令人惊叹的细胞(4) On average , however , a human cell is about twenty microns wide - that is about two hundredths of a millimeter , which is too small to be seen but roomy enough to hold thousands of complicated structures like mitochondria ,
and millions upon millions of molecules . In the most literal way , cells also vary in liveliness .
Your skin cells are all dead . It ' s a somewhat galling notion to reflect that every inch of your surface is deceased .
If you are an average - sized adult you are lugging around about five pounds of dead skin , of which several billion tiny fragments are sloughed off each day .
Run a finger along a dusty shelf and you are drawing a pattern very largely in old skin . Most living cells seldom last more than a month or so , but there are some notable exceptions .
Liver cells can survive for years , though the components within them may be renewed every few days . Brain cells last as long as you do .
You are issued a hundred billion or so at birth , and that is all you are ever going to get . It has been estimated that you lose five hundred of them an hour ,
so if you have any serious thinking to do there really isn ' t a moment to waste . The good news is that the individual components of your brain cells are constantly renewed
so that , as with the liver cells , no part of them is actually likely to be more than about a month old . Indeed , it has been suggested that there isn ' t a single bit of any of us - not so much as a stray molecule - that was part of us nine years ago .
It may not feel like it , but at the cellular level we are all youngsters .
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