Overview
This lecture covers the chemical classifications of hormones, their mechanisms of action, and key differences in how hormones interact with target cells.
Hormone Classifications
- Hormones are chemical messengers that travel through the blood to target cells.
- Three main hormone types: amino acid derivatives, peptide hormones, and lipid derivatives.
- Amino acid derivatives include catecholamines (epinephrine, norepinephrine, dopamine), thyroid hormones (T3, T4), and melatonin.
- Peptide hormones are chains of amino acids, divided into glycoproteins and short polypeptides/small proteins.
- Glycoproteins include thyroid-stimulating hormone (TSH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH).
- Short polypeptides/small proteins include antidiuretic hormone (ADH), oxytocin, insulin, glucagon, and parathyroid hormone.
- Lipid derivatives include steroid hormones (estrogen, testosterone, cortisol, aldosterone, calcitriol) and eicosanoids (leukotrienes, prostaglandins).
Hormone Transport and Metabolism
- Hormones are released into blood near capillaries.
- They circulate either freely (short-lived) or bound to plasma proteins (longer-lasting).
- Hormones are inactivated by binding to receptors, breakdown in liver/kidneys, or enzymatic degradation.
Hormone Mechanisms of Action
- Hormones act via fixed or mobile receptor mechanisms.
- Fixed receptor mechanism: Hormone binds to a membrane-bound receptor, activating internal signaling without entering the cell.
- Mobile receptor mechanism: Hormone passes through the membrane and binds to intracellular receptors, often affecting DNA transcription.
- Catecholamines, peptide hormones, and eicosanoids use fixed receptors.
- Steroid hormones and thyroid hormones use mobile receptors.
Effects of Hormones on Target Cells
- Hormones activate or inhibit enzymes, or open/close ion channels in target cells.
- Fixed receptors use a "second messenger system": hormone (first messenger) triggers production/amplification of internal second messengers.
- Amplification allows a small hormone signal to produce a large cellular response.
Key Terms & Definitions
- Hormone — Chemical messenger that regulates physiological processes by binding to specific cell receptors.
- Peptide hormone — Hormone composed of amino acid chains.
- Steroid hormone — Lipid-based hormone derived from cholesterol.
- Catecholamines — Amino acid-derived hormones including epinephrine, norepinephrine, and dopamine.
- Glycoprotein — Peptide hormone with carbohydrate (glucose) chains attached.
- Second messenger — Intracellular molecule amplified in response to hormone-receptor binding, relaying the signal inside the cell.
Action Items / Next Steps
- Review hormone classification chart and be able to group examples by category.
- Understand the difference between fixed and mobile receptor mechanisms.
- Prepare for test questions on hormone structure, function, and mechanisms.