Unit 2: The Molecules of Life

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2.1: Organic Molecules
2.2: Hydrocarbons
Hydrocarbons are organic molecules that consist exclusively, or primarily, of carbon and hydrogen atoms. They come in two flavors: (1) aliphatic hydrocarbons that consist of linear chains of carbon atoms and (2) aromatic hydrocarbons that which consist of closed rings of carbon atoms.
2.3: Fats
Fat molecules are made up of four parts: a molecule of glycerol (on the right) and three molecules of fatty acids.
2.4: Phospholipids
Phospholipids are fat derivatives in which one fatty acid has been replaced by a phosphate group and one of several nitrogen-containing molecules.
2.5: Cholesterol
The cholesterol molecule is a steroid that is essential to life. It has also been responsible for 17 Nobel Prizes, countless pages of reports in scientific journals and the popular press, and mounting anxiety on the part of health-conscious people. The human body contains about 100 g of cholesterol. Most of this is incorporated in the membranes from which cells are constructed and is an indispensable component of them. The insulating layers of myelin wound around neurons are rich in cholesterol.
2.6: Carbohydrates
Carbohydrates were once thought to represent "hydrated carbon". However, the arrangement of atoms in carbohydrates has little to do with water molecules. Starch and cellulose are two common carbohydrates. Both are macromolecules with molecular weights in the hundreds of thousands. Both are polymers built from repeating units, monomers, much as a chain is built from its links. The monomers of both starch and cellulose are the same: units of the sugar glucose.
2.7: Amino Acids
Amino acids are the building blocks (monomers) of proteins. 20 different amino acids are used to synthesize proteins. The shape and other properties of each protein is dictated by the precise sequence of amino acids in it.
2.8: Enantiomers
In three-dimensional (3D) space, the four covalent bonds of carbon atoms point toward the corners of a regular tetrahedron.
2.9: Polypeptides
Polypeptides are chains of amino acids. Proteins are made up of one or more polypeptide molecules. The amino acids are linked covalently by peptide bonds. The picture below shows how three amino acids are linked by peptide bonds into a tripeptide.
2.10: Proteins
Proteins are macromolecules. They are constructed from one or more unbranched chains of amino acids; that is, they are polymers. An average eukaryotic protein contains around 500 amino acids but some are much smaller (the smallest are often called peptides) and some much larger (the largest to date is titin a protein found in skeletal and cardiac muscle; one version contains 34,350 amino acids in a single chain!).
2.11: Rules of Protein Structure
The function of a protein is determined by its shape. The shape of a protein is determined by its primary structure (sequence of amino acids). The sequence of amino acids in a protein is determined by the sequence of nucleotides in the gene (DNA) encoding it. The function of a protein (except when it is serving as food) is absolutely dependent on its three-dimensional structure.
2.12: Glycoproteins
Glycoproteins have carbohydrate attached to them — a process called glycosylation.
2.13: Nucleotides
2.14: Proteomics
While we humans probably have only some 21 thousand genes, we probably make at least 10 times that number of different proteins. The great majority of our genes produce pre-mRNAs that are alternatively-spliced. The study of proteomics is important because proteins are responsible for both the structure and the functions of all living things. Genes are simply the instructions for making proteins. It is proteins that make life.

Contributors and Attributions

John W. Kimball. This content is distributed under a Creative Commons Attribution 3.0 Unported (CC BY 3.0) license and made possible by funding from The Saylor Foundation.
Thumbnail: Cellulose molecular structure (CC BY-SA 3.0 Unported; Pintor4257 via Wikipedia)