Comparing Prokaryotic and Eukaryotic Cell Types
The first cells to appear on Earth were prokaryotic cells. A prokaryote is an organism
made of a single prokaryotic cell. The earliest prokaryotes may have arisen more than
2.5 billion years ago. Bacteria are the most common types of prokaryotes. They are very
small cells with a simple structure. Prokaryotes do not have a nucleus. This means that
their DNA is not enclosed in a membrane inside the cell. Instead, prokaryotes have a
single loop of DNA that floats in the cell’s cytoplasm. Protein-making bodies called
ribosomes also form part of the cytoplasm. Like all cells, prokaryotes have a cell
membrane. All prokaryotes also have a cell wall surrounding the cell membrane. The
cell wall helps provide support and protection for the cell. An additional layer encloses
some prokaryotes; this layer is called the capsule. The capsule has a sticky surface
area, so it allows prokaryotes to cling to surfaces, such as your skin and your teeth, and
pretty much any surface for that matter.
Eukaryotic cells are more complex than prokaryotic cells. They all have a cell
membrane, ribosomes, and DNA as prokaryotic cells do. However, the DNA of
eukaryotic cells does not float freely in the cytoplasm. Instead, it is found in the nucleus,
an internal compartment bound by a cell membrane. The nucleus is one kind of
organelle found in eukaryotic cells. Organelles are structures that perform specific
functions. Most organelles are surrounded by a membrane. Some organelles have
membranes that form channels, which help transport substances from one part of the
cell to another part of the cell.
Comparing Plant and Animal Cells
