Phospholipids, which are important components of cell membranes, consist of a glycerol molecule attached to two fatty acid chains and one phosphate group (PO₄^–2):

 

 

    Like all fats, the hydrocarbon tails of phospholipids do not dissolve in water. However, phosphate groups do dissolve in water because they are polar. The different solubilities of the two ends of phospholipid molecules allow them to form the bilayers that make up the cell membrane.

 

 

    Steroids are the primary structure in hormones, substances that play important signaling roles in the body. Structurally, steroids are made up of four fused carbon rings attached to a hydrocarbon chain.

 

 

    The linked rings indicate that each carbon atom is attached to other carbon atoms that form multiple loops. Cholesterol, the steroid in the image above, is the central steroid from which other steroids, such as the sex hormones, are synthesized. Cholesterol is only found in animal cells.

 

 

    Cells use a class of compounds called nucleic acids to store and use hereditary . Individual nucleic acid monomers, known as nucleotides, consist of three main units: a nitrogenous base (a compound made with nitrogen), a phosphate group, and a sugar:

 

    There are two main types of nucleotides, differentiated by their sugars: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA nucleotides have one less oxygen than RNA nucleotides. The “deoxy” in deoxyribonucleic acid refers to the missing oxygen molecule. In terms of function, DNA molecules store genetic for the cell, while RNA molecules carry genetic messages from the DNA in the nucleus to the cytoplasm for use in protein synthesis and other processes.

 

    Within both DNA and RNA, there are further subdivisions of nucleotides by nitrogenous bases. For DNA, there are four kinds of nitrogenous bases:

1. adenine (A)
2. guanine (G)
3. cytosine (C)
4. thymine (T)

    The nitrogenous base of a nucleotide provides it with its chemical identity, so the nucleotides are called by the name of their nitrogenous base. RNA also has four nitrogenous bases. Three—adenine, guanine, and cytosine—are identical to those found in DNA. The fourth, uracil, replaces thymine.

 

 

    In 1953, James Watson and Francis Crick published the discovery of the three-dimensional structure of DNA. Watson and Crick hypothesized that DNA nucleotides are organized into a polymer that looks like a ladder twisted into a coil. They called this structure the double helix.

 

 

    Two separate DNA polymers make up each side of the ladder. The sugar and phosphate molecules of the DNA form the vertical supports, while the nitrogenous bases stick out to form the rungs. The rungs attach to each other by hydrogen bonding.