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Why is it said that bacteria have no membrane-bound organelles, when they often have one or more flagella?

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3386205/ "Cilia and flagella .. are membrane-bounded organelles with unique membrane,"

Also, there is a biological motor (I don't know whether it's considered part of a flagellum or considered to be outside the flagellum, driving it?), but the motor surely has parts, e.g. a flagellum motor -like some mechanical motors, has a part, a "stator". And surely these parts have membranes.

I understand there's one bacterium discovered, that does have a membrane-bound organelle mentioned here "There is at least one species of bacteria that has a clearly membrane-bound structure inside it called an acidocalcisome (I think that's how it's spelled). This organelle was.. discovered.. this is obviously a minor (albeit important for that bacterium) exception to the rule...that prokaryotes lack membrane-bound organelles."

Note- Alcamo's Fundamentals of Microbiology: Body Systems p124 this link from google books mentions other cases of membrane-bound organelles in bacteria including a discover as early as 1994. here

But putting aside the bacteria that have been discovered to contain membrane-bound organelles and going back to before those discoveries.. The texts that describe bacteria as having a flagellum, say that they don't have membrane-bound organelles. Why is this?

I suppose an answer might be that when people say that a bacterium contains no membrane-bound organelles, they mean except for any flagellum that it has. They're referring to the main body of the bacterium, like one might refer to the torso of a human.

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  • $\begingroup$ Let us continue this discussion in chat. $\endgroup$ – barlop Jan 4 '17 at 11:15
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    $\begingroup$ Explanation of my edits to the question. 1. Some words in biology are of Latin origin and if they end in -um in the singular generally form their plural with -a. Hence bacterium / bacteria, flagellum / flagella. 2. When a noun such as membrane is being used as part of a compound adjective such as 'membrane-bound' (describing organelle) it is hyphenated to avoid initial confusion when reading. This may appear pedantic, but clear communication of ideas is important in science, and if nobody points this out, people remain ignorant. $\endgroup$ – David Jan 4 '17 at 12:32
  • $\begingroup$ This question is unclear. Your question is underpinned by a question like"what are the differences between bacterial and eukaryotic flagella?". The literal question being asked "The texts that describe bacteria as having a flagellum, say that they don't have membrane-bound organelles. Why is this?" is subjective since each text may have different reasons and qualifiers. $\endgroup$ – James Jan 6 '17 at 3:33
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    $\begingroup$ The question boils down to 'what is an organelle'. I remember a heated yet interesting discussion in the past in the chat room about this topic. I think the lead was this question. But it's closed. Perhaps OP ('you') can edit the question to make it more clear. I'll vote to close but I'm happy to re-vote to re-open it again once edited. $\endgroup$ – AliceD Jan 6 '17 at 13:19
  • $\begingroup$ @AliceD the question you link to shows no research whatsoever. My question is far more specific and researched and is asking about a very specific contradiction. It is possible to write a general question what is an organelle and to research it well, but there would be other questions within that. My question is very specific and asks about a specific contradiction $\endgroup$ – barlop Jan 9 '17 at 23:11
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Prokaryotic flagellum is very different from eukaryotic flagellum: it's not surrounded by a biomembrane (as in eukaryotes) and its composition is distinct. Actually, they only have in common the name and the function:

The term flagella is ambiguous. It refers to bacterial structures composed of flagellin protein and to eukaryotic structures composed of microtubule proteins.

Source: https://www.ncbi.nlm.nih.gov/pubmed/7378551

It's clear that prokaryotic flagellum is just a protein-made structure that crosses the membrane and grows outside it, as represented in this image (notice the TEM at the bottom right):

enter image description here

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  • $\begingroup$ though that picture is not a cross-sectional view, and this wikipedia page on the flagellum, showing a cross-sectional view, identifies this one as being with bacteria i.e. prokaryotic i.imgur.com/JZVXEbc.png and it has a part labelled as stator and it looks quite elaborate. Also it says "The depicted type of flagellum is found in bacteria such as E. coli and Salmonella, and rotates like a propeller when the bacterium swims. " and "The bacterial flagellum is driven by a rotary engine (Mot complex) made up of protein, " $\endgroup$ – barlop Jan 4 '17 at 11:03
  • $\begingroup$ 1. The picture I shared is a cross section. 2. What does all this has to do with the answer? $\endgroup$ – user24284 Jan 4 '17 at 15:36
  • $\begingroup$ well, do you think the picture I linked to in my imgur link in my comment is a prokaryotic flagellum? if so, it seems to have parts, and do you not think those parts have membranes? $\endgroup$ – barlop Jan 4 '17 at 16:02
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    $\begingroup$ That's the plasma membrane! Bacteria lack endombranes, or membrane-bound organelles. That's the definition of prokaryotic cell, it's quite simple. $\endgroup$ – user24284 Jan 4 '17 at 16:07
  • $\begingroup$ Let us continue this discussion in chat. $\endgroup$ – barlop Jan 4 '17 at 16:15
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First, this is of some relevance..

https://www.karger.com/Book/Home/258738 says "The traditional view of biology divides living organisms into two major groups, the eukaryotes and the prokaryotes, the former having membrane-bound organelles, the latter lacking them. However, recent research has revealed that this view is blatantly in error. " (so that is a link confirming one of the things the question said).

Though, a knowledgeable biologist I spoke to said while indeed, that above mentioned distinction between prokaryotes and eukaryotes, is mistaken, there are still distinctions, a genetic distinction(nowadays the distinction would be genetic), and also observable distinction that prokaryotes don't have a nucleus and eukaryotes do. So the terms prokaryotes and eukaryotes are still relevant and not in error.

It may be that texts that say "membrane bound organelles" may mean "membrane bound organelles within the cytoplasm" But it seems even so, as I mention below, a bacteria's flagellum doesn't have a membrane.

Now, to elaborate in answer to the question itself -

I have spoken to a biophysicist familiar with microbiology, that seems pretty knowledgeable.. and he gave an answer that went straight to the point.

He said that the bacterial flagellum has no membrane, and none of its intricate internal components have any membranes.

As to what holds them together(given that it's not a membrane) - all the flagella stuff is protein, held together with protein-protein interactions, the same stuff that holds all protein complexes together, such as salt bridges, hydrophobic effect, etc.

Also the question mentions this

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3386205/ "Cilia and flagella .. are membrane-bounded organelles with unique membrane,"

It appears the above statement is wrong or misleading, see

https://www.ncbi.nlm.nih.gov/pubmed/7378551

"The term flagella is ambiguous. It refers to bacterial structures composed of flagellin protein and to eukaryotic structures composed of microtubule proteins and ATPase (tubulin and dynein). The fact that cilia are nearly identical to eukaryotic flagella and have nothing in common with prokaryotic flagella is not apparent from the terminology. It is proposed that the 30-year old suggestion of Smagina and reiterated by Kuznicki and others, be adopted: that cilia and eukaryotic flagella be called "undulipodia." The term flagella ought to be restricted to prokaryotic organelles, bacterial flagella and spirochaete axial filaments: solid structures composed of flagellin which protrude through the plasma membrane and lack intrinsic motility throughout their length. Undulipodia are defined as intrinsically motile intracellular structures showing a 9-fold symmetry in the pattern of arrangement of 24 nm diameter microtubules. They are limited to eukaryotes, members of the protoctist, animal and plant kingdoms."

The knowledgable biologist I spoke to then said that the distinction between eukaryotes and prokaryotes nowadays is done mainly by genetics. Nevertheless, (as mentioned), an observable distinction between eukaryotes and prokaryotes is that eukaryotes have a nucelus and prokaryotes do not have a nucleus. Furthermore, if any prokaryote was ever discovered to have a nucelus, then it'd require a reorganization of phylogenetic trees. Also, while phylogenetic trees use the term eukaryote, they don't tend to use the term prokaryote, though the term prokaryote is still relevant, and there is an organization called the ICSP(International Committee on Systematics of Prokaryotes), that deal with the naming of groups of prokaryotes.

Also, while it's not entirely clear to me if there was at any time a shift from observable characteristics, to genetic characteristics, in determining the difference between eukaryotes and prokaryotes, this page mentions something about the origin of the terms prokaryote and eukaryote. And I understand from the knowledgable biologist I spoke to that nowadays the distinction is genetic or mainly genetic. http://www.madsci.org/posts/archives/2000-04/956011333.Gb.r.html

added

I have spoken to another microbiologist. He thought that the term prokaryotes is not useful, it doesn't have an evolutionary basis.. And at least the term eukaryotes, cells with a nucleus, the cells may be quite related.. Whereas prokaryotes - cells without a nucleus - is defining something partly by what it's not, and it doesn't group organisms phylogenetically/evolutionarily. Prokaryote refers to bacteria and archaea, both generally unicellular. But archaea are evolutionarily and biochemically more similar to us(eukaryotes) than they are to bacteria. There's no valid reason to lump them with bacteria. Not evolutionarily or biologically. The most recent development is that eukaryotes likely evolved from archaea. Some newer models show the eukarya lineage as rooted in the archaean lineage. All of them show that they're more close than either is to bacteria though That's universally agreed Phylogenetically and also cytologically. The three domains of life are Bacteria, archaea, and eukarya. Eukaryotes still make sense as a grouping as they are a group of organisms more similar to themselves than to bacteria or archaea. And indeed they are identified genetically.

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  • $\begingroup$ There are some interesting topics here for discussion, particularly in the discussion with the microbiologist on how prokaryotes is often an unhelpful term for describing evolutionary distance. The key point is ...the bacterial flagellum has no membrane, and none of its intricate internal components have any membranes. This is already in @GerardoFurtado's succinct answer. $\endgroup$ – James Jan 31 '17 at 9:38
  • $\begingroup$ @James I notice Gerardo wrote "It's clear that prokaryotic flagellum is just a protein-made structure that crosses the membrane and grows outside it, " So what is a membrane made of then that distinguishes it from just protein, is a membrane..protein+fatty acid? Or is the definition more abstract like is it something that surrounds something, so e.g. in a sense the body has a membrane that is skin? $\endgroup$ – barlop Oct 11 '17 at 16:13

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