Physical & Chemical Properties of Water (SL IB Biology)

• Hydrogen bonds within water molecules allows for strong cohesion between water molecules
  • Allowing columns of water to move under tension (called mass transport) through the xylem of plants 
  • Enabling surface tension where a body of water meets the air, these hydrogen bonds occur between the top layer of water molecules to create a sort of film on the body of water
    • This layer is what allows insects such as pond skaters to move across the surface of water

• Water is also able to bond via hydrogen atoms to other molecules which are polar or charged, such as cellulose, which is known as adhesion
  • This also enables water to move up the xylem during transpiration
  • Water is drawn up narrow channels in soil, called capillary tubes, by means of capillary action
  • 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

Cohesion and adhesion in xylem diagram

Hydrogen bonding results in cohesion and adhesion forces in xylem which allows water molecules to flow through the plant in a continuous stream

• Biological molecules can be hydrophilic or hydrophobic (and sometimes both)
  • Hydrophilic = "water-loving"
  • Hydrophobic = "water-hating"
• 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
  • These molecules tend to join together in groups due to hydrophobic interactions where hydrogen bonds form between water particles but not with the non-polar molecule
• Because most biological molecules are hydrophilic and can be dissolved, water is regarded as the universal solvent

Water as a solvent diagram

Due to its polarity water is considered a universal solvent

Solvent properties of water

• Different solutes behave differently with water as a solvent
• Even though water is a universal solvent, different metabolites have different solubilities in water
• Different solutes have different hydrophobic and hydrophilic properties which affect their solubility in water

Highly soluble molecules

• Some molecules are highly soluble (e.g. sodium chloride, urea) and some are insoluble (e.g. fats) 
• 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
• Different transport mechanisms have evolved to assist in the transportation of the less soluble molecules

Insoluble molecules

• 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 have hydrophobic hydrocarbon tails which forms the hydrophobic core of cell membranes

Less soluble molecules

• A low solubility molecule such as oxygen requires assistance through combining with haemoglobin, to allow more oxygen to be carried than directly in blood plasma
  • 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
  • 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

• Specific heat capacity is a measure of the energy required to raise the temperature of 1 kg of a substance by 1^oC
• 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
• 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
• The advantage for living organisms is that it:
  • Provides suitable, stable aquatic habitats since water temperatures will change more slowly than air temperatures
  • Is 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
• Artic and sub-artic 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

By NOAA Seal Survey, Public domain, Wikimedia

A ringed seal (Pusa hispida) in its native habitat

Thermal conductivity

• Thermal conductivity refers to 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
• The black-throated loon (Gavia arctica) is a species of diving bird which spends much time underwater catching its prey
• Their feathers trap an insulating layer of air, which assists them with regulating their body temperature

By Robert Bergman, Public domain, Wikimedia 

The black-throated loon (Gavia arctica)

• The seal on the other hand, 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

• Buoyancy refers to the ability of an object to float in water
• To overcome the problem of buoyancy, the black-throated loon 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
• For the ringed seal, 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

• Viscosity refers to 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
• The body shapes of both the loon and seal makes it easy for them to move through water
• Both organisms are adapted in their own way 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