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Chapter 6 How Cells Release Energy Snake © Gunter Ziesler/Photoshot Copyright © McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Cells Use Energy in Food to Make ATP Every organism requires a steady food supply to survive. All plants and animals, as well as many microbes, use food (such as glucose) and oxygen gas to produce ATP, an energy carrier used to power cell activities. Section 6.1 Bluebird: © Getty Images/Purestock RF Cells Use Energy in Food to Make ATP The process of using glucose and oxygen to produce ATP is called aerobic respiration. C6H12O6 + 6O2 6CO2 + 6H2O + 36ATP (Glucose) Section 6.1 Bluebird: © Getty Images/Purestock RF Cellular Respiration Is Linked to Breathing Inhaled oxygen is consumed in cellular respiration. Carbon dioxide, produced as a byproduct, is then exhaled. The cell uses the ATP formed during cellular respiration to do work, such as muscle contraction. Section 6.1 Mitochondrion: © Thomas Deerinck, NCMIR/Science Source Figure 6.1 Clicker Question #1 Do plants carry out cellular respiration? A. No, photosynthesis has the same function in plants as respiration has in animals and microbes. B. No, their energy needs are too small to require respiration. C. Yes, they require ATP like other living things, and respiration generates ATP. D. Yes, they require cellular respiration as a way to get rid of extra CO2. Flower: © Doug Sherman/Geofile/RF Clicker Question #1 Do plants carry out cellular respiration? A. No, photosynthesis has the same function in plants as respiration has in animals and microbes. B. No, their energy needs are too small to require respiration. C. Yes, they require ATP like other living things, and respiration generates ATP. D. Yes, they require cellular respiration as a way to get rid of extra CO2. Flower: © Doug Sherman/Geofile/RF Cellular Respiration Occurs in Three Stages ATP synthesis requires energy input. Cellular respiration releases energy from glucose in several steps. During glycolysis, glucose is split into two three-carbon molecules of pyruvate. The pyruvate molecules then enter a mitochondrion, where they are disassembled into carbon dioxide molecules during the Krebs cycle. Section 6.2 Figure 6.2 Cellular Respiration Occurs in Three Stages Glycolysis and the Krebs cycle transfer some of the potential energy in glucose to ATP. Meanwhile, electrons are transferred to NADH and FADH2. NADH and FADH2 unload electrons at the electron transport chain, where the potential energy in the electrons is used to produce more ATP. Section 6.2 Figure 6.2 Clicker Question #2 What happens to glucose’s carbon atoms during the overall process of aerobic respiration? A. They are donated to O2. B. They remain in the pyruvate molecules. C. They become part of ATP. D. They are released as CO2. Flower: © Doug Sherman/Geofile/RF Clicker Question #2 What happens to glucose’s carbon atoms during the overall process of aerobic respiration? A. They are donated to O2. B. They remain in the pyruvate molecules. C. They become part of ATP. D. They are released as CO2. Flower: © Doug Sherman/Geofile/RF Mitochondria Produce Most ATP Many of the reactions of cellular respiration occur in mitochondria. Mitochondria have two phospholipid bilayers: an outer membrane and an inner membrane. Section 6.3 Figure 6.3 Mitochondria Produce Most ATP Between the mitochondrial membranes is an intermembrane compartment. The space within the inner membrane is the mitochondrial matrix, which houses the reactions of the Krebs cycle. Section 6.3 Figure 6.3 Clicker Question #3 Where is the mitochondrial matrix? A. Outside the outer membrane B. Between the inner and outer membranes C. Inside the inner membrane Flower: © Doug Sherman/Geofile/RF Clicker Question #3 Where is the mitochondrial matrix? A. Outside the outer membrane B. Between the inner and outer membranes C. Inside the inner membrane Flower: © Doug Sherman/Geofile/RF Glycolysis Splits Glucose Glycolysis occurs outside of the mitochondrion, in the cytoplasm. During glycolysis, a glucose molecule is split into two three-carbon pyruvate molecules. The enzymes of glycolysis extract some of the potential energy stored in glucose. The process yields two ATP molecules and two electron-carrying NADH molecules. Section 6.4 Figure 6.4 Glycolysis Splits Glucose Glycolysis requires an input of two ATP to “activate” glucose. Section 6.4 Figure 6.4 Glycolysis Splits Glucose The activated glucose is then split into two 3-carbon molecules. Section 6.4 Figure 6.4 Glycolysis Splits Glucose Each of the 3-carbon molecules proceeds to the energy extraction reactions of glycolysis. Section 6.4 Figure 6.4 Glycolysis Splits Glucose First, each 3-carbon molecule is oxidized, producing two NADH molecules. Section 6.4 Figure 6.4 Glycolysis Splits Glucose Then, each 3-carbon molecule donates its phosphate groups to ADP molecules, producing ATP molecules via substrate-level phosphorylation. Section 6.4 Figure 6.4 Glycolysis Splits Glucose In substrate-level phosphorylation, an enzyme transfers a phosphate from a molecule to ADP. Section 6.4 Figure 6.15 Glycolysis Splits Glucose In total, four ATP are produced. Recall that two ATP were used to start the reactions. The net yield is two ATP. Section 6.4 Figure 6.4 Glycolysis Splits Glucose Note that these reactions do not require oxygen. Glycolysis can therefore occur in anaerobic conditions. Section 6.4 Figure 6.4 Glycolysis Splits Glucose Glycolysis yields two ATP molecules, two electron-carrying NADH molecules, and two pyruvates. Section 6.4 Figure 6.4 Glycolysis Splits Glucose Each glycolysis molecule has a name. Section 6.4 Figure 6.4 Clicker Question #4 If 8 glucose molecules enter glycolysis, the net products will be ____ pyruvate molecules and ____ ATP molecules. A. 2 … 2 B. 4 … 4 C. 8 … 8 D. 16 … 16 Flower: © Doug Sherman/Geofile/RF Clicker Question #4 If 8 glucose molecules enter glycolysis, the net products will be ____ pyruvate molecules and ____ ATP molecules. A. 2 … 2 B. 4 … 4 C. 8 … 8 D. 16 … 16 Flower: © Doug Sherman/Geofile/RF Aerobic Respiration Yields Many ATP The reactions of Krebs cycle and the electron transport chain require oxygen gas. These reactions yield much more ATP than glycolysis. Section 6.5 Aerobic Respiration Yields Many ATP The two pyruvate molecules produced in glycolysis undergo an oxidation reaction as they enter the mitochondrion (this is sometimes called the transition step). Section 6.5 Figure 6.5 Aerobic Respiration Yields Many ATP A carbon atom is stripped from each pyruvate, and leaves the cell as a carbon dioxide molecule. At the same time, NAD+ is reduced to NADH. Through this process, each pyruvate molecule is converted to an acetyl CoA molecule. Each acetyl CoA molecule then enters the Krebs cycle. Section 6.5 Figure 6.5 Aerobic Respiration Yields Many ATP Update figure During the Krebs cycle, the two acetyl CoA molecules are oxidized, yielding 4 CO2, 2 ATP, 6 NADH, and 2 FADH2. Section 6.5 Figure 6.5 Aerobic Respiration Yields Many ATP The Krebs cycle occurs in several steps. Acetyl CoA combines with a 4-carbon molecule, yielding citrate. Section 6.5 Figure 6.6 Aerobic Respiration Yields Many ATP Citrate is then rearranged and oxidized, yielding 3 NADH, 1 FADH2, and 1 ATP per turn. The ATP is produced via substratelevel phosphorylation. Section 6.5 Figure 6.6 Aerobic Respiration Yields Many ATP The original four-carbon molecule is re-created, and the cycle starts anew. Section 6.5 Figure 6.6 Aerobic Respiration Yields Many ATP Glycolysis Acetyl CoA formation Krebs cycle So far, aerobic respiration of one glucose molecule has yielded only four ATP. But 10 NADH molecules have been produced, as well as two FADH2. Section 6.5 Aerobic Respiration Yields Many ATP NADH and FADH2 donate their electrons to the electron transport chain, where energy from the electrons is used to produce many ATP. Section 6.5 Figure 6.7 Aerobic Respiration Yields Many ATP As electrons travel through the transport chain, carrier molecules use the potential energy of the electrons to transport hydrogen ions into the intermembrane compartment. Section 6.5 Figure 6.7 Aerobic Respiration Yields Many ATP At the end of the transport chain, electrons are donated to an oxygen atom, which combines with hydrogens to form water. Section 6.5 Figure 6.7 Aerobic Respiration Yields Many ATP The hydrogen ions move down their concentration gradient from the intermembrane compartment into the matrix through ATP synthase. Section 6.5 Figure 6.7 Aerobic Respiration Yields Many ATP ATP synthase produces ATP via chemiosmotic phosphorylation. Section 6.5 Figure 6.7 Aerobic Respiration Yields Many ATP The electron transport chain produces 34 ATP. Section 6.5 Figure 6.7 Cellular Respiration of One Glucose Yields 36 ATP Glycolysis Acetyl CoA formation Krebs cycle Electron transport 34 Section 6.5 Cellular Respiration of One Glucose Yields 36 ATP Glycolysis and Krebs cycle each produce 2 ATP, and the electron transport chain produces 34 ATP. Transporting NADH into the mitochondrion requires 2 ATP, making the total production of ATP equal to 36. Section 6.6 Figure 6.8 Other Food Molecules Enter the Energy-Extracting Pathways Proteins and fats are also used as energy sources for the cell. These molecules enter the energy-extracting pathways and produce ATP. Section 6.7 Avocado: © Digital Vision/Getty Images RF Figure 6.9 Fermentation Generates ATP Only in Glycolysis Organisms produce ATP in the absence of oxygen, as well. Glycolysis produces ATP and does not require oxygen. However, glycolysis does require NAD+, which is re-created in the electron transport chain of cells undergoing respiration. Section 6.8 Figure 6.10 Fermentation Generates ATP Only in Glycolysis In the absence of oxygen, a cell can re-create NAD+ other pathways, called anaerobic respiration and fermentation. In anaerobic respiration, NADH donates is oxidized at an electron transport chain that uses electron acceptor molecules other than O2. Fermentation uses pyruvate to oxidize NADH, producing alcohol, lactic acid, or other byproducts. Section 6.8 Figure 6.10 Fermentation Generates ATP Only in Glycolysis In alcoholic fermentation, NADH reduces pyruvate to ethanol. NAD+ is re-created. Section 6.8 Beer: © Adam Woolfitt/Corbis; Yogurt: © Scimat/Science Source In lactic acid fermentation, NADH reduces pyruvate to lactic acid. NAD+ is re-created. Figure 6.11 Fermentation Generates ATP Only in Glycolysis During fermentation, oxidation of a glucose molecule yields only 2 ATP. Section 6.8 Beer: © Adam Woolfitt/Corbis; Yogurt: © Scimat/Science Source Figure 6.11 Clicker Question #5 What is the main advantage of fermentation over aerobic cellular respiration? A. Fermentation generates ATP even if O2 is not present. B. Fermentation generates more ATP per glucose than aerobic cellular respiration. C. Fermentation does not generate toxic byproducts such as CO2. D. Fermentation gets rid of pyruvate, which would otherwise accumulate in the cell. Flower: © Doug Sherman/Geofile/RF Clicker Question #5 What is the main advantage of fermentation over aerobic cellular respiration? A. Fermentation generates ATP even if O2 is not present. B. Fermentation generates more ATP per glucose than aerobic cellular respiration. C. Fermentation does not generate toxic byproducts such as CO2. D. Fermentation gets rid of pyruvate, which would otherwise accumulate in the cell. Flower: © Doug Sherman/Geofile/RF Photosynthesis and Respiration Are Related Section 6.9