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nervous system guide

Caffeine Critters

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The environment challenges each living thing to react. It includes the air; water, heat, and light; and the chemicals that enter our bodies. Some chemicals look much like neurotransmitters to our bodies; we react to them in ways that can help or harm us.

You can explore the effects of some chemicals using a simple organism called a Daphnia.

In this exercise you'll explore National Science Education Content Standard C: Life Science - Structure and function

Middle School:

Specialized cells perform specialized functions in multicellular organisms. Groups of specialized cells cooperate to form a tissue, such as a muscle. Different tissues are, in turn, grouped together to form larger functional units, called organs. Each type of cell, tissue, and organ has a distant structure and set of functions that serve the organism as a whole.

Behavior is one kind of response an organism can make to an internal or environmental stimulus. A behavioral response requires coordination and communication at many levels, including cells, organ systems, and whole organisms. Behavioral response is a set of actions determined in part by heredity and in part from experience.

High School:

Multicellular animals have nervous systems that generate behavior. Nervous systems are formed from specialized cells that conduct signals rapidly through the long cell extensions that make up nerves. The nerve cells communicate with each other by secreting specific excitatory and inhibitory molecules. In sense organs, specialized cells detect light, sound, and specific chemicals and enable animals to monitor what is going on in the world about them.

What You'll Need:

  • A microscope that will enlarge your sample 100x
  • Two droppers
  • Tiny samples of two chemicals: caffeine and ethyl alcohol
  • A glass depression slide
  • A cover slip
  • Paper towels
  • A stopwatch
  • A culture of Daphnia

Also Good to Have:

  • A drop of a chemical to slow down the Daphnia

Keeping Safe:

Your teacher will give your group only 5 drops of each liquid. They are toxic (poisonous). In your room, your teacher should also have a special report on the dangers of each chemical, called an MSDS sheet. Never taste any chemicals in the lab. Wash your hands after touching the Daphnia culture.


A Daphnia is a tiny crustacean (in the group with shrimp) that has a clear outside skeleton and jointed legs. Like other Arthropods, its heart is on its back.

Put one Daphnia into the well of a cover slip. Give the critter a little water, but not much, or it will swim too quickly for you to observe it. (You may have to experiment a bit.) If your microscope has an electric light source, make sure it does not cause the water to evaporate or the Daphnia will die. Put a cover slip over the depression, and look carefully at your organism. Can you see the tiny, beating heart? Hint: It's on its back. Count the number of beats per minute. Then have each of your lab partners count the heart rate and average the results. Record the average in the table.

Next add one drop of caffeine solution to your Daphnia culture. Wait 30 seconds, then count the heartbeat three times. Finally, add a drop of alcohol solution. Repeat your counts.

Normal Caffeine Alcohol
Beats per minute:
  Trial 1      
  Trial 2      
  Trial 3      


1. When humans consume caffeine, how does it make them feel?

2. Did you see any evidence that the Daphnia reacted in the same way?

3. When humans consume alcohol, how does it make them feel?

4. Did you see any evidence that the Daphnia reacted in the same way?

5. Both alcohol and caffeine can be deadly poisons if consumed in large quantities. Why do you think this would be true?

6. What other chemicals might act like these in the body?


1. Humans experience increased heart rate, higher blood pressure, and greater alertness.

2.The heart rate increased.

3. Humans experience decreased heart rate and slow movement and reaction time.

4. The heart rate slowed down.

5. These chemicals contradict normal body responses for the nervous system.

6. Herbal ephedra is a stimulant. Acetylcholine and ibuprofen slow the nervous system.

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