Hormones

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Hormones

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are molecules that are released by endocrine glands directly into the blood.
They act as messengers , carrying a signal from the endocrine gland to a specific target organ or tissue.

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endocrine gland

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is a gland that secretes hormones directly into the blood.
They have not ducts.

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exocrine gland

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is a gland that secretes molecules into a duct that carries the molecules to where they are used.
e.g salivary glands secrete saliva into a duct.

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Target cells

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the target cells are those that posess a specific receptor on the plasma membrane. The shape of the receptor is complementary to that of the hormone molecule.
many similar cells together form a tissue.

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2 types of hormone:

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1. protein and peptide hormones and derivatives of amino acids (e.g adrenaline, insulin and glucagon)- do not enter the cell as protein is not soluble
2. Steroid hormones (e.g sex hormones) lipid based - have a direct affect on the DNA in the nucleus

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Adrenaline

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-amino acid derivative
-unable to enter the target cell
-must cause an effect on the cell without entering it.
-The adrenaline receptor has a complementary shape to that of the adrenaline molecule.
-the receptor is associated with an enzyme on the inner surface of the cell membrane- adenyl cyclase.
-adrenaline molecule binds with the receptor on the surface of the cell - FIRST MESSENGER.
- it then activates adenyl cyclase.
-the adenyl cyclase converts ATP to cyclic AMP. the cAMP is the SECOND MESSENGER.
-the cAMP can then cause an effect inside the cell by activating enzyme action.

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The adrenal medulla

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-found in the centre of the gland
-manufacture and release adrenaline in response to stress such as pain or shock.
-The effect of adrenaline is to prepare the body for activity. e.g
increase mental awareness, body hair to erect and dilate the pupils.

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The adrenal cortex

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-uses cholesterol to produce certain steroid hormones. These have a variety of roles in the body.
e.g The mineralocorticords help to control the concentrations of sodium and potassium in the blood.

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Pancreas

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a small organ lying below the stomach. It has both endocrine and exocrine functions.


exocrine function: the majority of cells in the pancreas manufacture and release digestive enzymes. The cells are found in small groups surrounding tiny tubules into which they secrete digestive enzymes. The tubules join to make up the pancreatic duct.

endocrine: the islets of langerhans contain different types of cells alpha cells and beta cells. The islets are well supplied blood capillaries and these hormones are secreted directly into the blood.

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insulin

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is the hormone released from the pancreas that causes blood glucose levels to go down

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Glucagon

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is the hormone that causes blood glucose levels to rise.

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If blood glucose rises too high:

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A high blood glucose is detected by the beta cells. In response the beta cells secrete insulin into the blood. The target cells are hepatocytes,muscle cells and some other body cells including the brain (membrane bound receptors for insulin). When the blood passes these cells insulin binds to the receptors. This activates the adenyl cyclase which converts ATP to cAMP. The cAMP then activates a series of enyme controlled reactions.

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effects on the cell caused by insulin

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-more glucose channels are placced into the cell surface membrane
-more glucose enters the cell
-Glucose in the cell is converted to glycogen for storage (glycogenesis)
-More glucose is converted to fats
-more glucose is used in respiration.

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If blood glucose drops too low:

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Detected by the alpha cells. In response the alpha cells secrete the hormone glucagon . Its target cells are the hepatocytes which possess the specific receptor for glucagon.

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The effects of glucagon:

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-conversion of glycogen to glucose (glycogenolysis)
-use of more fatty acids in respiration
-the production of glucose by conversion from amino acids and fats (gluconeogenesis).

This increases the glucose concentration.