Hydrogen Bonds (SL IB Biology)

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

• 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

Hydrogen bonds in a water molecule diagram

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.

Hydrogen bonds between water molecules diagram

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