Biological Molecules: Water Flashcards
Importance of Water
- Metabolite
- Solvent
- High specific heat capacity
- High latent heat of vaporisation
- Strong cohesion
Chemical Structure of Water
- Water consists of 2 hydrogen atoms and 1 oxygen atom, giving it the molecular formula H2O
- The hydrogen atoms are bonded to oxygen atoms through covalent bonds
- A covalent bond is one in which two atoms share the same pair of electrons
The Polar Nature of Water
• The electrons in the covalent bonds of water are not equally shared
- They lie closer to the oxygen nucleus than the hydrogen nuclei
- This is because the negative electrons are more attracted to the oxygen nucleus as there are more positively charged protons
• The unequal sharing of the electrons result in the oxygen atom being slightly negative and the hydrogen atoms being slightly positive
- Slightly negative / delta negative
- Slightly positive / delta positive
• The uneven distribution of charge across the water molecule makes it a polar molecule
- Polar molecules are molecules that have an uneven distribution of charge
Hydrogen Bonding in Water
- Hydrogen bonds are weak bonds between a slightly positive charged hydrogen atom in one molecule and a slightly negative charged atom in another molecule
- The slightly positive hydrogen atoms in one water molecule are attracted to the slightly negative oxygen atom in another water molecule
- Hydrogen bonding gives water some of its useful properties
Properties of Water: Metabolite
- Water is an important metabolite
- Many metabolic reactions involve hydrolysis and condensation
- Hydrolysis requires a water molecule to break a bond
- Condensation reaction releases a water molecule to make a bond
Properties of Water: Solvent
• Water is a good solvent
- Many substances in biological reactions are ionic e.g salt
• Oppositely charged ions are attracted to polar ends of a water molecule
- Slightly positive charged end of water is attracted to a negative ion
- Slightly negative charged end of water is attracted to a positive ion
• Ions will be totally surrounded by water molecules / dissolve
- Living organisms can take up useful substances (e.g mineral ions) dissolved in water and transport them around an organism’s body
Properties of Water: Latent Heat of Vaporisation
• Water has a high latent heat of vaporisation
- Lots of heat is used to change it from a liquid to a gas
- Water evaporates when the hydrogen bonds holding water molecules together are broken
- Takes a lot of energy (heat) to break the hydrogen bonds, meaning a lot of energy is used up when water evaporates
• Living organisms can use water loss through evaporation to cool down without losing too much water
- When water evaporates it carries heat energy away from a surface, which cools the surface and lowers the temperature
Properties of Water: Specific Heat Capacity
• Water has a high specific heat capacity
- Requires a lot of energy to raise the temperature of 1kg of water by 1°C
• This property is due to presence of lots of hydrogen bonds
- When water is heated, most heat energy is used to break hydrogen bonds between water molecules
- So there is less heat energy available to actually increase the temperature of water
• Water acts as a temperature buffer
- Makes water a good habitat as it remains at a fairly stable temperature
- This helps organisms maintain a constant internal body temperature
Properties of Water: Cohesion
• Water is very cohesive
- Cohesion is the attraction between molecules of the same type
- Water molecules are very cohesive (stick together) because they’re polar
• Strong cohesion helps water flow, making it good for transporting substances
- This supports movement of water up the xylem in plants
• High surface tension
- Surface of water acts as a thin elastic sheet when in contact with a gas (e.g air)
- Reason why sweat forms droplets
- Reason why pond skaters can ‘walk’ on the surface of a pond
