Engage: Ask students what they know about global warming, and answer questions as appropriate. Brainstorm ways in which global warming might affect different species, including people. Ask students to think about the impacts of increased temperature on an aquatic species. As a class, you will set up the yeast and temperature demonstration. To do this, fill each plastic water bottle about a third of the way with warm tap water. Place each water bottle in an appropriate water bath: it is recommended to do three temperatures (room temperature, ice water, and hot water) and a replicate of each. Then, take each of the six filled balloons and secure them over the top of the bottle. Invert the balloon and swirl it with the water inside of the bottle so that the yeast and sugar mix well. Allow the bottles to stand in their water baths for the rest of the period, examining them every 5-10 minutes and observing the change in size of the balloons. The balloons will fill with carbon dioxide from the respiration of the yeast.
 

Explore: Students will receive the materials above and will design an experiment to determine the effects of temperature on the heart rate of an aquatic organism (Daphnia magna). They should test at least two different temperatures and a control. Students should ask you for approval of their experimental procedure before conducting the experiment. Some students may increase the temperature above that which the Daphnia can survive (about 40 C); decide ahead of time with your students if they are comfortable with this possibility.
 

Explain: Temperature affects the metabolism of Daphnia since they are cold-blooded; consequently their body temperature will be the same as the temperature of the water they are floating in. The chemical reactions that occur in the cells of Daphnia are dependent on certain enzymes, or proteins, to help the reactions proceed. As you increase the temperature of the water, the metabolism of the Daphnia increases as well, because chemical reactions occur faster at higher temperatures. This means that the heart rate will speed up in order to provide oxygen to the cells as the metabolism increases. However, at 40 degrees Celsius, the enzymes break down, and the chemical reactions can no longer occur, so metabolism stops and the Daphnia dies. In its natural environment, Daphnia does not ever experience temperatures of 40 degrees, so this is unlikely to occur. However, they do experience changes in temperature in their natural environment, and their metabolism does increase or decrease as the temperature changes.
Temperature affects other organisms as well, and although you can measure stress on a larger animal, it would be unethical to test extreme temperatures. Discuss with students what might happen to other animals in a warmer ecosystem, beyond the effects on heart rate. What might happen to phytoplankton or zooplankton populations? Ask students to think about how growth rate increases with temperature, and whether there is an upper limit to what both plants and animals can tolerate. What about the animals that depend on these species? This lesson leads well into Lesson #3, “Long-term temperature changes in the Hudson River”.
 

Extend: Students could also test the effects of temperatures on a plant, such as lettuce seeds, and measure the root hair growth of those that have germinated.
 

Evaluate: Students should create a graph of their results and discuss it with the class.