Organic chemistry is the study of carbon compounds that contain only carbon and hydrogen (organic), as opposed to the inorganic compounds that are found in ordinary metals, minerals, and salts. Organic compounds may be regarded as all of the most common types of compounds found in living organisms and the atmosphere. Organic compounds, both natural and synthetic, are the basis of most of the world's modern industry, and are essential ingredients in the composition of living organisms. Industrial organic compounds include the essential oils and fats used in the flavor and fragrance industry, and the petrochemicals used to make plastics. Organic compounds are generally classified as either natural or unnatural. Natural organic compounds are those found in living organisms (such as starch and lactic acid) or produced by biological processes (such as cellulose and the amino acids). Unnatural organic compounds were first produced by humans during the late 19th century. They can be divided into two basic types: 1. Organic compounds containing carbon, hydrogen, and generally oxygen in other atoms. The carbon-carbon double bond (C=C) is a common structural feature. Examples include organic acids such as acetic acid, butanoic acid, propanoic acid, butyric acid, and valeric acid. Organic compounds containing carbon, hydrogen, and generally oxygen in other atoms are also known as aliphatic compounds. 2. Organic compounds containing carbon, hydrogen, and generally nitrogen in other atoms. Examples include amines such as ammonia, urea, and uric acid, and carbamates such as dimethylaminoethanol. Natural organic compounds also include many small molecules found in biological organisms and that are essential to their function. These molecules are called "biomolecules" (related to the Greek bios, life), and includes such compounds as amino acids, lipids, and nucleic acids. Organic chemists also study non-biological "organic" compounds, produced in laboratories or industry. They are often called "synthetic organic compounds", "artificial organic compounds", "organometallic compounds", or "organics".