Hormones
Introduction
Hormones are chemical messengers involved in cell signalling (the process of communication
between cells). They travel in the blood. Most hormones are globular proteins/peptides. These
hormones have receptor protein (things that detect the presence of a hormone) specific to their
target tissue/organ. These hormones are produce by the endocrine system. These hormones have a
slower onset of action which can last for days or even weeks.
The endocrine system
The endocrine system is what makes hormones. This comprises of endocrine glands as a separate
organ e.g. the adrenal glands and also glands dotted about in some organs e.g. stomach. Hormones
are released in response to a stimuli. This is an internal stimuli e.g. the presence of other hormones,
presence of some chemicals etc. and the hormone is secreted. These are inside the body. There are
three types of hormones classed in terms of their structure. These are also secreted in different
ways:1. Peptides/globular proteins- This group of hormones are secreted by exocytosis e.g. insulin
secretion by B cells.
2. Amines- This group is secreted by exocytosis as well at synapses e.g. the hormone adrenalin
is secreted in this way.
3. Steroids- This group can diffuse out of the plasma membrane because it is a lipid derivative
(made from cholesterol) e.g. the glucocorticoid cortisol secreted from the adrenal cortex.
There are many endocrine glands in the human body:
1. Pituitary gland-controls all other endocrine glands but itself is under the control of the
hypothalamus through its release of releasing hormones.
2. Pancreas-secrete insulin and glucagon.
3. Ovaries-secrete oestrogens.
4. Testes-secrete testosterone
5. Adrenal glands-secrete adrenalin from adrenal medulla
6. Kidney-secretes erythropoietin(EPO)
7. Thyroid gland- secrete thyroid hormones e.g. thyroxine
Hormonal control
Peptides/globular proteins and amine hormones have similar action at the target tissue/organ but
that doesn’t mean that they have the same effects on targets. These hormones are hydrophilic. This
means that they are not lipid soluble/ permeable through the phospholipid bilayer. They are also
too large to pass through a channel protein. This means peptide/globular proteins have to cell
signal from outside through a first/1ry and second/2ndry messenger system.
E.g. the action of adrenalin(e)/epinephrine
This the action of hydrophilic hormones.
The hormone adrenalin is an example of a
first messenger-chemicals which act to
spread signal around the body. When a
hormone binds to a complementary
protein receptor, it activates a (trimetric)
G-protein. So therefore this is an example
of a G-protein coupled receptor. When a
G-protein becomes activated by adrenalin,
it causes the Alpha subunit of it to
dissociate leaving a beta- gamma complex. It is this beta- gamma complex which activates the
plasma membrane enzyme called( adenyl/adenylate/adenylyl )cyclase. This catalyses a reaction In
the cytoplasm. ATP made from cellular respiration becomes the substrate and is converted to cyclic
AMP/cAMP. This acts as a second messenger-chemicals which spread a signal inside a cell. Cyclic
AMP acts to activate other enzymes in a metabolic pathway e.g. protein kinase A/ PKA or it acts to
activate proteins which initiate transcription.
Effects of adrenalin
This hormone is an example which is involved in the flight or fight response. This hormone has many
effects on the body, getting it prepared for stressful action:1. Makes hairs stand on end- contraction of erector (pili) muscles in the dermis act to raise
hairs. This makes a person more aggressive which is the point of the response
2. Activation of glycogen phosphorylase- This enzyme converts glycogen to
glucose(glycogenolysis) which then can be incorporated into glycolysis and subsequently
into aerobic respiration if oxygen is available to get a lot of ATP.
3. Contraction of smooth muscle in bronchioles- This acts to increase the surface area of
bronchioles so that they can allow more air in and allow more rapid gas exchange
4. Vasodilation in some organs including the brain and muscle.
5. Vasoconstriction in some organs including the gut.
6. Pupils dilate- Contraction of radial smooth muscle and relaxation of circular smooth muscle
allows in the iris of the eyes to allow for far vision.
7. Increases the cardiac output/CO of the heart= stroke volume/SV x heart rate/HR.
8. Inhibits peristalsis-contractions of smooth muscle in the gastro-intestinal tract (the mouth
to anus journey)
9. Increase in ventilation- There is more demand for oxygen now as there is a demand for
MORE ATP through oxidative phosphorylation(where most of the ATP is produced).
Adrenal medulla and cortex
Adrenalin is secreted from the medulla(inner region ) of the adrenal glands. There are steroid
hormones secreted from the cortex of the adrenal gland.
Cortex:This region uses cholesterol to produce a variety of steroid hormones e.g.:
1. Glucocorticoids- cortisol, which controls metabolism of proteins and carbohydrates in the
liver
2. Mineralocorticoids-aldosterone, which controls levels of sodium ions and potassium ions in
the blood.
Insulin secretion by beta cells
Insulin is a globular protein secreted from beta(B) cells from the pancreas when there is an increase
in blood glucose levels. The stimulus for the secretion is glucose as it can readily diffuse from the
blood into a b cell (as endocrine cells are embedded in blood capillaries). This triggers some events
that lead to insulin secretion.
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1. Embedded on the plasma membrane of a
B cell are ATP-gated/sensitive potassium ions
channel proteins and voltage-gated calcium ion
channel proteins.
2. In the resting state (when there is a
resting potential maintained), the ATP-sensitive
channels remain open the voltage gated channels
remain closed.
3. When there is a high concentration of
glucose in the blood plasma, it readily diffuses into
the b cell by facilitated diffusion through a GLUT
channel
Glucose is quickly metabolised in the glycolytic pathway/glycolysis to produce ATP(2 ATP
per glucose molecule). Because there are ATP-sensitive potassium ion channels embedded,
this leads to the closure of those channels.
Now there is a depolarisation of the plasma membrane from about -70mV to -35mV. The
change in voltage causes the opening of the voltage- gated calcium ion channel proteins
which were closed before this point.
Calcium ions diffuse down their concentration gradient into the cell where there are vesicles
containing insulin made by the rough ER and packaged by the Golgi body.
Calcium ions entering cause the release of insulin by exocytosis into the blood.
When there is a lower concentration of glucose, insulin secretion will stop as there is less
ATP made in the glycolytic pathway, therefore the ATP-sensitive channels open again.