Amoeba Definition & Meaning

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You’ve probably heard of amoebas but, without more research, you wouldn’t know what they are and what they do. Despite being so small, these single-celled organisms are very important for most living things on the planet, from animals to algae. So, what is an Amoeba?

Here we have a rundown of what amoebas are, how they are named, what they do, and why learning about them is important for anybody with a passing interest in biology, microbiology, and the environment.

The Definition Of Amoeba

Let’s start with the basics, like the definition of amoeba and what they are on a fundamental, molecular level. As we said, amoebas are unicellular organisms, so they are made from just one cell.

The word ‘amoeba’ comes from the old Latin word for ‘change’ or ‘alternating.’ This makes sense when you consider that amoebae are often shapeshifters, or at the very least can drastically change their shape since they lack a skeletal or muscular structure. Like the words ‘formula’ or ‘larva’, the plural form of amoeba is often amoebae. However, ‘amoebas’ is accepted as a common pluralization of the word.

Along with being unicellular, amoebas are also eukaryotic. Organisms are divided into eukaryotes and prokaryotes. Prokaryotes are simple single-celled organisms like bacteria while eukaryotes are either multicellular organisms or sophisticated unicellular organisms, like an amoeba. Yes, this means that human beings are eukaryotes too!

Most living beings are eukaryotes and, as one of the most fundamental living beings on the planet, amoebas are too. A lot of free-living and parasitic single-celled organisms, including amoeba, are combined under the loose, informal group referred to as protozoa.

The Sarcodina

Historically, amoebae have been categorized in a much different way. After the discovery of amoeboid organisms in 1755, the words used to describe them changed over the next century before settling on the word amoeba.

In 1841, the term ‘sarcode’ was first used to describe the protoplasmic structures of protozoa, but it came to represent the whole cell. From there, the phylum group Sarcodea was established, then changed to a taxon group – Sarcodina.

This changed in the late 1900s when it was confirmed that Sarcodina was not a monophyletic group. In simpler terms, they didn’t share the same common ancestors and features that the group was built around.

This meant that the discovered amoeba types needed to be divided into other taxonomic groups, of course. The main two supergroups full of amoeba and other eukaryotes are Amoebozoa and Rhizaria, with others being found in very broad groups like Stramenopile, Opisthokonts, and Excavata. Others exist in a league entire on their own, like Centroheliozoa, for example.

Amoeba Anatomy

They may be a single cell but amoebas are eukaryotes for a reason – because that cell is much more complex than the ones you’ll find in the average germ.

Cell Structure

Like most cells, the center is comprised of a nucleus. This is a compartment that contains a lot of a cell’s genetic data. Around that nucleus is a set of special structures – organelles – which are very versatile in what they can do.

Organelles are important cellular constructs that can transport proteins around the cell and produce energy so that the amoeba can keep moving. You may already know one type of organelle that’s found in nearly all cells, including yours – mitochondria. However, not all amoebas have mitochondria, some have mitosomes or hydrogenosomes instead. Also inside the cells are vacuoles. These are found in many cells and especially water-bound ones, where the vacuoles let water in and then expel it.

Then, like all other cells, amoebas have a membrane that combines its contents into a neat package and protects it from penetration by micro-debris. Some amoebas, called testate amoebas then have a shell made from chitin, calcium, or silica. Then other amoebae, called naked or gymnamoebae, don’t have this relatively hard covering.

Amoebas look exactly like the individual cells found in multicellular organisms when viewed from a distance. That’s why they’re thrown into our category as a eukaryote instead of being prokaryotes like the other single-celled organisms.

Pseudopodia

Amoebas even move like our white blood cells, through a process called pseudopodia. For the etymology fans out there, pseudopodia are the Latin word for ‘false feet’ which is a reference to the small growths that amoebas create. These are protrusions from the cytoplasm, which is part of the cell’s membrane and exists in a semifluid state. This helps it move and change shape at will, so the amoeba sends out protrusions that grip a surface and pulls the cell along as the back parts of the amoeba contract.

This facilitates movement, as adhesions at the front of the cell drag it forward. This is slow by our standards but, when you’re an amoeba, your world is a lot smaller and it’s a valid way to get around. The movement of an amoeba is important because it helps us distinguish amoebas from similar but simpler eukaryotic organisms like protists.

We can even differentiate between different forms of pseudopodia, which is important when identifying certain amoebas. There are four that we know of:

Lobopodia – shorter, blunter ‘leg’ protrusions.
Filopodia – very thin ‘leg’ protrusions.
Axopodia – uses axonemes, tiny tubular structures.
Rhizopodia – uses reticulopodia, filamentous structures.

Amoeba Diet

The pseudopodia of the amoeba aren’t just used for movement, they can also consume material. When you’re the size of an amoeba, there’s not much room for separate parts to eat with. That’s why they eat using their pseudopodia protrusions instead.

There are so many amoebae out there that their diet varies. Generally, they can consume smaller, lesser single-celled organisms like bacteria. Upon contact with the amoeba and its pseudopodia, the bacterium cell becomes bonded and then absorbed into the amoeba cell. Imprisoned within the amoeba, the bacterium cell’s genetic material is used by the amoeba.

Some soil-dwelling amoeba can ingest both liquids and solids, through processes called pinocytosis and phagocytosis. The solid food sources can be algae, worms, nematodes, and other forms of small protozoa. Amoebas are omnivorous, so they don’t care if they are consuming animal or plant matter. They can also be predatory or parasitic, predating on or living inside larger organisms.

Amoeba Sizes

The sizes of amoebae can vary drastically. At their smallest, amoeboid cells like Massisteria voersi are around 3 micrometers in diameter. There are certain bacteria cells that are that size. Meanwhile, neutrophils (our white blood cells) are approximately 12 to 15 micrometers, for size comparison.

Then there are larger species like Syringammina fragilissima, which make up the shells of deep-sea unicellular organisms called Xenophyophorea and measure in at around 200,000 micrometers – 20 centimeters. While that may sound exciting, they aren’t very interesting to look at when compared to spying on other amoebae from your microscope.

Why Amoebas Are Important

For human beings, amoebae are important because they’re most often responsible for diseases and other ailments. For example, amoebas from a certain species (E. histolytica) can cause amoebic dysentery in humans by occupying the colon wall and causing inflammation after disturbing the colon lining. These are, of course, parasitic amoebas. Parasitic amoebas can even make their way into the brain, though this is rare and virtually impossible to get unless you’re taking contaminated water up the nasal cavity.

When they aren’t causing diseases in humans and other organisms, they are thriving in soil and stagnant water all around us. Most of these amoeba types are beneficial to their environments, forming the molecular bedrock of the local ecosystem. As part of a soil food chain, they prey on bacteria and other small cells and then get preyed upon by nematodes and similar beings. Then those nematodes get eaten by insects, giving way to birds and mammals, and then us. Every food web begins at a microscopic level and, in those environments, you’ll find amoebae.

So, amoebae are vitally important in many environments, like many single-celled organisms. Where amoebae live in soil or stagnant water, they do a good job of clearing harmful bacteria and ensuring they don’t reproduce too much. Without amoebas, humans and other mammals would catch more diseases from surfaces in the wild.

Popular Amoeba Examples

If it isn’t clear enough already, amoebas cover a wide range of sizes, functions, and features. There are many amoebas out there, with many Here are three popular examples of amoebae that interact with human beings most often.

Entamoeba histolytica

We’ve already mentioned this one above, so we won’t get into too much detail. As said, it’s a parasitic amoeba that causes colitis if it makes its way into the human body. While small (varying between 15 and 60 micrometers) it’s estimated to have been responsible for as many as 50 million deaths worldwide. There can also be asymptomatic carriers who spread it without suffering any of the illnesses associated with it.

Amoeba proteus

The Amoeba proteus is a larger amoeba species that are closely related and resembles the Chaos genus and is sometimes called Chaos diffluens because of that. In 1755, when amoebas were first discovered by German naturalist August Johann Rosel von Rosenhof, it was probably the Amoeba proteus species. They measure in at approximately 220 to 760 micrometers.

Just like the old scientists of the 1700s and 1800s, modern labs and classrooms cultivate Amoeba proteus to study. Its accessible size and safety for humans make it a great candidate for studying amoebae.

Naegleria fowleri

Among amoeba and specifically parasitic amoeba, the Naegleria fowleri has a reputation. A cell doesn’t become known as the brain-eating amoeba for no reason, after all. While it identifies more with the Excavata group over true amoebas, this is a single-celled microorganism that can threaten the brain if it gets to it. As for how small they are, they span 8 to 15 micrometers.

As we mentioned earlier, this often happens when contaminated water from the wild (usually warm water) makes its way through your nasal cavity. Cases of the resulting and most often fatal infection, naegleriasis, have been documented worldwide from the USA to Pakistan. The latest US infections were identified in just 2021.

Conclusion

With those examples covered, you should now understand amoebas a lot better than you did a few minutes ago!

You haven’t just got the definition of amoeba and how it has changed over time – we have also covered how amoeba cells are structured, how they ‘eat’, how large they can be, and why they are important for human study despite being such a small part of our world.

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