Tadpoles must undergo a major metamorphosis to become frogs. This change includes reabsorption of the tail, growth of limbs, calcification of the skeleton, increase in rhodopsin in the eye, development of lungs, change in hemoglobin structure, and reformation of the gut from the long gut of an herbivore to the short gut of a carnivore. Amazingly, all of these changes are induced by thyroxine. What is the most likely explanation for such a wide array of effects of thyroxine?
A) There are many different forms of thyroxine, each specific to a different tissue.
B) Different tissues have thyroxine receptors that activate different signal transduction pathways.
C) Some tissues have membrane receptors for thyroxine, while other tissues have thyroxine receptors within the nucleus.
D) Different releasing hormones release thyroxine to different tissues.

Different tissues have thyroxine receptors that activate different signal transduction pathways

Explanation:

The thyroxine hormone is the most important hormone in frog metamorphosis. It has been shown that frogs in which the thyroid gland was removed showed very little growth afterwards.

Different tissues in frogs have different thyroxine receptors in them that activate different

signal transduction pathways.

B) Different tissues have thyroxine receptors that activate different signal transduction pathways.

Explanation:

As you saw in the question above, thyroxine is extremely important in the process of metamorphosis of tadpoles. It is secreted by the thyroid and its main function is to control the metabolic rate, which makes it able to act on the growth, maturation and development of tadpoles in different regions of the body, causing a series of changes not only in metabolism but also in physical structure of this animal.

To be able to act in so many different regions of the same body, it is assumed that different tissues in the body of the tadpole have thyroxine receptors that activate different signal transduction pathways.

If a tadpole's thyroid cannot secrete thyroxine, metamorphosis does not happen.

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