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Endocrine System for AP Biology (page 2)

By — McGraw-Hill Professional
Updated on Oct 24, 2011

Homeostasis

Homeostasis is the maintenance of balance. Hormones can work antagonistically to maintain homeostasis in the body. Two examples we will talk about are insulin/glucagon and calcitonin/PTH:

  1. Insulin/glucagon. Both are hormones of the pancreas and have opposing effects on blood glucose. Let's say that you eat a nice sugary snack that pushes the blood glucose above its desired level. This results in the release of insulin from the pancreas to stimulate the uptake of glucose from the blood to the liver to be stored as glycogen. It also causes other cells of the body to take up glucose to be used for energy. Sometimes if you go a long time between meals, your blood glucose can dip below the desired level. This sets glucagon into action and causes its release from the pancreas. Glucagon acts on the liver to stimulate the removal of glycogen from storage to produce glucose to pump into the bloodstream. When the glucose level gets back to the appropriate level, glucagon release ceases. This back-and-forth dance works to keep the glucose concentration in our bodies relatively stable over time.
  2. Calcitonin/PTH. Like glucose, the body has a desired calcium (Ca2+) level it tries to maintain. If it drops below this level, PTH is released by the parathyroid gland and works to increase the amount of Ca2+ in circulation in three major ways: (a) releaseof Ca2+ from bones, (b) increased absorption of Ca2+ by the intestines, and (c) increased absorption of Ca2+ by the kidneys. If the blood Ca2+ level gets too high, the thyroid gland releases calcitonin, which pretty much performs the three opposite responses to PTH's work: (a) puts Ca2+ into bone, (b) decreased absorption of Ca2+ by the intestines, and (c) decreased absorption of Ca2+ by the kidneys.

One last distinction I want to make before we move on is to touch on the difference between protein hormones and steroid hormones.

Protein hormones are too large to move into cells and thus bind to receptors on the surface of cells. In response to the binding of a protein hormone, a change occurs in the receptor that leads to the activation of molecules inside the cell, called second messengers, which serve as intermediaries, activating other proteins and enzymes that carry out the mission. The second messenger to know for this exam is cyclic adenosine monophasphate (cAMP), involved in numerous signal cascade pathways. Protein hormones activate cAMP through a multi-step process that begins with protein–hormone activation of relay proteins such as G proteins. These proteins are able to directly activate a compound known as adenyl cyclase, which in turn produces cAMP.

Since we discussed regulatory mechanisms earlier, it is important to point out that there are G proteins that function to inhibit cAMP and work antagonistically to hormones that activate cAMP.

Steroid hormones are lipid-soluble molecules that pass through the cell membrane and combine with cytoplasmic proteins. These complexes pass through to the nucleus to interact with chromosomal proteins and directly affect transcription in the nucleus of cells.

Practice problems for these concepts can be found at: Human Physiology Review Questions for AP Biology

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