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Students are aware after Lesson 3 that not all chemicals have the same effect on seed germination. In this lesson, students look at the variation among responses of seeds to the same dose of the same chemical. In reviewing their seed investigation, students note that sometimes not all seeds in the same bag responded in the same way. Students then use information about acetaminophen to learn more about dose and individual susceptibility. They learn that individual variability accounts for the different responses in the same types of organisms exposed to the same dose of a chemical. Students conduct an investigation into their own susceptibility to caffeine and compare individual responses.
The variety of responses among organisms exposed to the same dose of chemical is due to individual susceptibility. Dose and individual susceptibility play roles in all situations involving chemicals, including those involving medicines and caffeine.
After completing this lesson, students will
The human body is a complex organism. It responds to both external and internal conditions. For example, when the body gets too hot, it begins to cool itself by turning on its cooling system: sweat. When sensors in the brain detect that the body has cooled enough, the brain turns off the sweat glands.
Sometimes the body's balance is upset by chemical agents that put stress on different body functions. Some chemicals increase heart rate or sweating. Others decrease the rate of breathing. A chemical can have a wide-ranging effect or it can cause a very limited change in a particular organ of the body. A chemical is toxic if the body's response to it is an adverse one, such as headaches, nausea, rashes, convulsions, or death.
Chemicals vary widely in their toxic potential. The variation among chemicals is due to their chemical structure. A chemical's structure determines
Small differences in structure can cause big differences in toxicity.1
The effect of a chemical also depends on another set of variables. Organisms will have various responses to chemical exposures because each organism is unique. For example, if a population of people was exposed to the same amount of the same chemical, some people might have very severe reactions to the chemical while others might have no reaction at all. Those with a severe reaction are considered more "susceptible" to the chemical. Individual susceptibility varies but is influenced by age, gender, health status, genetics, and lifestyle.
 Photos: Corel |
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Children and elderly people often have an increased susceptibility to chemicals. For example, children have higher rates of respiration and consume more calories per unit of body weight than adults do. This can result in a greater exposure to and dose of a chemical for children compared with adults. The metabolism of chemicals also can vary because children's metabolic systems are still developing. In contrast, changes in physiologic functions, such as diminished kidney function, can impair the ability to excrete chemicals in an elderly person.
 Photos: Corel |
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Some people's susceptibility is related to their overall health. People with asthma may be more susceptible to air pollution. Lifestyle factors such as smoking, drinking, and drug use may affect a person's susceptibility to toxicants. On the other hand, people with good nutrition and health are more resilient, so they can metabolize and excrete toxic substances faster.
Chemicals sometimes affect males and females differently. For example, women absorb and metabolize alcohol differently from men. Women will generally have a higher blood alcohol concentration after ingesting the same amount of alcohol as men. While some of the difference in blood alcohol concentration can be attributed to the difference in size between men and women, scientists also have found that one of the enzymes that metabolize alcohol are less active in women than in men, causing a larger proportion of the ingested alcohol to reach the blood.2 When drinking heavily, women are more susceptible to alcoholic liver disease, heart muscle damage, and brain damage. In addition, pregnant women can share an exposure to a chemical with the fetus through the placenta, and women who are nursing may expose their babies to the chemical through breast milk.
Inherited differences among people account for some of the different ways people respond to chemicals. For example, some people are genetically more susceptible to poisoning from the insecticide parathion. Most commonly, people are exposed to parathion by eating foods grown with the use of the insecticide. Differences in individual reactions to the chemical depend on which of the two variants, or alleles, of a particular gene the individual carries. One variant results in low activity of the enzyme paraoxanase, and one variant results in high activity of the enzyme. Scientists have determined that the enzyme paraoxanase breaks down parathion into inactive, nontoxic products that are excreted in the urine. Those people who have the highly active enzyme and are exposed to parathion have mild symptoms such as abdominal discomfort, vomiting, and diarrhea, followed by headache and weakness, because their bodies are able to break down the parathion into nontoxic products at a higher rate. Those people with the gene that results in low activity of the enzyme can have much more severe symptoms when exposed to the same amount of parathion. These people may experience sudden collapse, coma, sweating, and difficulty breathing because their bodies cannot break down parathion quickly and the chemical remains in the body for a longer time.
Understanding the variety of responses that different individuals can have to different doses of a chemical is important to the study of pharmaceuticals. Written on the label of any drug are indications describing who can use the drug and how much to take. In addition, warning statements describe who should not use the drug. Such information is the result of the knowledge scientists have gained about the way the chemical affects a diverse population over time.
Caffeine is a chemical that most people ingest at some time in their lives, and many people ingest caffeine daily. People consume caffeine in coffee, tea, cocoa, chocolate, soft drinks, and some drugs. It is a naturally occurring chemical that comes from the coffee bean, tea leaf, kola nut, and cacao pod.
 Photos: Corel |
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As a central nervous system stimulant, caffeine in moderate doses can increase alertness, reduce fine motor coordination, alter sleep patterns, and cause headache, nervousness, and dizziness. In massive doses, caffeine is lethal. However, it is hard to get a lethal dose of caffeine just by ingesting it in food and drink: A lethal dose of caffeine is 170 milligrams of caffeine for every kilogram of body weight, or about 10 grams for an average-sized adult. One would have to drink 80–100 cups of coffee in rapid succession to reach that threshold.3
Caffeine is absorbed rapidly into the bloodstream from the gastrointestinal tract. It can exert its effects within 15 minutes after it is consumed and reaches maximum concentration in the bloodstream within about one hour. The blood distributes caffeine throughout the body, where the caffeine increases metabolic rate by about 10 percent. Once in the body, caffeine will stay around for hours: It takes about six hours for one-half of the caffeine to be eliminated. Gender affects a person's response to caffeine: Females metabolize caffeine 20–30 percent more quickly than do males.4
Because caffeine constricts the cerebral blood vessels, blood pressure rises and heart rate increases in the presence of caffeine. When people who regularly ingest caffeine stop ingesting it, they can suffer severe headaches because the blood vessels in their brain are dilating.
Research into the detrimental effects of caffeine has not uncovered any connections between ingestion of caffeine and heart disease, stroke, or cancer. There are some studies that show that large doses of caffeine, such as five to seven cups of coffee a day, can delay fertility.5
Many soft drinks popular among youth contain caffeine. The following table lists some soft drinks (12-ounce size) and the amounts of caffeine they contain.
| Soft Drink | Milligrams in 12 ounces |
|---|---|
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Jolt Cola |
71 mg |
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Josta |
58 mg |
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Mountain Dew |
55 mg |
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Surge |
51 mg |
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Diet Coke |
45 mg |
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Coca-Cola |
45 mg |
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Dr Pepper |
41 mg |
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Sunkist Orange Soda |
40 mg |
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Pepsi-Cola |
37 mg |
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Barq's Root Beer |
23 mg |
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7 UP |
0 mg |
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Minute Maid Orange Soda |
0 mg |
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Mug Root Beer |
0 mg |
| Source: Center for Science in the Public Interest.
Soft drinks and health: |
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Compared with other caffeinated drinks popular with adults, the caffeine content in soft drinks is lower. Coffee can contain between 80 and 175 milligrams of caffeine (per 7 ounces) depending on how it is brewed; espresso has 100 milligrams in just 1.5 to 2.0 ounces. Tea can contain 40–60 milligrams of caffeine (per 7 ounces). Iced tea contains 70 milligrams of caffeine in 12 ounces.
In this lesson, students can use information from their seed investigation to help them understand some of their own voluntary exposures to chemicals. When they take a medicine, they are exposing themselves to a chemical. When they follow the correct dosage for that medicine, they are taking advantage of what scientists know about how much exposure to the chemical results in the desired response. In addition, although they might not think of it in these terms, students are exposing themselves to the chemical caffeine each time they drink a caffeinated soft drink or eat a piece of chocolate. Students intuitively know that caffeine, a stimulant, affects some people more than others: It may keep some people awake at night while others can drink a caffeinated drink right before bedtime and still sleep. In this lesson, students measure their own heart rates after drinking a soft drink containing caffeine to see if they can quantify some of those individual differences.
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Activity 1 |
Master 4.1, Acetaminophen Dosage Chart |
1 transparency |
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Activity 2 |
Master 4.2, A Poisonous Dose? The Case History |
1 2-page transparency (optional) |
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Activity 3 |
Master 4.4, Parent Letter |
1 for each student |
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Extension Activity |
None |
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