Biology 12 Ms. Kim Lesson 5: Hormones Learning Goals · mechanism, a chemical that is affected at the end of the action pathway of a hormone controls the further action of the hormone

Biology 12 Ms. Kim Lesson 5: Hormones

Hormones Hormones acts as chemical messengers, enabling one part of the body to give instructions to another part of the body. Hormones are secreted by the cells and organs that compose the endocrine system. The primary function of the endocrine is to make and secrete hormones in order to regulate and coordinate the functions of organs throughout the body by contributing to the control of growth, development, reproduction, behavior, energy, metabolism, and water balance. The endocrine system uses hormones, with slower, longer acting responses to control organ functions. From other side, the endocrine system is controlled by the nervous system which can regulate the release of most hormones. Endocrine glands secrete hormones directly into blood or extracellular fluid, which are then circulated throughout the body. As a result, most body cells are constantly exposed to a wide variety of hormones. Only target cells will respond to a specific hormone because only these calls have the receptor proteins that recognize and bind to that type of hormone.

Learning Goals:

1. What are the two types of hormones? 2. What are the different pathways of hormones?

Biology 12 Ms. Kim The Cell Membrane The cell membrane is semipermeable, allowing the uptake of key nutrients and elimination of waste products, while maintaining a protected environment in which metabolic processes can occur. The lipid molecules in all biological membranes are highly dynamic or fluid, which is critical for membrane function. The lipid molecules exist in a double layer called a bilayer. The dominant lipids that are found in membranes are phospholipids, which contains two fatty acid tails, linked to glycerol and a phosphate group. This composition is important for membrane function. The fatty acid tails of phospholipids are very hydrophobic, whereas the phosphate containing head is hydrophilic. Two types of hormones How do hormones signal cells? There are two types of hormones, which differ in chemical structure and action:

(1) Protein hormones consist of chains of amino acids of varying length; are soluble in water (water-soluble) (2) Steroid hormones are derived from cholesterol and can diffuse from the capillaries into the interstitial

fluid and then into target cells (lipid-soluble) Pathway for water-soluble hormones Protein hormones are water soluble and cannot easily cross membranes, thus bind to receptors on the cell membrane.

• A hormone may be released from a cell (the first messenger)

• Protein hormones bind to receptor molecules/sites on the cell membrane, causing the receptor to change shape.

• Many protein hormones form a hormone-receptor complex that activates the production of an enzyme called adenylyl cyclase, that causes the cell to convert adenosine triphosphate (ATP) into cyclic adenosine monophosphate (AMP).

• Cyclic AMP acts as a second messenger, activating enzymes in the cytoplasm to carry out their normal functions

Biology 12 Ms. Kim Pathway for lipid-soluble hormones Steroid hormones are lipid soluble. They pass easily through the cell membrane and activate an internal receptor mechanism.

• A hormone diffuses from a cell • Steroid hormones attach to internal receptor

proteins in the cytoplasm or nucleus of the cell. • The hormone-receptor complex then attaches to a

segment of DNA that has a complementary shape, turning the gene’s action on or off; activation or deactivation of the gene changes the amount of protein that is synthesized

• If activated, the hormone activates a gene that sends a message to the ribosomes into the cytoplasm to begin producing a specific protein

Hormones and Feedback mechanisms The secretion of most hormones is regulated by negative feedback mechanisms. In a negative feedback mechanism, a chemical that is affected at the end of the action pathway of a hormone controls the further action of the hormone by inhibiting an earlier chemical reaction. A response in a feedback loop may be the production of a substance or a decrease in the production of a substance. Questions

1. What is a hormone? A chemical “messenger” that is produced in one part of the body, that regulates a process in another part of the body

2. Name and describe the two most common types of hormones, classified according to their molecular structure. Why can steroid hormones diffuse into a cell while protein hormones cannot?

Protein hormones are made of amino acid chains and are hydrophilic (note: think of the different functional groups and properties found in polypeptide chains); while steroid hormones are made of cholesterol and are hydrophobic (lipid). Steroid hormones are lipid-soluble, and cell membranes are made of phospholipids, and steroid hormone are thus able to diffuse through the cell membrane. Protein hormones are water-soluble and cannot pass through the membrane.

Biology 12 Ms. Kim Note: solubility depends on interactions. We know that “like dissolves like” and so polar molecules will be not attracted or form interactions with the hydrophobic barrier and thus be unable to pass. The protein hormone would prefer to interact/be more attracted to the aqueous environment outside of the membrane than the lipid-based hydrophobic core.

3. What are the main mechanisms that hormones use to produce their effects in target cells? Hormones only produce effects in specific target cells that contain the correct receptor molecules/proteins for that hormone. Protein hormones bind with receptor molecules on the cell membrane, while steroid hormones bind with internal receptors

4. If a particular hormone, such as the hormone that stimulates the development of facial hair, is released throughout the bloodstream, why does it not affect the entire cells in that way?

Only the cells of the face have the necessary receptors that would grow hair.

5. Testosterone is a hormone derived of cholesterol. Explain how testosterone would perform its intended action in a cell

Since testosterone is derived of cholesterol, it is a steroid hormone. It would use an internal receptor mechanism. It would easily pass through the lipid bilayer, and would activate a receptor inside the nucleus.

6. How are target cell’s activities changed if a gland produces too much of a particular hormone? Note: view slide on feedback mechanism (high levels of the hormone produced at the end of the pathway will inhibit previous pathways to stop the cycle à thyroxine will inhibit the release of thyrotropin, thus the thyroid gland will no longer receive a signal to release thyroxin)

Additional Reading (Nelson 2003) Read: Pg. 372-375

Next Class Lesson 6: Endocrine System (pg. 375-377)