The Endocrine System (Cambridge (CIE) A Level Biology)

The Endocrine System

• A hormone is a chemical substance produced by an endocrine gland and carried by the blood

  • They are chemicals which transmit information from one part of the organism to another and bring about a change

  • They alter the activity of one or more specific target organs

• Hormones are used to control functions that do not need instant responses

• The endocrine glands that produce hormones in animals are known collectively as the endocrine system

  • A gland is a group of cells that produces and releases one or more substances (a process known as secretion)

Endocrine System Diagram

The major endocrine glands in the body

• Hormones such as insulin, glucagon, ADH and adrenaline are cell-signalling molecules that are released into the blood

• Endocrine glands have a good blood supply because, when they make hormones, they need to get the hormones into the bloodstream (specifically the blood plasma) as soon as possible so they can travel around the body to the target organs to bring about a response

• Hormones only affect cells with receptors that the hormone can bind to

  • These are either found on the cell surface membrane or inside cells

  • Receptors have to be complementary to hormones for there to be an effect

• Hormones such as insulin, glucagon and ADH are peptides or small proteins

  • They are water-soluble and so cannot cross the phospholipid bilayer of cell surface membranes

  • These hormones bind to receptors on the cell surface membranes of their target cells, which activates second messengers to transfer the signal throughout the cytoplasm

• Hormones such as testosterone, oestrogen and progesterone are steroid hormones

  • They are lipid-soluble and so can cross the phospholipid bilayer

  • These hormones bind to receptors in the cytoplasm or nucleus of their target cells

Hormone Action Diagram

Hormones are synthesised and released into the bloodstream from a gland (such as the pancreatic gland) and circulate in the bloodstream, affecting target cells