D.5. Hormones and metabolism

Miltiadis-Spyridon Kitsos

Platon IB Diploma

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The official IB Diploma Biology guide

Essential idea: Hormones are not secreted at a uniform rate and exert their effect at low concentrations.

https://ibpublishing.ibo.org/server2/rest/app/tsm.xql?doc=d_4_biolo_gui_1402_1_e&part=3&chapter=6

Endocrine glands

Endocrine glands secrete hormones directly into the bloodstream

Endocrine glands are structures that secrete

chemical messages, called hormones, directly into

the blood. These messages are transported to

specific target cells

Types of hormones

• Steroids

• Proteins

• Glycoproteins

• Polypeptides

• Amines

• Tyrosine derivatives

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The mechanism of action of steroid hormones

Steroid hormones bind to receptor proteins in the cytoplasm o• the target cell to •orm a receptor-

hormone complex.

Steroid hormones can cross directly through the plasma membrane and the nuclear membrane and

bind to receptors

An example is estrogen. The receptor-hormone complex then serves as a transcription factor,

promoting or inhibiting the transcription of a certain gene.

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The receptor-hormone complex

The receptor-hormone complex promotes the transcription of specific genes

Transferred to specific target cells

Calciferol (steroid)-> binds to specific receptor in nucleus->promotes expression of calbindin (calcium

transport protein) -> absorption of calcium from the intestine.

http://www.thenutritiondr.com/files/CarbsProFat-Gluconeogenesis.jpg

Some steroids, such as cortisol, bind to receptors in the

cytoplasm and the receptor-hormone complex then

passes through the nuclear membrane into the nucleus

to effect transcription.

Cortisol may have different effects in different kinds of cells.

The receptor-hormone complex promotes the transcription of specific genes

The hormone may have different efects in different

cells and it may even have an inhibitory effect.

For example, when the steroid hormone cortisol

binds to its receptor in the cytoplasm of a liver cell

and enters the nucleus it activates many of the

genes needed for gluconeogenesis; i.e., the

conversion of fat and protein into glucose raising

blood glucose.

At the same time, it decreases the expression of the

insulin receptor gene, preventing glucose from

being stored in the cells and also raising blood

glucose.

In the pancreas, the cortisol receptor complex

inhibits the transcription of insulin genes.

Mechanism of action of peptide hormones

Peptide hormones bind to receptors in the plasma membrane of the target cell.

Protein hormones are hydrophilic so they

cannot pass through the membrane directly.

Instead they bind to surface receptors that

can trigger a cascade reaction mediated by

chemicals called second messengers.

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The role of secondary messengers

Binding of hormones to membrane receptors activates a cascade mediated by a second messenger

inside the cell

Second messengers: small water soluble molecules

spread throughout the cells

Relay messages

Calcium ions and cyclic AMP (cAMP) are the two

most common second messengers.

See how epinephrine (“fight or flight” hormones)

works

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http://highered.mheducation.com/sites/0072507470/student_view0/chapter17/animation__second_messenger__camp.html

The role of secondary messengers

Binding of hormones to membrane receptors activates a cascade mediated by a second messenger

inside the cell

+GTP(energy)

The pituitary hormones

Hormones secreted by the pituitary control growth, developmental changes, reproduction and

homeostasis.

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The anterior pituitary synthesizes and secretes a number

of hormones that control growth, reproduction and

homeostasis.

Examples include FSH and LH.

The posterior pituitary gland secretes oxytocin and ADH,

but these hormones are not produced there. Instead, they

are synthesized in unusual cells called neurosecretory

cells found in the hypothalamus.

The hormones travel down the axons

of the neurosecretory cells and are

stored at the ends of the axons, until

impulses pass down the axons from

the hypothalamus, stimulating

secretion.

http://highered.mheducation.com/sites/0072943696/student_view0/chapter10/animation__hormonal_communication.html

The role of the hypothalamus

The hypothalamus controls hormone secretion by the anterior and posterior lobes of the pituitary gland

1. The hypothalamus links the nervous

system to the endocrine system via

the pituitary gland.

1. Both the nervous system and

the endocrine system play a role

in homeostasis and in the control

of other processes including

reproduction.

3. The role of the hypothalamus

is to secrete releasing factors,

which stimulate the secretion of

the anterior pituitary gland

hormones. The releasing factors

are carried from the

hypothalamus to the anterior

pituitary gland by a portal vein.

4. Negative feedback

mechanisms (e.g., ADH)http://image.slidesharecdn.com/ch19lecturepresentation-140913124201-phpapp01/95/dr-b-ch-19lecturepresentation-11-638.jpg?cb=1410612186

Regulation of milk secretion

Control of milk secretion by oxytocin and prolactin

Prolactin is a hormone produced by the anterior

pituitary found

During pregnancy, high levels of estrogen increase

prolactin production but inhibit the effects of prolactin on

mammary glands.

The abrupt decline in estrogen and progesterone

following delivery removes this inhibition and the

production of milk begins.

However, the release of the milk after it is produced

depends on the hormone oxytocin.

Nursing by an infant stimulates the continued creation

of prolactin. It also stimulates oxytocin release.

Oxytocin stimulates the contraction of cells that

surround the structures holding the milk leading to the

ejection of the milk.

Positive feedback loops

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Injection of growth hormone by athletes

Some athletes take growth hormones to build muscles

Growth hormone is another polypeptide hormone

produced in the anterior pituitary.

The binding of growth hormone to liver cells stimulates

the release of insulin-like growth factor which circulates

in the blood and stimulates bone and cartilage growth.

Additional effects, one of which is increase in muscle

mass.

While it is clear that it leads to greater muscle mass, the

data is not clear that it leads to greater strength.

Another claim is that it allows tired muscles to recover

more quickly allowing an individual to train harder and

more often

The scientific research on the topic suggests that the

benefits provided in terms of enhanced performance

are small or non-existent compared to the risks of

injecting the hormone. For this reason, use of the drug

is banned by most international sporting federations.

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The negative effects of the use of the human growth hormone

Some athletes take growth hormones to build muscles

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Eradicating iodine deficiencyCooperation and collaboration between groups of scientists: the International Council for the Control of

Iodine Deficiency Disorders includes a number of scientists who work to eliminate the harm done by

iodine deficiency.

http://wisemensvitamins.com/wp-content/uploads/2010/12/iodine-deficiency-symptoms.jpg

Thyroid hormone refers to two similar

hormones derived from tyrosine.

Triiodothyronine (T3 ) contains three iodine

atoms and tetraiodothyronine (T 4 ) contains

four iodine atoms.

Correct functioning of the thyroid requires

iodine in the diet. If there is dietary

insufficiency, then there are a number of

consequences including a condition known

as goiter.

https://highered.mheducation.com/sites/9834092339/student_view0/chapter46/mechanism_of_thyroxine_action.html

The inability to produce the thyroid hormones because

of the absence of iodine means that the hypothalamus

and the anterior pituitary continuously stimulate the

thyroid and enlargement of the thyroid results.

Iodine defciency during pregnancy can affect fetal

nervous development and can lead to mental

retardation in children.

Eradicating iodine deficiencyCooperation and collaboration between groups of scientists: the International Council for the Control of

Iodine Deficiency Disorders includes a number of scientists who work to eliminate the harm done by

iodine deficiency.