Because we’re really, really small.

How big can a cell get?

A discussion about life and its limits (and a list of super cool cells)

Greetings! Long time no science.

I, like many of you, have been thinking a lot about the definition of life. I’ve also been thinking about how life is usually discussed among scientists (including but not limited to biologists). Since NASA and SETI officially study the possibility of life on other planets, talks of what we could find in the universe have become more complex and interesting.

What is the definition of life?

Physicist Erwin Schrödinger, wrote a whole popular science book about the topic titled: What is life? The Physical Aspect of the Living Cell.

When many of these discussions happen, we tend to talk about alien languages, cultures, technology and mathematical prowess, as though these things are crucial to consider in during our search.

What’s most commonly seen as the resounding quality of life (at least life worth studying) is a ‘consciousness’ or ‘self-awareness’, a concept I can’t say I understand scientifically.

Of course, it’s very exciting to think that such alien cultures might exist, but there’s also a quieter, equally fascinating discussion happening that’s based more closely on our ‘working definition’ (more like criteria) of life:

Growth. Reproduction. Metabolism. Homeostasis. Response to Stimuli. Organisation. Evolution.

To me, all of these criteria point to one, slightly broader answer. Life is chemistry! But, when we do think about biology’s accepted definition of life, many ask: ‘according to who’ or ‘what if we find life that doesn’t meet the definition of life’. To this, I’ve always thought there was a simple answer.

According to biologists…

Now, there’s definitely no magical property of biologist humans that makes them the ultimate authority on life. But, it can be important to make the distinction between a meaningful term and a useful term, and what the inherent limitations of defining words are.

When scientists typically discuss life, they’re foremost concern is often not: “what is the definition of this word in your language”. They’re looking for something that helps to answer questions and solve problems.

The term ‘living’ might not be the most meaningful, as it doesn’t satisfy most peoples craving to define what we feel and how we act and what makes us (non-genetically) distinct from the rest of living things.

It does, however, allow biologists to specialize in a specific subject. All things are made out of matter: atoms, subatomic particles, etc. Some humans have made the observation that some matter clumps behave differently.

They make and break other molecules so they can grow bigger, they can make copies of themselves that are identical to them, they can contract and swim, and in some cases, they learn and spread information without the direct use of nucleic acids. We call those matter clumps life!

Pretty… Not comforting, huh?

Well, this is a pretty basic, microscopic definition of life. It might not be the most philosophical, and it definitely doesn’t form an all encompassing theory of life, but it does help us answer pretty cool questions about pretty cool matter clumps and share our discoveries with a common vocabulary.

Any difference between big life and small life?

Big things like humans and elephants tend to require more specialized conditions than small things like bacteria. As I hope you’ll see with our Microbe Profiles, extremophilic microbes can live in some really ‘hostile’ conditions, so even if a planet has a thin atmosphere of methane and sulfur (like Earth’s once was!) we might find some microbes there.

I feel that thinking of life in these terms also helps us challenge assumptions of what life typically looks like — either a small single cell or a larger group of cells. But on other planets, there may be matter clumps that adhere to this definition, but are incredibly distinct morphologically.

The imaginative side of me wondered: Are there any limits to life morphologically? Are all macroscopic organisms made up of multiple cells? What limits how large single cells can grow? Are the limitations based on Earth’s conditions alone? I had to look to science.

What limits the size of cells?

A lot of things serve as cell size regulators! On Earth, at least, cells are firstly limited by available resources that they need to grow — things like nutrients (however the organism feeds) and water. After taking up these resources, they then have to transport them around the cell which could be passive, or active (requiring energy).

Molecular pathways seem to regulate how large a cell grows before it divides based on scarcity. In multicellular organisms, cells have to receive signals from other cells to avoid overcrowding and some studies have identified promising signaling pathways. However, despite over 100 years of study, many mysteries still lie in the chemistry that makes cells ‘decide’ to grow.

Adding to this complexity is the fact that there are two distinct components cells have to deal with when deciding to grow: proliferation (population growth) and growth (individuals getting larger).

I did, however, find one interesting study that documents a relationship between temperature and cell size. Some species exposed to cold environments grow slowly but end up larger as adults.This phenonmena is described as Bergmann’s rule, which shows the correlation in natural habitats. The temperature-size rule describes a similar relationship in lab reactions.

How big can single celled organisms get?

Finally! Unfortunately, at this point, we can’t give an universal answer to this question that isn’t just speculation. I can, however, show you some of my favorite massive unicellular organisms (MUO’s?). By the way, I saved my favorite for last!

Number one: Stentor

The Trumpet Protozoan

Stentor is a genus of organisms that are actual giants of unicellular organisms growing up to 2mm long, completely visible to the naked eye. They are quite possible the most trumpet shaped protozoans on Earth, and tiny fragments of one individual can grow into full sized adults. (By tiny fragments, we mean 1/100th the size of the individual, so that’s pretty cool).

Número dos: Chaos carolinensis

Amoebas are weird. All creds go here!

In addition to the simple, dystopian vibes of this species genus, this amoeba displays some pretty impressive growth with sizes up to 5mm long. This species is kind of the superstar of the entire genus, but they’re all scavengers who lurk at the bottom of fresh water habitats.

Number three: Acetabularia

They look more like paper plates to me… This photo is by Isao Inouye . You can see more of their photos online.

The mermaid’s wineglass, as it’s eloquently known as, are a group of green algae that have exceptionally large nuclei. Small fragments of a species within this genus can fuse to fragments of another species and grow perfectly well. That makes the genus the topic of tons of studies on growth and the nucleus.

Five: Gromia sphaerica

There really should be better pictures of this… The microbe (macrobe?) is the lump!

This is probably my favorite species of amoeba. They are grape-sized, spherical protists that live on the sea floor and can grow up to 1.5 inches in diameter. They’re native to the Arabian sea and have the game-changing ability to make these grooved tracks at the ocean floor. Not only does our hipster microbe challenge assumptions, but because the tracks like these can fossilize and reform, scientists have started to rethink how they view fossil evidence. It’s possible that track fossils weren’t made by multicells after all!

SIX: Ventricaria ventricosa

GLORIOUS

AKA Bubble Algae, AKA sailors’ eyeball, AKA amazing, beautiful algae that lives in the ocean (that location seems to be a trend). Isn’t it just beautiful? They like tropical and subtropical areas but it’s suggested that they can be found, living in solitude among coral rubble, in virtually ever ocean in the world. The literature that details how a single cell manages to grow to such massive sizes just… doesn’t seem to exist. We do know that it has multiple nuclei and chloroplasts as well as defined cytoplasmic ‘domains’, although apparently, the domains can’t survive independently and aren’t considered single cells. Still, there’s definitely a degree of team effort and division of labor.

So how large can a cell get?

Sorry about that. Unfortunately, I couldn’t find anyone who has come up a shareable figure based on anything but speculation. But, who know. Maybe one of us will.


I hope you enjoyed this discussion piece! If you did, tap the heart to show microbes your love. Have a different perspective on life’s definition? Want to share some of your favorite massive organisms? I’d love to hear from you in the comments below.


Don’t take my word for it! View the full list of citations and additional resources here.


If you liked this science story, you might also like this post about the ecosystem of the skies.


Hi! I’m flagellate. When I’m not swimming in agar, or writing about my favorite microbes, you can find me making up stories about the end of the world, and helping science get funded.