Unicellular Organisms

By Olivia Zimmerman

A unicellular organism is an organism that consists of only one cell.

What Are The Types?

There are many different types of unicellular organisms. Historically the simple celled organisms have been called monads. The main groups of unicellular organisms are bacteria, archaea, protozoa, algae, and fungi.

Eukaryotes & Prokaryotes

All of those listed are separated into two general categories: prokaryotic and eukaryotic. Prokaryotes lack a nucleus and include bacteria and archaea. While Eukaryotes possess a nucleus which include some of the unicellular organisms. Eukaryotes also possess organelles which is where life sustaining activities take place.


Unicellular organisms come in many shapes such as spheres, oblong, or spiral. Single-celled organisms get food in different ways. In some organisms the cell membrane forms a sac, or vacuole, around the food. One of the most important substances that passes through the cell membrane is water, which provides key nutrients for the cell's survival. Some unicellular organisms are disease causing agents called pathogens. Pathogens are often found in water, frequently as a result of fecal matter from sewage discharges, leaking septic tanks, and runoff from animal feedlots. Unicellular organisms vary in size, with the smallest bacteria measuring only a third of a micron (300 nanometers) across, ranging up to the titanic plasmodial slime molds, which can grow to 20 cm (8 in) across. The largest unicellular organisms may have millions of nuclei scattered throughout the cellular envelope. To observe some of the smallest unicellular organisms requires an electron microscope, while the very largest can be seen with a microscope or anything that enhances images.


1. Cells are referred to as microorganism or microbes because they cannot be seen with out using a microscope.

2. Prokaryotic single-celled organisms were the first living things on earth.

3. Scientists have discovered giant amoebas the size of a grapefruit. (It is 20 cm in diameter.)

4.  There are 6 Eukaryotic "super groups" only 4 of which are single-celled.

An Amoeba On A Microscope Slide

I tried to contact Professors at the University of New England but I could not contact them. The sources I used were Wikipedia, www.science20.com, www.online.une.edu/science, and last I used www.averyschools.net.