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stevo247

What is "alive"?

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What I mean is that we have a sense for what is alive, but whenever we try to put a definition to it, it excludes things that we considered alive. What is the difference between the first living cell and its parent? Where does life begin? When you get to the beginning of life (or the end of life, as in the case of viruses), you can't draw a definite line separating life from "almost-life".

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The NASA definition of life: "Life is a self-sustained chemical system capable of undergoing Darwinian evolution."

 

Does 'self-sustained' imply that that the lifeform actively, or passively consumes energy/matter to live? Is there anything that science isn't sure whether to classify as alive or not alive?

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Isn't the actual classification of a virus still disputed among biologists?

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Isn't the actual classification of a virus still disputed among biologists?

 

Somewhat, but that is because they don't all agree on the definition of life. Viruses have no or almost no metabolism, and no catabolism. This restricts their ability to reproduce such that they can only reproduce with the help of (living, or at least mostly intact) cells. So there are many aspects to viruses that don't conform to the usual definition of life.

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I was thinking about the difference between a corpse and a living body. The most notable difference, before putrefaction, is the absence of motion.

 

I wonder if spontaneous motion is a fundamental quality of being alive?

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Uh. good question... actually, on one hand, it makes sense -- motion means there is energy. No motion - no energy left... so, it sounds okay. But I think it's good to note that "external" motions (or lack thereof) shouldn't be an example -- there are many times that people don't move at all while still being perfectly alive. From sleeping to certain types of brain damage..

 

But what you're saying makes sense if you consider "inner" movements as well as external ones.

 

I've heard once that this tendency to declare someone dead by external look (and crude devices) was one of the things that led to the creation of zombie myths in south american and some african countries..

 

Looks can be decieving, but your point seems to have merit if you look at the *entire* body (inside and out).

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What is the difference between the first living cell and its parent? Where does life begin? When you get to the beginning of life (or the end of life, as in the case of viruses), you can't draw a definite line separating life from "almost-life".

 

If you go the protein first route of abiogenesis, then there is a definite line. It is when the proteins form a cell. That cell is alive. Now, if you look at the RNA World hypothesis, there is no definite line.

 

I was thinking about the difference between a corpse and a living body. The most notable difference, before putrefaction, is the absence of motion.

 

I wonder if spontaneous motion is a fundamental quality of being alive?

 

That's a tough one. Remember, the "corpse and a living body" refers to a multicellular organism. Even when you have a corpse, not all the cells in that corpse are dead. For humans, the criteria for being dead is lack of electrical activity in the brain. Let's face it, lack of motion is not necessarily dead for people. You can have no gross movemnt of the body, the heart stopped, and breathing stopped and the person is still "alive" in that you can do CPR and all the motion is restored.

 

Now, at the molecular level, yes, living things all have molecules that are in motion. BUT, so do non-living things. If you make motion a "fundamental quality", then fire is alive. So are rocks, because the atoms in the crystals vibrate (move)!

 

Remember, we want something that all living things have but all non-living things don't have. So you have to test your criteria not only against living things to see if they have them, but look to see if non-living things also have them.

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That would fall under the common requirement "response to stimuli".

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Remember, we want something that all living things have but all non-living things don't have. So you have to test your criteria not only against living things to see if they have them, but look to see if non-living things also have them.

 

Trouble with this is that we would have to distinguish living from non-living a priori. Which is the basic problem I have when it comes to the definition of life.

 

Edit, I missed that one:

Viruses are a little bit special. Most people consider viruses as going from non-life to life. However' date=' the reality is backwards to that. Viruses are going from life to non-life. That is, viruses started out as bacterial parasites and have evolved to get rid of a lot of non-esssential functions. Non-essential for an obligate parasite, that is.

[/quote']

 

Actually this is not that clear. I think I read somewhere a paper stating what you wrote, though I cannot remember it anymore. However, it was one of the speculations of viral origins. The issue has, to my knowledge, not been resolved yet.

In addition to the two theories mentioned above:

1) viruses started off as organism and were reduced to viruses and

2) viruses predate cellular life;

3) there is the notionthat viruses originated from fragments of genetic material (which ist most commonly preferred by molecular geneticists as it so neatly fits into the selfish gene view)

 

However all hypotheses have their weaknesses:

1) all other reduced intracellular parasites from cells retained cellular character and no intermediates between cells and viruses are known;

2) all viruses require a cellular host and

3) no example are put forward how genetic material can acquire a protein capsid, also many viral proteins lack homologues in one of the domains (which of course can be a result of a lack of data)

 

However, other hypotheses circumvent the problems of the above hypotheses by assuming that viruses might arose around, or before LUCA, either in a RNA or RNA/DNA transition world either by reduction of RNA proto-cells, or by splitting off from proto-cells.

However, as the specifics of the RNA/DNA world are shrouded in speculations I think that placing the elusive origins of viruses there only adds to the mystery and just obfuscates contradicting data.

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Remember, we want something that all living things have but all non-living things don't have. So you have to test your criteria not only against living things to see if they have them, but look to see if non-living things also have them.

 

That would fall under the common requirement "response to stimuli".

 

I agree. And I think that spontaneous movement is an essential feature of the stimulus-response activity.

 

But I wonder if this now implies the need for some sort of nervous system to detect external conditions. What is a stimulus in relation to a response? Is it a sensation?

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That would fall under the common requirement "response to stimuli".

 

It's time to discuss the terms "necessary" and "sufficient". When looking at characteristics you have a list of characteristics that are "necessary" for life, but the total list has to be "sufficient". That is, "necessary" means that you can't leave it off the list but "sufficient" means that the list is complete, you don't need anything else.

 

In this case "movement" is necessary but not sufficient. My problem was that "movement" was proposed as "sufficient".

 

Trouble with this is that we would have to distinguish living from non-living a priori. Which is the basic problem I have when it comes to the definition of life.

 

That is done for us by nature. We observe that some entities are very different from others (living from non-living). Now comes the tasks of listing the necessary conditions that distinguish life from non-life and then deciding when that list is sufficient. When we have that list, we have a "definition". And we can test that list since each entry on the list is a hypothesis: response to stimuli is a necessary and sufficient condition for something to be alive. We test that against observations living and non-living things. Thus we come to the conclusion that response to stimuli is necessary but not sufficient.

 

In addition to the two theories mentioned above:

1) viruses started off as organism and were reduced to viruses and

2) viruses predate cellular life;

3) there is the notionthat viruses originated from fragments of genetic material (which ist most commonly preferred by molecular geneticists as it so neatly fits into the selfish gene view)

 

The first 2 are the theories mentioned above. The first is that viruses are the ultimate parasite and evolved from parasites as unnecessary components were eliminated by natural selection.

 

However all hypotheses have their weaknesses:

1) all other reduced intracellular parasites from cells retained cellular character and no intermediates between cells and viruses are known;

2) all viruses require a cellular host and

3) no example are put forward how genetic material can acquire a protein capsid, also many viral proteins lack homologues in one of the domains (which of course can be a result of a lack of data)

 

You can view mitochondria and chloroplasts as intermediates. In both cases, many of the necessary components are coded by the nucleus and made in the cytoplasm.

 

However, other hypotheses circumvent the problems of the above hypotheses by assuming that viruses might arose around, or before LUCA, either in a RNA or RNA/DNA transition world either by reduction of RNA proto-cells, or by splitting off from proto-cells.

 

Do you have a citation for this reduction hypothesis?

 

How does that work? After all, you still have the same problem you noted in #3 above: how could they aquire protein making ability in an RNA world?

 

And I think that spontaneous movement is an essential feature of the stimulus-response activity.

 

Actually, "spontaneous" movement is irrelevant to a stimulus-response. And the response does not always have to be "movement" as in the "organism moves"

 

But I wonder if this now implies the need for some sort of nervous system to detect external conditions. What is a stimulus in relation to a response? Is it a sensation?

 

This is where the protocells are so cool. When stimulated either by "prodding" or changes in their chemical environment, they have an action potential identical to that of a neuron!

 

Przybylski AT. Excitable cell made of thermal proteinoids. Biosystems 1985;17(4):281-288.

Vaughan G, Przybylski AT, Fox SW. Thermal proteinoids as excitability-inducing materials. Biosystems. 1987;20(3):219-23.

Ishima Y, Przybylski AT, Fox SW. Electrical membrane phenomena in spherules from proteinoid and lecithin. Biosystems. 1981;13(4):243-51.

 

So, single cells can act as a "nervous system". Steve, if you limit life to "movement" as in the whole organism and requiring a nervous system, you are going to eliminate all single celled life.

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Originally Posted by stevo247

And I think that spontaneous movement is an essential feature of the stimulus-response activity.

 

Actually, "spontaneous" movement is irrelevant to a stimulus-response. And the response does not always have to be "movement" as in the "organism moves"

 

How can there be a response without something moving? When something responds, doesn't something move?

 

But I wonder if this now implies the need for some sort of nervous system to detect external conditions. What is a stimulus in relation to a response? Is it a sensation?

 

This is where the protocells are so cool. When stimulated either by "prodding" or changes in their chemical environment, they have an action potential identical to that of a neuron! So, single cells can act as a "nervous system". Steve, if you limit life to "movement" as in the whole organism and requiring a nervous system, you are going to eliminate all single celled life.

 

I was actually thinking about an amoeba when I wrote what I did. That's why I said "some sort of" nervous system. As far as I know, an amoeba does not have an organized nervous system, yet it clearly responds to stimuli, with movement.

 

Do you have a good link for learning about protocells. Interesting!

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Originally Posted by stevo247

Originally Posted by lucaspa

Actually, "spontaneous" movement is irrelevant to a stimulus-response. And the response does not always have to be "movement" as in the "organism moves"

 

How can there be a response without something moving? When something responds, doesn't something move?

 

Stevo, we are back to "necessary" and "sufficient". Unless you are at absolute zero, "something" is always moving. Atoms vibrate, molecules move in brownian motion, etc. So saying "something move" doesn't help you. The water in a stream moves; is the stream alive? If you change the gradient, the water doesn't move as fast. Is that "response to stimuli"? It involves motion.

 

Yes, in a response, something must move. But that, by itself, won't give you life. In an action potential, the organism does not move. Instead, there is movement in the proteins making up the cell membrane such that Na ions move into the cell and K ions move out.

 

As far as I know, an amoeba does not have an organized nervous system, yet it clearly responds to stimuli, with movement.

 

Which means the amoeba does not have "some sort of nervous system". You can't use the phrase to mean "anything that detects stimulus". "Nervous system" means specialized cells -- neurons.

 

Do you have a good link for learning about protocells. Interesting!

 

Start here:

http://www.theharbinger.org/articles/rel_sci/fox.html

http://www.siu.edu/~protocell/

 

Then you are going to have to move to old-fashioned hardcopy. :)

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I'd say that another attribute of life is to have positive and negative feedback loops, which make their response to stimuli more "interesting" than that of a non-living thing.

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Stevo, we are back to "necessary" and "sufficient". Unless you are at absolute zero, "something" is always moving. Atoms vibrate, molecules move in brownian motion, etc. So saying "something move" doesn't help you. The water in a stream moves; is the stream alive? If you change the gradient, the water doesn't move as fast. Is that "response to stimuli"? It involves motion.

 

I'd say that another attribute of life is to have positive and negative feedback loops, which make their response to stimuli more "interesting" than that of a non-living thing.

 

Perhaps the nature of the movement we are talking about is different than say a hammer hitting a nail (i.e. a mechanical reaction).

 

The nature of “a response” implies some sort of functional, co-ordinated, organized, and puposeful manner of movement, relating to the integrity of the organism.

 

Yes, in a response, something must move. But that, by itself, won't give you life. In an action potential, the organism does not move. Instead, there is movement in the proteins making up the cell membrane such that Na ions move into the cell and K ions move out.

 

I would say that the movement of an action potential, in and of itself, does not constitute “a response”. It appears to me, to be part of the “stimulus package”, until a functional response is initiated, which would involve an organized, co-ordinated, purposeful movement.

 

 

I was actually thinking about an amoeba when I wrote what I did. That's why I said "some sort of" nervous system. As far as I know, an amoeba does not have an organized nervous system, yet it clearly responds to stimuli, with movement.

 

 

Which means the amoeba does not have "some sort of nervous system". You can't use the phrase to mean "anything that detects stimulus". "Nervous system" means specialized cells -- neurons.

 

I would think that the amoeba’s ability to sense the environment, detect stimulus, and respond accordingly, is considered to be, at least, a primitive representation of a nervous system. How that is done without neurons, I have no idea. But however it is done, I would say that it is laying the groundwork for the development of a nervous system.

 

There appears to be a significant qualitative difference between the movement associated with a mechanical reaction and an organized, functional, co-ordinated, purposeful response.

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Originally Posted by Mr Skeptic

I'd say that another attribute of life is to have positive and negative feedback loops, which make their response to stimuli more "interesting" than that of a non-living thing.

 

Perhaps the nature of the movement we are talking about is different than say a hammer hitting a nail (i.e. a mechanical reaction).

 

If this is so, then you need to make a new criteria for being alive than simple "movement", don't you? Which is what we have been saying all along. :)

 

Now, positive and negative feedback loops are not necessarily tied to "response to stimuli". Many of these are in metabolism and simply have the product interfere with the production -- negative feedback. That isn't really "response to stimuli" but an internal control to keep the cell from producing too much of something.

 

The nature of “a response” implies some sort of functional, co-ordinated, or puposeful manner of movement, relating to the integrity of the organism. I would say that the movement of an action potential, in and of itself, does not constitute “a response”. It appears to me, to be part of the “stimulus package”, until a functional response is initiated, which would involve a co-ordinated, purposeful movement.

 

How do you mean "movement"? Are you talking about movement of the entire organism? Part of it? You seem to, since you dismiss the action potential itself as being a "response".

 

Yet the action potential is a very specific response to a very specific set of stimuli. Only a very small set of stimuli elicit an action potential. You seem to want that action potential to connect to a muscle so that some part of the organisms "moves" in space. That, of course, requires a multicellular organism.

 

So how would you say that a unicellular organism was alive? Would you require the entire organism to move in space to a new location? How about just adjust it's cellular shape? How about a pancreas cell secreting more insulin in response to an increased extracellular concentration of insulin? No "movement" of the pancreatic cell or change in cell shape, but how could you deny that this is a response to a stimulus?

 

All in all, I think you are being too restrictive with your criteria of movement for "response to stimuli". IOW, you are eliminating too many recognized and accepted responses to stimuli.

 

I would think that the amoeba’s ability to sense the environment, detect stimulus, and respond accordingly, is considered to be, at least, a primitive representation of a nervous system.

 

It's not. A "nervous system" is defined to require specialized cells to to these tasks. What the amoeba has isn't even a "precursor" to a nervous system because the amoeba is unicellular. You are trying to force your ideas on to nature instead of looking at nature and changing your ideas accordingly.

 

What the amoeba has is "response to stimuli". Having proteins in the cell membrane that change shape when bound by the ligand. These cell surface receptors are attached to actin. When the receptor changes shape, this pulls on the actin molecules which in turn pull on the other end of the actin, causing the amoeba to change shape and move.

 

"Nervous system" is used to describe a situation where cells transmit signals from one cell to another by depolarizing the neural cell.

 

Here is amoeba movement: http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=cooper.section.1790#1804

 

Scroll down the page until you get to "Cell Crawling".

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Perhaps the nature of the movement we are talking about is different than say a hammer hitting a nail (i.e. a mechanical reaction).

 

If this is so, then you need to make a new criteria for being alive than simple "movement", don't you? Which is what we have been saying all along.

 

It appears to me, that living things have a very particular “way of moving”. If I was to reduce that manner of movement into its simplest form, it would have to be expansion and contraction, or pulsation. I don’t think non-living things pulsate.

 

The nature of “a response” implies some sort of functional, co-ordinated, or puposeful manner of movement, relating to the integrity of the organism. I would say that the movement of an action potential, in and of itself, does not constitute “a response”. It appears to me, to be part of the “stimulus package”, until a functional response is initiated, which would involve a co-ordinated, purposeful movement.

 

How do you mean "movement"? Are you talking about movement of the entire organism? Part of it? You seem to, since you dismiss the action potential itself as being a "response".

 

 

I think I was considering the nature of a reponse in terms of the whole organism. After giving it some thought, I can see how the stimulation of a sensory receptor could be considered a response. There could be a stimulus without activating a sensory receptor, i.e. no response.

 

Yet the action potential is a very specific response to a very specific set of stimuli. Only a very small set of stimuli elicit an action potential.

 

I now agree that the response mechanism is initiated when the action potential is achieved and then transmitted. Of course, I think its transmitted in a pulsatory manner, but I don’t know if thats ever been determined.

 

You seem to want that action potential to connect to a muscle so that some part of the organisms "moves" in space. That, of course, requires a multicellular organism.

 

Doesn’t the action potential eventually connect to something and do something? If it doesn’t, what’s the point? Input/output. Sense/respond.

 

So how would you say that a unicellular organism was alive? Would you require the entire organism to move in space to a new location?

 

I would say that it was alive by its ability to sense stimuli and respond accordingly.

 

How about a pancreas cell secreting more insulin in response to an increased extracellular concentration of insulin? No "movement" of the pancreatic cell or change in cell shape, but how could you deny that this is a response to a stimulus?

 

I’m not sure yet how to consider something like a pancreas cell. Its a single cell, but its part of an organ, that’s part of a larger functioning body, compared to an amoeba that exists as an independent functioning unit. However, I find it hard to believe that a pancreas cell functions without moving. Does it expand and contract?

 

 

I would think that the amoeba’s ability to sense the environment, detect stimulus, and respond accordingly, is considered to be, at least, a primitive representation of a nervous system.

 

It's not. A "nervous system" is defined to require specialized cells to to these tasks. What the amoeba has isn't even a "precursor" to a nervous system because the amoeba is unicellular. You are trying to force your ideas on to nature instead of looking at nature and changing your ideas accordingly.

 

What the amoeba has is "response to stimuli". Having proteins in the cell membrane that change shape when bound by the ligand. These cell surface receptors are attached to actin. When the receptor changes shape' date=' this pulls on the actin molecules which in turn pull on the other end of the actin, causing the amoeba to change shape and move.

 

"Nervous system" is used to describe a situation where cells transmit signals from one cell to another by depolarizing the neural cell.[/quote']

 

I would say that the process of sensing the environment, detecting stimulus, and responding accordingly, in a functional, co-ordinated, and puposeful manner of movement, relating to the integrity of the organism, is shared by an amoeba and by multicellular organisms, in a functionally identical manner.

 

There is clearly a functional relationship between the stimulus/response behavior of an amoeba and the nervous system of a multicellular organism:

 

http://www.unmc.edu/Physiology/Mann/mann_i.html

"However, we do have some idea of the development of nervous systems. The first organisms were very likely single cells like the amoeba. Because single-celled animals have all of the characteristics of matter, they must have conductivity and excitability. In this way, these first single-celled organisms were also the first nervous systems, doing many of the things more complex systems do."

 

If matter has coductivity and excitability, wouldn’t the “response” of a living organism involve something more than just conducting an action potential? Would non-living matter, demonstrating conductivity and excitability, be considered as responding? Even though the action potential appears to be a fundamental aspect of the response mechanism, it appears at least similar to the conductivity and excitability of non-living matter? What is it about an action potential that makes it a "response", compared to the conductivity and excitablity of non-living matter?

 

This is where the protocells are so cool. When stimulated either by "prodding" or changes in their chemical environment, they have an action potential identical to that of a neuron!

 

So, single cells can act as a "nervous system".

 

It looks like we may agree more than it would appear.

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If you look at the blood cells in the human body, which show ameboid motion, they are singles cells with the entire DNA of a human being. They are us, if we existed as a single cellular organism, capable of individual adaptation. Like most single cellular organisms, they have extra DNA they don't use.

 

This is interesting, because theoretically, if DNA manipulation was far more advanced, one could make an entire human out of one of these single cell critters. We could make a complete dog out its its own single cell version. It would be interesting if other single cell critters, using the same DNA manipulation technique, would become a new multicellular critter.

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You can define living things in thousand ways, but in the limit of "dead and living stuff" It becomes kind of a philosophycal problem. Jeje. So maybe in a philosophy forum youll get more interesting awnsers.

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Why would philosophy be able to give more interesting answers? It is scientists that are directly studying life and all the processes that life carries, and it science that provides the right answers.

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Why not?? It's not all about science, even if we believe it is.
Well because I don't think that only sitting and thinking about it will be enough to get a clear and goood explanation of what alive is.

 

And yes, it is all about science.

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If you are trying to come up with a scientific definition of life, then it is all about science. If you are trying to come up with a philosophical definition of life, then it is all about philosophy. It can be assumed that we are trying to get a scientific definition of life, as this discussion is on a website called scienceforums.net.

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It appears to me, that living things have a very particular “way of moving”. If I was to reduce that manner of movement into its simplest form, it would have to be expansion and contraction, or pulsation. I don’t think non-living things pulsate.

 

but the protocells don't even "pulsate" during the action potential! Yet they are responding to stimuli. The protocell does not "pulsate" in that the cell membrane would move (and neither do neurons). The cell membrane remains in the same place.

 

Back to pancreatic cells. They don't "pulsate" either, but they do respond to low glucose by secreting insulin. As to non-living things, oil droplets pulsate in response to the movement of the water molecules in which they are suspended.

 

Once again let me say this: "movement" isn't a good criteria for living things because 1) too many non-living things move and 2) some living things don't.

 

Doesn’t the action potential eventually connect to something and do something? If it doesn’t, what’s the point? Input/output. Sense/respond.

 

It doesn't have to. It's still a response of the cell: depolarize the cell membrane. In the peripheral nervous system, the action potential causes muscle cells to contract. But that is simply a use of the response, not the response itself.

 

I would say that it was alive by its ability to sense stimuli and respond accordingly.

 

Well, we are getting closer to "respond to stimuli". But be careful about that adverb "accordingly". That is making a judgement call based on a human idea of what is "accordingly". You don't want to do that.

 

Now, is "response to stimuli" sufficient to define a living thing? We can say that for something to be "alive" that it is necessary to respond to stimuli. But can non-living things respond to stimuli? Of course. Fire does, for instance. So do storms. In both cases movement of air causes the fire or storm to move in the direction of the wind. So both "respond to stimuli". But neither are alive.

 

So you need additional criteria that the thing must also have in order to be alive.

 

However, I find it hard to believe that a pancreas cell functions without moving. Does it expand and contract?

 

Nope. It just sits there surrounded (and held in place by) it's extracellular matrix. Let's take an even more radical example: an osteocyte. Osteocytes are the cells within bone. Completely surrounded by the bone (rock) and can't move. When stimulated by parathyroid hormone, they respond by taking calcium out of the bone and moving it from one osteocyte to another until it can be dumped in the blood.

 

I would say that the process of sensing the environment, detecting stimulus, and responding accordingly, in a functional, co-ordinated, and puposeful manner of movement, relating to the integrity of the organism, is shared by an amoeba and by multicellular organisms, in a functionally identical manner.

 

Sorry, but there are too many examples where "purposeful manner of movement" doesn't happen. Responses don't always involve movement. Also, remember that fire responds in "a functional, co-ordinated, and purposeful manner of movement" in a wind. Back to sufficient. "movement" in terms of actually moving the cell/organism is neither necessary nor sufficient to define life.

 

There is clearly a functional relationship between the stimulus/response behavior of an amoeba and the nervous system of a multicellular organism:

 

http://www.unmc.edu/Physiology/Mann/mann_i.html

"However, we do have some idea of the development of nervous systems. The first organisms were very likely single cells like the amoeba. Because single-celled animals have all of the characteristics of matter, they must have conductivity and excitability. In this way, these first single-celled organisms were also the first nervous systems, doing many of the things more complex systems do."

 

You needed to continue the quote and read this one more carefully. Notice it said "single cells like the amoeba". He wasn't talking about amoeba themselves. Notice he didn't demonstrate conductivity and excitability in the amoeba. Notice that the source I provided didn't talk about conductivity and excitability. If you continue the quote: "As animals became more complex, it became more efficient to differentiate cells into functional types. Different tissues appeared and, with them, nerve cells." If you go back up to the top of the page you see "Finally, a nervous system is the aggregate of all nerve cells within a single organism. It happened that, in the course of evolution, all but the simplest animals have resorted to the use of a nervous system of some sort to organize and carry out their behaviors."

 

Taking things out of context is not a good way to argue. It may "prove" your point, but what you want to do is look for truth, not prove your point. So, using the source you posted, I'll stand by the claim that nervous systems is an aggregate of all nerve cells. Amoeba do not have a nervous system although they can respond to stimuli.

 

If matter has coductivity and excitability,

 

Not all matter does, however. The graphite in your pencil has neither.

 

 

Even though the action potential appears to be a fundamental aspect of the response mechanism, it appears at least similar to the conductivity and excitability of non-living matter?

 

I would say "no". Conductivity in copper, for instance, means moving electrons from one atom to another. The "conductivity" and "excitability" of a nerve cells involves a disparity of atoms from one side of a membrane to another. Not at all similar. The only "simularity" is a movement of charge and that is too vague matter.

 

What is it about an action potential that makes it a "response", compared to the conductivity and excitablity of non-living matter?

 

Changes in the proteins in the cell membrane that open up channels in the membrane. non-living matter doesn't have either proteins or membranes.

 

It looks like we may agree more than it would appear.

 

Not really. Notice that I put "nervous system" in quotes to denote that it really isn't like one. Single cells that can have an action potential. But that doens't make them a nervous system.

 

You can define living things in thousand ways, but in the limit of "dead and living stuff" It becomes kind of a philosophycal problem. Jeje. So maybe in a philosophy forum youll get more interesting awnsers.

 

I agree with noz. You can't define living things in a thousand ways that fit the evidence. That's the difference between doing philosophy the way you do it and doing science.

 

Even in philosophy, the definition must fit the "evidence". In most cases, the "evidence" is the cases you which the definition to apply to and avoid the cases you don't want the definition to apply to. There are constraints even in philosophy.

 

But, of course, we are talking about biology here. How do we tell biologically "living" things from non-living things?

 

Everyone is making this much more complicated than it is. People have thought about the subject before. In order to be "alive" an entity must have all four of the following criteria:

1. Metabolism

2. Growth

3. Response to stimuli

4. Reproduction

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Back to pancreatic cells. They don't "pulsate" either, but they do respond to low glucose by secreting insulin.

 

How about a pancreas cell secreting more insulin in response to an increased extracellular concentration of insulin? No "movement" of the pancreatic cell or change in cell shape, but how could you deny that this is a response to a stimulus?

 

I find it hard to believe that a pancreas cell functions without moving. Does it expand and contract?

 

Nope. It just sits there surrounded (and held in place by) it's extracellular matrix.

 

 

This is from a paper titled "Insulin signalling through ultradian oscillations":

 

"Periodic oscillations appear to be a characteristic of insulin secretion at various different levels. Very rapid pulsations are seen in the isolated b-cell and islet, while rapid (10- to 15-min) pulsations are seen both in the intact organism and in the isolated pancreas."

 

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WG5-4G23KWF-4&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=d7f83ab25463c4e1ed9000ae3f1ae252

 

 

Let's take an even more radical example: an osteocyte. Osteocytes are the cells within bone. Completely surrounded by the bone (rock) and can't move.

 

 

This is from a paper titled "Osteocytes as multifunctional cells":

 

Osteocytes can move

 

"Evidence is accumulating that osteocytes are more active than previously known. Dallas and colleagues will show at this meeting that osteocyte cell body movement occurs within lacunae and that extension and retraction of dendrites can occur within canaliculi. These observations were made possible by the recent generation of transgenic mice with green fluorescent protein (GFP) expression targeted to osteocytes19 and with time-lapse dynamic imaging. Calvaria from these mice were used to image living osteocytes within their lacunae20. These studies have revealed that, far from being a static cell, the osteocyte is highly dynamic."

 

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1796957

 

It would appear that the “fact” of the immobility of the pancreas cell and the osteocyte is not etched in stone, and may in fact be altogether wrong.

 

As a layman, with almost no formal scientific education, it’s almost comical that I would even consider disputing these issues with someone of your educational backround and profession. I respect your opinions. If I have misunderstood these references or taken them out of context, please clarify them for me. I just think that pulsation is fundamental to living organisms.

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