1
$\begingroup$

I am trying to clarify my understanding of how unicellular organisms are alive, given my existing background (computational modelling, unrelated to biology). During my thoughts on subject I have came to a conclusion that any unicellular organism, given it's undamaged, and has enough resources (so it does not undergo autolysis), must be always alive.

Is my conclusion true? In other words, is it possible to have completely undamaged cell that appears healthy, but yet is dead? And by "dead cell" I mean a cell that does not move, does not consume any resources, does not respond to any external stimulus an alive cell would respond to, does not divide, etc.

$\endgroup$
  • $\begingroup$ Would a frozen cell meet your definition? $\endgroup$ – arboviral Nov 9 '16 at 15:55
  • 3
    $\begingroup$ Your definition of "dead cell" seems to include dormant cells and spores, which, technically, are not dead organisms. So you might want to reconsider your definition: cell death is something irreversible, while dormancy and sporulate state are not. Anyway the notion of "healthy dead cell" seems a bit oxymoric... $\endgroup$ – Flo Nov 9 '16 at 16:32
  • 1
    $\begingroup$ @toriningen, Flo makes a very good point. Assuming you are committed to your definition of "dead cell", changing "dead" to "quiescent" would be advisable. healthy =/= dead $\endgroup$ – Michael_A Nov 9 '16 at 23:17
4
$\begingroup$

There is a great bit on the podcast "The Infinite Monkey Cage" wherein Brian Cox recurrently asks "When is a strawberry dead?"

If you think about it, it is a difficult question: when a berry is growing on a plant, it is easy to say it is living because the cells are actively dividing and functioning. But when the berry reaches maturity, the autosomal cells no longer divide. This is called Programmed Cell Death or apoptosis. This is a physiological process. However, one could still argue the strawberry is alive in the sense that the seeds are still viable and can actively produce new strawberry plants. These cells are quiescent. What though, if a strawberry is crushed to smithereens or a disease infects all of the cells? In this case the "death" of the cell has ceased function as in apoptosis, but this is externally mediated. This type of non-programmed cell dysfunction is called necrosis.

So, in direct answer to your question, you must taken into account the mechanism (internal or external) for cessation of function and the permanency. Internal and permanent = apoptosis. Internal and temporary = quiescent. External and permanent = necrosis. External and temporary = ...maybe injury?

To be clear, I've used a multicellular plant as an example (because it is easier to relate to than yeast), but the principles are the same for unicellular organisms.

@Flo made the good suggestion to add references for those interested:

Buttner et al. (2006) Why yeast cells can undergo apoptosis: death in times of peace, love, and war. Journal of Cell Biology

Gordeeva et al. (2004) Apoptpsis in unicellular organisms: mechanisms and evolution. Biochemistry

$\endgroup$
  • $\begingroup$ OP was mentioning unicellular organisms. I'm not sure they are concerned by apoptosis. Do you have a reference for apoptosis among unicellular organisms? $\endgroup$ – Flo Nov 10 '16 at 7:50
  • $\begingroup$ @Flo Yes, it is still defined as apoptosis in unicellular organisms. I've added a reference and a note for clarity. $\endgroup$ – et is Nov 10 '16 at 12:33
0
$\begingroup$

Dead means either at physically decaying stage or at least no longer able to react whatever the future changes of its environment (contrarily to dormancy). To be in such state, an apparently healty cell would need to have had his epigenetic "program" stuck in some dead-lock bug (I don't know if this exist). I wouldn't say that this states his then an "healty" state.

Now maybe what you do seek for is dormancy state. But since it's waiting for some environmental signal, you could assimilate this to "lacking of a resource".

$\endgroup$
  • $\begingroup$ What do you mean by "epigenetic 'program'"? Why would such a phenomenon need to be epigenetic rather than genetic? $\endgroup$ – et is Nov 17 '16 at 23:57
  • $\begingroup$ Because the whole expression mechanism is involved. (comprising how DNA locations are packed or exposed to transcription. + the various expression factors). $\endgroup$ – Fabrice NEYRET Nov 18 '16 at 9:11
  • $\begingroup$ I can imagine a scenario, as you've proposed, in which a genetic mutation creates downstream effects that disallowed a cell to react to a environmental stimuli, while still preserving pathways for all other homeostatic functions (so, essentially "healthy"). In this case, the issue would be strictly genetic and digital, and not epigenetic. $\endgroup$ – et is Nov 18 '16 at 21:22
  • $\begingroup$ Then if it is maintaining homeostasy, what would you call it "dead" ? $\endgroup$ – Fabrice NEYRET Nov 19 '16 at 11:55
  • $\begingroup$ I only said it would have all mechanisms for homeostatic function, not necessarily in homeostasis, nor actively maintaining homeostasis. In this case I would call it quiescent. See my answer above in which I propose four, more exact alternatives to "dead." $\endgroup$ – et is Nov 21 '16 at 4:42

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.