ib bio

• Water


  The medium for life
• Origin of water on Earth (HL)


  1. First cells evolved in a watery environment

  2. Close to hydrothermal vents in the Earth's crust

  3. Water and solutes got trapped within a membrane

  4. Chemical reactions began occurring within the membrane-bound structure

  5. Led to the evolution of cells
• Water in its 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
• Hydrogen bonding


  Plays 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
  • Structure of proteins
  • Tensile strength of cellulose and collagen
  • Interactions between mRNA and tRNA during protein synthesis
  • Surface effects on membranes between polar phosphate groups and water
• Water


  • Composed of atoms of hydrogen and oxygen
  • One atom of oxygen combines with two atoms of hydrogen by sharing electrons (covalent bonding)
  • Oxygen atom attracts the electrons more strongly than the 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 separation of charge due to the electrons in the covalent bonds being unevenly shared is called a dipole
  • Water is therefore a polar molecule

  
• Hydrogen bonds form between the positive and negatively charged regions of adjacent water molecules

  
• Hydrogen bonds


  • Weak when there are few, but form a strong structure when there are large numbers present
  • Cause many of the properties of water molecules that make them so important to living organisms
• Polarity of water molecules

  Allows hydrogen bonds to form between adjacent water molecules

  
• Cohesion


  Hydrogen bonds within water molecules allows for strong cohesion between water molecules
• Adhesion


  Water is able to bond via hydrogen atoms to other molecules which are polar or charged, such as cellulose
• Capillary action


  1. Water is drawn up narrow channels in soil, called capillary tubes

  2. Spaces between cellulose fibres in plant cell walls can also draw water from xylem vessels by capillary action and allow water to flow through plant tissue
• 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

  
• Because most biological molecules are hydrophilic and can be dissolved, water is regarded as the universal solvent
• Solubility


  • Highly soluble molecules can be easily transported in solution within organisms
  • Less soluble molecules require assistance through combining with other molecules to allow more to be carried
• Oxygen is less soluble at body temperature (37ºC) than at 20ºC
• Oxygen is sparingly soluble but soluble enough to allow it to dissolve in oceans, rivers and lakes for aquatic animals to breathe
• Enzymes


  • Require water in order to hold their shape and improve their stability, enabling them to catalyse reactions in aqueous solutions
  • Hydrogen bonds will often facilitate the binding of the enzyme active site and its substrate molecule, forming an enzyme substrate complex
• Hydrophobic


  Insoluble in water
• Less soluble molecules


  Require assistance through combining with other molecules (e.g. oxygen combining with haemoglobin) to allow more to be carried than directly in blood plasma
• Haemoglobin can bind oxygen to allow sufficient oxygen to be transported to all body cells
• Enzyme action in water

  • Most enzymes require water in order to hold its shape and improve its stability
  • This enables them to catalyse reactions in aqueous solutions
  • Hydrogen bonds will often facilitate the binding of the enzyme active site and its substrate molecule
  • This forms an enzyme substrate complex
• Specific heat capacity


  • A measure of the energy required to raise the temperature of 1 kg of a substance by 1 C
  • Water has a higher specific heat capacity (4200 J/kg/°C) compared to air (1000 J/kg/°C), meaning a relatively large amount of energy is required to raise its temperature
  • The high specific heat capacity is due to the many hydrogen bonds present in water
  • It takes a lot of thermal energy to break these bonds and a lot of energy to build them, thus the temperature of water does not fluctuate greatly
• Advantages of water's high specific heat capacity for living organisms


  • Provides suitable, stable aquatic habitats since water temperatures will change more slowly than air temperatures
  • Able to maintain a constant temperature as water is able to absorb a lot of heat without wide temperature fluctuations
  • This is vital in maintaining temperatures that are optimal for enzyme activity
• Arctic and sub-arctic species, such as the ringed seal (Pusa hispida) are able to survive throughout the year due to stable sea temperatures
• The density of ice is lower than the density of liquid water, which means that ice floats on water
• This forms a habitat for the seals both on the floating ice sheets, as well as below the ice
• Thermal conductivity


  • The ability of a substance to conduct heat
  • The thermal conductivity of water is almost 30 times higher than that of air, which makes air a very good insulator for organisms living in colder climates
• Seal


  Relies on a layer of fat called blubber to insulate it from the outside air
• Ice in its environment will also form an insulating layer above the water, since the thermal conductivity of ice is much lower than liquid water
• This increases the sea temperature below the ice as thermal energy is trapped
• Buoyancy


  The ability of an object to float in water
• How the black-throated loon overcomes buoyancy


  • Has solid bones, unlike the hollow bones that most bird species have to assist them with flight
  • This increases the weight of the bird and compresses air out of the lungs and feathers during a dive
• How the ringed seal overcomes buoyancy


  • The layer of blubber under its skin will improve the buoyancy of the animal, along with providing a layer of insulation against the cold temperatures of its habitat
• Viscosity


  The resistance of a fluid to flow
• The viscosity of water is much higher than air, which enables the black-throated loon to fly through the air without much friction
• Adaptations for movement through water


  • The seal has flippers to propel itself
  • The loon uses its webbed feet to push against the water and the lateral location of its feet reduces drag as it moves through water
• Extraplanetary origin of water

  • Water is crucial for the existence of life but when Earth formed around 4.5 billion years ago, conditions were too hot for water vapour to condense into liquid water
  • This has led scientists to believe that Earth's water must have originated from somewhere else