IB DP Biology: SL复习笔记2.1.4 Water

Properties of 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 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 TableTable_-22-Water-molecules_-In-living-organisms

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

COhesion = water particles sticking to each otherADhesion = water particles sticking to other materials

Hydrophilic & Hydrophobic

  • 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

Water-is-a-solvent-1

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 heatWater-as-a-Coolant

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

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 "sweating" is "transpiration"!

Focus on Water as a Solvent

  • 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

 

 

 

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