The endocrine system communicates through hormones secreted directly into the bloodstream. Target cells have receptors that bind specific hormones, and similar cells group into target tissues. There are two types of hormones: protein/peptide hormones that cannot enter cells, and steroid hormones that can enter cells and affect DNA in the nucleus. Adrenaline is an example of a protein hormone that binds receptors to activate the enzyme adenyl cyclase, which converts ATP to cyclic AMP and causes effects inside the cell. The adrenal medulla releases adrenaline in response to stress, while the adrenal cortex produces steroid hormones involved in mineral balance and metabolism.

1. THE ENDOCRINE
SYSTEM

2. The endocrine system is another communication system. It uses
blood circulation to transmit its signals. The signals released by the
endocrine glands directly into the blood are called hormones.
 Endocrine gland  secretes hormones directly into the blood
and have no ducts
 Exocrine gland  secretes molecules into a duct which carries
them to where they are used

4. TARGET CELLS
Target cells are those that possess a specific receptor on their plasma
membrane. The shape of the receptor is complementary to the shape
of the hormone molecule. Many similar cells group together to form
target tissues.

5. HORMONES
There are two types of hormones
Protein and Peptide:
(and derivatives of amino acids)
Examples: adrenaline, insulin,
glucagon.
These are not soluble in the
phospholipid membrane and so
don’t enter the cell
Steroid:
Examples: sex hormones
These enter the cell and
have a direct effect on the
DNA in the nucleus

6. ADRENALINE
Adrenaline is an amino acid derivative and so can’t enter the target
cell. So, it may cause an effect inside the cell without entering it.
Adenyl Cyclase is the enzyme associated with the receptor for
adrenaline. It is found on the inside of the cell surface membrane.

7. THE ACTION OF ADRENALINE
1. Adrenaline (first messenger) in the blood binds to its specific receptor on
the cell surface membrane.
2. When it binds to the receptor, it activates adenyl cyclase.
3. The adenyl cyclase converts ATP to cyclic AMP (second messenger)
4. The cAMP then causes an effect inside the cell by activating enzyme action

8. THE ADRENAL
MEDULLA
This is found in the centre of the gland. The cells in it release, after manufacturing,
adrenaline in response to stress (e.g. pain and shock) to prepare the body for activity
Some Effects…
 Dilate pupils
 Body hair erects
 Inhibit gut action
 Increase mental awareness
 Increase heart rate

9. THE ADRENAL CORTEX
 This uses cholesterol to produce certain steroid hormones.
 Roles in the body…
 Mineralocorticoids: (e.g. adolsterone) to help control the
concentration of sodium and potassium in the blood
 Glucocorticoids: (e.g. cortisol) to help control the metabolism of
carbohydrates and proteins in the liver