2.1.4 Water

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 a higher 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)
• Water is present on Earth in all three physical states (solid, liquid and gas) thanks to this characteristic
  • 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 skin

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

• 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

• 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 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
  • 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

• 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)
• Highly soluble metabolites simply travel dissolved in the blood plasma
  • eg. 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 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