All living things are made of cells, which are the basic units of life. There are two main types of cells: prokaryotic cells which lack membrane-bound organelles and a nucleus, and eukaryotic cells which have a nucleus and membrane-bound organelles. Organelles such as the nucleus, mitochondria, endoplasmic reticulum, and chloroplasts allow cells to carry out essential functions like DNA storage, energy production, protein transport, and photosynthesis.