Water (DP IB Biology: SL)

Water as the medium for life

• The first cells evolved in a watery environment
• This is believed to have been in the deep oceans, close to hydrothermal vents in the Earth's crust
• Some water and solutes got trapped within a membrane
• Chemical reactions began occurring within the membrane-bound structure
• This 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 look for evidence of water to suggest that life could have occurred there

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

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

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