• Hydrogen bonding plays an important role between many biological molecules
• Some key functions include:
  • Dissolving of solutes?in water
  • The?cohesion?and?adhesion?of water molecules
    • These properties allow water to move up the trunks of really tall trees
  • Base-pairing?between the two strands of DNA
  • Structure:
    • Hydrogen bonds help to form part of the?secondary?and?tertiary?levels of structure in proteins
    • The hydrogen bonds found between strands of?cellulose?and?collagen?give those molecules their?tensile strength
  • Interactions between?mRNA?and?tRNA?during?protein synthesis
  • Surface effects?on membranes between?polar phosphate groups?and?water

Hydrogen bonding in water

• Hydrogen bonding is a fundamental property of?water
• Water is of the?utmost biological importance
  • It is the?medium?in which all metabolic reactions take place in cells
  • Between 70% to 95% of the mass of a cell is water
  • Water is?so fundamental to life?that astronomers look for?signs of water?on other planets and moons, as indicators of possible extra-terrestrial life
  • As 71% of the Earth’s surface is covered in water it is a?major habitat?for organisms
• Water is composed of atoms of hydrogen and oxygen
  • One atom of oxygen combines with two atoms of hydrogen by sharing electrons (covalent bonding)
• Although water as a whole is electrically neutral, the?sharing?of the?electrons?is?uneven?between the oxygen and hydrogen atoms
  • The oxygen atom attracts the electrons more strongly than the hydrogen atoms, resulting in a?weak negatively charged region?on the oxygen atom (δ-) and a?weak positively charged region?on the hydrogen atoms(δ+), this also results in the molecule's asymmetrical shape
• This separation of charge due to the electrons in the covalent bonds being unevenly shared is called a?dipole
• When a molecule has?one?end that is negatively charged and?one?end that is positively charged it is also a?polar molecule
• Water is therefore a?polar molecule

The covalent bonds of water make it a polar molecule

• Hydrogen bonds?form between water molecules
  • As a result of the polarity of water,?hydrogen bonds form?between the positive and negatively charged regions of adjacent water molecules
• Hydrogen bonds are weak, when there are few, so they are?constantly breaking and reforming
• However, when there are large numbers present they form a strong structure
• Hydrogen bonds?cause many of the properties of water molecules, that make them so important to living organisms:
  • Excellent?solvent?– many polar substances can dissolve in water
  • A relatively?high specific heat capacity
  • A relatively?high latent heat of vaporisation
  • Water is less dense when a solid (ice floats, allowing aquatic life to flourish beneath)
  • Water has?high surface tension?and cohesion
  • It acts as a?reagent

The polarity of water molecules allows hydrogen bonds to form between adjacent water molecules

• Both?methane?(CH4) and?water?(H2O) are?small, covalently-bonded molecules
• Methane is the?simplest organic compound
• They have similar molecular weights, 16 and 18 respectively
• But?water is polar and so forms hydrogen bonds, whereas methane is?not polar?and so does not form hydrogen bonds
• Water is liquid?at room temperature, whereas?methane is a gas
  • Other compounds, with a small molecular weight, such as ammonia (NH3) and carbon dioxide (CO2) are gases at room temperature
    • These compounds also?do not form hydrogen bonds
  • This shows how?water molecules are held together by hydrogen bonds, whereas the other gas molecules are free to move around in the gaseous state
  • Methane is a?fuel?with a?high energy content?in its bonds, whereas water is a?final product?of combustion and respiration
• Understanding the molecular properties of methane underlines?the significance of hydrogen bonding?in water

Methane’s structure determines its properties

Comparison of the Properties of Water and Methane Table