Hormones: Chemical Messengers

• Hormones are chemical messengers secreted by endocrine glands that regulate the functions of other cells.

Classes of Hormones

• Amino-acid hormones include amine, peptide, and protein hormones, are generally hydrophilic, and bind to plasma membrane receptors.
• Steroid hormones are derived from cholesterol, are hydrophobic, and bind to receptors in the cytosol or nucleus.

Hormone Transport Through the Blood

• Hormones travel as free unbound chemicals or as bound chemicals attached to plasma protein carriers.
• Free hormones:
  • These are small, amino acid–based hormones are hydrophilic.
  • They travel freely through the water-based plasma of blood.
• Bound hormones
  • They form complexes with binding proteins in plasma
  • They are usually hydrophobic.
  • They allow hydrophobic hormones to be transported safely through the watery environment of blood.
  • They serve as a reservoir of hormones that can be released when needed.
  • They help prevent large fluctuations in the concentration of free hormone in plasma.
  • They extend the lifespan of a hormone in the blood.

Target Cells and Receptors

• Hormones affect only particular cells (target cells) that contain protein receptors to which hormones bind to.
  • Their three-dimensional shapes are highly specific for the hormone molecule.
  • They can bind even when the hormone concentration is extremely low.
  • They are either embedded in the plasma membrane or reside within the cytosol or nucleus of the target cell.
• Receptor location largely depends on the chemical structure of the hormone:
  • Hydrophilic hormones cannot readily cross the plasma membrane and generally interact with receptors embedded in the target cell’s plasma membrane.
  • Hydrophobic hormones can cross through the plasma membrane and generally interact with receptors in the cytosol or nucleus.
• Target cells control their interaction with hormones by:
  • Up-regulation, wherein target cells produce and display a greater number of receptors in response to a decrease in hormone level.
  • Down-regulation, wherein target cells decrease the number of receptors displayed in response to prolonged exposure to a high hormone level.