ABSTRACT
A human body is a complex, living machine. It is built of a few types of chemical element: carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus. These elements combine into a few types of large molecule: nucleic acids, proteins, carbohydrates, and lipids. From these building blocks arise our 260 or so different types of cell, which assort and assemble into four types of tissue, which in turn interact to form our organs and organ systems. The nucleic acids DNA and RNA control the activities of our bodies through the production of proteins. Some proteins are like building materials, such as the stretchy connective tissue protein elastin or the scaly skin protein keratin. Very important are the enzymes, which are proteins that control the rates of the hundreds of chemical reactions that keep us alive and well.
