Unicellular vs multicellular

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pwnell
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Unicellular vs multicellular

Post by pwnell »

I cannot seem to find a proper explanation on what unicellular means in the context of an unicellular organism.

In specific, does unicellular refer to the fact that the organism only has one cell instance, or does it refer to an organism that may have one or more cells but the cells are the same kind (type) of cell?

rjlittlefield
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Post by rjlittlefield »

As written in Wikipedia,
A unicellular organism, also known as a single-celled organism, is an organism that consists of only one cell, unlike a multicellular organism that consists of multiple cells.
...
Prokaryotes, most Protista, and some fungi, are unicellular. Although some of these organisms live in colonies, they are still unicellular. These organisms live together, and each cell in the colony is the same. However, each individual cell must carry out all life processes to survive. In contrast, even the simplest multicellular organisms have cells that depend on each other to survive.
Reading between the lines of this description, I see an implicit definition that "unicellular and colonial" would refer to organisms that are capable of surviving as individuals but are usually found in colonies, where "multicellular" could be used to describe a form that is only viable in collections of two or more cells even if those cells are undifferentiated.

However, that last situation is so much of a gray zone that I would not trust any one-word description to be correctly interpreted by most readers.

At the moment I cannot think of any such beast, though I would be surprised if they do not exist. Maybe some bacteria in their biofilm-producing state, though that seems like stretching the concept of "organism".

--Rik

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Post by rjlittlefield »

Come to think of it, I'm curious about what prompts the question.

What type of beast are you thinking of?

--Rik

pwnell
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Post by pwnell »

Hi Rik,

Thanks for your response. I was thinking of Euglenoids in specific, which are supposed to be unicellular if I am correct. Since I am not a biology major I was wondering how they can have the complex structure but be unicellular? Does that mean the whole organism is one cell?

Maybe I need to read up on the definition of a cell as I always thought of them as tiny structures that build up an organism - much like atoms are the building blocks of molecules and molecules of higher structures.

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Post by rjlittlefield »

pwnell wrote:I was thinking of Euglenoids in specific, which are supposed to be unicellular if I am correct. Since I am not a biology major I was wondering how they can have the complex structure but be unicellular? Does that mean the whole organism is one cell?
Ah! Sorry, my mistake -- I was thinking of a question that was at quite a different level from yours.

Yes, in Euglenoids the entire organism is one cell.

You raise an interesting question about just what a "cell" is. One way of thinking about it is that there's always some kind of barrier between the organism and its environment. Whatever that barrier is, if it goes around the organism but does not pass through it so as to split the organism into two or more regions, then the organism is unicellular. If the barrier does split the organism into two or more regions, then it's multicellular, and the barrier is called the "cell wall". But the cell wall is just sort of like the wall around a medieval city -- it restricts what can go in and out of the cell, but there's still a huge amount of structure and subdivisions inside the cell, just like there would be buildings and closets and cupboards inside the city.

For some introduction to this, take a few minutes to watch say http://www.youtube.com/watch?v=1Z9pqST72is.

The interesting thing about pond water is how much overlap there is between the unicellular and multicellular organisms. A large stentor or amoeba might be 1 mm in length, and it's unicellular because its barrier doesn't divide it. Meanwhile a small rotifer might be 0.1 mm, but it's multicellular because there are internal divisions that are substantially the same as what separates at least the rotifer's gut from the water of its environment. (The exterior of the rotifer is likely made of some tougher stuff, so the definitions get hard to put in a few words.)

I hope this helps!

--Rik

pwnell
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Post by pwnell »

Well that is exactly what I was looking for. Since I am an engineer, I tend to be very exact with my interpretations of what is written. A single cell and a single cell type can easily be interchanged in the casual written language.

But your explanation is perfect - it explained exactly what I needed to know. A very stupid and basic question, I know, but I learn quickly!

Thanks.

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Post by Pau »

pwnell wrote: A very stupid and basic question...
Not stupid at all, this is an important question on Biology. In many cases the difference is obvious (a man vs a free living bacterium) but in others like filamentous and colonial algae (Volvox is a nice example in between both categories for example), slime moulds and even sponges the situation is not so clear.

Protists and in special Ciliates are really interesting because their awesome cellular complexity, they have so big repertory of organelles outside the typical cell components shown in typical textbooks. They took a different evolutionary pathway than animals: complexity with large complex cells with specialized organelles vs complexity by means of many specialized cells.
Pau

pwnell
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Post by pwnell »

Pau wrote:Protists and in special Ciliates are really interesting because their awesome cellular complexity, they have so big repertory of organelles outside the typical cell components shown in typical textbooks. They took a different evolutionary pathway than animals: complexity with large complex cells with specialized organelles vs complexity by means of many specialized cells.
I guess it is just my bad luck that by taking photos of protists (which are very common), I start my biology education with the outlier cases! Fascinating and I hope to learn lots more.

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Post by rjlittlefield »

but in others... the situation is not so clear.
Indeed! I recall in one of my college biology courses being surprised to hear that a particular alga was unicellular. The reason for my surprise was that at the time I was familiar with the beast, and I knew that it came in large flat sheets about the size of my hand. When I asked for clarification, the instructor explained that the thing is just missing the internal divisions. It has gobs of nuclei all over the place, but nothing to obviously separate one region from another. When I asked what happened when the structure got torn (imagining that the insides would all leak out), he replied that's no problem, it just builds a new wall to patch over the break. Might be handy even, if the torn off bit happens to land in a spot where it can keep growing.

--Rik

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Post by Ken Ramos »

I found slime mold plasmodia to be good models for cellular study. I once had one that I kept in a petri dish for about a month or so, named “Slick” and it attained a size of about 30mm while in captivity. Plasmodia are interesting to study because for one they can grow to enormous size and the protoplasmic amoeboid flow is easily observed with a 10x hand lens or a good dissecting microscope. They are coenocytic or multinucleate, trophic and quite motile and as Rik mentioned of the alga, if the cell wall is broken or the plasmodia becomes torn apart, the individual pieces repair themselves and continue to grow. There are two types of slime mold cells here, acellular and cellular slime molds, it is the former that I refer to and if you are a bio-mathematician the cellular molds are really fascinating for those who understand such things and if I am not mistaken a number of computer programs are based on cellular slime mold behavior.

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