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What is a Unicellular Organism?

Michael Anissimov
Updated May 21, 2024
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A unicellular organism is any life form that consists of just a single cell. This group includes most life on Earth, with bacteria serving as the majority. The main groups of single celled life are bacteria, archaea (both prokaryotes), and the eukaryota (eukaryotes). The differences between the prokaryota and eukaryota are significant: eukaryotes possess a nucleus, while prokaryotes lack it, and eukaryotes possess a range of subcellular organs called organelles, while prokaryotes are very minimal.

People can observe the larger unicellular organisms, such as amoebae, by using the higher settings on a light microscope. Bacteria are so small that they just appear as dots under such magnification. To gather them for observation, a person can place a cover slip on the surface of pond water, and leave it overnight. By the next morning, many organisms will have grown entire colonies on the bottom of the slip. They replicate fast: colonies can double their size in between 30 minutes and a few hours.

Unicellular organisms as diverse as they are ubiquitous. The oldest forms of life, they existed 3.8 billion years ago, if not longer. They pursue a variety of strategies for survival: photosynthesis (cyanobacteria), chemotrophy (many archaea), and heterotrophy (amoeba). Some have flagella, little tails they use for locomotion, or lobopods, extensions of the cellular skeleton (cytoskeleton), which appear as bloblike arms. The flagella of the unicellular ancestors of humans is retained all the way up into the animals, where it makes an appearance as flagellated sperm.

Of all the six eukaryote supergroups, four are exclusively composed of single cell organisms. Only the opisthokonts, consisting of animals, fungi, and close relatives, and the archaeplastids, consisting of both unicellular and multicellular plants, are exceptions. These organisms vary in size, with the smallest bacteria measuring 300 nanometers across, ranging up to the titantic plasmodial slime molds, which can grow to 8 inches (20 cm) across. The largest may have millions of nuclei scattered throughout the cellular envelope. To observe some of the smallest requires an expensive electron microscope, while the very largest can be seen with the naked eye.

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Michael Anissimov
By Michael Anissimov , Writer
Michael Anissimov is a dedicated All The Science contributor and brings his expertise in paleontology, physics, biology, astronomy, chemistry, and futurism to his articles. An avid blogger, Michael is deeply passionate about stem cell research, regenerative medicine, and life extension therapies. His professional experience includes work with the Methuselah Foundation, Singularity Institute for Artificial Intelligence, and Lifeboat Foundation, further showcasing his commitment to scientific advancement.

Discussion Comments

By anon994432 — On Feb 08, 2016

Can anyone help me? What do unicellular organisms look like?

By bear78 — On Dec 17, 2012

@anon72078-- They have many different functions. For example, the bacteria in our gut helps with digestion.

By candyquilt — On Dec 16, 2012

@turquoise-- What you described is correct. Many unicellular organisms reproduce through asexual reproduction. They do this by replicating their DNA and splitting it into two. The process of this split is called "binary fission."

Some unicellular organisms don't use fission, but rather budding. The difference is that budding produces a smaller cell rather than one that is equal in size. Unicellular organisms that reproduce are called a mother cell and the smaller one is called the daughter.

I hope this helps. You should also check out unicellular organism fission pictures. You can see clearly how they divide. It is cool.

By turquoise — On Dec 15, 2012

How do unicellular organisms multiply exactly?

I know that bacteria sort of split into two and as each new bacteria also splits into two, they quickly go from one bacteria to two, to four, to eight, to sixteen and so forth.

I think this is really cool. I want to know how they do this.

By anon149786 — On Feb 05, 2011

this website is great and so helpful. i found all my info about cells here and i posted it on my project and got an A. I thank this website a lot!

By anon122257 — On Oct 27, 2010

amazing website, helps me finish my biology assignments.

By anon112097 — On Sep 19, 2010

the article is so good. i got it on the first read, easy in terms. good one!

By anon101397 — On Aug 03, 2010

This site is so great.I found my reports here, thanks to this site. It helped me a lot.

By anon91640 — On Jun 23, 2010

This is really great because it is helping me finish my assessment tasks and it's all about this stuff.

By anon90973 — On Jun 19, 2010

nice article, but is there any info for multicellular organisms? i need it for some lame project my teacher asked us to do.

By anon89242 — On Jun 09, 2010

I like this.

By anon72078 — On Mar 21, 2010

what is a unicellular organism's responsibility? By the way, the article is good.

By anon53335 — On Nov 20, 2009

this article is a very good article. it gives a lot of information about unicellulars and stuff.Please enjoy what I said. Thank you!

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By anon52222 — On Nov 12, 2009

good article

By anon49453 — On Oct 20, 2009

was not the numulith the largest unicellular organism?

By anon48678 — On Oct 14, 2009

thank you for all your help. it has given me a lot of information.

By anon46430 — On Sep 25, 2009

i had to study this for a weird science writing assignment. i guess it was helpful.

By anon45326 — On Sep 15, 2009

what is the unique part of a unicellular organism?

By anon42439 — On Aug 21, 2009

Could you give more information?

By anon38564 — On Jul 27, 2009

i like this stuff. i have a science project in my school. it is about unicellular organisms. i'd like to know more about this.

By anon35535 — On Jul 06, 2009

nice article, Brief and precise. Manoj Bhatt

By anon24698 — On Jan 16, 2009

This article is great!

Michael Anissimov

Michael Anissimov


Michael Anissimov is a dedicated All The Science contributor and brings his expertise in paleontology, physics, biology...
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