ORGANISM: An organism has several organ systems that work together. ORGAN SYSTEM: An organ system is a group of organs working together to perform a specific job. ORGAN: Organs are a group of tissues working together to complete a specific task. TISSUE: Tissues are a group of similar cells working together. CELL: Cells are the smallest unit of life in a living organism. In multicellular organisms each cell has a specific job. EX: Humans, Animals, Plants ORGAN SYSTEM EX: Circulatory System Skeletal System ORGAN EX: Heart Lungs Skin TISSUE EX: Skin Tissue Bone Tissue CELL EX: Blood Cells Bone Cells Skin Cells