Introduction: Principal molecules of the living world

Living organisms are built from a small number of large, repeating classes of molecules, and this chapter studies three of the most important: carbohydrates, proteins and nucleic acids. Proteins and cellulose (a carbohydrate) are the primary structural materials of organisms -- proteins form muscle, skin and connective tissue in animals, while cellulose builds the rigid cell wall of plants. Plants manufacture carbohydrates from carbon dioxide and water by the unique process of photosynthesis, and they use the minerals absorbed through their roots, together with nitrogen compounds, to build proteins. Lipids (fats and oils) are the main component of vegetable oils and of milk fat, though this chapter does not study lipids in detail. Nucleic acids -- DNA and RNA -- constitute the genetic material of every organism: they store and transmit the hereditary information that specifies how an organism is built and how it functions, generation after generation. The rest of this chapter takes up carbohydrates, then proteins, then nucleic acids in turn, examining how each is built up from simpler repeating units and how that structure explains its chemical and biological behaviour.