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In one of my fictitious planets, there are large cells that reside in the ocean and are 3 feet (almost 1 metre) in diameter. The cells themselves are aggregates of many amoeboid cells, have only one cell membrane, but many nuclei.

My question is: is such a large size for a single celled organism possible? If not, what modifications might I need to make to make it more plausible?

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as a reference: giant grape amoebas – vanillagod Feb 18 at 16:03
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Yay for a question with more +1s than the answer! – Caleb Woodman Feb 18 at 17:33
    
See also this answer. It covers some of the issues and gives examples of giant cells. – JDługosz Feb 18 at 18:19
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This is an awesome question... – James Feb 18 at 20:47
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I'm upvoting this question and Ray's answer just for introducing me to Acetabularia. – Whelkaholism Feb 19 at 11:49
up vote 36 down vote accepted

Many far-fetched things you try to come up with, nature did it first. They already exist macroscopic unicellular algae, although not so big, they are spectacularly big. Just have a look at these two.

As you can see from the links, they are very different from each other (in fact, one of them is mononuclear).

Valonia Ventricosa

This would be the easiest approach. Searching for differences between it and other more common, microscopic unicellular beings, I found out that thay are almost identical. So just:

  • Make it bigger.
  • Multiply the cell components, simply put more of them.

And that would be pretty much it. According to the description in the question, I think this is the approach you were looking for. Just a big unicellular blob (Warning! TV Tropes link).

Acetabularia

This other approach is more complex. In fact, even if it is unicellular and mononuclear, it behaves more like a plant, with a plant-like stalk and root-like rhizoid. With the difference that it has a very big nucleum in the base.


Another one thanks to Nathaniel:

Xenophyophore

There is this other one which looks more like a sponge. It lives at the sea bottom, just like the other two, but in deeper waters. It feeds like the amoebas. From the wikipedia link:

These giant protozoans seem to feed in a manner similar to amoebas, enveloping food items with a foot-like structure called a pseudopodium.

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Can you add a little more to your answer from the linked articles? Otherwise this is really just a comment. – bowlturner Feb 18 at 16:16
    
@bowlturner I saw the question and couldn't keep myself from answering, I'll extend when I arrive home. – Ray O'Kalahjan Feb 18 at 16:18
    
@bowlturner Done, thanks. I hope that's enough explanation. – Ray O'Kalahjan Feb 18 at 18:05
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@jpmc26 What is so wrong about TVTropes? – Kelly Thomas Feb 19 at 14:03
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See also en.wikipedia.org/wiki/Xenophyophore , which can grow even bigger. I didn't know about Valonia Ventricosa or Acetabularia - the photos of them are mind-blowing. – Nathaniel Feb 20 at 8:34

No.

Reason 1: Surface area to volume ratio

The volume increases much faster than the surface area of the cell. Cells are small for a very good reason, they need to diffuse nutrients into and waste out of the cell surface for all of the internal volume.

The volume inside your cell would require a lot of energy and produce a lot of waste, much more than the relatively small surface area could hope to transport in and out.

As cited in this paper:

Another possible factor limiting cell growth is shown in Fig. 8: the rates at which a protein can diffuse across it. Eqs. 11 and 13 show that every factor of 10 increase in linear size of a cell leads to a factor of 100 increase in the time required for proteins to diffuse across the cell’s length.

Emphasis mine

Reason 2: Protection

A cell wall is not a very strong boundary. It's literally a bilayer of phospholipids relatively loosely held together. You can't make such a sheet of phospholipids arbitrarily large and not run into problems. It wants to form smaller surface areas (think of water droplets and why three-meter ones don't spontaneously form). If you try to make this cell membrane much more durable and thicker you run into even more problems regarding Reason 1.

Just take Nature's advice and make a multicellular organism. It's far more efficient.


How to solve it.

If you must have it be one cell then it simply can not be a sphere. It can be 3 feet in one dimension, but must have many deep folds and thin areas that increase the surface area to a reasonable ratio.

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THANK YOU for bringing up the surface-area-to-volume ratio problem. – Schwern Feb 18 at 19:40
    
Also consider Laplace's law. The membrane must grow twice as fast in thickness as compared to the radius to keep the tensile force constant. That, or the cell will burst. Of course a very thick membrane is kind of pointless if survival is a goal. – Damon Feb 20 at 23:35
    
The surface area to volume issue is only present if the cell increases size in three dimensions while retaining the same shape. A three foot diameter pancake-shaped cell or a ball of filaments wouldn't run into the same problem. – ckersch Mar 21 at 18:03

Slime molds form plasmodia:

A plasmodium is an amoeboid, multinucleate and naked mass of cytoplasm that contains many diploid nuclei. The resulting structure, a coenocyte, is created by many nuclear divisions without the process of cytokinesis which in other organisms pulls newly-divided cells apart.

(Wikipedia)

Hemitrichia serpula, from Wikipedia

They are already macroscopic, so it would not be much of a stretch to grow them to three feet.

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Yes, but it could be argued otherwise rather easily...

A single cell of such a size is, in my belief, totally possible, but not necessarily likely. To tart, we would have to consider the various parts of a cell and decide if at such a size they wouldn't be organelles any more, but rather organs.

  • The cell membrane is made of fat and protein, which at a certain size must? be controlled/contained in the form of cells.
  • The centrosome, or microtubule organizing center, has a bunch of tubules, which at their size are just nicely arranged biological particles, like all else. But at a certain size, according to the previous premise, it is hard to control or maintain a shape or desired form without structure.
  • On that note, we have the cell's skeletal structure, called the cytoskeleton. This piece, perhaps not an organelle, holds up the desired structure of your cell so that it doesn't get squished, punctured, or otherwise maimed or non-useful. At a size of a small animal skeleton, it is extremely arguable whether it could exist.
  • The cytoplasm, the jelly-like interior of the cell, could very easily be recreated, no doubt about that, using any feasible semi-liquid, including real jelly, for funsies.
  • On the other extreme, we have the mitochondrion. these are pill/bean shaped "power plants" which convert glucose into ATP(Adenosine Triphosphate), providing the whole cell with energy. It consists of multiple folds and projections, and even at its size, it very nearly pushes the boundaries of a basic organelle. At a three-foot proportional size, it would almost have to be made of a series of cells, or made by them, because synthesizing something that complex requires a whole different biological level of creation.
  • Or say for argument's sake that you are making a three-foot plant cell. Bring out the chloroplasts! The way that these are made to collect sunlight and output energy is complex and deliberately micro-scaled, the reason you see them in large quantities, and at the same size, regardless of the cell, for the most part.
  • And other organelles, such as the nucleus(brain), golgi apparatus and lysosomes are pretty much the same; possible, but not using the same materials as we see in a "typical" cell.

My conclusion

From my fairly large amount of cell knowledge, I would have to say that yes, you could, with a tiny amount of magic and some improvised materials. But personally, I would just have everybody be microscopic, making cells look big, and maybe... just maybe introducing a whole new problem.

Cell Anatomy (Enchanted Learning)

Cytoskeleton (Wikipedia)

Chloroplasts (Nature.com)

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I think a plant cell makes it easier. The arbitrarily thick cell wall solves the structural integrity problem. – Joshua Feb 18 at 19:16
    
Very true, @Joshua. But not entirely. Integrity of jelly-floating pieces, even if contained in a box, is still a big deal, and hard(er) to solve. – Caleb Woodman Feb 18 at 19:20

Egg yolks are single cells, and ostrich eggs are pretty big.

There was a news story about giant fossilized dinosaur eggs found in Chechnya a few years back which where up to 3 feet wide. There is some doubt whether they were really eggs (especially since the largest verified dino egg is football sized*), and no one is guessing what laid them if they are actually eggs, but it is interesting.

So giant cells aren't unheard of here on earth. I don't see any reason why one couldn't develop in an alien sea if conditions were right.

* I don't know if they are talking about American gridiron football or European soccer football, but since it was found in France...

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Yes, but life on this planet wouldn't be very complex

What you describe is a large, unicellular organism, that is buoyant, slow moving, and lives in a nutrient rich environment whose other participants are similar if not smaller. This could also be fungal in nature

Options:

  • An amoeba like cell with an ovaloid disposition
  • A fungal entity, such as a mold or yeast, technically a Coenocytic hyphae
  • A branch like cell that is 3 feet in one dimension, but thin in the others, and relies on length to reach that measure. Neurons and nerve calls would be the analogous but smaller version.

Keep in mind, none of these will be fast, or particularly energetic, almost certainly passive, most likely fragile, your planet may need a lower or higher gravity, and an abundant solvent to act as suspension, almost certainly in liquid form with plenty of depth. These things will need to be near the surface for light. If they're to float they must be buoyant. Otherwise they will need to sit on the floor of the sea which limits depth, and they're unlikely to be mobile as a result.

Your environment will need a suitable energy source, most likely the sun, and your oceans can't be too violent as these organisms will not be hardy. Anything like fish or animal life will be an impediment to them existing at all.

Precedents:

  • Multinucleate cells are not uncommon, for example, human muscle tissue, fungi, slime moulds, etc
  • Human nerve cells can reach 1m in length
  • Giant grape amoebas and slimes, and other moulds
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The largest single-cell organism is probably the Xenophyophore which has been known to grow up to 20cm (roughly 8 inches). From the outside they don't look like single cells because they build a hard shell around themselves by gluing sediment (basically fine sand) to form coral-like structures.

The size you want is only around 4½ times bigger so it's not too much of a stretch.

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Maybe.

Giant cells actually exist. They occur when many cells fuse together, often to fight an infection. They typically reach a maximum size of 120 micrometers - nowhere near your size - but I suggest that it might be possible in an extreme case.

Potential reasons for the formation of a giant cell of bacteria:

  • A disease that threatens all bacteria that live on their own.
  • Cells fuse together in part of an embryo in response to something, and when the embryo is born, part of it is fused together.

These are pretty far-fetched, but I wouldn't write them off entirely as impossible.

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