water

• First cells

  Evolved in a watery environment, close to hydrothermal vents in the Earth's crust
• Evolution of cells

  1. Water and solutes got trapped within a membrane

  2. Chemical reactions began occurring within the membrane-bound structure
• Water in liquid state

  Allows dissolved molecules to move around, so they are easily able to collide and react with each other
• Most life processes occur in water
• The link between water and life is so strong that scientists looking for life on other planets and moons look for evidence of water to suggest that life could have occurred there
• Hydrogen bonds


  Play an important role between many biological molecules
• Key functions of hydrogen bonding

  • Dissolving of solutes in water
  • Cohesion and adhesion of water molecules
  • Base-pairing between the two strands of DNA
  • Forming part of the secondary and tertiary levels of structure in proteins
  • Giving tensile strength to cellulose and collagen
  • Interactions between mRNA and tRNA during protein synthesis
  • Surface effects on membranes between polar phosphate groups and water
• Hydrogen bonding in water

  A fundamental property of water
• Water is the medium in which all metabolic reactions take place in cells</b>
• Between 70% to 95% of the mass of a cell is water
• Water is so fundamental to life that astronomers look for signs of water on other planets and moons, as indicators of possible extra-terrestrial life
• 71% of the Earth's surface is covered in water, making it a major habitat for organisms
• Water composition

  One atom of oxygen combines with two atoms of hydrogen by sharing electrons (covalent bonding)
• Polarity of water

  Oxygen atom attracts electrons more strongly than hydrogen atoms, resulting in a weak negatively charged region on the oxygen atom (δ-) and a weak positively charged region on the hydrogen atoms(δ+), this also results in the molecule's asymmetrical shape
• Hydrogen bonds in water

  • Form between the positive and negatively charged regions of adjacent water molecules
  • Weak when there are few, but form a strong structure when there are large numbers present
• Hydrogen bonds in water

  • Allow for strong cohesion between water molecules
  • Enable surface tension where a body of water meets the air
  • Allow water to move up the xylem of plants through mass transport
  • Allow water to bond to other polar or charged molecules (adhesion)
  • Enable capillary action to draw water up narrow channels in soil and plant cell walls
• Polar molecules and molecules with positive or negative charges can form hydrogen bonds with water (and dissolve) so are generally hydrophilic
• Non-polar molecules with no positive or negative charge, cannot form hydrogen bonds with water so are generally hydrophobic
• Water is regarded as the universal solvent because most biological molecules are hydrophilic and can be dissolved
• Solubility of molecules in water

  Highly soluble (e.g. sodium chloride, urea), insoluble (e.g. fats), and varying degrees of solubility in between
• Highly soluble molecules can be easily transported in solution within organisms (e.g. salts, glucose, amino acids)
• Even the amino acids with hydrophobic R groups are soluble enough to be freely transported in water
• Non-polar, hydrophobic molecules cannot dissolve in water
• The function of certain molecules in cells depend on them being hydrophobic and insoluble (e.g. phospholipids forming the hydrophobic core of cell membranes)
• Less soluble molecules like oxygen require assistance through combining with haemoglobin, to allow more to be carried than directly in blood plasma
• Oxygen is less soluble at body temperature (37ºC) than at 20ºC, but soluble enough to allow it to dissolve in oceans, rivers and lakes for aquatic animals to breathe
• Most enzymes require water in order to hold their shape and improve their stability, enabling them to catalyse reactions in aqueous solutions
• Hydrogen bonds often facilitate the binding of the enzyme active site and its substrate molecule, forming an enzyme substrate complex
• Specific heat capacity

  A measure of the energy required to raise the temperature of 1 kg of a substance by 1oC
• Water has a higher specific heat capacity (4200 J/kg/oC) compared to air (1000 J/kg/°C), meaning a relatively large amount of energy is required to raise its temperature
• what are the benefits of water having a high specific heat capacity?

  • Provides suitable, stable aquatic habitats
  • Allows water to absorb a lot of heat without wide temperature fluctuations, vital for maintaining optimal enzyme activity
  • Enables Arctic and sub-Arctic species like the ringed seal to survive throughout the year due to stable sea temperatures
• what is thermal conductivity?

  The ability of a substance to conduct heat
• what is buoyancy?

  The ability of an object to float in water
• what is viscosity?

  The resistance of a fluid to flow
• what are the adaptations animals have for movement through water?

  • Seals have flippers to propel itself
  • Loon uses webbed feet to push against the water
• what is an adaptation the ringed seal has?

  a layer of blubber under its skin to improve the buoyancy and keep it insulated during the cold temperatures
• what is an adaptation that the black-throated loon has?

  solid bones to assist them with flight and to overcome the problem of buoyancy