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How do we define life?

  1. Oct 23, 2007 #1
    I read that DNA is where life begins and therefore Viruses are hard to fight b/c they are RNA. Anyways...how is this claim supported? Aren't we programmed in certain way? Including electrons which carry information. How come everything is not a life? Or better yet, how can anything be life?
  2. jcsd
  3. Oct 23, 2007 #2
    Life is seen (by us who have the property of being alive), as a thing that moves around (mostly) looking for food, reproduces (copies itself), and processes external stimuli in order to have a "map" of itself and its surroundings.
    The simplest concept of something "alive" is a thing that uses stored energy. It uses it to find more energy to store. This is an ongoing process, so its always a trade-off. Viruses don't carry around much energy except for the bit they use to get inside a cell or bacterium. Viruses co-opt other forms of life that follow the above general outline (they go around looking for more energy to store).
  4. Oct 24, 2007 #3

    jim mcnamara

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    There are several slightly differing definitions of life, depending on what the guy doing the writing is dealing with.

    For example, blue-green alage are considered alive by first definition definition above, but some folks would quibble about the 'stimulus' & 'moves' parts of the definition. And as Phred mentions there are things like viruses and prions that defy the first definition completely.

    You'll have to learn that in Biology there is no one perfect definition for most broad terms, like 'what is life', you have to pick one that suits what you are working with, then convey it to anyone reading your work.

    As long as you say 'this is my definition' and stick to it, then the most anyone else can say is:' such and such species/thing/gizmo is outside your definition'. What Phred & I showed with Phred's #1 definition. Notice we did not say it was wrong - just that it could be quibbled with: it didn't fit with viruses.
  5. Oct 25, 2007 #4
    You could say that having stored energy is a necessary condition, but it needs to use it both to store more, and (if it can), move around. Some things seem to be only like an arc of the "life" circle.
  6. Oct 25, 2007 #5

    jim mcnamara

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    Quibble: I'm not sure you could say that prions store energy.
    The [tex]PrP^{sc}[/tex] protein acts as a catalyst or maybe enzyme, to transform
    [tex]PrP^{c}[/tex] into [tex]PrP^{sc}[/tex]

    I'm not also sure that prions are life anyway.
  7. Oct 25, 2007 #6


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    As jim says - it depends on who you ask.
    Chemists talk about respiration and energy. Maths/CS people talk about inheriting characterstics. The definition about having DNA is a bit circular - if aliens turned up with TNA they would still be alaive.

    Not sure if prions/enzymes can pass-on favourable characterstics and so be considered alive.
  8. Oct 26, 2007 #7
    Aren't these opinions? What the factual theories in existence considering this issue?
  9. Oct 26, 2007 #8
    Fact 1. Life uses energy
    Fact 2. Life stores energy (storing energy requires energy)
    Fact 3. Life reproduces itself (this uses energy)
    Fact 4. Life maintains some 'map' of itself and its environment (this requires energy too)
    Fact 5. Some life is mobile. (guess what mobility requires)
  10. Oct 26, 2007 #9
    I would just like to modify the use of the word "mobile" into "responsive".
    Not all forms of life are mobile (except when they grow in size) but what is meant by the term mobile is that a living organism has to be responsive to external stimuli.
  11. Oct 26, 2007 #10

    jim mcnamara

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    Skhandelwal -

    What you are seeing is exactly what you were told. It is not opinion, although it looks kinda like we are just throwing stuff out there.

    Do you understand Math? If you know what the Axiom of Choice is, then you will understand what is going on here. There is a theorem - Goodman's Thoerem.

    Goodman proved that if the axiom of choice vx3yA(x, y) ->
    3fVxA(x, f(x)) is added to intuitionistic arithmetic (here
    x, y9 and / are functionals of finite type), then no new arithmetic
    theorems are obtained.

    This is basic Math. The idea is that accepting or rejecting certain Axioms gives you different results - or not. We are essentially defining biological postulates, not voicing opinions necessarily. Read about Koch's postulates which define what is required to demonstrate the existence of a pathogen.

    It is NOT opinion. It is the fact that it's diffcult to create a definition of life that always works because the huge number of wierd things we find over time. Nobody really knew about prions or even suspected them prior to 1975, for example. They bent the rules.
  12. Oct 26, 2007 #11
    So what you guys are trying to tell me is that we don't have an exact definition of life....the more life we discover, the better our definition gets. However, so far, scientists make us look "superior" to nonliving things b/c we have the ability to make a choice. But may be, we don't make choices at all...we are programmed that way in our DNA.
  13. Oct 26, 2007 #12

    jim mcnamara

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    S -

    Yes, you got it. I don't know if defintions get "better" or not but they do change. Your idea of better may not be what mine is.
  14. Oct 26, 2007 #13
    Skhandelwal: this issue is something we still can't really say too much about. Except that we know neurons grow and connect to other neurons, and this is 'controlled' by signalling molecules -ultimately DNA 'produces' a neuron's environment but a neuron is also affected by the external world...
  15. Nov 2, 2007 #14
    I may have missed it, but I didn't see anyone make this correction yet. Viruses don't all have RNA. They can have DNA too (like HepB, for example). And that's hardly what makes them hard to fight -- I thought it was more that viruses are basically parasites feeding off our cells, so 'killing' them is hard to do without killing our own cells.

    Also: Viruses may or may not be life. We may or may not have free will.
  16. Nov 5, 2007 #15
    I'm only a high school student, but having recently studied the nervous system I would have to say that there is free will, if free will is defined as the ability for the brain to analyze senses and make create different options, in some cases each with a probability of living, for example if u are on the edge of a cliff, and you look down, ur eyes perceive the depth and height at which you are and this information is transmitted to ur CNS(i'm pretty sure that this isn't an automated response) and you have the choice of going closer, which increases ur probability of falling(which is going to equal death in this hypothetical situation) and backing off which will increase ur chances of survival. So now u make the choice of stepping back, if u did the opposite you are taking a risk factor. Which surprisingly is also a tendency that humans have, since evolutionary speaking living things must take risks to survive sometimes(this tendency is what drives gambling addictions and the rush u get when u win). Now the only reason why you might step forward is because of an advantage you would gain from doing this, such as respect from the community(i.e. your friends being impressed with you). If there is no advantage, then you brain will tell you to back off, and if you do the opposite that usually means ur crazy. I would consider this free will, because u do have the choice. Free will really comes into play in situations where there isn't an advantage, such as taking a left or right at an intersection(and you u don't know where either leads), where u just have to guess, and that is what free-will really is, the ability to take risks and adapt

    Hope this isn't too off-topic, kinda was just inspired to write after the last few posts. Everything i said of course can be debated, most advanced brain functions are still being researched more fully, so its tough to say anything about the 'big' questions. I also agree with post #8 by Phred and #9 by Applebite
    Last edited: Nov 5, 2007
  17. Nov 5, 2007 #16
    Right. Free will is something we perceive ourselves (and other forms of life) as having because it is an advantage to be able to choose (between chasing your lunch or waiting somewhere for it to come to you, for example). Or alternatively, life is compelled to do this (there is no choice), but this is because life needs to learn about things...
  18. Nov 7, 2007 #17
    I'm not really sure if that's how most would define free will. The argument against free will is that when the brain is "analyzing" and "creating options" all that's going on is some chemical reaction and some electric forces being passed around. It's not a "soul" that is doing the thinking, it's just a sac of chemicals acting solely on the laws of physics and chemistry.

    If I make a robot that can sense its environment and create different options for how to react to what it senses, does it then have free will? Absolutely not -- it simply follows exactly what I programmed into it. That the brain is more complex to the point where we can't fully know what's going on doesn't mean that it's not analagous to my robot's software.
  19. Nov 7, 2007 #18
    re: ganstaman

    Yes, i agree with you, thats because robots do a linear process form(i.e. sense --> analzyze --> respond(action)) and the human mind is a cyclic cycle, right? So that means humans will sense something like heat and move away, then sense some more and continue doing this cycle. whereas the robot would just sense the fire move away and be done with that process. So the robot separates each process and the human kinda integrates multiple processes together and puts them on repeat?
  20. Nov 7, 2007 #19
    There is a sense, or an apprehension we have of "something" that does the thinking. Except of course, we can view thinking as a response, rather than a 'self-initiated' process by some "independent" being. This last (which is pretty much the classical view) then begs the question: where is this independent self? What is behind it (is there a higher "self of selves", and etc.)?
    I personally think that this sense we have (of being independent of our thinking) is due to our advanced brain, which has so much circuitry that we have the capacity, unlike most other animals, to diverge from immediate concerns, and think (a lot) about things that aren't to do with finding food, or shelter, or getting a fire going, or any of the ongoing tasks that life requires. Once we started to just think, for the sake of it, we diverged from the usual environmentally-directed thought patterns, and learned to philosophise. This explains our belief in our 'free' ability to think and act because of it, rather than just responding (but it's probably an illusion).
    Last edited: Nov 7, 2007
  21. Nov 7, 2007 #20


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    1] metabolizes - consumes fuel and excretes waste
    2] reproduces - makes copies of itself
    3] responds to external stimuli
    4] grows
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