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How does a cell function?(in principle)

  1. Nov 12, 2007 #1
    First of all, why do cell die? Should it live forever? I am guessing that by the time cell indicates it needs food, it is already hungry, and the time it takes food to get to the cell...it suffers damage...and it is this small damage that keeps the cell aging till it dies.

    I read somewhere that the reason cell reproduce b/c it is too large and its membrane is not big enough to transfer sufficient food supply. But I don't get this...how would the size matter?

    And if this is why cell reproduce...then why do we reproduce?(biologically)
     
  2. jcsd
  3. Nov 12, 2007 #2

    jim mcnamara

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    Sometimes a simple question is difficult to answer. You actually need to finda high biology book and read the chapters on Cell structure, Cell metabolism, just to get started.

    These are short, bad answers.
    1. Cells die from trauma, environmental problems, starvation or from "cell cycle kinetics" - deciding they are "old" and need to die.

    2. Cells "decide" to divide for a lot of reasons. In the case you mention, as cell size doubles, from 1 cm on a side to 2 cm on a side, the volume of cytoplasm goes up faster than double. ie, a cube 1cm on a side has a surface area of 6 cm2 and a volume of 1 cm3. a cube 2cm on a side has a suface area of 24 and volume of 8.
    So we went from 6 -> 1 to 24 -> 8. This means the membrane has to suck up more goodies. one sq cm of cell membrane has to "feed" three times as much cytoplasm.

    3. Why reproduce? the simplest answer is so that the species continues to live.
    Reproduction allows species to change genetic makeup of a population of individuals over time so the species as a whole can be better adapted to the environment, for example.
     
  4. Nov 12, 2007 #3
    So you mean cells commit suicide? Wow
    Also, are you indicating that they are immortal? Fascinating
    (die from external causes)

    I still don't get the membrane sucking up part....how the a smaller size solve the problem?
     
  5. Nov 12, 2007 #4

    mgb_phys

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    It's a good way of handling the build up of toxins or errors in the operation - like a watchdog timer in computers.

    In theory single celled organisms are immortal, they split in two - both of the offspring are 'it', they split etc... however many die you could consider all the existing offspring to be the same person as the original.

    Imagine an office where there are windows along all the walls, you can open the windows to let in air and light.
    In a small office all the windows are within a few metres so you get lots of air. Now imaging exapnading that office to the size of an aircraft hanger. You have a lot more windows but they are now 100m away from you, so sitting at your desk you don't get as much air.
     
  6. Nov 12, 2007 #5
    (I am still following the analogy) Won't the windows get bigger too, making up for the distance by the greater force of air.

    Here how I see it....small office have many windows...big office has one huge opening....a lot more air is coming.
     
  7. Nov 12, 2007 #6

    jim mcnamara

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    No. Let's try this window size=membrane area size in square meters
    room size is the volume of the room (or cell) in meters cubed

    Code (Text):

    length of one side      window size    room size            window size / room size
    1                             6                      1                   6
    2                            24                     8                   3
    3                            54                     27                  2
    4                            96                     64                  1.5
    5                           150                    125                 1.2
    100                        60000             1000000               0.6
     
    Notice the ratio column As the sides get bigger each unit volume of air inside the room gets "less" from the outside, the numbers get smaller, less air comes in relative to the total volume of air in the room.
     
    Last edited: Nov 12, 2007
  8. Nov 12, 2007 #7

    mgb_phys

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    Scaling laws are a very important feature in biology that a lot of biologists don't 'get'.
    There is a famous essay by JBS Haldane - "On being the right size" http://irl.cs.ucla.edu/papers/right-size.html

    It has a famous quote about falling - as you make something bigger it's surface area goes as the square of size but it's weight goes as the cube. Air resistance increases with the surface area, so a mouse can fall without going very fast when it hits the ground but an horse can't. A similair rule tells you why cargo ships are large, the cargo goes up as the volume ( the cube of the size) but the resistance, and so fuel used, goes as the area ( sqaure of the size )

    Imagine stacking cubes ( or childrens building blocks ) if you make a pile of 8 blocks you have a volume 8 times bigger than a single block but a surface area only 4 times bigger (count the faces). So you cannot get as much stuff through the surface as before.


    Haldane explained it better 80 years ago - read the paper.
     
  9. Nov 12, 2007 #8
     
  10. Nov 12, 2007 #9
    Google "apoptosis."
     
  11. Nov 12, 2007 #10

    mgb_phys

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    That's the word I was looking for but couldn't remember how to spell it!
     
  12. Nov 15, 2007 #11
    Spirochete:

    Regarding the immortality of single-celled microbes, it doesn't happen, and here's why:

    When you grow bacteria up in batch culture (that is to say, in a flask, with a pre-defined medium and set nutrient level, seal the system, don't add anything,) the bacteria go through five distinct phases:

    1) The lag phase, where the microbes adjust their protein expression to the nutrient composition of the provided medium,

    2) The exponential growth phase (also for some diabolical reason called log phase, but I hate this :-( ), where the microbes behave as if they were growing in an infinite system, without bound,

    3) The stationary phase, when the microbial population begins to be starved for nutrients, as it is too large to be sustained on those nutrients provided,

    4) The death phase, where around 99% of the population dies for the same reason, and finally

    5) Long-term stationary phase, where the primary nutrient consists of dead bacterial biomass.

    Now, there exist dyes that stain only actively dividing cells. (I forget what they're called at the moment.) If you use these dyes to stain a culture in the exponential growth phase, most of them stain positive - this makes sense, since during exponential growth phase, the cells are dividing as fast as they can. However, once you slide into stationary phase, the stain starts showing up negative, and in death phase it's obviously even worse.

    But the tricky bit is that if you stain a culture in long-term stationary phase, you find that most of the cells that are alive are dividing again! In fact, if you take the same batch culture, and plate out samples from when it is 5 days old, and again when it is 10 days old, and 20 days old, etc., and you look at their most basic properties like color, shape, and size, they are all different. Additionally, if you inoculate that 20-day-old culture into the 5-day-old culture, the 20-day-old culture will take over the 5-day-old culture; it will reproduce more successfully and drive the 5-day-old culture into extinction. It's been shown that the later cultures are more efficient at taking up amino acids from the environment (read: the remnants of dead cells) than the earlier cultures are.

    The moral of this story is that long-term stationary phase is only "stationary" with respect to total cell concentration, not cell division, growth, or death. What appears to be stationary phase in terms of cell density is really a succession of individual populations, each out-competing the one before it. Strains that are advantaged in such environments are said to exhibit the Growth Advantage in Stationary Phase (GASP) phenotype; you can Google-scholar the topic for more info.
     
    Last edited: Nov 16, 2007
  13. Nov 17, 2007 #12
    I've taken a general microbiology class and lab, so I'm familar with the stages of growth that a culture goes through (lag, log, stationary, death). You added some nuance to that though. It makes sense, now that you say it, that cells which survive well in a stationary phase would be adapted to eat up their dead neighbors.

    But ultimately what you're referring to is evolution. Given limited resources, natural selection plus mutation continuously selects for better and better suited bacteria. So there are still outside factors which are preventing a single paticular cell from surviving indefinitely. In this case, other cells around the dying cell are better suited to extract nutrients from the environment, or better able to tolerate the toxic byproducts of their own metabolism. It's not as if the dead cells are dying of old age.

    The situation I'm proposing isn't really testable. But imagine a single cell in a culture, where it is continously maintained with just enough resources to survive, but not enough resources to compell it to reproduce. The question people ask is: could it live forever?
     
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