All currently known living organisms on Earth are composed of matter. Matter is anything that has mass and takes up space. Living things are made up of atoms and molecules, which are the basic building blocks of matter. Atoms are the smallest units of matter that still retain the properties of an element. Molecules are formed when two or more atoms bond together. Living things are composed of a wide variety of molecules, including proteins, lipids, carbohydrates, and nucleic acids. These molecules are used to build cells, which are the basic units of life. Cells are made up of a variety of organelles, which are specialized structures that perform specific functions. Organelles are surrounded by a cell membrane, which is a thin layer of material that separates the cell from its surroundings. Living things also contain a variety of other materials, such as water, minerals, and gases.