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Essay - Free Response 1989

Describe negative and positive feedback loops, and discuss how feedback mechanisms regulate each of the following:

a. The menstrual cycle in a nonpregnant human female. (FSH, Estrogen, LH, and Progesterone control of egg development uterine lining development, absence of HCG and the reactivation of FSH production & loss of Progesterone)
b. Blood glucose levels in humans. (increase in blood glucose, release of insulin, greater permeability to glucose in membranes of muscle cells - adipose cells - liver cells, decrease in blood sugar below optimu causes release of glucagon which causes stored glycogen in muscles and liver to be broken down to glucose and released into blood)

Using your text book and the Internet for help, answer the questions below.
You may want to check one or more of the many animations available at MedBroadcast.com or Visual Content for Health Care for additional support.for additional support.

Endocrine System :

All animals depend upon the use of chemicals to enable the body to regulate and adjust the internal environment. Glands are special tissues that are responsible for producing chemicals that will initiate some change in metabolism.

Exocrine glands secrete the chemicals that they produce through small tubes or ducts directly into the organ where the metabolic activity is to take place. (Salivary glands, digestive glands)

Endocrine glands have no ducts. They secrete chemical signals into the blood stream which are carried throughout the body. The chemicals produced by endocrine glands are referred to as HORMONES. The endocrine system is a system common to most higher animals, such as mammals and birds. It's purpose is to maintain the body in its normal state (called homeostasis). It consists of 4 components:

  1. The hypothalamus at the base of the brain
  2. The Pituitary at the base of the hypothalamus
  3. The endocrine organs (such as adrenal glands)
  4. The target tissues (such as the kidney)

The hypothalamus is nereve tissue. Its role is to sense changes in the internal environment and stimulate the Pituitary (Master Gland) to release specific hormones that activate other endocrine glands in the body.

Hormones released into the blood will alter the activity of any cell in the body that possess a compatible receptor. For many hormones (peptides) the receptor is fixed to the surface of the cell membrane. For others, the receptor may be in the cytoplasm (steroids) or attached to the DNA in the nucleus (thyroxine).

1. What is the significance of having nerve tissue associated with the endocrine system?

2. What chemical characteristic of steroids make it possible for them to move through the plasma membrane and bind to intracellular receptors?

Once bound to the receptor chemical reactions occur leading to the increase in cAMP. The presence of cAMP activates kinase enzymes that triggers a significant change in cell metabolism

The human endocrine system (along with the nervous system) attempts to maintain the optimum internal environment needed to support specific activity of the body. Feedback loops insure that activation or reversal of cell metabolism can be achieved and the optimum condition is established. Thyroxine is a hormone produced by the thyroid gland. It causes cells to increase metabolism (increase the rate of chemical activity - using larger amounts of ATP and converting chemical energy into heat energy.

3. Discuss the secretion of thyroxine in response to body temperature and briefly describe how it employs a feedback loop in order to maintain an optimum internal environment.

The diagram above shows the glands associated with the human endocrine system. Each of these glands secretes one or more hormones into the circulatory system.

4- Choose a hormone of special interest and IN DETAIL describe the sequence of events that leads to it's release, how it effects target cell function, and how this change in cellular function ultimately leads to a reduction in the amount of the hormone being released. (Negative Feedback!!!)

5- List and BRIEFLY describe 5 different diseases that occur as a result of an endocrine gland malfunctioning.

The menstrual cycle offers exquisite example of how the endocrine system uses feedback mechanisms to establish optimum conditions for insuring pregnancy.

Preparation of the uterus neds to be perfectly timed for the fertilized egg. If the egg is not fertilized more energy can be saved by allowing the uterine wall to be broken down than to attempt to maintain it indefinately. Hormone levels in a mature female fluctuate rather dramatically and as a result work to establish a relatively predictable cycle that increases the likelihood of pregnancy and carrying the embryo to term.

6- Why is there a continuous and somewhat dramatic increase in Estradiol (estrogen) from day 3 through day 13.

7- What does the dramatic rise in LH at day 14 trigger?

8- The presence of Progesterone along with estrogen causes a significant decrease in the production of FSH and LH. What is the benefit of turning off the production of FSH & LH after day 14.

9- If fertilization does not occur the corpus luteum atrophies and ceases the production of progesterone. What does the dramatic drop in progesterone promote?

10- A fertilized egg produces a hormone called HCG (human chorionic gonadotropin). this hormone maintains the activity of the corpus luteum. What is the significance of this?

11- Identify four hormones produced by plants and briefly state what their role is.

12- What is the significance of the "Juvenile Hormone" produced by arthropods that exhibit complete metamorphosis (egg -> larva -> puppa -> adult)? How might this hormone, or others like it, be used for pest control purposes?

13- Pheremones are hormones released into the air. Describe at least two different examples of how pheremones are used by organisms to initiate changes in behavior.