IB DP Biology: SL复习笔记2.1.3 Hydrogen Bonds

Hydrogen Bonds

  • 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

A-water-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

Hydrogen-bonds-between-water-molecules

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

Comparison: Water & Methane

  • 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

Structure-of-Methane

Methane’s structure determines its properties

Comparison of the Properties of Water and Methane Table

Comparison-of-the-Properties-of-Water-and-Methane-Table

Exam Tip

It is important to know where the hydrogen bonds form between water molecules (oxygen of one water molecule to the hydrogen atom of another).

 

 

 

 

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