More than half of the organic compounds in cells are proteins, which play an important function in almost every cellular process. Proteins, for example, provide structural support to the cell in the cytoskeleton and make up many of the hormones that send messages around the body. Enzymes, which regulate chemical reactions in the cell, are also proteins.

 

 

    Proteins are made up of monomers called amino acids. The names of many, but not all, amino acids end in -ine: methionine, lysine, serine, etc. Each amino acid consists of a central carbon atom attached to a set of three designated groups: an atom of hydrogen (–H), an amino group (–NH₂), and a carboxyl group (–COOH). The final group, designated (–R) in the diagram below, varies between different amino acids.

 

 

    It is possible to make an infinite number of amino acids by attaching different compounds to the R position of the central carbon. However, only 20 types of R groups exist in nature, so there are only 20 naturally occurring amino acids.

 

 

    All proteins are made of chains of some or all of these 20 amino acids. The bond formed between two amino acids by dehydration synthesis is known as a peptide bond.

 

 

    A particular protein has a specific sequence of amino acids, which is known as its primary structure. Every protein also winds, coils, and folds in three-dimensional space in specific and predetermined ways, taking on a unique secondary (initial winding and coiling) and tertiary structure (overall folding). In harsh conditions, such as high temperature or extreme pH, proteins can lose their normal tertiary shape and cease to function properly. When a protein unfolds in harsh conditions, it has been “denatured.”

 

 

    Lipids are carbon compounds that do not dissolve in water. They are distinguished from other macromolecules by  hydrocarbon chains—long strings of carbon molecules with hydrogens attached. Such chains do not dissolve well in water because they are nonpolar.

 

 

    Triglycerides consist of three long hydrocarbon chains known as fatty acids attached to each other by a molecule called glycerol.

 

    Because they include three fatty acids, fats and oils are also known as triglycerides. As you might expect by this point, glycerol and each fatty acid chain are joined to each other by dehydration synthesis.

 

    Some fats are saturated, while others are unsaturated. These terms refer to the presence or absence of double bonds in the fatty acids of fats. Saturated fats have no double bonds, whereas unsaturated fats contain one or more such bonds. In general, plant fats are unsaturated and animal fats are saturated. Saturated fats are generally solid at room temperature, while unsaturated fats are typically liquid.