Cells make up the smallest level of a living organism such as yourself and other living things. The cellular level of an organism is where the metabolic processes occur that keep the organism alive. That is why the cell is called the fundamental unit of life. A cell is the smallest unit of a living thing. A living thing, whether made of one cell (like bacteria) or many cells (like a human), is called an organism. Thus, cells are the basic building blocks of all organisms. Cells make up the smallest level of a living organism such as yourself and other living things. The cellular level of an organism is where the metabolic processes occur that keep the organism alive. That is why the cell is called the fundamental unit of life. The size of cells does not vary within the organisms. However, the number of cells varies from one organism to another. A larger animal like an elephant will have more number of cells as compared to a smaller animal like a rat. However, the size of the cell will be the same.