Prokaryotic Cells

By the end of this lesson and the next, you should be able to:

• Name examples of prokaryotic and eukaryotic organisms
• Compare and contrast prokaryotic cells and eukaryotic cells
• Describe the relative sizes of different kinds of cells
• Explain why cells must be small

Introducing Prokaryotic Cells

Cells fall into one of two broad categories: prokaryotic and eukaryotic. Only the predominantly single-celled organisms of the domains Bacteria and Archaea are classified as prokaryotes (pro- = “before”; -kary- = “nucleus”). Cells of animals, plants, fungi, and protists are all eukaryotes (ceu- = “true”) and consist of eukaryotic cells.

Components of Prokaryotic Cells

All cells share four common components:

1. a plasma membrane, an outer covering that separates the cell’s interior from its surrounding environment;
2. cytoplasm, consisting of a jelly-like cytosol within the cell where you can find other cellular components;
3. DNA, the genetic material of the cell; and
4. ribosomes, which synthesize proteins.

However, prokaryotes differ from eukaryotic cells in several ways.

A prokaryote is a simple, mostly single-celled (unicellular) organism that lacks a nucleus, or any other membrane-bound organelle. We will shortly come to see that this is significantly different in eukaryotes. Prokaryotic DNA is found in a central part of the cell: the nucleoid (see image below).

This figure shows the generalized structure of a prokaryotic cell. All prokaryotes have chromosomal DNA localized in a nucleoid, ribosomes, a cell membrane, and a cell wall. The other structures shown are present in some, but not all, bacteria. Image Attribution: OpenStax Biology

Most prokaryotes have a peptidoglycan cell wall and many have a polysaccharide capsule (see image above). The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The capsule enables the cell to attach to surfaces in its environment. Some prokaryotes have flagella, pili, or fimbriae. Prokaryotes use flagella for locomotion. Pili are used to exchange genetic material during a type of reproduction called conjugation. Bacteria use fimbriae to attach to a host cell.