"I think of a prokaryote as a one-room efficiency apartment and a eukaryote as a $6 million mansion," says Erin Shanle, a professor in the Department of Biological and Environmental Sciences at Longwood University, in an email interview. "The size and separation of functional 'rooms,' or organelles, in eukaryotes is similar to the many rooms and complex organization of a mansion. Prokaryotes have to get similar jobs done in a single room without the luxury of organelles."
One reason this analogy is helpful is because all cells, both prokaryotes and eukaryotes, are surrounded by a selectively permeable membrane which allows only certain molecules to get in and out — much like the windows and doors of our home. You can lock your doors and windows to keep out stray cats and burglars (the cellular equivalent to viruses or foreign materials), but you unlock the doors to bring in groceries and to take out the trash. In this way, all cells maintain internal homeostasis, or stability.
"Prokaryotes are much simpler with respect to structure," says Shanle. "They have a single 'room' to perform all the necessary functions of life, namely producing proteins from the instructions stored in DNA, which is the complete set of instructions for building a cell. Prokaryotes don't have separate compartments for energy production, protein packaging, waste processing or other key functions."
In contrast, eukaryotes have membrane-bound organelles that are used to separate all these processes, which means the kitchen is separate from the master bathroom — there are dozens of walled-off rooms, all of which serve a different function in the cell.
For example, DNA is stored, replicated, and processed in the eukaryotic cell's nucleus, which is itself surrounded by a selectively permeable membrane. This protects the DNA and allows the cell to fine-tune the production of proteins necessary to do its job and keep the cell alive. Other key organelles include the mitochondria, which processes sugars to generate energy, the lysosome, which processes waste and the endoplasmic reticulum, which helps organize proteins for distribution around the cell. Prokaryotic cells have to do a lot of this same stuff, but they just don't have separate rooms to do it in. They're more of a two-bit operation in this sense.
"Many eukaryotic organisms are made up of multiple cell types, each containing the same set of DNA blueprints, but which perform different functions," says Shanle. "By separating the large DNA blueprints in the nucleus, certain parts of the blueprint can be utilized to create different cell types from the same set of instructions."