NOS: Use theories to explain natural phenomena; the theory that hydrogen bonds form between water molecules explains the properties of water

• When scientists observe natural phenomena they try to come up with?credible theories?to explain their occurrence
• A theory is presumed to be correct?if it explains the observation, is supported by?experimental evidence?and has not been falsified
• As hydrogen bonds are not visible, scientists can never say for certain that they exist, however there is strong experimental evidence to suggest that they do
• The presence and number of?hydrogen bonds?between?polar?water molecules helps to explain water's unique properties

The properties of water

Solvent

• As water is a?polar molecule?many ions (e.g. sodium chloride) and covalently bonded polar substances (e.g. glucose) will dissolve in it
  • This allows?chemical reactions?to occur within the?cytoplasm?of cells (as the dissolved solutes are more chemically reactive when their individual molecules are free to move about)
  • Metabolites?can be transported efficiently (except non-polar molecules which are hydrophobic)

  Water molecules 'surround' individual solute particles to ensure each solute particle is isolated from others

  • This explains why?solutions are clear?- we can't see individual molecules that are separated from their crystal structures

  This is also why concentrated solutions have a lower?water potential?or?osmolarity

  • Because many water particles are 'occupied' in keeping a solute molecule in solution,?fewer water molecules are free to diffuse?across partially permeable membranes

  Water has a high 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?high?specific heat capacity of 4200 J/kg/^oC 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 habitats
  • 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

    Water in blood plasma is also essential in transferring heat?around the body, helping to?maintain a fairly constant temperature, especially at body extremities eg. fingertips

    • As blood passes through more metabolically active (‘warmer’) regions of the body, heat energy is absorbed but the temperature remains fairly constant
    • Water in?tissue fluid?also plays an important regulatory role in maintaining a constant body temperature

     

  Water has a high latent heat of vaporisation

• In order to change state (from liquid to gas) a large amount of thermal energy must be absorbed by water to break the?hydrogen bonds?and allow individual gas particles to escape (evaporate)
• This explains water's?high boiling point?(100°C)
• • Ice,?liquid water?and?water vapour?all play a vital role in the biosphere
  • This is an advantage for living organisms as?only a little water is required?to evaporate for the organism?to dissipate a great amount of heat
  • This provides a?cooling?effect for living organisms, for example, the transpiration from leaves or evaporation of water in sweat from the skinWater is present on Earth in?all three physical states (solid, liquid and gas) thanks to this characteristic

Properties of Water & its Role in Living Organisms Table

Cohesion and adhesion

• Hydrogen bonds between water molecules allows for strong?cohesion between water molecules
  • Allowing?columns of water?to move (called?mass transport) through the xylem of plants and through blood vessels in animals
  • 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 hydrogen bond to other molecules, such as cellulose, which is known as?adhesion

  • This also enables water to move up the?xylem?during?transpiration
  • Cohesion and adhesion both contribute to water forming a?meniscus?in glassware, where water molecules adhere to polar molecules in the glass
  • Water adheres to the xylem walls (made of lignin) by?capillary action

  Exam Tip

• 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

• Some large molecules have?different groups?with?different characteristics
  • Phospholipids?have hydrophilic (phosphate group) heads and hydrophobic (hydrocarbon chain) tails. This dual character is a key feature in the structure and function?of?cell membranes

Due to its polarity water is considered a universal solvent

Focus on Water as a Coolant

• Water's high latent heat of vaporisation makes it an excellent coolant
• Animals?have evolved?sweating?(perspiration) as a way of disposing of excess heat generated through physical activity
• The?hypothalamus?detects changes to blood temperature and when temperatures rise, it stimulates the secretion of sweat
• Small droplets of water are secreted from?sweat glands?onto the skin's surface
• Vasodilation?of arterioles just beneath the skin carries more blood close to the surface
• Sweat (mainly water, also contains salts and other solutes)?evaporates, carrying the excess heat away into the surrounding air and reducing the temperature of the organism
• Water's high latent heat of vaporisation allows only?small volumes of water to be needed to carry away a lot of heat

The excess heat carried in blood causes the evaporation of sweat from the skin surface

Water as a coolant in plants

• Plants?transpire
• • A tree cannot seek shade, because it?requires light energy?for photosynthesis
  • A tree is also?immobile?and provide shade for other organisms
  • A?transpiration stream?of water flows up the tree, from roots to xylem to leaves, throughout the day
  • Water?evaporates inside the spongy mesophyll layer?of leaves, so water vapour can?diffuse out?via the stomata
  • For example, a large oak tree can absorb around 500 litres of water per day from the soil, around?90% of which is evaporated in transpiration?to dissipate heat
    • The remainder is used to keep cells turgid and as a raw material for photosynthesis

     

    A large tree will stand in direct sunlight all day, so?will absorb a huge amount of heat (as infra-red radiation from the Sun) on a hot day

• 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 metabolites

• Some are?highly soluble?(eg. sodium chloride, urea), some are?insoluble?(eg. fats) and some have?intermediate solubility?(eg. oxygen and certain amino acids with a large R group)
• • • Even the amino acids with hydrophobic R groups are soluble enough to be freely transported in watereg. salts, glucose, amino acids

    Highly soluble metabolites simply travel dissolved in the?blood plasma

• Different transport mechanisms have evolved to assist in the transportation of the?less soluble metabolites

Less soluble metabolites

• A low solubility metabolite 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 enough 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

  Insoluble?metabolites like fats require?emulsification, and transport in?lacteals, or by being converted to?soluble phospholipids

• Cholesterol, which is insoluble, is?converted to lipoproteins?by combining? with proteins