Types of Hormones

• Steroid hormones are lipophilic (fat-loving) – meaning they can freely diffuse across the plasma membrane of a cell
• Steroid hormones bind to receptors in either the cytoplasm or nucleus of the target cell, to form an active receptor-hormone complex
• An active receptor-hormone complex will move into the nucleus and bind directly to DNA, acting as a transcription factor for gene expression
• Examples of steroid hormones

  estrogen, progesterone, testosterone
• Peptide hormones are hydrophylic and lipophobic (fat-hating) – meaning they cannot freely cross the plasma membrane
• Peptide hormones bind to receptors on the surface of the cell, which are typically coupled to internally anchored proteins
• The peptide hormone receptor complex activates a series of intracellular molecules called second messengers, which initiate cell activity
• In signal transduction, the external signal (hormone) is transduced via internal intermediaries
• Examples of second messengers

  cyclic AMP (cAMP), calcium ions (Ca2+), nitric oxide (NO) and protein kinases
• The use of second messengers enables the amplification of the initial signal
• Examples of peptide hormones

  insulin, glucagon, leptin, ADH and oxytocin