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I would like to apologise first, this is going to be a long one. Im gonna try and put down all the notes to cover the entire A2 topic on hormones- which is pretty extensive, so dont kill yourself trying to read this all in one go. Ill split it down into three or four main topics which should be easier to digest- enjoy. Tom 

The Endocrine System~

This is one (of the many) communication systems in the body, which uses blood circulation to transport its signals. The blood is pumped all round the body and therefore any signal put in the blood will be carried around with it- all over the body. The signals released by the endocrine system are called hormones

Hormones are released directly into the blood stream from glands known as the endocrine glands - which are ductless glands. This means that they consist of a group of cells that produce and release the hormone straight into the blood. This is done by secreting them into capillaries that flow through the gland. 

Exocrine glands are the opposite system, which do  not create hormones. They have a tube, or duct, which carries a secretion (enzymes, for example) to a specific location. (Salivary glands -> Saliva -> the mouth) 

Target Cells/Tissues~ 

In order for a hormone to have an effect on an organ/tissue/cell, it (the organ/tissue/cell) must have a specific complimentary receptor on its plasma membrane for the hormone to bind to. If the cell does not have a receptor for a hormone, it is unable to cause a response. Simple. All hormones have  a different molecular shape and therefore a different specific complimentary receptor and this allows them to target specific target cells/tissues with the correct receptor. In this way the endocrine system can be used to send signals all over the body, or to specific target organs. 


Now, we've used the word 'hormone' a lot in the previous couple of paragraphs, without actually stating what one is. There are two types of hormone: 

-Protein, peptide and derivatives of amino acid hormones. (for example, adrenaline, insulin and glucagon) 

-Steroid hormones. (for example, the sex hormones: testosterone etc...) 

The two types of hormones work in different ways. Protein based hormones are not soluable in lipids and therefore CANNOT cross the phospholipid bilayer and enter the cell. As a result they have to have a system that allows them to cause a response on the inside of the cell- more on that later... Steroid hormones are soluble in lipids and therefore can pass through cell surface membranes no problem, causing a response themselves. 


Adrenaline is an amino acid derivative and is therefore unable to cross the cell surface membrane and cause a response within a cell on its own. Target cells have a specific complimentary receptor on their cell surface membrane which is complimentary to the shape of the adrenaline molecule: 


The diagram above outlines the action of adrenaline- NB, the AMERICAN term for adrenaline


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