This simple cartoon shows general similarities and differences between eukaryote and prokaryote cells.
Cells: The Building Blocks of Life!
The Cell Theory states that all living things are made of cells, which are the basic units of life, and that cells come from other cells.
Prokaryotic cells have no nucleus or organelles enclosed within membranes. Species in the domains Archaea and Eubacteria have prokaryotic cells.
Eukaryotic cells have a nucleus and organelles that are surrounded by membranes. Each organelle does a specific cell function. All species in the Eukaryota domain (protists, fungi, plants, and animals) have eukaryotic cells. Individual protists are small and have only one cell, while individual plants and animals can have trillions of cells. Complex creatures like humans have special cells for particular functions such as carrying oxygen around the body, digesting food, or making bone.
One theory about how cells originally formed states that the first life on Earth consisted of several types of tiny, simple prokaryotic cells. Over time, prokaryotes with different specializations became engulfed within larger prokaryotes. This was beneficial to both the larger prokaryotes who were able to perform more functions with help from the smaller ones trapped within and the smaller prokaryotes who were protected within the larger one. Over millions of years the smaller prokaryotes became the organelles within eukaryotic cells.