Give a simple explanation of how the class will try to indirectly observe yeast respiration by providing molasses as a source of sugar, and devising a way to try to capture the carbon dioxide gas that will be given off as the yeast cells respire.

Demonstrate how to set up the yeast respiration chambers, first using water to show how to remove air bubbles from the small test tube. Be sure to ask students why there should be no air present at the onset. Next demonstrate the same thing, using the 10% molasses solution instead of water. Be sure to caution students about pressing too hard on the stoppers.

Then show students the warm yeast solution, and demonstrate how to add yeast cells to the test chamber. This is done by first gently stirring the solution, then filling the pipette with some of it. (Since the yeast cells will settle to the bottom of the beaker, it is important to stir the solution first.) Gently squeeze exactly 6 drops of the yeast solution down the side of the large test tube. Do not put the tip of the pipette into the molasses solution. Advise students to practice dropping 6 drops back into the beaker of yeast solution before attempting to add them to their test tubes. The drops should be uniform in size and they should not contain air bubbles.

Demonstrate how to place a square of plastic wrap over the top of the chamber and smooth it around the tube's circumference. Point out that this is to keep out insects and other airborne contaminants. Also demonstrate how they can mark their chambers with their initials or some other system you have devised. (This can be done at the onset if you prefer.)

Allow students to practice with water before they actually set up their test chambers. As students finish preparing their test tubes, have them carefully wipe any liquid from the outside of the tubes with damp paper towels, and place plastic wrap over the tops. Then place the completed chambers in a test tube rack.

When all students have finished, place the racks where they will not be disturbed. Avoid very warm or very cool locations.

Have students examine their test tubes to see if anything happened. Depending largely on temperature (but don't tell students that!) there may be only small bubbles present in the smaller tube or it may be nearly filled. It is okay if the smaller tubes float up as they fill with carbon dioxide and dislodge the plastic wrap. Be sure to avoid calling the bubbles produced "air" bubbles, since they are not filled with air. Ask students what the difference is between the composition of air and the bubbles produced by their yeast cells. If they don't know the composition of air, make it an assignment to find out.

Have students measure the heights of the gas bubbles (to the nearest millimeter) each day and record them. Continue this on a daily basis until nearly all of the small tubes are completely filled with carbon dioxide. Students can then graph their results using a bar graph or an x-y scatter plot, with time on the x-axis (dependent variable) and height of gas bubble on the y-axis (independent variable). Allow students a few minutes to compare their graphs with those of their classmates.

Ask students to describe what happened physically in their test tubes. Then ask them to describe how what physically happened gives evidence for what was happening with the yeast cells in their tubes.

Some students may have noticed the tiny bubbles floating upward from the bottom of the tubes, but seen that not all of these were trapped inside the smaller, inverted test tube. Some of the tiny bubbles escaped outside the smaller tube and rose to the surface of the molasses solution, where they were released into the air at the top of the chamber. So is the size of the gas bubble inside the smaller test tube an accurate representation of the amount of yeast respiration that occurred? It isn't, but it is safe to assume that the ratio of gas trapped to the gas that escaped is consistent across all the test chambers. This is because the yeast cells, which are denser than the molasses solution, sank to the bottom of the large test tubes when they were first added. The curved bottom of the tube caused them to occupy the center of the tube, where their population was visible, and where most of the carbon dioxide they produced would rise into the smaller inverted tube.

Students might also notice (from their graphs or the bubbles themselves) that during the first 24-48 hours (again, depending on ambient temperature), most of the bubbles grew relatively slowly in height, but then suddenly grew much bigger the following day or days. Give students some time to think about why that might be. If they need help, ask them how yeasts reproduce. If they don't make the connection, explain that one cell buds a new one to make two cells, those two become four, those four become eight, etc. A diagram sketched on the board will help. Also sketch a graph of a theoretical population size, with time on the x-axis and population sizes of 2, 4, 8, 16, and 32 on the y-axis. Ask if this graph is similar in shape to the graphs of the gas bubble heights. Make sure students make the connection that not only were the yeasts respiring in their test chambers, but because they had an adequate food supply (glucose from the molasses), they were also reproducing.

Ask students how they know that the bubbles produced were actually made of carbon dioxide produced by the yeasts. Without a chemical analysis of the gas, they actually don't know. Also ask them how they know that if they created an identical test chamber of molasses solution, but without adding the yeast cells, they wouldn't get the same result - a gas bubble that grew over time. Until they've done the experiment they don't know.

This is why scientists use controls when they do experiments in order to rule out other explanations of their results. In this case the control would be an identical test chamber set up without the addition of yeast cells, and placed alongside the experimental chambers in the rack for the duration of the experiment. If a gas bubble appeared and grew over the next few days, it could not be concluded that the gas bubbles in the test chambers were due to yeast respiration. Instead, it would be more likely that a chemical reaction within the molasses solution produced the bubbles. But if no gas bubble appeared in the control chamber, the most likely explanation for the growing bubble in the test chambers would be the production of carbon dioxide by respiring yeast cells.