Water is the most abundant molecule in cells at 70% of the cell mass.

Most organic molecules fall into one of four classes:

Class	Monomers	Polymers
Carbohydrates	sugars	polysaccharides
Protein	amino acids	proteins and polypeptides
Nucleic Acids	nucleotides	DNA and RNA
Lipids	fatty acids	(membranes = non-covalent structures)

Polymers of the first three classes make up 80-90% of the total dry mass of a cell. Polymers are made by the condensation (Fig. 2) of monomers at the end of the polymer chain. This releases water and is also known as dehyration synthesis. Breaking of polymer covalent bonds to release monomers requires the addition of water across the bond and is called hydrolysis (Fig. 2).

Polymers of nucleotides are characterized by their number of monomers:
DNA - 1000 nucleotide pairs = 1 kilobase pair = 1 kbp
RNA - 5000 nucleotides = 5 kb

Polymers of amino acids are often characterized by their molecular weight as compared to the element hydrogen:
1 H atom = 1 dalton (Da)
1 C atom = 12 Da
An average amino acid weighs 110 Da
An average protein = 100 amino acids =`66,000 Da = 66 KDa

Molecular distances are measured in Angstroms and nanometers (nm) where:
1 nm = 10 Angstroms = 10^-9 meters