree of solubility, carried by the blood plasma
• Sodium Chloride


  Ionic compound, freely soluble in water, dissolving to form sodium ions (Na+) and chloride ions (Cl-), carried in the blood plasma
• Oxygen


  Non-polar molecule, due to small size it is soluble in water but only just, water becomes saturated with oxygen at relatively low concentrations, as temperature increases the solubility of oxygen decreases, at body temperature very little oxygen can be carried by the plasma, too little to support aerobic respiration, hemoglobin in red blood cells carry the majority of oxygen, hemoglobin has 4 binding sites for oxygen
• Hydrophobic


  Molecules that 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)
• Fats


  Large, non-polar molecules, insoluble in water, they are carried in blood inside lipoprotein complexes (in the plasma)
• Cholesterol


  Molecules are hydrophobic, apart from a small hydrophilic region at one end, this is not enough to make cholesterol dissolve in water, they are carried in blood in lipoprotein complexes (in the plasma)
• Lipoprotein complex


  Outer layer consists of single layer of phospholipid molecules, hydrophilic phosphate heads of the phospholipids face outwards and are in contact with water, the hydrophobic hydrocarbon tails face inwards and are in contact with the fats, cholesterol molecules are positioned in the phospholipid monolayer - hydrophilic region facing outwards, proteins are also embedded in the phospholipid layer
• Water


  • High specific heat capacity (4.2 Joules is required to raise the temperature of 1 g of water by 1°C)
  • High heat of vaporisation (amount of energy needed to change from a liquid to a gas or vapour)
  • 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
• Water


  • Viscosity - is a measure of the items resistance to flow and is related to how much energy is required to change its shape, more viscous than air
  • Thermal conductivity - a measure of how well heat is able to be transferred through a substance from regions of hot to cold, water has high thermal conductivity compared to other liquids and air
  • Buoyancy - the force that counteracts the force of gravity, the high density of water creates high buoyancy enabling less dense materials to float, cold water is more dense so sinks to the bottom, air is less dense than water so less buoyant
• Black throated loon - found in the Scottish highland lochs (summer) and sheltered coasts in winter
• Ringed seal - found in the cold artic water, rarely come to land but stay where there are pack ice and ice floes, it maintains a stable body temperature with a thick layer of fat/ blubber
• Origin of water on Earth

  Initially Earth consisted of a surface of hot molten magma unable to form or maintain water, over a long period of time the surface cooled down, during this time asteroids struck the surface of the earth bringing hydrated minerals that released water, hydrated minerals are solid minerals that have molecular bonds with water molecules
• Evidence for extraplanetary origin of water

  All bodies of water consists of two forms - normal water (more common) and heavy water - consisting of an isotope of hydrogen with a neutron, the ratio of normal to heavy water hydrogen is similar to those found on many asteroids
• Reasons for retention of water on Earth

  Earth's temperature allows water to change phase readily (from solid – liquid – gas), Earth has a gravitational pull to keep water on its surface
• Relationship between search for extraterrestrial life and presence of water

  To support life a planet must have water and therefore exist in an area of the solar system that allows water to exist in a liquid state, requirements are: distance for ideal temperature to maintain liquid water, atmosphere to prevent water escaping and protect against harmful radiation, planetary size to create sufficient gravitational pull