Some examples of substances that were once living material include **wood**, **cotton**, **wool**, **leather**, **paper**, **food**, and **biofuels**. Fossil fuels, such as coal, oil, and natural gas, were once living material. They are formed from the remains of plants and animals that lived millions of years ago. These organisms died and were buried under sediment, which over time, turned into rock.
Living organisms are composed of a wide variety of substances, including proteins, carbohydrates, lipids, nucleic acids, and minerals. While some of these substances are exclusively found in living organisms, others can be found in non-living matter as well.
For example, **proteins** are large molecules made up of amino acids that are essential to all living organisms. They are involved in many biological processes such as catalyzing metabolic reactions and transporting molecules across cell membranes.
