Properties of Water
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 1oC
- 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
Examiner Tip
COhesion = water particles sticking to each other. ADhesion = water particles sticking to other materials