Flashcards in Chapter 18 Deck (127)
a mediator molecule that is released in one part of the body but regulates the activity of cells in other parts of the body; most enter the bloodstream; bind to receptors on or in the target cells
secrete products into ducts that carry the secretions into body cavities, into the lumen of an organ, or to the outer surface of the body; i.e. sudoriferous (sweat), sebaceous (oil), mucous and digestive glands
secrete their products (hormones) into interstitial fluid surrounding the secretory cells rather than into ducts, from the interstitial fluid, hormones diffuse into the blood which carries the hormones to the target cells
protein that only specifically recognize and bind to one hormone
if a hormone is present in excess, the number of target cell receptors may decrease; makes target cell less sensitive to the hormone
when a hormone is deficient, the number of target cell receptors may increase; makes target cell more sensitive to a hormone
most endocrine hormones; pass from secretory cells that make them into interstitial fluid and into the blood
act locally on neighbouring cells or on the same cell that secreted them without first entering the blood stream
local hormones that act on neighbouring cells
local hormones that act on the same cell that secreted them
lipid soluble hormones
soluble in lipids; include steroid hormones, thyroid hormones and nitric oxide
derived from cholesterol; each is unique due to the presence of different chemical groups attached at various sites on the four rings at the core of its structure (allows for different function)
synthesized by attaching iodine to the amino acid tyrosine
gas; both a hormone and a neurotransmitter
water soluble hormones
soluble in water; include amine hormones, peptide and protein hormones and eicosanoid hormones
synthesized by removing a molecule of CO2
peptide and protein hormones
amino acid polymers
protein hormones with attached carbohydrate groups ; i.e. thyroid stimulating hormone
derived from arachidonic acid (20 C fatty acid)
prostaglandins and leukotrienes
two types of eicosanoid hormones
3 functions of transport proteins
1. make lipid soluble hormones temporarily water soluble (increasing solubility in blood)
2. they retard passage of small hormone molecules through the filtering mechanism in the kidneys (slowing the rate of hormone loss in the urine)
3. provide a ready reserve of hormone (already ready in the bloodstream)
0.1-10% of the molecules of a lipid soluble hormone are not bound to a transport protein
Action of Lipid-Soluble Hormones
1. A free lipid-soluble hormone molecule diffuses from the blood, through interstitial fluid, and through the lipid bilayer of the plasma membrane into a cell.
2. If the cell is a target cell, the hormone binds to and activates receptors located within the cytosol or nucleus. The activated receptor–hormone complex then alters gene expression: It turns specific genes of the nuclear DNA on or off.
3. As the DNA is transcribed, new messenger RNA (mRNA) forms, leaves the nucleus, and enters the cytosol. There, it directs synthesis of a new protein, often an enzyme, on the ribosomes.
4. The new proteins alter the cell’s activity and cause the responses typical of that hormone.
Action of Water Soluble Hormones
1. A water-soluble hormone (the first messenger) diffuses from the blood through interstitial fluid and then binds to its receptor at the exterior surface of a target cell’s plasma membrane. The hormone–receptor complex activates a membrane protein called a G protein. The activated G protein in turn activates adenylate cyclase.
2. Adenylate cyclase converts ATP into cyclic AMP (cAMP). Because the enzyme’s active site is on the inner surface of the plasma membrane, this reaction occurs in the cytosol of the cell.
3. Cyclic AMP (the second messenger) activates one or more protein kinases, which may be free in the cytosol or bound to the plasma membrane. A protein kinase is an enzyme that phosphorylates (adds a phosphate group to) other cellular proteins (such as enzymes). The donor of the phosphate group is ATP, which is converted to ADP.
4. Activated protein kinases phosphorylate one or more cellular proteins. Phosphorylation activates some of these proteins and inactivates others, rather like turning a switch on or off.
5. Phosphorylated proteins in turn cause reactions that produce physiological responses. Different protein kinases exist within different target cells and within different organelles of the same target cell. Thus, one protein kinase might trigger glycogen synthesis, a second might cause the breakdown of triglyceride, a third may promote protein synthesis, and so forth. As noted in step 4 , phosphorylation by a protein kinase can also inhibit certain proteins. For example, some of the kinases unleashed when epinephrine binds to liver cells inactivate an enzyme needed for glycogen synthesis.
6. After a brief period, an enzyme called phosphodiesterase inactivates cAMP. Thus, the cell’s re- sponse is turned off unless new hormone molecules continue to bind to their receptors in the plasma membrane
when a water soluble hormone binds to its receptor at the outer surface of the plasma membrane, it acts as the first messenger
the first messenger (the hormone) then causes the production of a second messenger inside the cell, where hormone specific stimulated responses take place
cyclic AMP (cAMP)
common second messenger
a membrane protein that activates edentate cyclase
converts ATP into cyclic AMP