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Lesson Flow Engage ➙ Explore ➙ Reflect ➙ Assess ➙ Make Career Connections ➙ Variations Engage (20 minutes) Discuss the concept of cellular respiration and put it in context with respiration (breathing). What does your body need to survive and function? Can you give some examples? Nudge the discussion as needed so that oxygen, food, water, and energy are all mentioned. Why do we need all of these things? For example, can you explain why we need to eat? [You might need to prompt students to get to the answer with questions like "What happens if you do not eat for a long time?" or "Do you think you will feel more energized and can run around faster and longer with or without food?" et cetera.] We need food and oxygen to generate energy that our body can use for growth, repair and movement. If we do not eat, our body will eventually run out of energy, will not be able to function anymore, and will starve. What happens inside our body to the food we eat? The food we eat is digested in our stomach and digestive system. That means it is broken down into smaller components (such as proteins, vitamins, carbohydrates, and fats) that can be absorbed by our body to be processed. Some of these processes release energy that our body can use for functions such as moving, thinking, or growing. What kind of nutrients or compounds do you think foods contain that help give our body energy? How does our body transform food into energy? [Students may not know the answer—that is fine, and a good time to formally introduce cellular respiration.] Many foods that we eat can be converted by our body to carbohydrates or sugars such as glucose, which is the most important starting compound to make energy. The process that our body uses to make energy from glucose is called cellular respiration. It happens continuously in the cells of our body to provide energy to us nonstop. Respiration, the process of breathing, is not the same as cellular respiration, although both are related. Write down the chemical equation for cellular respiration and explain it to your students: C6H12O6 + 6 O2 → glucose (from food) oxygen (from air) (in exhaled breath) (used by body) (used by body) carbon dioxide water 6 CO2 + 6 H2O + energy Explain to your students that cellular respiration is a two-stage process. First, glucose is broken down into smaller molecules, which happens in the cytoplasm of our cells and produces some energy. The second stage takes place in the "powerhouses of our cells," the mitochondria. Here, the smaller molecules from glucose are broken down further and, in combination with oxygen, make the end products of cellular respiration carbon dioxide, water, and energy. This step is the major energy contributor during cellular respiration. Make the link between cellular respiration and breathing (respiration) by discussing the following questions. Where do you think the oxygen that we need for cellular respiration comes from? The oxygen for cellular respiration comes from the air (which contains about 20% oxygen). This is the reason why we have to breathe—to provide oxygen to the cells for breaking down glucose to generate energy. What do you think happens to the by-products of cellular respiration, carbon dioxide and water? Both, carbon dioxide and water, are carried away in our bloodstream. The water usually leaves our body through sweating or is transported to our kidneys and ends up in our urine. The carbon dioxide eventually ends up in the breath that we exhale. Explain your students that you will do a series of experiments to measure the end product of cellular respiration (carbon dioxide) that is present in their exhaled breath. Optionally, you can show your students this introductory video to this lesson plan experiment: Google Classroom Tell your students that in their experiments, they will investigate what happens to cellular respiration when they get more active and need more energy—like when exercising. Use the following questions for them to make predictions. Write the predictions down to compare to the results after the experiment. What would happen if your body suddenly needed a lot of extra energy (for example, when you exercise)? Do you think the cellular respiration rate would change? What does that mean for the amounts of carbon dioxide and water being produced? Would the amounts increase or decrease? Once the prediction is made, help students understand how the indicator solution is related to carbon dioxide and cellular respiration by asking questions and doing two short demonstrations. Carbon dioxide is an acidic gas. Can you think of a liquid that has carbon dioxide in it? Is that liquid acidic, basic, or neutral? Soda has carbon dioxide. Adding carbon dioxide to a liquid makes it acidic. Knowing that carbon dioxide is acidic, and makes water acidic, how could we measure how much carbon dioxide we exhale? Knowing that carbon dioxide acidifies a solution, we can measure the amount of carbon dioxide being produced by the time it takes to make a neutral solution acidic. We can use pH indicators that change color dependent on if the solution is acidic, neutral, or basic. An example of such an indicator is bromothymol blue. Demonstrate how the indicator solution (bromothymol blue) changes color depending on if the solution is acidic, neutral, or basic. Place the three cups with indicator solution in front of the class so everybody can see them. Let your students notice the color and tell them that this is a neutral solution of just water (plus indicator). Add one teaspoon of acid (vinegar) to one of the cups or beakers and let your students observe the color change (to green/yellow). Tell them that you added an acid to the indicator solution. Add enough of the prepared baking soda solution to one of the other cups to make the color change from green to blue and let your students observe the color change. Tell them that you added a base to the indicator solution. Let your students compare the different colors and recall which solution is acidic, basic, or neutral. Show that the indicator solution also turns green/yellow (acidic) when you breathe into it through a straw, due to the carbon dioxide in your breath. Explain that during their experiment they will monitor the color of the solution using cell phones and Google's Science Journal app. Introduce Google's Science Journal app to your students, specifically the light sensor. Alternatively, you can assign the relevant tutorials on this Science Journal tutorial page to them before class. Let the students locate the light sensor on the phone they will be using for their experiments. Demonstrate how the light sensor readings are affected by the different colors of the solution. In the Science Journal app, open the light sensor. Then place each of the solutions (acidic, basic, and neutral) on top of the light sensor, one after the other, and communicate the light sensor readings for each solution to your students. The values should decrease in the order of yellow, green, and blue. Can someone explain why we can use the light sensor readings to monitor the color change of the solution? When light passes through the beaker or cup with the indicator solution, some portion of the light will be absorbed by the solution itself. That means the light sensor can only measure the amount of light that gets through the solution to the sensor. If the color of the solution gets lighter, for example, because it changes color from blue-green to green-yellow (as it does during the experiment), less light will be absorbed by the solution and the readings of the light sensor will increase. This way, the light sensor readings will tell us when the color of the indicator solution changed. Explore (30 minutes) Based on previous discussions, let each student formulate his/her hypothesis on how cellular respiration rates (or the amount of carbon dioxide in your breath) changes after exercising. Divide the class into groups of 2-4 students and inform them that each group will conduct an experiment to measure the amount of carbon dioxide in their breath before and after exercising, to test their hypotheses. Each student should perform the whole experiment (before and after exercising) once. Within an experiment, students should divide tasks. For example, one student could prepare the indicator solution, another could operate the phone, a third one exhales into the indicator solution, and optionally, a fourth student could measure the color change with an additional stopwatch. Students should then rotate through each task so everyone can have a turn measuring the amount of carbon dioxide in their breath. Walk the students through the experimental procedure described below. (A slideshow is available that you can use to guide your students through the experiments.) warning purple icon Make sure the students use a one-directional check valve with their straws when exhaling into the indicator solutions to prevent any accidental ingestion of the solution. The valve will only allow air flow in one direction, which prevents any suction of the indicator solution into the straw. Experimental Procedure To familiarize students with the experimental procedure, first let them practice exhaling through a straw into just water while monitoring the light intensity with Google's Science Journal app as described in the following steps. Assign one student at a time to perform the experiment. Students should take turns so everyone in the group can perform the experiment once. Fill a 9 oz cup or 150 mL beaker two thirds with water. Cover the cup with plastic wrap and secure it with a rubber band. With a straw, poke two holes into the plastic wrap. One should be located closer to the rim of the cup. Prepare the straw by cutting it into two pieces and inserting the valve in the top part as shown in Figure 2. Each student needs to prepare his/her own straw. They can use one valve each, or alternatively, students of one group can share a valve. In this case, the valve needs to be passed from student to student during the experiment so each one can insert it into their individual straw. Prepare the straw by inserting the safety valve. Figure 2. Inserting the check valve into the top part of the straw. Stick the bottom of the straw through the hole that is closer to the rim of the cup. Open the Science Journal app on the phone and clearly label your experiment. Make sure to select the light sensor for your measurements. Lean the phone against a box or books with the light sensor facing sideways towards the cup. Then place the beaker or cup in front of the light sensor. Put the flashlight in front of the cup so it shines through the dye solution directly onto the light sensor as shown in Figure 3. Experimental setup for measuring the amount of carbon dioxide in the exhaled breath using Google's Science Journal app. Figure 3. Experimental setup for the experiments. Note that the light sensor reading will be dependent on the position of the flashlight with respect to the phone. Students might want to try different positions to get the highest sensor readings. However, they should not move the flashlight or the phone while recording data. Confirm that the sensor readings are stable. Then, press the record button in the Science Journal app, take a deep breath and start exhaling through the straw into the water for as long as you can. Try to exhale from your lungs and place the end of the straw close to the light sensor—but do not block the sensor with the straw! Then inhale through your nose and then continue blowing into the water. Because of the bubble formation, the sensor readings will temporarily fluctuate. This will not affect the results of the experiment. Stop recording after about one minute and review the light sensor readings. The maximum sensor readings should not change much, as shown in Figure 4, but you should see fluctuations due to bubble formation, which temporarily affects the absorption properties of the solution. Example graph for measuring light intensities when exhaling into just water. Figure 4. Example graph of measured light intensity for exhaling into just water without indicator. The x-axis is time in minutes:seconds [min:s] and the y-axis shows light intensity in lux. The red line shows the general trend of the data, not taking into account the fluctuations from creating bubbles in the solution. Now, let your students do the experiment with indicator solution instead of water. Fill a 9 oz cup or 150 mL beaker two thirds with water. Add about 2 mL, or half a teaspoon of 0.04% bromothymol blue indicator solution. The solution should be green or blue dependent on the pH of your water. If the solution is already yellow or very light green, try a different water. Repeat steps a.ii–a.viii. Students can reuse their individual prepared straws and valves. Stop recording once the maximum sensor readings level off and are stable for more than 20 seconds. The light intensity should start to stabilize once the indicator solution has changed color from blue-green to green-yellow. An example graph is given in Figure 5. Example graph for measuring light intensities when exhaling into the indicator solution. Figure 5. Example graph of measured light intensity for exhaling into the indicator solution. The x-axis is time in minutes:seconds [min:s] and the y-axis shows light intensity in lux. The red line shows the general trend of the data, not taking into account the fluctuations from creating bubbles in the solution. Let each student repeat the same experiment, following step b. again. Each one should use his/her own straw. The safety valve can be shared. Make sure they clearly label and organize their data in the Science Journal app. Note that the initial light intensity does not need to be exactly the same for each experiment, since you are just measuring the amount of time it takes for the light reading to level off. Once all trials for the first experiment have been completed, repeat steps b. and c. again, but before the experimenter starts exhaling into the indicator solution, he/she should do jumping jacks or a similar exercise for one minute. Even though the students might feel a little winded, they should try to exhale into the straw the same way they did during the experiment before exercising. Let the students prepare a new straw for this step to avoid accidental swapping of straws. The safety valves can be reused. Lesson Completion Let the students clean up their experimental supplies. All the solutions can be disposed of in the sink. lesson plan clock icon If time is short, break here and have students analyze their data the next day of class. Troubleshooting Depending on what kind of water you use, the pH can be different. The pH of water is usually in the range of 6.5–8.5. If your tap water results in a dark blue color when adding the indicator solution, meaning it is slightly basic, you can still use it for the experiment. Ideally, distilled or deionized water should have a neutral pH and should result in a blue-green color with bromothymol blue. If your water has a light green or yellow color with the indicator, you cannot use it for this experiment and you should try a different water source. If your graph only shows light intensity changes in large, discrete steps instead of a smooth continuous curve (as shown in Figure 1), remember to make sure that you are blowing bubbles into the solution close to the light sensor to create short-term light intensity fluctuations. This will prevent the sensor from going into "sleep mode," which can affect the readings. If you still cannot get a smooth curve, your phone's light sensor might not have a high enough resolution for this project, as described in the Prep Work section, and you will need to try a different phone. Make sure the phone, beaker or cup, and flashlight all stay in the same place during each trial. For example, be careful not to bump them when exhaling into the solution. Moving them around can have a significant impact on the light readings and will affect your data. Make sure your flashlights have fresh batteries. If the batteries are dying, light intensity might actually decrease during your experiment as the light gets dimmer. The light sensor can be affected by shadows. Make sure people are not moving around nearby and casting variable shadows on the phone during each trial. Reflect (15 minutes) Throughout the data analysis process, encourage the students to question their data. You could ask questions like: How reproducible was your data? Did all your trials before or after exercising result in a similar pattern? Did all groups observe the same trend in their results? The amount of exhaled carbon dioxide, or actual times needed to change the color of the indicator solution, will probably vary from student to student. However, all groups should have observed the same trend between their data from before versus after exercising. What are possible sources of variation in your data? For example, did you see a difference depending on which student conducted the experiment? Were your flashlight and phone always positioned the same way throughout each experiment? Did you notice any light interference from the surroundings that could have influenced the data? Let each group review their recorded data in the Science Journal app. They should enter their results in the data tables provided in the student worksheet. How can you determine when the indicator changed color from your data recorded by the Science Journal app? Remind the students that the light sensor recorded the light intensity, which is dependent on the color of the solution. The lighter the color, the more light can get through to the sensor and the light intensity increases. Once the neutral solution changes to acidic due to the increase of carbon dioxide, the indicator changes color from blue-green to green-yellow. Once the indicator is green-yellow, it should not change color anymore. They can determine the reaction time by finding the endpoint of color change, or the time at which the light sensor readings first leveled off, in each graph. Dragging the cursor along the x-axis will give time and measured value for each individual data point. In the example graph, shown in Figure 6, the color change is completed at 24.2 seconds, which is when the maxiumum sensor reading stops changing (as indicated by the red line in Figure 6). Instructions are also given in the student worksheet. cellular respiration ambient ligh lux Figure 6. Example graph showing how students can determine the time when the color change of the indicator solutions is completed from their Science Journal data. X-axis shows time in minutes:seconds [min:s] and the y-axis shows light intensity in lux. The red line shows the general trend of the data, not taking into account the fluctuations from creating bubbles in the solution. Discuss the results with the students and ask them to interpret the meaning of their data. How many of you observed a faster color change after exercising? All students should raise their hands. If there is a group that has opposite results, do some joint troubleshooting to find out what led to their results. Everyone should have seen the same trend (faster color change after exercising) in their experiments. Why do you see a faster color change in the indicator solution after exercising? A faster color change of the indicator solution from neutral (blue-green) to acidic (greenyellow) means that more carbon dioxide was exhaled into the solution during the same time period, turning the solution acidic faster. What does a faster color change tell you about your cellular respiration rate? More carbon dioxide in your exhaled breath means that cellular respiration rates increase when exercising, as your body needs more energy to perform the exercises. If you have time, you can combine all the groups' average color change times before and after exercising into a scatter plot on the board to reflect the entire class's data. This way they can also see the trend of the data clearly, showing that after exercising, more carbon dioxide is produced due to a higher cellular respiration rate. Did the amount of exhaled carbon dioxide vary between different experimenters? Even if people are doing the same activities, it does not mean that their cells respire at the same rate. Every person's metabolism functions slightly different so the amount of exhaled carbon dioxide can vary from person to person even if they are doing the same thing. Assess You can use this quiz to assess student learning after the activity; quiz is available in online and pdf formats: Online quiz, assignable in Google Classroom Quiz (pdf) and answer sheet (pdf) Make Career Connections Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world. scientist performing experiments Biochemist Biochemists study the chemical composition of living things, including our bodies, and explore how they develop, function, or react to compounds such as drugs, hormones, or food. In fact, it was a biochemist that found out about cellular respiration and discovered how glucose is transformed into energy that we can use. female doctor talking to elderly patient Physician Physicians examine patients, perform diagnostic tests, and treat injuries or diseases. Just like in this activity, they need to know and find out how chemical processes inside our body can be affected by certain things such as exercise or drugs. This allows them to diagnose diseases or prescribe the right drugs to treat a patient. Therapist checking lung function. Respiratory Therapist Respiratory therapists deal with respiration every day. They care for patients with breathing disorders, provide therapeutic treatments and perform diagnostic procedures. In a pulmonary function test, for example, they measure how much air and how quickly you exhale, similar to what you did in this activity. Such tests allow them to assess the function of your respiratory system and diagnose and treat any abnormalities. Lesson Plan Variations In addition to monitoring the amount of CO2 in their exhaled breath, you can let your students count the number of breaths they take in a certain amount of time and record their heart rates before and after exercising. The results will enable them to assess how the heart rate and breathing rate are related to cellular respiration. This will allow students to explore how body functions are linked, as well as identifying how chemicals flow through our body, for example, how oxygen in our blood is pumped from the lungs through our heart to our muscles. Make the connection between cellular respiration and photosynthesis, which is the production of sugar molecules (glucose) and oxygen from carbon dioxide and water. You can demonstrate the photosynthesis reaction by adding an aqueous plant such as Elodea to the indicator solution after it changed color. The excess carbon dioxide, which resulted in the color change, will be taken up by the plant when exposed to direct sunlight and will be transformed into oxygen and glucose. When enough carbon dioxide is used up, the indicator color will change back to green, which indicates that carbon dioxide has been removed from the solution. You can even demonstrate the importance of sunlight for photosynthesis by running several parallel experiments in which you expose some plants to sunlight and keep others in the dark. A color change should only happen in the presence of sunlight. This variation allows the students to understand how organisms in nature depend on each other in a diverse ecosystem. Explore additional activities you and your students can do with Google's Science Journal app.