All living things perform basic life processes of interaction, nutrition, and reproduction. Living things range from unicellular organisms like bacteria to multicellular organisms like plants and animals. All organisms are made of cells, which are the basic units of life. Cells contain organelles and perform life processes. Multicellular organisms organize cells into tissues, organs, and systems. Scientists classify all living things into five kingdoms - Monera, Protista, Fungi, Plantae, and Animalia - based on their characteristics.