General ---------Biology ------------------Online------------------   ---- Ch.  2 :  The Chemical Context of Life

If two polar molecules are close to each other, then their opposite charged portions will attract, forming a weak attraction between the two polar molecules.  This weak attraction is called a "hydrogen bond," and is illustrated below as a dash line.

Warning: a covalent bond between a hydrogen atom and another atom, such a oxygen or carbon, is NOT a hydrogen bond. 

In this course, hydrogen bonds will always be illustrated as dash or dotted lines. 

Hydrogen bonds are very important to biochemistry because they cause organic molecules to contort into specific shapes.  In the piece of a protein molecule shown below, everywhere oxygen appears there is a polar covalent bond.  Oxygen's electronegativity causes a negative region near the oxygen and a corresponding positive region near the atom sharing electrons with the oxygen.  If the molecule is large and flexible enough,  positive and negative regions may attract each other causing the molecule to contort and twist into a shape that is characteristic of the protein.  The shape of an organic molecule, such as a protein, is critical to its biochemical function.

Summary: You should understand 

• hydrogen bonds form between positive and negative regions of molecules
• hydrogen bonds are weak chemical bonds and illustrated with dash lines
• hydrogen bonds can form between different regions of large organic molecules causing the molecule to contort and twist into a characteristic shape
• the shape of a molecule is critical to its biochemical function