Dear Student,
A cell is the smallest unit that perform all life processes. All living organisms whether made up of one cell or more have some basic degree of organisation. Complex multicellular organism at the highest level the organism is made up of organ system, that carry out certain function in an organism. Cells work together in hierarchical manner group of cells make tissues, tissues make organs, organs make systems, systems when work in co-ordination form an individual. So at the fundamental level cell is the basic structure.
Regards