Prokaryotic cells are microscopic in size. They’re typically measured in micrometers.

One micrometer is equal to one millionth of a meter. Most prokaryotic cells are in the range of 1 to 10 micrometers in length or width. Tens of thousands of average-sized prokaryotes could fit in one layer on the period at the end of this sentence.

One of the most obvious features of a prokaryotic cell is its overall shape. Most prokaryotic cells fall into one of three categories: Ball-shaped cells are called cocci; singular, coccus. Rod-shaped cells are called bacilli; singular, bacillus. And cells with a twisting, helical shape are called spirilla, singular, spirillum. A few species have cell shapes that don’t quite fit into these categories. Cell shape is a morphological trait that helps taxonomists identify a bacterial species.

The defining characteristic of prokaryotes is that they lack a membrane-enclosed nucleus. The single chromosome, usually circular, is tightly wound and compact. The region of the cytoplasm containing the chromosome is called the nucleoid. Other smaller pieces of DNA, called plasmids, may also be present in the cytoplasm. Each plasmid contains only a few genes.

Prokaryotes lack the membrane-enclosed organelles of eukaryotes. There are no mitochondria, no chloroplasts, no endoplasmic reticulum, no Golgi apparatus. Many of the functions of these organelles take place at the prokaryotic cell’s plasma membrane. A few prokaryotic species do have infoldings of the plasma membrane, where certain biochemical reactions occur.

Prokaryotes have nothing corresponding to the eukaryotic cytoskeleton. However, almost all prokaryotes have additional layers around the plasma membrane that give their cells structural support and provide additional protection against unfavorable conditions in their environment.

Most prokaryotes have a distinctive cell wall surrounding the plasma membrane. The cell wall is somewhat stiff, and partially permeable. The cell walls of eubacteria are made of a polymer called peptidoglycan, which is composed of amino acids and sugars, and forms a meshlike network around the cell. Eubacteria can be divided into two categories using the Gram stain, a technique developed by the Danish physician Hans Christian Gram in the late nineteenth century. This staining procedure discriminates between cells based on properties of their cell walls.

To stain the bacteria, we treat a sample with the dye crystal violet, then with iodine.

Then we rinse the cells with alcohol. Finally, we use a pink counterstain to see the cells that didn’t retain the violet dye. Eubacteria that retain the violet dye are called Gram-positive. They are characterized by cell walls with a thick layer of peptidoglycan. Eubacteria that do not retain the violet dye are called Gram-negative. These cells have a much thinner peptidoglycan wall, and a second, outer, lipid bilayer membrane!

Now see if you can match a description of cell wall type to the result of a Gram staining procedure. Drag the correct diagram of the outer layers of a eubacterial cell next to the image of the Gram-stained sample. Click Submit to check your answer. Click Jump Ahead to skip this step.

Sorry, that’s not correct.

That’s correct!

The violet cells are Gram-positive: they have a thick peptidoglycan wall but no outer membrane. The pink cells didn’t retain the violet stain, so they’re Gram-negative. They have a thin peptidoglycan wall and an outer membrane.

On top of cell walls and outer membranes, many prokaryotes secrete sticky substances to form a capsule around the cell. The capsule is an extra layer of protection, and it also helps prokaryotic cells stick to surfaces and to each other.

Now that we’ve seen the basic structure of the prokaryotic cell, let’s explore some of the lifestyles and activities of prokaryotes.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education