• large (some are made up over 10,000 atoms),
• complex (complex shapes with groves, tunnels and protuberances), 
• diverse (made up of many different atoms and smaller molecules).

.
Insulin: a protein

This is a computer generated model of the protein insulin.  Each colored sphere represents an atom. The white atoms are carbon.  Carbon forms the backbone of organic molecules, such as proteins, carbohydrates, fats, and DNA.  Go to Ch. 5 in your text and look at Figures 5.3 &  5.10.  Observe how the carbon atoms form the "backbone" of these sugars and fats.

We start Ch. 4 with a discussion of how organic molecules arise.  This discussion extends to the question of the origin of life on earth, i.e. where did the first organic molecule come from?  Then we look at methane (CH₄), the smallest organic molecule.  Next we build larger molecules by adding one carbon at a time.  The Chapter ends by looking at the "functional groups" that determine the emergent properties we associate with familiar organic molecules such as sugars, amino acids and alcohol's. 

Ch. 4 is critical background for our study of carbohydrates, fats (lipid), proteins and nucleic acids (DNA) and for our study of the chemistry of life.