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About Hormones and what they do
The essence of multicellularity is the coordinated interaction of the
various kinds of cells that make up the body. Cells communicate with
each other by chemical signals.
Three kinds of chemical signaling can be distinguished |
autocrine –
the cell signals itself through a chemical that it synthesizes and then
responds to. Autocrine signaling can occur solely within the
cytoplasm of the cell or by a secreted chemical interacting with
receptors on the surface of the same cell |
paracrine – chemical signals that diffuse into the area and interact with receptors on nearby cells. |
Examples are
- The release of cytokines that cause an inflammatory response in the area
- The release of neurotransmitters at synapses in the nervous system
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endocrine
the chemicals are secreted into the blood and carried by blood and
tissue fluids to the cells they act upon. |
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| Kinds of Hormones |
| There are two major classes of hormones - proteins, peptides, and modified amino acids steroids. |
 These
hydrophilic (and mostly large) hormone molecules bind to receptors on
the surface of "target" cells; that is, cells able to respond to the
presence of the hormone. These receptors are transmembrane proteins.
Binding of the hormone to its receptor initiates a sequence of
intracellular signals that may alter the behavior of the cell (such as
by opening or closing membrane channels) or stimulate (or repress) gene
expression in the nucleus by turning on (or off) the promoters and
enhancers of the genes.
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| This is the sequence of events |
| The hormone binds to a site on the extracellular portion of the receptor. |
| The receptors are transmembrane proteins. |
Many (but not all) pass through the plasma membrane 7 times, with their
N-terminal exposed at the exterior of the cell and their C-terminal
projecting into the cytoplasm. |
Binding of the hormone to the receptor activates a G protein associated
with the cytoplasmic C-terminal. |
This initiates the production of a "second messenger". The most common
of these are cyclic AMP, (cAMP)which is produced by adenylyl
cyclase from ATP (shown here), and inositol 1,4,5-trisphosphate
(IP3) |
The second messenger, in turn, initiates a series of intracellular
events (shown here as short arrows) such as phosphorylation and
activation of enzymes release of Ca2+ stores within the cytoplasm. |
In the case of cAMP, these enzymatic changes activate the transcription
factor CREB (cAMP response element binding protein) |
Bound to its response element 5' TGACGTCA 3' in the promoters of genes
that are able to respond to the hormone, activated CREB turns on gene
transcription. |
The cell begins to produce the appropriate gene products in response to
the hormonal signal it had received at its surface.
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| Steroid Hormones |
Steroid
hormones, being hydrophobic molecules, diffuse freely into all cells.
However, their "target" cells contain cytoplasmic and/or nuclear
proteins that serve as receptors of the hormone. The hormone binds to
the receptor and the complex binds to hormone response elements -
stretches of DNA within the promoters of genes responsive to the
hormone. The hormone/receptor complex acts as a transcription factor
turning target genes "on" (or "off")
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| Hormone Regulation |
| The levels of hormones circulating in the blood are tightly controlled by three homeostatic mechanisms: |
| When one hormone stimulates the production of a second, the second suppresses the production of the first. |
Example:
The follicle stimulating hormone (FSH) stimulates the release of
estrogens from the ovarian follicle. A high level of estrogen, in turn,
suppresses the further production of FSH. Antagonistic pairs of
hormones. |
Example:
Insulin causes the level of blood sugar (glucose) to drop when it has
risen. Glucagon causes it to rise when it has fallen. Hormone secretion
is increased (or decreased) by the same substance whose level is
decreased (or increased) by the hormone. |
Example: a rising level of Ca2+ in the blood suppresses the production of the parathyroid hormone (PTH). A low
level of Ca2+ stimulates it.
Hormone Transport - Although a few hormones circulate simply dissolved
in the blood, most are carried in the blood bound to plasma proteins.
For example, all the steroid hormones, being highly hydrophobic, are
transported bound to plasma proteins.
MORE INFORMATION
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