Reading Preview. Cellular Respiration. Overview of Cellular Respiration. Glycolysis. Essential Questions

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year. Similarly, humans and other living organisms need reliable food sources to supply energy to survive and grow.

1 Cellular Respiration Living organisms obtain energy by breaking down organic molecules during cellular respiration. Real-World Reading Link Monarch butterflies must constantly feed on nectar from flowers to provide energy to sustain themselves during their winter migration to parts of Mexico and California each year. Similarly, humans and other living organisms need reliable food sources to supply energy to survive and grow. Overview of Cellular Respiration Recall that organisms obtain energy in a process called cellular respiration. The function of cellular respiration is to harvest electrons from carbon compounds, such as glucose, and use that energy to make ATP. ATP is used to provide energy for cells to do work. The overall chemical equation for cellular respiration is shown below. Notice the equation for cellular respiration is the opposite of the equation for photosynthesis. Reading Preview Essential Questions What are the stages of cellular respiration? What is the role of electron carriers in each stage of cellular respiration? What are the similarities between alcoholic fermentation and lactic acid fermentation? Review Vocabulary cyanobacterium: a type of bacterium that is a photosynthetic autotroph New Vocabulary Cellular respiration occurs in two main parts: glycolysis and aerobic respiration. The first stage, glycolysis, is an anaerobic process. Anaerobic processes do not require oxygen. Aerobic respiration includes the Krebs cycle and electron transport and is an aerobic process. Aerobic processes require oxygen. Cellular respiration with aerobic respiration is summarized in Figure 1. anaerobic process aerobic respiration aerobic process glycolysis Krebs cycle fermentation Figure 1 Cellular respiration occurs in the mitochondria, the energy powerhouse organelles of a cell. Glycolysis Glucose is broken down in the cytoplasm through the process of glycolysis. Two molecules of ATP and two molecules of NADH are formed for each molecule of glucose that is broken down. Follow along with Figure 2 as you read about the steps of glycolysis.

2 Figure 2 Glucose is broken down during glycolysis inside the cytoplasm of cells. Summarize the reactants and products of glycolysis. First, two phosphate groups, derived from two molecules of ATP, are joined to glucose. Notice that some energy, two ATP, is required to start the reactions that will produce energy for the cell. The 6-carbon molecule is then broken down into two 3-carbon compounds. Next, two phosphates are added and electrons and hydrogen ions (H + ) combine with two NAD + molecules to form two NADH molecules. NAD + is similar to NADP, an electron carrier used during photosynthesis. Last, the two 3-carbon compounds are converted into two molecules of pyruvate. At the same time, four molecules of ATP are produced. Reading Check Explain why there is a net yield of two, not four, ATP molecules in glycolysis. VOCABULARY WORD ORIGIN Glycolysis comes from the Greek words glykys, meaning sweet and lysis, meaning to rupture or break Krebs Cycle Glycolysis has a net result of two ATP and two pyruvate molecules. Most of the energy from the glucose is still contained in the pyruvate. In the presence of oxygen, pyruvate is transported into the mitochondrial matrix, where it is eventually converted to carbon dioxide. The series of reactions in which pyruvate is broken down into carbon dioxide is called the Krebs cycle, or the tricarboxylic acid (TCA) cycle. This cycle also is referred to as the citric acid cycle. Steps of the Krebs cycle Prior to the Krebs cycle, pyruvate first reacts with coenzyme A (CoA) to form a 2-carbon intermediate called acetyl CoA. At the same time, carbon dioxide is released and NAD + is converted to NADH. Acetyl CoA then moves to the mitochondrial matrix. The reaction results in the production of two carbon dioxide molecules and two NADH molecules. Follow along in Figure 3 as you continue reading about the steps of the Krebs cycle.

Figure 3 Pyruvate is broken down into carbon dioxide during the Krebs cycle inside the mitochondria of cells. Trace Follow the path of carbon molecules that enter and leave the Krebs cycle.

3 Figure 3 Pyruvate is broken down into carbon dioxide during the Krebs cycle inside the mitochondria of cells. Trace Follow the path of carbon molecules that enter and leave the Krebs cycle. The Krebs cycle begins with acetyl CoA combining with a 4-carbon compound to form a 6-carbon compound known as citric acid. Citric acid is then broken down in the next series of steps, releasing two molecules of carbon dioxide and generating one ATP, three NADH, and one FADH 2. FAD is another electron carrier similar to NAD + and NADP +. Finally, acetyl CoA and citric acid are generated and the cycle continues. Recall that two molecules of pyruvate are formed during glycolysis, resulting in two turns of the Krebs cycle for each glucose molecule. The net yield from the Krebs cycle is six carbon dioxide molecules, two ATP, eight NADH, and two FADH 2. Ten NADH and two FADH 2 move on to play a significant role in the next stage of aerobic respiration. Study Tip Clarifying Statement Work with a partner to read the text and discuss unfamiliar words and difficult concepts. Write a clarifying statement to summarize the Krebs cycle. Electron Transport In aerobic respiration, electron transport is the final step in the breakdown of glucose. It also is the point at which most of the ATP is produced. High-energy electrons and hydrogen ions from NADH and FADH 2 produced in the Krebs cycle are used to convert ADP to ATP. As shown in Figure 4, electrons move along the mitochondrial membrane from one protein to another. As NADH and FADH 2 release electrons, the energy carriers are converted to NAD + and FAD, and H + ions are released into the mitochondrial matrix. The H + ions are pumped into the mitochondrial matrix across the inner mitochondrial membrane. H + ions then diffuse down their concentration gradient back across the membrane and into the matrix through ATP synthase molecules in chemiosmosis. Electron transport and

chemiosmosis in cellular respiration are similar to these proc esses in photosynthesis. Oxygen is the final electron acceptor in the electron transport system in cellular respiration.

4 chemiosmosis in cellular respiration are similar to these proc esses in photosynthesis. Oxygen is the final electron acceptor in the electron transport system in cellular respiration. Protons and electrons are transferred to oxygen to form water. Figure 4 Electron transport occurs along the mitochondrial membrane. Compare and contrast electron transport in cellular respiration and photosynthesis. Overall, electron transport produces 32 ATP. Each NADH molecule produces three ATP and each group of three FADH 2 produces two ATP. In eukaryotes, one molecule of glucose yields 36 ATP under ideal conditions. Prokaryotic cellular respiration Some prokaryotes also under go aerobic respiration. Because prokaryotes do not have mitochondria, there are differences in the process. The main difference involves the use of the prokaryotic cellular membrane as the location of electron transport. In eukaryotic cells, pyruvate is transported to the mitochondria. In prokaryotes, this movement is unnecessary, saving the prokaryotic cell two ATP, and increasing the net total of ATP produced to 38. Careers in Biology Bioenergeticist A researcher who studies energy transfers in cells is a bioenergeticist. Some bioenergeticists study mitochondria and their relationship to aging and disease. VOCABULARY SCIENCE USAGE V. COMMON USAGE Concentration Science usage: the relative amount of a substance dissolved in another substance The concentration of hydrogen ions is greater on one side of the membrane than on the other. Common usage: the directing of close, undivided attention The student s concentration was focused on the exam. Anaerobic Respiration Some cells can function for a short time when oxygen levels are low. Some prokaryotes are anaerobic organisms they grow and reproduce without oxygen. In some cases these cells continue to produce ATP through glycolysis. However, there are problems with solely relying on glycolysis for energy. Glycolysis provides only two net ATP for each molecule of glucose, and a cell has a limited amount of NAD +. Glycolysis will stop when all the NAD + is used up if there is not a process to replenish NAD +. The anaerobic pathway that follows glycolysis is anaerobic respiration, or fermentation. Fermentation occurs in

the cytoplasm and regenerates the cell s supply of NAD + while producing a small amount of ATP. The two main types of fermentation are lactic acid fermentation and alcohol fermentation.

This involves the transfer of high-energy electrons and protons from NADH. Skeletal muscles produce lactic acid when the body cannot supply enough oxygen, such as during periods of strenuous exercise.

5 the cytoplasm and regenerates the cell s supply of NAD + while producing a small amount of ATP. The two main types of fermentation are lactic acid fermentation and alcohol fermentation. Lactic acid fermentation In lactic acid fermentation, enzymes convert the pyruvate made during glycolysis to lactic acid, as shown in Figure 5. This involves the transfer of high-energy electrons and protons from NADH. Skeletal muscles produce lactic acid when the body cannot supply enough oxygen, such as during periods of strenuous exercise. When lactic acid builds up in muscle cells, muscles become fatigued and might feel sore. Lactic acid also is produced by several microorganisms that often are used to produce many foods, including cheese, yogurt, and sour cream. Figure 5 When oxygen is absent or in limited supply, fermentation can occur. Compare and contrast lactic acid fermentation and alcohol fermentation. Alcohol fermentation Alcohol fermentation occurs in yeast and some bacteria. Figure 5 shows the chemical reaction that occurs during alcohol fermentation when pyruvate is converted to ethyl alcohol and carbon dioxide. Similar to lactic acid fermentation, NADH donates electrons during this reaction and NAD + is regenerated. 2.2.b 1.e.(...) Based on Real Data* Interpret the Data Data Analysis Lab How does viral infection affect cellular respiration? Infection by viruses can significantly affect cellular respiration and the ability of cells to produce ATP. To test the effect of viral infection on the stages of cellular respiration, cells were infected with a virus, and the amount of lactic acid and ATP produced were measured. Data and Observations

6 Think Critically 1. Analyze how the virus affected lactic acid production in the cells. 2. Calculate After 8 hours, by what percentage was the lactic acid higher in the virus group than in the control group? By what percentage was ATP production decreased? 3. Infer why having a virus such as the flu might make a person feel tired. Data obtained from: El-Bacha, T., et al Mayaro virus infection alters glucose metabolism in cultured cells through activation of the enzyme 6-phosphofructo 1-kinase. Molecular and Cellular Biochemistry 266: d 2.2.b 2.2.c Photosynthesis and Cellular Respiration As you have learned, photosynthesis and cellular respiration are two important processes that cells use to obtain energy. They are metabolic pathways that produce and break down simple carbohydrates. Figure 6 shows how these two processes are related. Recall that the products of photosynthesis are oxygen and glucose, the reactants needed for cellular respiration. The products of cellular respiration, which are carbon dioxide and water, are the reactants for photosynthesis.

Figure 6 Photosynthesis and cellular respiration form a cycle in which the products of one metabolic pathway form the reactants of the other metabolic pathway.

7 Figure 6 Photosynthesis and cellular respiration form a cycle in which the products of one metabolic pathway form the reactants of the other metabolic pathway. Review Lesson Summary Many living organisms use cellular respiration to break down glucose. NADH and FADH 2 are important electron carriers for cellular respiration. In the absence of oxygen, cells can sustain glycolysis by fermentation. Vocabulary Review Define each vocabulary term in a complete sentence. 1. Krebs cycle 2. anaerobic process 3. fermentation 4. aerobic 5. glycolysis Understand Main Ideas 6. Summarize the stages of cellular respiration. 7. Identify how many carbons from one glucose molecule enter one round of the Krebs cycle. 8. Explain how high-energy electrons are used in electron transport. 9. Describe the role of fermentation in maintaining ATP and NAD + levels. Use the figure below to answer questions 10 and 11.

10. Which organelle is illustrated in the figure? A. Golgi apparatus B. mitochondrion C. nucleus D. endoplasmic reticulum 11. Which process does not occur in the organelle illustrated above? A. glycolysis B.

lactic acid fermentation 13. What is produced when the electrons leave the electron transport chain in cellular respiration and bind to the final electron acceptor for the chain? A. H 2 O B. O 2 C.

8 10. Which organelle is illustrated in the figure? A. Golgi apparatus B. mitochondrion C. nucleus D. endoplasmic reticulum 11. Which process does not occur in the organelle illustrated above? A. glycolysis B. Krebs cycle C. conversion of pyruvate to acetyl CoA D. electron transport 12. Which is not a stage of cellular respiration? A. glycolysis B. Krebs cycle C. electron transport chain D. lactic acid fermentation 13. What is produced when the electrons leave the electron transport chain in cellular respiration and bind to the final electron acceptor for the chain? A. H 2 O B. O 2 C. CO 2 D. CO 14. In which molecule is most of the energy of glucose stored at the end of glycolysis? A. pyruvate B. acetyl CoA C. ATP D. NADH Constructed Response 15. Short Answer Discuss the roles of NADH and FADH 2 in cellular respiration. 16. Short Answer In cellular respiration, where do the electrons in the electron transport chain originate? What is the final destination of the electrons? 17. Short Answer Why do your muscles hurt for some time after a large amount of strenuous exercise? Think Critically 18. Compare and contrast the two types of fermentation.

19. Explain The end products of cellular respiration are CO 2 and H 2 Where do the oxygen atoms in the CO 2 originate? Where does the oxygen atom in H 2 O originate? 20.

Biology 22. How many ATP, NADH, and FADH 2 are produced in each step of cellular respiration? How is the number of ATP produced different from the net ATP available? Summative Assessment 23.

9 19. Explain The end products of cellular respiration are CO 2 and H 2 Where do the oxygen atoms in the CO 2 originate? Where does the oxygen atom in H 2 O originate? 20. What is the advantage of aerobic metabolism over anaerobic metabolism in energy production in living organisms? 21. Compare and contrast electron transport in photosynthesis and cellular respiration. Biology 22. How many ATP, NADH, and FADH 2 are produced in each step of cellular respiration? How is the number of ATP produced different from the net ATP available? Summative Assessment 23. What are the chemical equations for photosynthesis and cellular respiration? Analyze the relationship between photosynthesis and cellular respiration using the equations for both processes. 24. Biology Write an article using what you know about the relationship between photosynthesis and cellular respiration to explain the importance of plants in an ecosystem. Document-Based Questions Cadmium is a heavy metal that is toxic to humans, plants, and animals. It is often found as a contaminant in soil. Use the data below to answer questions about the effect of cadmium on photosynthesis in tomato plants. Data obtained from: Chaffei, C., et al Cadmium toxicity induced changes in nitrogen management in Lycopersicon esculentum leading to a metabolic safeguard through an amino acid storage strategy. Plant and Cell Physiology 45(11): What was the effect of cadmium on leaf size, chlorophyll content, and photosynthesis rate? 26. At what concentration of cadmium was the largest effect on leaf size observed? On chlorophyll content? On photosynthesis rate? 27. Predict the effects on cellular respiration if an animal eats contaminated tomatoes.

Living organisms obtain energy by breaking down organic molecules during cellular respiration.

Section 3: Living organisms obtain energy by breaking down organic molecules during cellular respiration. K What I Know W What I Want to Find Out L What I Learned Essential Questions What are the stages