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NOVA scienceNOW: Picky Eaters or Superior Tasters?

Classroom Activity

Activity Summary
Students investigate the genetic basis of taste by testing their ability to taste a bitter compound, and by sampling food.

Learning Objectives
Students will be able to:

• define the term "taste receptor" (a structure on the surface of a cell that binds to specific molecules and initiates a signal)

• explain how genes can influence our taste perception

• describe how taste and smell combine to create a flavor sensation

Suggested Time
45 - 60 minutes

Multimedia Resources

• Picky Eaters QuickTime or Windows Media Video
• The Sense of Taste QuickTime Video
• Howard Hughes Medical Institute (HHMI) Taste Lecture Video

Additional Materials

• PTC paper (order from a biological supply company)
• one type of baby food for the class to taste (e.g., peas, carrots)
• a spoon to dispense the baby food
• small paper cups for samples of baby food (one per student)
• straws or small spoons for tasting the food sample (one per student)
• mint oil (e.g., peppermint, spearmint, wintergreen, or eucalyptus, available at grocery stores) or a tin of strong-smelling breath mints
• cotton swabs (one per student)
• Picky Eaters Student Handout (PDF), one per student

Do all people experience taste in the same way? Some people savor the flavor of beets while others grimace in disgust. What accounts for the difference?

Our sense of taste is rooted in our tongue, which is covered with taste buds. Each taste bud is a cluster of taste cells. Each taste cell has specific proteins on its surface that act as taste receptors, detecting sweet, bitter, sour, salty, or umami, a savory taste sometimes described as "meaty." These taste receptors are shaped in such a way that food molecules can fit into them. When a food molecule fits properly into a receptor, a signal is sent to our brain, and we experience a particular flavor.

Scientists are still teasing apart the many factors that can influence our sense of taste, but genes play a key role. Genes, segments of DNA, are transcribed into messenger RNA (mRNA) and then translated into amino acids. Hundreds of amino acids, linked in a specific order, fold intoparticular 3-D structures to create proteins. Differences in taste-receptor genes can result in different amino acid sequences that give taste-receptor proteins different shapes. Because the sense of taste depends on food molecules fitting into taste-receptor proteins that send signals to the brain, differences in receptor shapes can result in differences in taste perception.

Genetic differences might render specific taste receptors ineffective or prevent them from forming in the first place. Individual genetic make-up determines, for example, the number of sweet receptors, the sensitivity of bitter receptors, and the brain's ability to convert sour sensory information. Genetic variation might make you crave sweets as a child, but enjoy bitter flavors as an adult. Scientists are still sorting out the details.

Taste is different from flavor perception. Our sense of taste is centered on our tongue, but our perception of flavor is strongly influenced by smell. Food temperature and texture, as well as moisture in the mouth and the residual taste of food we just ate, all affect how we experience food.

Taste and flavor perception are examples of how genetics can influence our behavior. Someone with the gene for the bitter taste receptor will likely avoid bitter foods. In other words, that person's genetic make-up is influencing his or her choice to avoid broccoli. However, genes do not entirely direct our behavior. We still choose what we eat, based on cultural practices, available food choices, marketing, and knowledge of what's healthy and what's not. The same is true of other genetic links to behavior. For example, specific genes are linked to drug addiction, but family stability and peer groups also influence whether or not someone becomes addicted to drugs.

In this activity, students will investigate the genetic basis of taste by testing their ability to taste a bitter compound (PTC) and by sampling food.

• order PTC paper from a biological supply company
• Make copies of the Picky Eaters handout (one per student).
• Bookmark the Web pages:
  • Picky Eaters Quick Time or Windows Media Video
  • The Sense of Taste QuickTime Video
  • HHMI Taste Lecture Video
• obtain cotton swabs (one per student) and mint oil or strong-smelling breath mints
• obtain small paper cups for the taste test (one per student)
• obtain one type of baby food for the class to taste, and a spoon for serving
• cover the container's label so the food's identity is hidden

1. Informally survey students about their taste preferences. For example, ask students to raise their hand if they:

   • love ice cream
   • enjoy eating broccoli
   • hate spicy, hot, or strongly flavored foods
   • have a hard time deciding what to order at a restaurant
   • always ask for sauces and dressings on the side
   • dive right into their food after being served
   • carefully taste each food on their plate
   • add a lot of salt
   • have ever been called a picky eater

2. Explain that their responses might be based on their genes. Show the following videos:

   • The NOVA scienceNOW segment Picky Eaters. You can stream it from the NOVA scienceNOW Web site at http://www.pbs.org/wgbh/nova/scienceNOW (10 minutes)
   • The Sense of Taste at: http://www.pbs.org/teachers/connect/resources/888/preview/, which describes the connection between taste buds and the brain (4 minutes)
   • HHMI Taste Lecture Video, http://www.hhmi.org/lectures/webcast/ondemand/97webcast1/taste.html which describes taste buds and receptors in more detail (6 minutes)

3. If appropriate for your students, point out the genetics underlying taste. For example:

   • Genes are segments of double-stranded, complementary DNA, made of the nucleic acids adenosine (A), thymine (T), guanine (G), and cytosine (C).
   • To decode DNA, the complementary strand of DNA is transcribed into messenger RNA (mRNA)
   • To translate mRNA into amino acids, every three nucleic acids (called codons) are decoded using a chart
   • Amino acids, in a specific sequence and 3-D structure, form proteins

4. Tell students that special proteins on the surface of taste cells in our tongue help us detect specific flavors. These proteins (which are made up of amino acids) are called taste cell receptors. Mention that students will test factors influencing their ability to taste and to perceive flavor.

5. Give each student a copy of the Picky Eaters handout and a piece of PTC paper. Have them follow the instructions on the handout and answer the questions. For STEP 3, help students tally class PTC responses. Draw the following chart on the board:

   	Strong Taster	Non-Taster
   Number of students
   Percentage of the class

6. For STEP 5 on the handout, give each student a spoonful of baby food in a cup and a cotton swab dipped in mint oil (e.g., peppermint, spearmint, wintergreen, or eucalyptus) or a very strong breath mint. Do not reveal the identity of the food. Students should smell the mint as they taste the food. Next, they should taste the food sample while holding their nose.

7. When students have finished the taste test, reveal the identity of the food and tally the number of correct responses. A possible statement describing the difference between taste and flavor is: Taste is sensed by our tongues, whereas flavor is constructed by the brain, based on stimuli from the tongue and nose.

8. Discuss the role of genetics in our sense of taste and perception of flavor. Ask:

   • Did all students who were able to taste PTC experience the same bitter flavor?
   • Were tasters who could taste PTC better at identifying the food sample?
   • How do genes influence our food choices?
   • Even if our genes predispose us to dislike certain foods, can we still choose to eat healthy meals?

   Point out that generic difference, such as number of taste buds, play a role in flavor perception and ability to smell. Other external factors, such as temperature, can affect how we experience food.

• A. Ask students to design and carry out their own taste test, involving one or more of the following:

  • different flavors (e.g., bitter, sweet, salty)
  • sight (e.g., use food coloring to change the color of clear soda)
  • texture (e.g., prepare and compare puréed and whole foods)
  • temperature (e.g., taste the same food hot, cold, and room temperature)

  Challenge students to create tests that weigh one variable at a time, use experimental controls, and collect quantitative data.

• Discuss how genes can influence our behaviors (such as food choice), and ask students whether genes determine our choices. Our susceptibility to become addicted to certain drugs, including alcohol and cigarettes, seems to have a genetic basis. Understanding our family history is important so that we are aware of our own vulnerabilities. Point out that although our genes might make us more likely to engage in certain behaviors, many factors, such as our parents, our peers, the media, and cultural norms, influence our choices.

  • According to one study, smokers who were less sensitive to bitter taste (PTC) rated taste as a strong reason for smoking, and those who were sensitive to bitter taste were less likely to smoke for taste.
  • One genetic variation found in over 40 percent of people is associated with impulsivity, low self-control, binge drinking, and substance use.
  • Brain cells have receptors for dopamine, a molecule that makes us feel good. Research has shown that people with a particular gene for a certain type of dopamine receptor (called DRD2) are more likely to be addicted to alcohol or cocaine.

STUDENT HANDOUT ANSWER KEY

1. Predictions will vary.
2. Descriptions will vary. Some students will describe a strong bitter taste, whereas others will taste nothing at all. A few will describe tasting a mild bitter taste.
3. The class might categorize their responses as "tasters" and "non-tasters" or include categories that address the strength of the taste. In general, about 75% of the class should be tasters.
4. The Taster allele is dominant, and tasters are more common in the class. If a student can taste PTC, then the student can deduce that he or she probably has the dominant taster allele, but likely cannot distinguish the second allele. If a student is a non-taster, then he or she probably has two recessive alleles (homozygous recessive). Note that a person's alleles cannot be determined with absolute certainty unless a DNA test is done.
5. Answers will vary.
6. Possible statement: "Taste is sensed by our tongues, whereas flavor is constructed by the brain, based on stimuli from the tongue and nose."

1. The chart in STEP 4 shows only the two most common alleles for the PTC receptor gene. There are also several rare alleles. Do you think that anyone in your class has the rare alleles? Why or why not? (Yes, it is possible that students in the class have rare alleles. In particular, a taste described as other than bitter may indicate the presence of rare alleles. However, we cannot know a person's alleles with certainty unless a DNA test is done.)
2. List three reasons that can help explain why it is important to distinguish different tastes. (Different flavors give us different information about our food. For example, sweet flavors signal the presence of sugar, which provides energy. Bitter flavors might warn us that a plant is toxic. Sour might tell us that fruit is not ripe. Salt provides electrolytes. Depending on our needs, certain foods might be more nutritionally valuable than others. Also, having a diminished sense of taste may let a person eat a wider array of food. Evolutionarily, having a plentiful food supply is an advantage.)
3. How did holding your nose and smelling the mint affect the flavor of the food sample? (Blocking the nose and smelling the mint made it difficult to identify the food, demonstrating that our senses of smell and taste work together to develop our perception of how food tastes. Our brain combines the taste and smell signals to construct our perception of a food's flavor.)
4. Genes are only one factor that determine your food preferences. What else might affect food preferences? (Culture, age, food choices available, what foods you're accustomed to eating, marketing and advertising.)

ASSESSMENT

Use the following rubric to assess each team's work.

The Picky Eaters activity aligns with the following National Science Education Standards (see books.nap.edu/html/nses).

Grades 9-12
Life Science Standard C

• The Cell

Classroom Activity Author

Alison Fromme and WGBH Educational Outreach staff