Biol 61 – The Endocrine System
We will not be discussing all the hormones mentioned in this Chapter.
Focus on the topics and hormones that were
discussed in lecture.
The endocrine system is the system of organs/tissues in animals that secrete chemical signals called
the blood (Fig. 45.10).
organs and glands make hormones
– chemical signals that are released into the
blood, and which act to coordinate the activity of tissues and organs.
Endocrine glands are ductless
– the cells
secrete the hormone directly into the blood.
Contrast this with exocrine
glands with ducts
(that often exit
to the outside of the body, such as sweat glands, salivary glands, tear ducts, bile duct, etc.).
Many hormones are
used for homeostasis – keeping the internal environment of a body as constant as possible (for us this would involve
maintaining the right levels of glucose in the blood, the right blood volume, the right blood pressure, etc.).
Hormones are important because they are used by a body for
: an organ in one part of the body can
secrete a hormone signal that can, in theory, “talk” to any other organ and tissue of the body, giving it new
Of course, not all cells will respond to a particular hormone in the blood, so hormones have specific
target tissues that they regulate.
Certainly not all hormones are involved in homeostasis.
For example, the human
sex hormones play a role in regulating the growth, development and maintenance of the male and female
reproductive systems, as well as controlling reproductive cycles and sexual behavior.
Structure of Hormones
What kinds of molecules are hormones?
Hormones tend to be fairly large molecules, and types of hormones
, which tend to be short polypeptides, just a few amino acids long up to considerably
longer (this group includes ADH, insulin and glucagon), or single amino acids and their derivatives (examples
include epinephrine & norepinephrine).
(compounds derived from cholesterol).
hormone class includes aldosterone (a class of “mineralocorticoid”), estrogen, testosterone and the
glucocorticoids (which regulate aspects of metabolism).
Thyroxine is also a hydrophobic hormone that is
actually a derivative of an amino acid (that is also hydrophobic).
Most hormones are hydrophilic, water-soluble compounds that can travel easily in the blood but can’t cross cell
How do these hormones signal their target cells?
They bind to receptor proteins on the surface of
their target cells
(see Fig. 45.5a/45.6).
These membrane-bound receptor proteins have an extracellular region that
binds to the hormone, but they also contain an intracellular region that can act as an enzyme to affect the activity of
other cellular proteins and can activate transcription factors, which can alter gene expression within the target cell.
How does a steroid hormone act to stimulate its target cells?