Cells are microscopic in size, requiring microscopes for visualization .
About 10 human cells could be stacked on top of one another in the thickness of a piece of paper. Human red blood cells are about 7.5 micrometers wide, and are about 5-100 times as large as a bacterial cells . There is no typical cell shape . There are two main categories of cells- prokaryotic and eukaryotic cells. Prokaryotes are cells lacking a nucleus , that is lacking a membrane bound structure packaging their cellular genetic material as occurs with eukaryotic cells.