Re: Plant and animal cell replacement question

On 28 Nov 2003 05:57:52 -0800, [EMAIL PROTECTED] (David Oderberg)
wrote:
>r norman <[EMAIL PROTECTED]> wrote in message news:<[EMAIL PROTECTED]>...
>> On 26 Nov 2003 06:27:28 -0800, [EMAIL PROTECTED] (David Oderberg)
>> wrote:
>> 
>> >Is it the case that all plants and animals, including humans, turn
>> >over every single one of their cells during the course of their mature
>> >life? One hears stories to the effect that human cells change in their
>> >totality every 7 years, or (on the contrary) that certain neurons do
>> >in fact remain in the human being until the person's death, etc. But
>> >I've never seen an authoritative statement of the facts.
>> >
>> Nerve cells, in particular, are not ordinarily replaced.  Until
>> recently, it was believed that mammalian neurons were not produced in
>> adults at all, ever.  Now, it is known that there are least a few
>> brain centers where at least a few new neurons are generated.  Still,
>> the large bulk of the billions of neurons in our brain do not get
>> replaced.
>> 
>> Many invertebrates have identfiable and named neurons, each with a
>> specific function.  Some vertebrates have some similar cells --
>> Mauthner cells in fish and amphibians, for example.  The lobster
>> cardiac ganglion has nine neurons.  These things are constant
>> throughout life -- the cells don't change.
>> 
>> Some invertebrates have absolutely fixed cell number.  The roundworm
>> Caenorhabditis elegans, for example, has 556 cells in the newly
>> hatched larval stage and 959 somatic cells in the adult.  These are
>> not replaced during the lifetime of the animal.
>> 
>> It is quite likely, though, that te cells themselves completely turn
>> over during their life so that a cell that remains is made of
>> completely new proteins and organelles.  The DNA does not turn over,
>> though.

>Thank you for this. Do I understand you correctly in saying that the
>proteins and organelles would change even in those animals that
>retained exactly the same cells during their adult life? Would this be
>a general truth?
>

The is a constant synthesis of new proteins and breakdown of old
proteins in all cells -- a process called protein turnover.  In fact,
there is a specific mechansm to degrade damaged or unused proteins
involving a special protein called ubiquitin (because it is so
widespread) that "marks" other proteins for degradation and a special
cell structure called a proteosome to do the dirty work.  The amino
acids are recycled to rebuild proteins or else they are metabolized.
Free radicals in a cell are a major source of damage which is why
antioxidants are so important to us. The lifetime of various proteins
may be hours or days to weeks and months and years.

Lipids are also subject to turnover, as is even the mineral in bone.
DNA is never replaced, but there are DNA repair enzymes that work to
fix problems.   I don't really know if EVERYthing in the body (except
DNA) is replaced, but certainly a lot of things are. There are
organisms (like Mayflies) that have such short short lifetimes that
probably the molecules in those cells aren't around long enough to be
replaced.

The question remains -- what do you mean by the identity of an object?
If I replace every molecule in your body with another identical one,
are you the same person?  There is an old joke about a museum with
George Washington's original hatchet -- the one that (according to an
almost certainly false legend) he used to chop down the cherry tree.
"Of course, the handle has been replaced three times and the head
twice, but it is still the same hatchet!"   I have heard that Chinese
view ancient buildings with great respect, still they perform routine
maintenance to keep them in good working order.  So you rebuild the
damaged roof and the walls and repair the floors and replace the
broken furniture so the building can remain functional.  An ancient
building may have no part left that is ancient.  Still, the "building
itself" is still there.