The genetic material floats freely in the cytoplasm of the cell. A plasma membrane surrounds both eukaryotic and prokaryotic cells. Prokaryotes are thought to be the most primitive type of cell. Cells come in a variety of shapes and sizes, ranging from a tiny bacterial cell to a large nerve cell extending between two body parts. Some organisms, such as bacteria and protists, consist of only a single cell.
Tissues work together and form organs.
Finally, organs work together performing a job and make up an organ system. An example of an organ system is your digestive system. A cell is like a miniature body containing tiny organs, called organelles. The nucleus is an organelle that acts as the command center for the cell. The mitochondria is like a power plant providing the cell with energy.