Energy and Life Read the lesson title aloud to students.

Learning Objectives Describe why ATP is useful to cells. Describe what happens during the process of photosynthesis. Click to show each of learning objective. To prepare for this lesson, show students a living plant. Ask: What do you think would happen if the plant were watered but kept away from light for several weeks? Answer: The plant would probably turn yellow and not grow much. Ask: How do plants use light to survive and grow? Answer: They use the energy from sunlight to perform photosynthesis and to produce food. Explain to students that photosynthesis is a plant process that uses the energy of sunlight to convert water and carbon dioxide into high-energy sugars and oxygen. Tell students that in this chapter they will learn how plant cells capture and use the energy in sunlight.

Adenosine Triphosphate (ATP) Adenosine triphosphate (ATP) is one of the most important compounds that cells use to store and release energy. adenine ribose 3 phosphate groups Tell students that ATP is the basic energy source used by all types of cells. Work with students to break down the chemical components of adenosine triphosphate. Ask for volunteers to come forward and write in any parts of the compound that they know. They should be able to easily identify the three phosphate groups but may need assistance with the other components. Click to reveal each part of ATP. Ask: How would you change ATP into ADP (adenosine diphosphate)? Answer: Take away the third phosphate group and the bond that holds it to the molecule. Ask: Where is energy stored in the molecule of ATP? Answer: In the chemical bonds that hold the phosphate bonds together. Makes sure students understand that chemical bonds are represented in the figure by the thin black lines.

Adenosine Diphosphate (ADP) Adenosine diphosphate (ADP) has two phosphate groups instead of three. adenine ribose 2 phosphate groups Explain to students that the difference between ADP and ATP is the key to the way in which living things store energy. Based on what they learned about the components of ATP, ask for volunteers to come forward and write the names of the parts of ADP. Once students have completed their work, click to reveal each part of ADP and confirm what they have written.

Storing Energy When a cell has energy available, it can store small amounts of it by adding phosphate groups to ADP molecules, producing ATP. Make the following analogy: ADP is like a rechargeable battery that powers the machinery of the cell. Ask: Why would you compare ADP to a partially charged battery instead of a fully charged one? Answer: Like a partially charged battery, ADP can store more energy. In the case of ADP, when a third phosphate group is added, it stores this energy in the form of a chemical bond.

Releasing Energy When a cell needs energy, it can release it by breaking the bond between the second and third phosphate groups in ATP. Continue the analogy: ATP is like a fully charged battery. It can release energy when the bond between the second and third phosphate groups is broken. Click to show the removal of a phosphate group from ATP.

ATP and Batteries Explain that the fact that ATP can easily release and store energy by breaking and reforming bonds between phosphate groups makes it exceptionally useful as a basic energy source for all cells. Ask: How would you describe the light being produced by the flashlight shown? Answer: The light looks very bright. If possible, bring in a flashlight or other battery-powered device, a rechargeable battery, and a charger. Turn on the device and tell students it is operating like a cell carrying out cellular activities. Ask: What activity in the cell is like this device using energy from the battery? Answer: It is like the cell using energy from ATP as it releases a phosphate group. Ask: What activity in the cell is like the charger recharging the battery? Answer: It is like the process that adds a phosphate group to ADP, which can “recharge” ATP.

ATP Production Cells must produce ATP. In photosynthesis, plants convert the energy of sunlight into chemical energy stored in the bonds of carbohydrates. Explain that cells are not “born” with a supply of ATP—they must somehow produce it. Ask: Where do living things get the energy they use to produce ATP? Answer: It comes from the chemical compounds that we call food.

Heterotrophs and Autotrophs Organisms that make their own food are autotrophs. Organisms that obtain food by consuming other living things are heterotrophs. Explain to students that, ultimately, nearly all life on Earth depends on the ability of autotrophs to capture and convert the energy from sunlight to synthesize molecules of high-energy carbohydrates—sugars and starches—that can be used as food. Ask: What is the name of the process that autotrophs use to synthesize molecules of high-energy carbohydrates by trapping light energy? Answer: Photosynthesis. Ask: What are the heterotrophs in the image shown? Answer: The heron and the fish