Endocrine signals initiate responses in target cells that vary depending on the signal transduction pathways of the responding cell. Autocrines and paracrines act locally. Hormones are chemical signals released by endocrine cells into the extracellular fluid, where they diffuse into the blood and travel to distant target cells. The receptors for water-
learning outcomes
You should be able to:
Describe the structural feature of hormones that determines whether they act on surface or cytoplasmic receptors.
Explain how different target cells in the body can respond differently to a single hormone.
Discuss scientific thinking about the evolutionary history of hormones in multicellular animals.
Why are the effects of epinephrine more rapid than the effects of estrogen?
Epinephrine is a derivative of an amino acid, transported in solution in the blood, and acts on cell surface receptors to trigger an intracellular signaling cascade that produces an immediate response. Estrogen is a steroid hormone that is transported in the blood in combination with a protein carrier. Since it is lipid soluble, it diffuses into target cells to bind to an intracellular receptor that is then translocated to the nucleus and alters gene expression. Responses to the hormone are mediated through the products of that gene expression, all of which takes more time than the actions of epinephrine.
Why is the presence of a particular hormone in two species not an indication of the closeness of their evolutionary relationship?
The presence of the same hormone in two species is not an indication of closeness of their evolutionary relationships because hormone structures are highly conserved even as their functions change. Therefore distantly related species can share the same hormones.
How can a single hormone have two different effects in the same species?
A single hormone can have different effects in the same species depending on the types of cells that express its receptors and also on the nature of the receptors themselves, which can activate different signal transduction pathways.
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The advantage of hormone signals is that they can originate from a localized source but reach cells in all areas of the body and coordinate their activities. The disadvantage of hormones is that they are relatively slow in delivering and in terminating their messages. Thus they are not good for communicating rapidly changing information. In contrast, the nervous system sends messages to specific target cells, and its messages are rapid and can be turned off quickly. In addition, the nervous system has access to a great diversity of both external and internal information that indicates the state of the body and threats to homeostasis. The different characteristics of each of these informational systems make it advantageous for them to work together.