Define cellular level of organisation, tissue level of organisation and organ level of organisation

Cellular level of organisation - There is only one cells or a loose aggregate of cells that work independently, without any coordination

Example - Amoeba, Paramoecium

Tissue level or organisation - A group of cells that are structurally/ functionally similar work together to perform a common function

Example - Nervous tissue, Epithelial tissue 

Organ level of organisation - One or more tissues work together to achieve the functions of the organ

Example - heart, lungs