Hydrogen bonds

Lesson 2 : Water, pH and Buffers

Hydrogen bonds

Due to polar covalent bonds --> attraction of water molecules for each other.

Creates hydrogen bonds - attraction of one slightly positive hydrogen atom of one water molecule and one slightly negative oxygen atom of another water molecule.

Each water molecule can form hydrogen bonds with four other water molecules.

Hydrogen bonds give water a high melting point.

Density of water decreases as it cools --> water expands as it freezes--> ice results from an open lattice of water molecules --> less dense, but more ordered.

Hydrogen bonds contribute to water’s high specific heat
(amount of heat needed to raise the temperature of 1 gm of a substance 1oC) - due to the fact that hydrogen bonds must be broken to increase the kinetic energy (motion of molecules) and temperature of a substance --> temperature fluctuation is minimal.

Water has a high heat of vaporization - large amount of heat is needed to evaporate water because hydrogen bonds must be broken to change water from liquid to gaseous state. Due to its polar nature it has high surface tension, high adhesion properties.

  1. Water can interact with and dissolve other polar compounds and those that ionize. Water is a very strong solvent and is often referred to as the universal solvent. Solubility of organic molecules in water depends on polarity and the ability to form hydrogen bonds with water.
  2. Substances that dissolve in water, e.g. salts, sugars, acids, alkalis, and some gases – especially oxygen, carbon dioxide (carbonation) – are known as "hydrophilic" (water-loving) substances, while those that do not mix well with water (e.g. fats and oils), are known as “hydrophobic” (water-fearing) substances.

    Functional groups on molecules that confer solubility include: carboxylates, protonated amines, amino, hydroxyl and carbonyl.

    As the number of polar groups increases in a molecule, so does its solubility in water.

  3. All the major components in cells (proteins, DNA and polysaccharides) are also dissolved in water.
  4. Pure water has a low electrical conductivity, but this increases significantly with the dissolution of a small amount of ionic material such as sodium chloride.
  5. Water is vital both as a solvent in which many of the body's solutes dissolve and as an essential part of many metabolic processes within the body. Metabolism is the sum total of anabolism and catabolism. In anabolism, water is removed from molecules (through energy requiring enzymatic chemical reactions) in order to grow larger molecules (e.g. starches, triglycerides and proteins for storage of fuels and information). In catabolism, water is used to break bonds in order to generate smaller molecules (e.g. glucose, fatty acids and amino acids to be used for fuels for energy use or other purposes). Water is thus essential and central to these metabolic processes. Therefore, without water, these metabolic processes would cease to exist.
  6. Water is also central to photosynthesis and respiration. Photosynthetic cells use the sun's energy to split off water's hydrogen from oxygen. Hydrogen is combined with CO2 (absorbed from air or water) to form glucose and release oxygen. All living cells use such fuels and oxidize the hydrogen and carbon to capture the sun's energy and reform water and CO2 in the process (cellular respiration).


  7. Water is also central to acid-base neutrality and enzyme function. An acid, a hydrogen ion (H+, that is, a proton) donor, can be neutralized by a base, a proton acceptor such as hydroxide ion (OH?) to form water. Water is considered to be neutral, with a pH (the negative log of the hydrogen ion concentration) of 7. Acids have pH values less than 7 while bases have values greater than 7.
  8. Macromolecular components (i.e. proteins) assume shapes in response to water.
  9. Most metabolic machinery operates in an aqueous environment.
  10. Other noncovalent interactions in biomolecules: There are major non-covalent forces involved in the structure and function of biomolecules:
  11. Hydrogen bonds: More important when they occur between and within molecules --> stabilize structures such as proteins and nucleic acids.

    Hydrophobic interactions: Very weak. Important in protein shape and membrane structure.

Last modified: Monday, 16 January 2012, 11:00 AM