HORMONAL COMMUNICATION
(a) Endocrine communication by hormones
● To include secretion of hormones into the blood, transport by the blood, and detection by
target cells or tissues.
The endocrine system is a system used for communication around the body. It hormones as
signals and the circulatory system to transport them. Endocrine glands secrete hormones
directly into the blood—they consist of a group of specialised cells that manufacture and secrete
the hormone directly into the blood in the capillaries running through the gland, with no visible
ducts. Examples include, the pituitary gland, thyroid gland, thymus, adrenal glands, pancreas,
and ovaries/testes.
Steroid hormones—such as oestrogen or testosterone—are lipid soluble, and therefore can
diffuses across the plasma membrane and enter the cell and nucleus, to have a direct effect on
the DNA in the nucleus. However, their action is still specific, and they act only on target cells
with complementary nuclear or cytosolic steroid hormone receptors. The steroid-receptor
hormone complex enters the nucleus of the target cell and binds to another specific receptor on
the chromosomal material. Binding stimulates the production of mRNA molecules, which code
for the production of proteins, often enzymes.
Protein and peptide hormones, and derivatives of amino acids—such as adrenaline, insulin and
glucagon—are no soluble in the lipid membrane and do not enter the cell. Protein bind must bind
to a specific complementary receptor site in the plasma membrane of a target cell, triggering the
release of a secondary messenger which effects a change in the cell. This allows specific cells to
be targeted, such as the action of adrenaline on cells in the central nervous system and tissues
innervated by the peripheral nervous system, such as the heat, smooth muscle and skeletal
muscle.
Non-steroid hormones act as primary, or first, messengers that bind to the cell surface
membrane and effect a change inside the cell, through the action of a secondary messenger.
One example is cyclic Adenosine Mono-Phosphate (cAMP), which activates proteins. When a
signalling molecule, such as the protein hormone glucagon or insulin, binds to receptor sites in
the plasma membrane of a target cell, it activates a transmembrane protein, which activates a G
protein. This activates the membrane protein adenylyl cyclase, catalysing the formation of cAMP
from ATP. cAMP phosphorylates protein kinase A, which phosphorylates a number of other
proteins, facilitated by the hydrolysis of ATP. This activates enzymes in the cytoplasm which
perform glycogenolysis. PKA also phosphorylates CREB, which acts as a transcription factor,
altering gene expression at a transcriptional-level. At each stage, the number of product
molecules increase, producing a cascade effect, augmenting the action of the enzyme.
(b) The structure and functions of the adrenal glands
● Adrenal glands as an example of endocrine glands, to include the hormones secreted by
the cortex and medulla and their functions.
The adrenal glands are an example of an endocrine gland. Each gland is divided into the outer
adrenal cortex and the inner adrenal medulla. Both regions are well supplied with blood vessels
and produce hormones which are secreted directly into the capillaries. The cortex secrete
hormones that are vital to life, whilst the medulla produces non-essential hormones which help
the body react to stress.
