CHAPTER 7: How Cells Harvest Energy

1 CHAPTER 7: How Cells Harvest Energy CHAPTER 7: How Cells Harvest Energy Essential Knowledge Covered Learning Objectives & Science Practices Covered Activities and Labs Overview of Respiration , , LO [SP , SP ] LO [SP ] Respiration, the Play (TM p. xx) Glycolysis: Splitting Glucose , The Oxidation of Pyruvate Produces Acetyl-CoA The Krebs Cycle , The Electron Transport Chain and Chemiosmosis , LO [SP , SP ] LO [SP ] Scientific Thinking (TM p. xx) Modeling the Electron Transport Chain (TM p. xx) Brewing Root Beer (TM p. xx) Baking in Biology (TM p. xx) Energy Yield of Aerobic Respiration LO [SP ] Yeast Respiration (TM p. xx) Regulation of Aerobic Respiration AP Investigation 6: Cellular Respiration (TM p. xx) Oxidation Without O2 Catabolism of Proteins and Fats Evolution of Metabolism CHAPTER 7 Cumulative Review AP CHAPTER Review (SE pp.)

2 144 145) AP Review Questions (SE pp. 145 146) Focus Review Guide CHAPTER 7 (pp. 55-65) Connect Banks ( CHAPTER 7) CHAPTER SUMMARY FOR TEACHERS From the purring of a cat to a biology student reading these words, Energy drives life s activities. As described in CHAPTER 6, biological systems utilize free Energy to grow, to reproduce, and to maintain dynamic homeostasis and organisms employ various strategies to capture, use, and store free Energy . In CHAPTER 7, students explore two of these strategies: fermentation and cellular respiration. CHAPTER 7 suggested pacing is 5 days and includes AP Investigation 6. The harvesting of Energy via cellular respiration is a universal process, occurring in all organisms, from archaea and bacteria to complex multicellular eukaryotes. Whereas autotrophs capture free Energy from the environment, heterotrophs Harvest free Energy from carbon compounds produced by other organisms.

3 The processes of photosynthesis and respiration, however, are interdependent in their reactants and products. Cellular respiration and fermentation Harvest free Energy from sugars in multistep pathways ( , glycolysis, the Krebs cycle, and the electron transport chain) to phosphorylate ADP into the most common Energy carrier, ATP. Respiration can occur under both anaerobic and aerobic conditions; however, the process is much more efficient when oxygen is present. Cellular respiration is one of the most difficult topics in biology because students (and teachers) can get lost in the minutiae and lose track of the overriding principles. Instead of memorizing the steps in the complex pathways, students should focus on the major concepts that are highlighted in Big Idea 2, section , of the AP Biology Curriculum Framework.

4 Key concepts include how Energy from fuel molecules is transferred to ATP to supply free Energy for life processes; how that goal is achieved in different organisms and under different conditions (anaerobic vs. aerobic); the key reactants and products only in terms of where they fit into the flow through the pathways; how the pathways integrate with each other and where they occur in the Cells ; and the evidence that supports cellular respiration as an evolutionarily conserved core process. KEY CONCEPTS FOR STUDENT FOCUS The fundamental differences between fermentation and cellular respiration: the inputs, the conditions that trigger each process, and the products of each process That glycolysis is a universal metabolic process in both prokaryotes and eukaryotes The summary of each stage of aerobic respiration.

5 The inputs and products of each, where each stage occurs in the cell, and the efficiency of ATP production in each That glycolysis oxidizes glucose to produce 2 pyruvate molecules and a net yield of 2 ATP How pyruvate is transported from the cytosol to the Krebs cycle in the mitochondria The value of the Krebs cycle is in the production of the electron carriers NADH and FADH2, which deliver electrons to the electron transport chain and, ultimately, oxygen How the electron transport chain harvests electrons from NADH and FADH2 to fuel chemiosmosis The enzymatic role of ATP synthase in the synthesis of ATP from ADP and Pi, and how a proton gradient across the mitochondrial membrane is needed to drive this reaction How the double-membrane structure of the mitochondria enable their function in chemiosmosis TEACHING STRATEGIES CHAPTER 7 presents important information on cellular respiration and fermentation.

6 Allowing 5 days for both lecture and lab should be sufficient for this instruction. Below is a suggested schedule: Day 1: Glycolysis and fermentation Day 2: Cellular respiration: Krebs cycle Day 3: Cellular respiration: Electron transport chain and efficiency of cellular respiration Days 4 & 5: AP Investigation 6: Cellular Respiration (from AP Biology Investigative Labs: An Inquiry-Based Approach) *If time is an issue, you can choose sections of the lab investigation that are appropriate for your students, or you can modify an existing lab. In either case, the key is to engage students in an inquiry-based investigation about cellular respiration. A SUGGESTED APPROACH As was previously stated, cellular respiration is one of the most difficult topics in AP Biology. This CHAPTER is one in which students (and teachers) can get lost in the details and lose track of the overarching principles.

7 It is often mistakenly approached as an exercise in memorizing the steps of glycol ysis, the Krebs cycle, and the electron transport chain, as well as emphasizing an accounting of exact numbers of ATPs. High school students do not have to know the biochemical details of each of these pathways. They will face that challenge in college biochemistry courses. To set the stage for this topic, three principles need to be established: The point is to make ATP. First, discuss how the structure of ATP allows it to serve as short-term Energy sto rage. ATP is unstable the third phosphate group is easily donated to other molecules (repelled by the two other negative phosphate groups) and releases enough Energy to drive the endergonic reactions of life. In biology, moving hydrogens moves electrons; moving electrons moves Energy .

8 Students will see hydrogens move from fuel molecules to NAD and FAD, and these molecules will be called electron carriers. A disconnect can occur for students if they do not understand that electrons do not move on their own around biological syste ms. They are carried by hydrogen ions, and this is how organisms move Energy from one molecule to another. Oxidation and reduction reactions are coupled. Oxidation will be seen in respiration in a number of ways, such as removing a hydrogen from a molecule, or cleaving off a carbon and bonding it to oxygen, yielding CO2. Each time this occurs, ask your students what other molecule was reduced. It will help them track the electron carriers NADH and FADH2. With regard to the pathways, a comparative approach is beneficial glycol ysis vs. Krebs cycle vs.

9 The electron transport chain and can be carried through when photosynthesis is introduced in the next CHAPTER . Do not get sidetracked in counting the exact number of ATP molecules that are generated in each pathway or an exact accounting of the conversion of NADH and FADH2 to ATP. This is focusing on unnecessary details for this level of study. The critical concept in ATP accounting is that glycolysis and the Krebs cycle produce minimal ATP, and the electron transport chain produces roughly 10 times more. Although the details of required concepts are outlined in section of the AP Biology Curriculum Framework, the following tips might be helpful, especially if this is the first times students are introduced to the material. (Remember that students are not expected to memorize the steps in the pathways; however, if provided with a visual representation such as a diagram, they should be able to interpret, revise, or refine it.)

10 For example, if given a diagram of the Krebs cycle, students should be able to account for CO2 production or identify the step in which ATP is produced by substrate-level phosphorylation.) Glycolysis This is the earliest evolutionary step for harvesting Energy from organic molecules. An evolutionary perspective is useful to tie this unit to evolution and taxonomy; all organisms perform glycolysis, and the genes for glycolytic enzymes are highly conserved. (Note that Essential Knowledge (EK) refers to the fact that organisms share many conserved core processes and features that evolved and are widely distributed among organisms today. Learning Outcomes Review poses an important question regarding this concept.) Fuel molecules (glucose) are oxidized in small steps to produce a little bit of ATP and a little bit of NADH.