Anna Cajiga SME 301 Taste and Smell Summary (#10) May 2, 2005
Title Does Smell Really Affect Taste? Benchmark Area: Life Science Category: Cells Benchmark: Explain why and how selected specialized cells are needed by plants and animals (Middle School). Misconception There is no relation or connection between taste and smell. Background There is a common misconception that there is no relation or connection between taste and smell. However, this activity demonstrates that the sense of smell plays a large role in the quality of our sense of taste. The nose and the mouth connect at the back of the throat, so when we place food in our mouth, the vapors from that food are able to enter the nose. Since there are many more receptor cells, or specialized cells that allow our brains to interpret the taste and smell of the chemicals in various substances, in the nose than there are in the mouth, our sense of smell is much more sensitive than our sense of taste. Therefore, what our brain interprets as the flavor of a food is really a combination of our brain’s interpretation of both the taste and the smell of that food. Materials Ketchup Mustard Maple Syrup Barbeque Sauce Hot Sauce Honey 12 Spoons Blindfold Glass of Water Directions 1. Distribute a dime-sized drop of ketchup into two spoons. 2. Repeat step one with the mustard, maple syrup, barbeque sauce, hot sauce and honey, using a different spoon each time. Make sure your taste tester does not know what the condiments are. 3. Place the blindfold on the taste-tester so they cannot see. 4. Have the taste-tester pinch their nostrils together to plug their nose. 5. Ask the taste-tester to lick one of the spoons with ketchup on it and try to identify the condiment. Record the results. 6. Repeat step five with the mustard, maple syrup, barbeque sauce, hot sauce and honey, making sure you have the taste-tester take a drink of water and wait at least 15 seconds between sampling each condiment. 7. Now, using the remaining six spoons, have the taste-tester sample each condiment without their nose plugged. Make sure they keep their blindfold on, and that they continue to drink water and wait 15 seconds between sampling each condiment. Record your results. 8. Compare the results from when the taste-tester had their nose plugged to when they did not. 9. Wash the spoons, and repeat the entire activity using a different taste-tester. Results After completing this activity, students will observe that when the taste-tester has their nose plugged, it is generally much more difficult for them to distinguish between the various condiments than it is when their nose is not plugged. They also may notice that, with both their nose plugged and unplugged, some taste-testers are more able to distinguish between and identify the condiments than other taste-testers are. Discussion Taste and smell are two ways that our bodies get information about the world around us. In order for us to experience both taste and smell, we need receptor cells. These cells are a vital part of taste and smell and they can affect the quality of our senses. Receptor cells are double ended cells that are needed for us to be able to taste and smell. Receptor cells convert contact with the chemicals in the air or in our food into electrical signals. These electrical signals are then sent along sensory nerves to the taste or smell center of the brain. The brain converts these electrical signals into tastes and smells, thus allowing our bodies to interpret what it is that we are tasting or smelling. In the mouth, the receptor cells are called taste buds. Taste buds are contained inside tiny bumps called papillae. Papillae and taste buds are mainly located on the tongue, but can also be found in the throat, the epiglottis, and on the palate. In all, we have several thousand taste buds in our mouths and throats. When these taste buds come in contact with a chemical, electrical signals are sent along sensory nerves to the gustatory, or taste, center of the brain. In the nose, receptor cells are called olfactory receptors. We have between 5 and 20 million of these receptor cells in our noses. The olfactory receptors are located in the olfactory epithelium, which is found in the upper nasal passage. This postage-stamp-sized area is covered in tiny hairs called cilia. The olfactory receptors are located on the end of the cilia and connect to the olfactory bulb, the area of the brain that sends messages to the smell center in the brain. This means that when the olfactory receptors come in contact with a chemical in the air, electrical signals are sent from the receptors to the olfactory bulb, then along sensory nerves called olfactory nerves to the smell center of the brain, where the electrical signals are decoded into a certain type of smell. Both taste buds and olfactory receptors need to come in contact with a chemical in order for a taste or smell to be sensed. However, before a chemical can be “received” by these receptor cells, it must first be dissolved in a liquid. In the mouth, the chemicals in the food we eat are dissolved in the saliva. This allows the taste buds to convert the chemicals into electrical signals that our brains can understand. In the nose, the liquid is known as mucus. Mucus covers the cilia found in our noses. When chemicals come in contact with this mucus, they become dissolved in the liquid, so the olfactory receptors on the cilia are able to convert the chemicals into electrical signals to be sent to the smell center of the brain. While many people believe that smell and taste are independent of each other, the two senses are in fact interrelated. This is because when we are eating, we also smell the food, both through our nostrils and through the connection between the mouth and nose located at the back of the throat. Since we have many more olfactory receptors than taste buds, our sense of smell is much more sensitive than our sense of taste. Our brains interpret the flavor of a food as a combination of both the taste detected by the taste buds and the smell detected by the olfactory receptors. Therefore, we need both of these factors to determine the “taste” of a food. This is apparent in the above activity. When participants were unable to smell a condiment in addition to tasting it, it was much more difficult for them to recognize the flavor and identify the condiment. This activity illustrates that the sense of smell can affect the sense of taste. However, there are many other factors that can also affect the quality of both taste and smell. We know that receptor cells are needed for taste and smell to be detected, but the number of receptor cells a person has affects the quality of their senses. Some people are born with more receptor cells in their nose and mouth than other people are. This means that there are more cells to come in contact with chemicals in food and in the air and more electrical signals are produced and sent to the brain. This results in people who have more receptor cells having improved senses of taste and smell. These people are known to scientists as “super-tasters”. You may have noticed in the above activity that some people were more able to identify the condiments than other people were. It is very likely that the person who was less able to identify the condiments had less taste buds and olfactory receptors than the person who was more able to identify the condiments. In relation to many animals, for example, dogs, humans actually have very weak senses of taste and smell because these animals have many more receptor cells. Receptor cells are constantly dying and being replaced. This means that food preferences can change over time. A food that at one point tastes bad to us may later be appealing, because the taste buds in our mouths have been replaced with new ones. However, as people grow older, the receptor cells begin to die off without being replaced. This means that the number of receptor cells in both their nose and mouth slowly becomes less and less. This results in them having less receptor cells, meaning their senses are weaker, which is why young people are more likely to have stronger senses of taste and smell. We know that receptor cells convert chemicals into electrical signals and that the number of receptor cells can affect the quality of our senses, however, not all receptor cells are the same. There are some receptor cells that are more sensitive to certain chemicals that are converted into particular tastes or smells. For example, in the nose, there are about 20 different kinds of receptor cells. Each of these is more sensitive to certain chemicals in the air that produce different kinds of smells. On the tongue, there are four types of receptor cells. These are each more sensitive to a different taste – sour, salty, sweet, or bitter. All of the other tastes that our bodies can sense are simply different combinations of these four tastes. In the past, the general view on taste buds was that the location of these four types of taste buds was very specific. Researchers created a model of the tongue illustrating a strict location for each type of taste bud. The model looks like this:
Tongue
Sour Bitter
Salty Sweet However, in recent times, researchers are coming to realize that while certain taste buds are in fact more sensitive to certain chemicals, or tastes, the location of these taste buds is not as clear cut as this model suggests. Instead, they suggest that the location of these taste buds is more generalized. For example, researchers now believe that just because an area of the tongue is designated the “sweet” area, this does not mean that there are not taste buds for salty, sour and bitter in this area. While certain areas of the tongue may have higher numbers of a specific type of taste bud, all the types of taste buds can actually be found all over the tongue, epiglottis, palate and throat. Some researchers have also recently suggested that there may be a fifth type of taste that some receptor cells are sensitive to called umami, which is described as a meaty-flavor. However, the existence of umami taste buds has not been proved, and is a highly debated topic. Understanding more about taste and smell allows us to explain many observations from our every day lives. For example, anyone who has ever had a cold has noticed that when your nose is stuffed, you cannot smell as well. This is because the excess mucus that is produced when you have a cold forms a thick coat over the cilia, and therefore also over the olfactory rods. While mucus is usually helpful because it dissolves the chemicals in the air and allows the olfactory rods to detect the chemicals, when there is too much mucus the chemicals cannot get through all the liquid to the olfactory rods. This means that fewer chemicals are being detected by our receptor cells, less electrical signals are being produced and sent to the smell center of the brain, and subsequently, our sense of smell is weaker. You may also notice that when you have a cold, food often does not taste as good. This is because your sense of smell is weaker, and since smell plays a large role in the perceived flavor of the foods we eat, our ability to distinguish between different tastes is also weakened by the excess mucus in our nose. Similarly, many older adults do not enjoy eating as much as they used to, saying food just does not taste the same. Instead of being caused by excess mucus, this is caused by the fact that as people age, their taste and smell receptors die, so they have fewer receptor cells in their bodies. This means that their ability to taste and smell is reduced, which makes eating less enjoyable. Lastly, understanding that certain areas of the tongue may have higher numbers of a specific type of taste buds explains why many substances that would be lethal or poisonous to humans and animals, for example household cleaners, are often given a bitter flavor. This is because the back of the tongue is more likely to have high amounts of bitter taste buds, and the back of the tongue is also where our gag reflex is activated. When the bitter poisons touch the bitter taste buds at the back of the tongue, the gag reflex can be triggered and the poisons will be projected from our bodies before they can do us any harm. Classroom Resources 1. Taste and Smell Videos from the NOVA Mystery of the Senses Video Series These two, hour-long videos are some of the most engaging and interesting on the market today, as they present vital information about taste and smell by capturing students’ attention and taking a look at the science, history and cultural values that influence these senses, rather than just reiterating meaningless facts that students would find easy to ignore. 2. Dr. Margot Weinberg is a pediatrician who, in simple and straightforward language, would be able to provide an elementary school classroom with an interesting presentation about how our bodies detect different tastes and smells and gain information about the world around us, using real life examples from her experiences as a doctor. 3. Smelling and Tasting (Senses and Sensors) by Alvin Silverstein This book is a great resource for older elementary children to use in further research about taste and smell because it pairs in-depth, scientific information with descriptive photographs and diagrams so students are able to guide themselves as they learn about how humans are able to taste and smell and then make comparisons between the abilities of humans and those of animals. Credits I found this activity at
The Human Body – An Illustrated Guide to Its Structure, Function and Disorders by Charles Clayman, MD
The Random House Library of Knowledge: The Human Body, Your body and How it Works by Ruth Dowling Bruun, MD and Bertel Bruun, MD
Reader’s Digest: How the Body Works – 100 Ways Parents and Kids can Share the Miracle of the Human Body by Steve Parker.