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Textbook: Biology- Exploring Life
Chapter 6: A Tour of the Cell.
There are two basic kinds of cells. One kind- a prokaryotic cell- lacks an nucleus and
most other organelles. Bacteria and another group of organisms called the archaea are
prokaryotic cells. Prokaryotic organisms appear earliest in Earth’s fossil record. In
contrast, a eukaryotic cell has a nucleus and other internal organelles bounded by
membranes. Protists, fungi, plants, and animals consist of eukaryotic cells. Organisms
with eukaryotic cells appeared later in Earth’s history.
The major difference between these two main classes of cells is indicated by their names.
The word eukaryotic is from the Greek eu meaning “true” and karyon meaning “kernel”.
The kernel refers to the nucleus that eukaryotic cells have and prokaryotic cells lack.
Referring to Figure 6-6, the nucleus is the largest organelle. You can see many other
types of organelles outside the nucleus, surrounded by membranes of their own.
The smaller cell in the micrograph is a bacterium, an example of a prokaryotic cell.
Without a true nucleus and the organelles of eukaryotic cells, prokaryotic cells are much
simpler in structure. The DNA in a prokaryotic cell is concentrated in an area called the
nucleoid region, which is not separated from the rest of the cell by a membrane, as is the
case in a eukaryotic ell. Also, note the difference in size between these two cells. Most
bacteria are 1 to 10 micrometers in diameter, whereas eukaryotic ells are typical 10 to
100 micrometers in diameter.