• 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)

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?as when they make hormones they need to get them 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

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