Page 76 - The Cell in 40 Topics

hanks to the amazing systems we examined in the preced-
ing chapter, the hormone thyroxin is secreted at a guaran-
teed level. In addition, however, yet another extraordinary
system maintains the level of thyroxin in the blood stable, in the face of any
emergency situation.
The molecules of thyroxin that the thyroid gland releases into the
bloodstream are attached to a transporter molecule specially created for
that task alone. Traveling through the bloodstream in that form, they are
unable to perform their function so long as they are bonded to the trans-
porter molecule. Out of any 10,000 thyroxin molecules in the blood, only
four will be present in an unattached form. These are the four molecules of
thyroxin that affect cells' metabolic rates. 12
When these free thyroxin molecules enter the cells, new thyroxin mol-
ecules separate from the transporters to replace them. Thus the thyroxin
molecules attached to the transporter molecules serve as a kind of storage
depot, and the requisite thyroxin is always kept ready for immediate use
(Figures 61 and 62).

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