Biologists recognize five levels of cell organization. The lowest and simplest is the cellular level. Organisms here are either unicellular organisms, like protozoa, or colonial organisms. Colonial organisms are composed of single-celled individuals that stay together to sustain the life of the whole colony. Next on the scale of complexity is the level occupied by tissue. A tissue is a group of similar cells that perform a similar activity. Tissues that are organized around a common function together make up an organ, the third level of complexity. The fourth level of organization is the system, formed by a group of organs that together perform a specific bodily process. The fifth and highest level is the organism level, in which body systems work together in a structure capable of independent life.
