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NOVA scienceNOW: Obesity

Classroom Activity

Learning Objective | Materials | Background | Procedure | Activity Answer | Links & Books | Standards  Print this Teacher's Guide (PDF, 7 pages)

Activity Summary
Students make hormone-receptor models to understand how hormones bind to specific receptors before initiating a cellular response.

Learning Objectives
Students will be able to:

• make a hormone-receptor model that shows how hormones only enter specific tissues.

• describe properties common to tissues that respond to hormones: the presence of a hormone-specific receptor, and the triggering of a cellular response once the hormone binds to the receptor.

• discuss how the hormone leptin plays a role in appetite.

• string
• signs labeled with the parts involved in the leptin feedback cycle (see Step 3)

• copy of the "Hormones and Receptors" student handout ( HTML)
• copy of the "Model Diagrams" student handout ( HTML)
• access to print and Internet U.S. map resources
• 4 twist-ties
• clear, pint-size plastic container (e.g., soda bottle)
• funnel that fits the container
• 20 ml vegetable oil
• water
• 2 pipe cleaners
• sealable plastic bag
• 8 cotton swabs
• Foam ball (or apple or orange)
• piece of clay (about 3 cubic centimeters)
• 10-15 toothpicks

Background
Hormones are produced by endocrine glands (e.g., pituitary, pineal, thyroid, thymus, adrenal, pancreas, ovary, and testes) and some non-endocrine organs (e.g., brain, lungs, kidneys, liver, thymus, skin, adipose tissue, and placenta). The endocrine system is a precisely controlled chemical communication system that regulates many physiologic processes in the body. Important parts of the endocrine system include:

• glands or tissues that produce and secrete hormones,
• tissue (referred to as target tissue: see Key Terms) and receptors responsive to specific hormones,
• chemical products produced by target tissue in response to hormones, and
• the resulting action of these products on the body.

Some hormones can act on many cell types, while others act on just one type. And, hormones regulate cells in different ways: some affect the very cells that make the hormone; other hormones regulate neighboring cells; still others regulate cells at a distance from where they are made. Hormones that regulate cells at a distance travel through the bloodstream. Consequently, these hormones come in contact with many different organs, tissues, and cell types. Yet, they only trigger a response when there is a functional receptor with a strong affinity for that hormone. Once the hormone binds to a receptor, cellular processes, such as changing which genes in a cell are active or triggering the production of a protein, are initiated.

A target tissue's response to hormones is regulated by hormone concentration (there must be enough hormone to initiate a response), the number of receptors on the cell membrane, and the affinity the receptor and hormone have for each other. Hormone receptors are often large proteins with a specific structure, including a place for a hormone to attach (i.e., the binding site) and a part that produces chemicals that stimulate a cellular response. In addition to producing chemicals that stimulate a cell's response, target cells also often make substances that provide a feedback signal, which directs the cells producing a hormone to either increase or decrease the hormone level. Disease can result when hormone production is too high or low or when hormone receptors are blocked or nonfunctional.

Leptin is a hormone produced by the body's fat tissue (adipocytes), and, typically, the amount made is proportional to the amount of fat tissue. Leptin helps regulate food intake and control how fats are distributed and stored in the body. Under normal circumstances, when a person's fat level drops, less leptin is produced. The part of the brain called the hypothalamus detects this decreased signal, and this decrease stimulates a person's desire to eat. Similarly, when the fat level returns to or exceeds a person's usual level, the leptin level rises. This increased signal reduces a person's desire to eat.

One form of obesity in humans is due to a mutation in the gene responsible for leptin production. More common, however, is a mutation in the genes that encode for leptin receptors. In both cases, the amount of leptin available to body tissues is very different for people with a mutation than for those without it. So even after eating a full meal, people with these mutations can still feel hungry, making it easy to overeat and to gain weight. Mice that are unable to produce leptin become tremendously obese. Leptin injections cure this condition. This technique also works on those few people born with an inability to produce leptin. But since it is much more common to have a problem with the leptin receptor, most obese people do not lose significant amounts of weight when given leptin injections.

In this activity, students make models that illustrate hormone-receptor systems. These models underscore the need for the hormone and receptor to be matched in order for them to be able to bind strongly to one another. The models also show how mutations interfere with the functioning of a hormone-receptor system.

Key Terms

endocrine system: A chemical communication system in the body that controls physiologic processes, such as growth, development, and appetite.

hormones: Chemical messengers made by one type of cell that regulate the function of other types of cells (and sometimes the cell that made the hormone). Hormones have a specific structure that can bind to specific receptors, allowing for precise control of the cells they regulate.

leptin: A hormone produced by fatty tissue (adipocytes) that acts on the brain, ultimately influencing appetite, body fat storage, and other physiologic processes.

mutation: A change in a gene that alters the message carried by the gene. The gene product (often a protein) may be altered so that it functions differently or not at all.

plasma membrane: The outer membrane of animal cells, made of a lipid bi-layer. It controls the movement of substances in and out of the cell.

receptor: A molecule on the plasma membrane or inside a cell that binds to a specific substance, such as a hormone, and triggers a cellular response. For peptide hormones, receptors are in the plasma membrane. For steroid and thyroid hormones, the receptors are in the cytoplasm and nucleus.

target tissue: A tissue with a receptor for a specific hormone that responds chemically to the hormone.

1. After viewing the segment, have students direct you in making a diagram on the board that shows how hormones such as leptin function. As you make the diagram, review key vocabulary terms. Include information such as:

   • glands and tissues produce and secrete hormones (Leptin is produced in fatty tissue.)

   • target tissues and target cells (because of their receptors) respond to specific hormones (Cells in the hypothalamus and liver have specific receptors for leptin.)

   • hormones stimulate target tissues and cells to produce chemical products (Once stimulated by a hormone, target tissues and cells produce proteins and other kinds of molecules.)

   • the chemical products produced by target tissues and cells affect the body in particular ways. (Leptin affects appetite, fat storage, and fat metabolism.)

2. Remind students that models play an important role in helping people understand systems, abstract concepts, and processes that are difficult to experience directly. Develop a list of subjects that have been made familiar through the use of models, such as DNA, the atom, the solar system, plate tectonics, the cell, and the human body. Have students explain ways each model helps people better understand a concept, system, or process. In addition, discuss the limitations of each model. Tell students that, in the activity, they will be making models of hormones and receptors to help them understand how hormones work and why they act only on specific tissues.

3. 

   Have students create a simplified model of the leptin pathway. Distribute string (representing the circulatory system) and signs (labeled adipose tissue, hypothalamus, and liver). Discuss how the model represents the components listed in Step 1.

   
   

4. Discuss the model by asking questions, such as:

   • Why it is important that a hormone enter the correct tissue? (The body needs the hormone response to occur in a particular place.)

   • How can a hormone enter some tissues but not others? (A hormone can only enter tissues and cells with receptors that correspond to that hormone. Hormones and receptors are specific for each other the way a key opens a specific lock.)

   • What might happen if hormones and receptors were not precisely matched? (If a hormone were to attach strongly to just any receptor, a wide variety of tissues and cells would respond to the hormone, and body functions would not be precisely regulated.)

   • What might happen if a target tissue were to receive too much or too little hormone? (The tissue's response would be excessive or inadequate, which could lead to problems or disease.)

5. Divide the class into teams. Distribute the materials and student handouts. Tell students to do Steps 1-5, making the different hormone-receptor models.

6. After students complete their models, ask each team to present one to the class. Discuss how mutations affecting hormone concentration in the bloodstream or the ability of a receptor to respond to a hormone might affect a person with the mutation. After discussing the models, have students answer the four questions on the handout.

7. As an extension or for homework, have students research Type 1 and Type 2 diabetes. While both involve irregularities in hormone production or hormone receptors, the way each type manifests itself and how they are treated are very different.

Write a possible outcome or consequence of the following mutations or conditions.

• A mutation alters the amount of hormone produced: (When little or no hormone is available to bind to the receptor, there will be little or no hormone-activated product made in the target cell. When too much hormone is produced, the target cell may produce an excess of hormone-activated product. Both situations may cause problems or disease.)

• A mutation prevents a hormone from binding to the receptor: (With the receptor unable to bind to a hormone, no hormone-activated product will be made at the target cell. This may cause problems or disease.)

• There is no cell or tissue to produce a hormone: (Without any hormone, no hormone-activated product will be made at the target cell. This may cause problems or disease.)

Write at least two strengths and two limitations of each model system.

Student Handout Questions

1. Which model best represents a hormone-receptor system? Why? (Answers will vary.)

2. Which mutation or condition might be "corrected" by hormone injection? (The two mutations or conditions that might be aided by hormone injections are: a mutation that blocks hormone production and a condition where there is no tissue to produce a hormone.)

3. Which mutation or condition might respond to a transplant of the tissue able to produce the needed hormone? (The two mutations or conditions that might be aided by a transplant are: a mutation that blocks hormone production and a condition where there is no tissue to produce a hormone.)

4. Which mutation or condition would most likely respond to gene therapy (i.e., injecting genes that can help the body produce functional products)? (The first two situations would benefit if functioning, healthy genes could be incorporated into a patient's DNA. The third would not, because the necessary tissue to utilize the gene does not exist in the first place.)

Web Sites

NOVA scienceNOW
http://www.pbs.org/wgbh.nova/sciencenow/3313/03.html
Offers an array of obesity-related resources, including additional activities, streamed video, and reports by experts.

Chipping Away at Leptin's Effects
www.hhmi.org/news/friedman.html
Describes how researchers have identified genes that are specifically regulated by the hormone leptin.

Hormones, Receptors, and Control Systems
arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/basics/index.html
Includes an overview of endocrinology and information on hormones, receptors, and target cells.

Leptin's Legacy
www.practicingsafescience.org/bulletin/mar2003/leptin/leptin2.html
Reviews research on leptin's role in the body.

Molecular Studies of Food Intake and Body Weight
www.hhmi.org/research/investigators/friedman.html
Discusses studies that focus on leptin's action and how leptin is regulated.

Books

Biology Coloring Book
by Robert D. Griffin. Collins, 1986.
Provides detailed diagrams of human body structures and systems.

The Concise Encyclopedia of the Human Body
by David Burnie. Dorling Kindersley, 1995.
Presents detailed illustrations and informative text and captions.

The "Hormones and Receptors" activity aligns with the following National Science Education Standards (see books.nap.edu/html/nses).

Grades 5-8
Science Standard C
Life Science

• Structure and function in living systems
• Regulation and behavior

Grades 9-12
Science Standard C
Life Science

• The cell
• The molecular basis of heredity

Classroom Activity Author

Developed by WGBH Educational Outreach staff.