IB Biology SL

Revision Notes

2.1.4 Water

Properties of Water

The polarity of water molecules and the presence and number of hydrogen bonds between water molecules give water its unique properties

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 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

  • The specific heat capacity of a substance is the amount of thermal energy required to raise the temperature of 1kg of that substance by 1°C. Water’s specific heat capacity is 4200 J/kg/°C
  • 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 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 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

Water molecules_ In living organisms, downloadable AS & A Level Biology revision notes

Cohesion and adhesion

  • Hydrogen bonds between water molecules allows for strong cohesion between water molecules
    • This allows columns of water to move (called mass transport) through the xylem of plants and through blood vessels in animals
    • This also enables 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

COhesion = water particles sticking to each other

ADhesion = water particles sticking to other materials

Hydrophilic & Hydrophobic

Biological molecules can be hydrophilic and hydrophobic

  • Hydrophilic = “likes water”
  • Hydrophobic = “dislikes water”
  • Polar molecules can form hydrogen bonds with water so are generally hydrophilic
  • Non-polar molecules cannot form hydrogen bonds with water so are generally hydrophobic
  • 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

 

 

Water is a solvent, downloadable AS & A Level Biology revision notes

Due to its polarity water is considered a universal solvent

Water: 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
  • 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
  • Water’s high latent heat of vaporisation allows only small volumes of water to be needed to carry away a lot of heat
  • Plants transpire
  • A large tree will stand in direct sunlight all day, so will absorb a huge amount of heat (as infra-red radiation) on a hot day
    • A tree cannot seek shade, because it requires light energy for photosynthesis
    • A tree is also immobile and IS the 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

Exam Tip

Sweat and transpiration have a lot of parallels in keeping animals and plants cool. This is why the French use the same word for both; the French word for “sweat” is “transpiration“!

Water: Universal Solvent

Different solutes behave differently with water as a solvent

  • Even though water is a universal solvent, different metabolites have different solubilities in water
  • 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
  • Different transport mechanisms have evolved to assist in the transportation of the 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
  • 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
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