It is said that the human body turns over all cells or molecules in 7 year cycles. This is not quite correct, because there are different organs changing at different speeds. My interest is what is the scale of change and especially, is it possible to measure and how fast do:
1) cells change
2) molecules change
3) atoms change
High, medium and max rate organ examples would be appreciated.
1) cell growth
You should look into chemotherapy and cancer medicine in general. Because chemo is mostly effective because it kills fast dividing cells, this has been worked out reasonably well. the 7-10 year number is not really correct, some cells are replaced a lot more slowly.
This is why hair often falls out in cancer treatment, because the follicle cells are growing quickly. Neurons divide very slowly - if at all - and often are never replaced. Fat cells are in between - probably replaced in the 7-10 year range. Heart cells are replaced albeit quite slowly - less than 1% per year, which implies that many cells are with you your entire lifetime.
2) atoms/molecules change
The cell itself is in a continuous state of flux, but different parts of the cell, like cells in the body, change at different rates. Some proteins which make up the cell matrix or the DNA in the nucleus are replaced very rarely (through repair or rearrangement of the chromosome for instance) and most of the chromosome DNA is with the cell for the entire life of the cell.
Most proteins are labelled for degradation and are recycled after a few hours of function. Metabolic compounds such as sugars or salt might drift in and out of the cell continuously, maybe turning over in an hour or so. Fats can be incorporated into the cell and last for years I think.
