Water

• Water is made up of two hydrogen atoms covalently bonded to an oxygen atom. While the covalent bonding involves a share of electrons, they are not shared equally between the atoms resulting in polarity.
• Polarity is the slight difference in charge that occurs at different poles of the molecules. Oxygen (due to having a higher electronegativity) attracts the electrons more strongly resulting in a slightly negative charge. Hydrogen atoms have a weaker attraction to the electrons resulting in a slightly positive charge.
• The polarity of water allows it to form weak associations with other polar molecules or charged ions. The slightly negative poles will attract the slightly positive poles and vice versa.
• Hydrogen bonds are intermolecular associations formed as a consequence of the polar covalent bonds within water molecules.

  These bonds form when a positively charged hydrogen atom is attracted to a negatively fluorine, oxygen or nitrogen atom of another molecule.

  Hydrogen bonds are stronger than standard polar associations due to the particularly high electronegativity of F, O and NHydrogen bonding is responsible for several unique properties of water – including thermal, cohesive and solvent properties.
• Water is cohesive - a water molecule is able to stick to other water molecules via the formation of hydrogen bonds. The hydrogen bonding between hydrogen molecules allows the liquid to resist low levels of external force. (It creates surface tension)

  The high surface tension of water makes it sufficiently dense for certain smaller organisms to move along its surface.
• Water striders are small insects that can move along the surface of water due to its high surface tension. Water striders distribute their weight via their long legs that have surface hairs that trap air and increase buoyancy.
• Water is also adhesive- a water molecule is able to stick to other polar or charged molecules via the formation of polar associations. Attraction to polar or charged surfaces (such as cellulose walls) allows water to flow in opposition to gravitational forces- this is capillary action.
• What are the physical properties of water?

  Buoyancy, viscosity, thermal conductivity, specific heat capacity
• How do the adaptations of the black-throated loon and ringed seal differ based on their environments?

  Black-throated loon:

  • Lighter bones for buoyancy
  • Rear legs for swimming
  • Interlocking feathers for insulation

  
  

  Ringed seal:

  • Denser bones for submersion
  • Streamlined body for swimming
  • Thick blubber and fur for insulation
• What is solvation?

  The process by which solvent molecules surround and interact with solute molecules.
• What is a solute?

  Any substance (usually a solid) that is dissolved by a solvent molecule to form a solution. Solutes that are able to be dissolved by a solvent are considered to be soluble. (insoluble substances won't form a solution)
• Why is water referred to as the universal solvent?

  Due to it's capacity to dissolve a large number of substances. Water can dissolve any substance that contains charged particles (ions) or electronegative atoms (polarity). The polar regions of the water molecule associate with molecular surfaces that have an opposing charge, forming dispersive hydration shells.
• Metabolic Medium

  The capacity of water to dissolve a large variety of substances makes it an important medium for metabolic reactions

  • Solutes dissolved into aqueous solutions are more likely to collide with enzymes and undergo necessary chemical reactions
  • Water can also promote enzyme activity by absorbing heat from exothermic reactions and maintain acid-base neutrality
• Substances that are involved in metabolic reactions can be described as being either hydrophilic or hydrophobic

  • Substances that freely associate and readily dissolve in water are characterised as hydrophilic (‘water loving’) – this includes all polar molecules and ions
  • Substances that do not freely associate or dissolve in water are characterised as hydrophobic (‘water-hating’) – this includes all large non-polar molecules (lipids)
• Transport Medium

  The movement of water-soluble substances in plants and animals involves different transport systems

  • In animals, the blood plasma transports dissolved solutes – including amino acids, simple sugars, wastes (urea) and a small amount of gases (O2 and CO2)
  • In vascular plants, mineral ions are transported via xylem vessels while dissolved nutrients are transported via the phloem
• Certain substances are not water-soluble and cannot be freely transported within an aqueous environment

  • In animals, lipids are packaged with proteins to form water-soluble lipoproteins that can be transported via the blood
  • The mechanism of lipid transport is not well understood in plants, but may involve conjugation to amino acids and transport via the phloem
• What is buoyancy?

  • Buoyancy is the upward force applied to an object in a medium and is determined by the density of the medium
  • As water is more dense than air, it applies a greater upward force which allows objects to float in water
  • The capacity of an object to float in water will be determined by its relative weight (heavier objects will sink)
• What is viscosity?

  • Viscosity is a measure of a fluid’s tendency to flow (more viscous fluids are more resistant to flow)
  • Water is more viscous than air as it can form hydrogen bonds which increase the friction of flowing molecules
  • Additionally, water can dissolve many solutes and these solutes can increase the viscosity of the solution
• What is thermal conductivity?

  • Thermal conductivity is a measure of a medium’s ability to move heat across a temperature gradient
  • Water absorbs and transfers heat more readily than air because water particles are packed more tightly together
• What is specific heat capacity?

  • Specific heat capacity describes the amount of energy required to raise the temperature of 1g of substance by 1ºC
  • Water has a higher specific heat capacity than air (highest of any liquid) as its hydrogen bonds require additional energy to break
  • The high specific heat capacity of water makes it an effective coolant in sweat (sweat absorbs a lot of thermal energy resulting in a high latent heat of vaporisation)
• The different physical properties of water and air influence the various adaptations and behaviours of terrestrial and aquatic organisms

  • The black-throated loon (Gavia arctica) and the ringed seal (Pusa hispida) both live in the Arctic and spend time on land and in the water
• Black-Throated Loon

  • The loon has lighter (less dense) bones, allowing it to float on water (due to buoyancy) – however, the bones are not hollow like in many other birds of flight (allowing it to dive under water)
  • Loons have difficulty walking on land because their legs are located at the rear to better propel them through water (higher viscosity)
  • The loon’s feathers form an interlocking structure that functions as a barrier to water, preventing heat loss (water has higher thermal conductivity)
• Ringed Seal

  • The seal has denser bones than the loon, allowing it to stay submerged upon diving (less buoyant)
  • The seal possesses a streamlined body to better propel them through water (higher viscosity than air)
  • Ringed seals have an outer coat of fur that traps air for waterproofing and also has a thick layer of blubber to prevent heat loss while in water
  • Ringed seals do not have many effective cooling mechanisms because water temperatures are generally stable (due to specific heat capacity) – this makes the seal particularly vulnerable to climate change