Biology Commonwealth of Virginia

Content/Teacher Notes

Imagine that you are standing on the second floor of a building. You have potential energy due to your position. You can convert it to kinetic energy all at once by jumping to the floor below, or you can release it a little at a time by walking down the stairs.

The problem is that the release of the chemical bond energy of carbohydrates in a cell must be done in a manner that does not destroy the cell during the process. In cellular respiration, the energy in glucose is released in small amounts instead of all at once.

Cellular Respiration: In cells, potential energy in the chemical bonds of food is transferred to the bonds of a substance called “adenosine triphosphate” (ATP). The potential energy of ATP is readily converted into kinetic energy to do work, such as electrical work (nerve impulse), transport work (heart, active pumps), chemical work, or mechanical work (muscle contraction).

C₆H₁₂O₆ + 6O₂  6CO₂ + 6H₂0 + 38 ATP

A better representation of cellular respiration is shown below. The process involves dozens of intermediate steps requiring enzymes and the addition of two ATP molecules to start the reaction. To accomplish this, the cell uses special proteins called “enzymes.” Enzymes are organic catalysts (proteins) that lower the energy of activation for reactions.

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  Adenosine triphosphate (ATP) has three phosphate groups attached to it: A-P~P~P.
  
• 
  Adenosine diphosphate (ADP) has two phosphate groups attached to it: A-P~P.
  
• 
  Adenosine monophosphate (AMP) has a single phosphate group: A-P.
  

Before starting these activities, review the processes of photosynthesis and cellular respiration. Then introduce cellular energy and ATP (adenosine triphosphate [tri = three]) by showing some rechargeable batteries. Ask students what rechargeable batteries do and how they work. Use an analogy between ATP and the batteries. As the batteries are used, they give up their potential energy until all of it has been converted into kinetic energy and heat/unusable energy. Spent rechargeable batteries can be used again only after the input of additional energy. ATP is the higher energy form (like the recharged battery), while ADP (adenosine diphosphate [di = two]) is the lower energy form (like the used-up battery). When the terminal (third) phosphate is cut loose, ATP becomes ADP, and the stored energy is released for use in some biological process. The input of additional energy (plus a phosphate group) “recharges” ADP into ATP.

Introduction

1. Ask students where ATP comes from. Show an overview of photosynthesis with an animated tutorial. Possible Web sites for tutorials include the following:

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  http://www.fw.vt.edu/dendro/forestbiology/photosynthesis.swf
  
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  http://www.johnkyrk.com/photosynthesisdark.html
  

2. Remind students that carbohydrates are named for the number of carbon atoms they contain and that their names usually end in the letters ose.

Procedure

Activity 1. Relative Amount of Energy in Carbohydrates

Safety Note: Students must wear lab coats and protective eyewear! They must perform the experiment in an approved fume hood.

1. Put on safety glasses.

2. Place a test tube with 10 mL of room-temperature water in a clamp, and attach it to a ring stand.

3. Measure and record the temperature of the water.

4. Place a sugar cube on a heat-resistant dish under the test tube.

5. Light the sugar cube with the lighter, and let it burn as completely as possible.

6. Measure and record the increase in heat of the water.
Activity 2. Relative Amount of Energy in the Bonds of ATP

Safety Note: Students must wear lab coats and protective eyewear! They must complete this experiment in an open area: outside is best.

1. Put on safety glasses.

2. Label the large wooden block with an A (adenosine).

3. Label each of the other three with a P (phosphate group).

4. Use three rubber bands to represent the chemical bonds holding the phosphates to the adenosine molecule. The first rubber band goes around the adenosine block and one phosphate group. The second goes around the adenosine and two phosphate groups. The third goes around the adenosine and all three phosphate groups.

5. Cut the rubber bands with scissors to simulate the breaking of the bonds. Start with the band encircling all three phosphate groups.

6. Measure the distances the rubber bands traveled, which represent the amounts of energy in the molecule.

Safety Note: The trajectory of a rubber band can be totally unpredictable. Be prepared! Keep safety glasses on at all times.

Observations and Conclusions

1. Have the students view an animated tutorial on glycolysis, such as that at http://www.johnkyrk.com/glycolysis.html. Discuss reactions to the tutorial.

2. Have students state how much the temperature of the water rose in Activity 1. Based on the fact that a calorie is equal to the amount of energy needed to heat 1 mL of water 1°C, have students calculate how many calories were in the sugar cube.

3. Have students state which rubber band traveled the farthest in Activity 2. Have them explain why in terms of potential and kinetic energy.

4. Ask: “Is there more or less energy in ADP (two phosphate groups) than in ATP? Which would you want to have the most of if you were going to run a race?”

5. Ask: “Is phosphate important in a diet? Why, or why not?” Have students research foods that are high in phosphate.

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