AP BIOLOGY – ANIMAL HORMONES (CHAPTER 45)
YOU MUST KNOW:
 Two ways hormones affect target organs
 The secretion, target, action, and regulation of the hormones we learned
 An illustration of both positive and negative feedback in the regulation of homeostasis
by hormones.
I.
II.
Overview:
 In animals, hormones are chemical signals that are secreted into the extracellular
fluid and carried by circulatory system. Hormones communicate messages
within the body.
 A hormone may reach all parts of the body but only cells with the appropriate
receptors are able to take in the messages that are carried by hormones and
respond to the hormones (target cells)
The Interaction of the Endocrine and Nervous Systems
 The nervous system conveys high-speed electrical signals along specialized cells
called neurons that result in rapid responses by muscle contraction or other
sudden changes.
 Endocrine system – a collection of hormone producing and secreting cells that
coordinate slower, longer-lasting responses. The endocrine system is
responsible for developmental processes and metabolic processes.
 Endocrine glands are ductless glands that secrete their hormones into the
extracellular fluid than the bloodstream.
 Certain cells act as neurosecretory cells – specialized nerve cells that release
hormones. Some chemicals can also serve as both hormones and
neurotransmitters (such as epinephrine). The nervous system also frequently
acts as a regulator of endocrine glands.
 In biological control systems the following pathway is followed:
i. Receptor – detects a stimulus and sends the information to the control
center
ii. Control center – processes the information and sends a signal to the
effector
iii. Effector – respond to the stimulus

In endocrine pathways the efferent signal is a hormone or neurohormone that
acts of particular effector tissues and result in various physiological changes.
The three types of pathways show examples of these control systems:


Both systems (nervous and endocrine) use negative feedback mechanisms to
control their function. In this case the effector sends information back to the
center to lower the effectiveness of the message sent from there. This process
prevents the overreaction of control mechanism to a simple stimulus.
III.
Hormones and Cell Signaling
 Three major classes of hormones in vertebrates are
o Proteins and peptides – usually water-soluble molecules with cell
membrane receptors
o Amines (derived from amino acids) – usually water-soluble molecules
with cell membrane receptors
o Steroids – usually nonpolar molecules with intracellular receptors
 Only cells with specific receptors that fit the shape of the hormone molecules
are able to respond to the hormone molecules. The response will be generated
through a signal transduction pathway:

A particular hormone may cause a different response in each type of target cells
because the cells have different receptors that will result in a different signal
transduction pathway.
IV.



Paracrine Signaling by Local Regulators
Local regulators convey messages between neighboring cells within seconds or
milliseconds after release.
Binding on special receptors, these signal molecules trigger special events inside of
the target cells.
Some specific local regulators:
o Cytokines – these proteins are secreted by several cell types of the immune
system to regulate the function of lymphocytes.
o Growth factors – stimulate cell reproduction and cell differentiation
o Nitric oxide (NO) – This gas is highly reactive and very unstable so it breaks
down quickly. When blood oxygen levels fall, endothelial cells in the blood
vessel walls synthesize and release NO. NO than activates an enzyme that
relaxes smooth muscle cells in the walls of blood vessels. This in turn dilates
the vessels and improves blood flow in the given tissue area. NO is also
important in male sexual function, because it increases blood flow into the
penis that results in an erection. (Think back to our case study) NO can also
act as a neurotransmitter and can be secreted by white blood cells to kill
bacteria and cancer cells.
o Prostaglandins (PGs) – These are modified fatty acids that are released from
most types of cells and regulate nearby cells in a wide range of ways. In
semen, it helps the sperm cells to reach the egg by contracting the smooth
muscle of the uterine wall. During childbirth it contracts uterine muscles to
induce labor. In the immune system they help to induce fever and
inflammation, induce pain sensation. It helps the aggregation of platelets
during blood clotting. Different prostaglandins can act opposite to each
other (antagonistically) to contract or relax capillaries in the alveoli of the
lungs.
V.
The Hypothalamus and Pituitary System

The hypothalamus is located at the base of the brain. It receives information from
nerves throughout the body and from other parts of the brain.
 The hypothalamus contains two sets of neurosecretory cells whose secretions are
either stored in the pituitary gland or regulate the hormone production of the
pituitary gland that is a bean-sized organ right below the hypothalamus.
 The pituitary gland has two parts:
o Posterior pituitary gland (neurohyophysis) – is an extension of the
hypothalamus
o Anterior pituitary gland (adenohypophysis) – develops from a fold of tissue
at the roof of the embryonic mouth.
A. The Hormones of the Posterior Pituitary Gland:
 It does not produce its own hormones but stores and secretes two hormones that
are made by some neurosecretory cells of the hypothalamus. The axons of the
neurosecretory cells carry the hormones to the posterior pituitary where it is stored
until release.
 Antidiuretic hormone (ADH) – is a peptide hormone that acts on the kidneys. It
increases water retention and decreases urine volume. With its action, ADH
regulates the osmotic concentration of the blood. This hormone is usually released
when the blood plasma becomes too concentrated with salts and proteins.

Oxytocin – another peptide hormone that has target cells in the uterine muscle cells.
During childbirth, this hormone is released to cause the contraction of the smooth
muscle of the uterus and causes the mammary glands to release milk after giving birth.
Oxytocin is a good example of positive feedback regulation. The more muscle
contraction activates more oxytocin release, that result in more forceful muscle
contraction.
B. The Hormones of the Anterior Pituitary
 The anterior pituitary consists of endocrine cells that synthesize and secrete at least six
different hormones. Several of these regulate the function of other endocrine organs.
 Hormones that regulate other endocrine organs are called tropic hormones.
 The anterior pituitary is also regulated by the hypothalamus with various releasing and
inhibiting hormones. These hormones are released into capillaries in the base of the
hypothalamus and travel through short portal vessels that form a second capillary bed
in the anterior pituitary. Every hormone in the anterior pituitary is controlled by at least
one tropic hormone from the hypothalamus.
 Negative-feedback loops are vital in the regulation of these hormones.
 The hormones of the anterior pituitary are the following:
o FSH (Follicle-stimulating hormone) and luteinizing hormone (LH) – Structurally,
these hormones are glycoproteins. These hormones are also called
gonadotropins because they stimulate the activity of male and female gonads
(testes and ovaries). These hormones in females stimulate estrogen production
and ovulation and in males stimulate testosterone and sperm cell production.
o Thyroid-stimulating hormone (TSH) – Structurally, these hormones are
glycoproteins. They act on the normal development of the thyroid glands and on
the production of thyroid hormones.
o Adrenocorticotropic hormone (ACTH) – Structurally, this is a peptide hormone.
ACTH stimulates the production and secretion of steroid hormones by the
adrenal cortex.
o Prolactin (PRL) – a peptide hormone that has different effects in different
vertebrate classes. In mammals, they regulate mammary gland growth and milk
synthesis. However, it also has a wide range of functions in other vertebrate
organisms.
o Beta-endorphins – these peptide hormones bind to receptors in the brain and
dull the perception of pain. (Runners high)
o Growth hormone – Peptide hormone that act on a wide variety of target tissues.
Its main function is to stimulate bone and cartilage growth. It also increases
blood glucose levels. Hypersecretion of this hormone in childhood result in
gigantism, while in adulthood it results in acromegaly.

VI.
Hyposecretion of GH in childhood results in pituitary dwarfism (proportional, but small).
This disorder can be treated today with hormone therapy.
Thyroid Hormones

The thyroid gland consists of two lobes located on the front (ventral) surface of the
trachea.

This gland produces two very similar hormones. Both of these hormones are similar to
tyrosine amino acid with iodine atoms attached to them. Triiodothyronine (T3) has
three iodine atoms attached to it, while tetraiodothyronine (thyroxine or T4) has four
iodine atoms attached to them. Both of these hormones bind to the same receptor in
the nucleus of the target cell (intracellular receptor).
The production and release of these hormones is controlled by the hypothalamus and
the anterior pituitary gland with negative-feedback loops.


Thyroid hormones are responsible for development and maturation (ex: tadpole into a
frog, bone forming and nerve branching during embryonal development). It is also
important in homeostatic functions such as maintaining normal blood pressure, heart
rate, muscle tone, digestion, reproductive functions. T3 and T4 generally increases
oxygen consumption and cellular respiration in cells.

Hyperthyroidism can result in high body temperature, sweating, weight loss, irritability,
high blood pressure, goiter that is warm to the touch or in the worst case can lead to
Graves’ disease. It is treatable with hormone therapy and surgery.
Animation in secretion, regulation, hypothyroidism: http://biologyinmotion.com/thyroid/

Hypothyroidism can lead to cretinism in infants (retarded skeletal growth and poor
mental health). In adults, it can result in weight gain, lethargy and intolerance of cold.
It can also result in goiter that is cold and hard to the touch. It is a treatable condition
with hormone therapy.

Calcitonin – a hormone that is important in maintaining normal Ca ion concentration by
lowering blood calcium levels. This hormone stimulates calcium ion intake into bones
and reduce the reabsorption of calcium from the kidneys back into the bloodstream.
This hormone is antagonistic to parathyroid hormone.
VII.
Hormones of the Parathyroid Gland
 The parathyroid glands are four small structures that are embedded in the surface of
the thyroid gland. Their main hormone is the parathyroid hormone (PTH).
 PTH is responsible for increasing blood calcium ion concentration by making bone cells
break down and release more Ca2+ into the blood stream, by stimulating the kidneys to
reabsorb more Ca2+ ions into the blood stream, stimulating the liver the activate more
inactive vitamin D into its active form. Vitamin D will help to absorb more Ca ions from
the intestine.
 PTH’s antagonistic hormone is calcitonin.
VIII. Control of Blood Glucose Levels
 The islets of Langerhans of the pancreas are responsible for its endocrine function. The
alpha cells of the islets are responsible for releasing glucagon, while the beta cells are
responsible for releasing insulin. These two peptide hormones are antagonists of each other.

When blood glucose levels rise above normal, beta cells are stimulated to release insulin.
Insulin stimulates liver cells to take in more glucose and store it as glycogen. It also stimulates
other cells of the body to take in more glucose.

When blood glucose levels are too low, alpha cells are stimulated to release glucagon. Glucagon
makes the liver break down glycogen and release more glucose into the blood stream.

Diabetes mellitus – a disease where glucose homeostasis fails with very serious
consequences. People with diabetes mellitus excrete glucose because their kidneys are
not able to reabsorb all the glucose from the blood. Because they excrete glucose in the
urine, they also release more water than normal. As a result people with diabetes
mellitus excrete larger volumes of urine and feel constantly thirsty. In the meantime,
cells are starving for glucose and use more fat for cellular respiration. The fat
metabolites can lower blood pH to dangerous, life threatening levels. There are two
types of diabetes mellitus with different causes, but each is marked by high blood
glucose levels:
o Type I diabetes: is an autoimmune disorder, because the immune system
destroys the beta cells of the pancreas with this the body loses its ability to
produce insulin. This type of diabetes usually occurs in childhood.
o Type II diabetes: may be somewhat inherited but excess body weight and lack of
exercise can also result in Type II diabetes. In this case, insulin is either deficient
or the target cells have a reduced responsiveness to insulin because of changes
in insulin receptors. This type appears later in life.
IX.
Adrenal Hormones
 These glands are located on the top of the kidneys. They are the combination of two
different glands. The outer portion is the adrenal cortex, the inner portion is the
adrenal medulla. Adrenaline action animation (review on cell signaling):
http://courses.washington.edu/bchem442/Adrenaline.html

The adrenal medulla is originated from the nervous system and has neurosecretory
cells. Its hormones are synthesized from an amino acid and are amine hormones.
Epinephrine and norepinephrine are also neurotransmitters in the nervous system. Both of
these hormones act as stress hormones and give the body an energy boost by acting on a wide
range of tissues. They increase blood glucose and fatty acid levels, so cells can take in these and
use them as extra energy sources. They increase the action of the circulatory and respiratory
systems to get the body ready for a fight-or-flight response.

The hormone production of the adrenal medulla is regulated by the hypothalamus through
nerve impulses.

The adrenal cortex consists of true endocrine cells and releases steroid hormones.
Corticosteroids help to maintain homeostasis when the body experiences stress over a long
period of time. They influence glucose metabolism by increasing glucose production from other
organic molecules. Cortisol (glucocorticoid) also has an anti-inflammatory effect.

Other corticosteroids like aldosterone stimulates cells in the kidneys to reabsorb sodium and
water back into the bloodstream.

The production of these hormones is regulated by the hypothalamus through releasing
hormones, which will stimulate ACTH production in the anterior pituitary. ACTH will increase
the hormone production of the adrenal cortex. However, high levels of corticosteroids act as
inhibitors of the ACTH production (negative-feedback).
X.
Gonadal Sex Hormones
 The gonads (testes and ovaries) secrete three groups of sex hormones (androgens,
estrogens and progestins). All three types are found in both males and females but in
different proportions. These steroid hormones affect the growth and development,
regulate the reproductive cycle and sexual behavior.
 Testes mainly synthesize testosterone (type of androgen), that stimulates the
development and maintenance of the male reproductive system. These hormones will
determine that a fetus will develop into a male. During puberty, testosterone is also
responsible for the development of secondary male characteristics.
 Estrogens are the main hormones that are by the ovaries. They are responsible for
forming the female reproductive system and the female secondary sex characteristics.
 Progestins such as progesterone are also produced by the ovaries and are responsible
for forming the uterine lining and maintaining it during pregnancy.
 The regulation of androgens and estrogens occurs through releasing hormones of the
hypothalamus and the FSH and LH of the anterior pituitary.
XI.
Melatonin and Biorhythms
 The pineal gland – is a small mass of tissue in the center of the brain.

This gland synthesizes and releases an amine hormone called melatonin. This hormone
regulates functions related to light and to seasons marked by changes of day length. It
also regulates biological rhythms related to reproduction. Because this hormone is
secreted at night, during the winter, more melatonin is secreted. The concentration of
melatonin influences other brain regions that are related to biological clocks.
Review, study check: http://www.zerobio.com/drag_oa/endo.htm
Great overview of everything:
http://www.interactivephysiology.com/login/endodemo/systems/systems/endocrine/index.html
XII.
Hormonal Regulation of Insect Development
 Three hormones play important role in molting and metamorphosis in insects:
o Brain hormone – produced by neurosecretory cells in insects’ brain stimulates
the production of another hormone called ecdysone
o Ecdysone – produced by endocrine glands in insects promotes molting and the
development of adult characteristics.
o Juvenile hormone – an antagonistic of ecdysone and brain hormone that is
secreted by a pair of small endocrine glands behind the brain. This hormone
retains juvenile and larval characteristics. To develop into a pupa or an adult,
juvenile hormone levels must decrease.
THE FOLLOWING IS A PLANT HORMONE, PLEASE DO NOT MIX IT UP:
AUXIN


The term auxin is used for any substance that promotes the elongation of coleoptiles but
the natural auxin is IAA. It is released at the tip of the coleoptiles and seems to be
transported directly through parenchyma cells and always moves from the tips to the base.
This one directional transport is due to the arrangement of transport proteins in the cells’
cell membrane.
Auxin mostly stimulate cell elongation at certain concentrations but also can inhibit cell
elongation on other concentrations.


Proton pumps seem to play a major role in the growth of cells in response to auxin. Auxin
stimulates the plasma membrane’s proton pumps and increases the voltage (concentration
of H+ ions) across the membrane. It also lowers the pH in the cell wall. As the cell wall
becomes more acidic, it activates enzymes called expansins that break the hydrogen
bonds between cellulose microfibrils loosening the cell wall. An increase in membrane
potential also increases the intake of ions and water will passively follow, which increases
the turgor of the cells. Auxin also alters gene expression and increases protein production
in the cell.
Auxin also causes the roots to branch or enhances vegetative propagation of plant cuttings.