All known living things are composed of one or more cells, and they contain carbon compounds and water. All known life reproduces independently and passes down heritable genetic information to offspring in the form of nucleic acids.
The simplest forms of life are the archaea and bacteria, which are single-celled organisms that do not contain a cell nucleus. Viruses and viroids are simpler than archaea and bacteria, but they are not universally defined as being forms of life. They require host cells to reproduce and thus do not meet the independent reproduction requirement of life. Potential artificial life forms do not meet the standard definition of life.
All living organisms synthesize proteins from amino acids. Nucleic acids provide cells with protein coding instructions; all modern life uses DNA as its primary nucleic acid and RNA as its secondary one, but some viruses use RNA as their sole nucleic acid. All living cells are also surrounded by a cell membrane formed from phospholipids.
Every living organism requires water and some form of energy. This energy can be either electromagnetic radiation or chemical energy. Chemical energy is obtained from consuming inorganic molecules or other living organisms, while electromagnetic radiation is most commonly obtained from sunlight.