Chapter 8 Harvesting Energy Glycolysis and Cellular Respiration ATP is the Universal Energy Source Photosynthesizers get energy from the sun Animals get energy 2nd or 3rd hand from plants or other organisms Regardless the energy is converted to the chemical bond energy of ATP Main Types of Energy Releasing Pathways Anaerobic pathways Aerobic pathways Evolved first Don t require oxygen Start with glycolysis in cytoplasm Completed in cytoplasm Evolved later Required oxygen Start with glycolysis in cytoplasm Completed in mitochondria Overview of Aerobic Respiration 4 ATP 2 ATP net CYTOPLASM 2 ATP MITOCHONDRIA glucose Glycolysis e H 2 pyruvate 2 NADH 2 NADH 8 NADH 2 FADH2 e H e H e H e Electron Transfer Phosphorylation 2 CO2 4 CO2 2 ATP 32 ATP Krebs Cycle H water e oxygen Typical Energy Yield 36 ATP 8 1 How Do Cells Obtain Energy Most cellular energy is stored in the chemical bonds of energy carrier molecules like adenosine triphosphate ATP Cells break down glucose in two stages 1 2 Glycolysis liberates a small quantity of ATP Cellular respiration produces far more ATP Author Animation Overview Photosynthesis and Respiration Photosynthesis Provides the Energy Released by Glycolysis and Cellular Respiration energy from sunlight 6 CO2 6 H2O 6 O2 C6H12O6 photosynthesis cellular respiration glycolysis ATP Fig 8 1 The Role of Coenzymes for Cellular Respiration Small molecules associated with an enzyme that participates in enzymatic catalysis NAD FAD NADH FADH2 are exampls NAD and FAD accept electrons and hydrogen to become NADH and FADH2 Deliver electrons and hydrogen to the electron transfer chain Oxidation versus Reduction Oxidation refers to the loss of electrons When a molecule becomes oxidized it loses an electron s and becomes more positively charged Na Na e Reduction refers to the gain of electrons When a molecule becomes reduced it gains an electron s and becomes more negatively charged F e F Glucose C6H12O6 is a key energy storage molecule All cells metabolize glucose for energy In humans energy is stored as long chains of glucose called glycogen or as fat These storage molecules are converted to glucose to produce ATP An overview of glucose breakdown The first stage of glucose breakdown is glycolysis begins by splitting glucose a 6 carbon sugar into two molecules of pyruvate a 3 carbon sugar 2 ATP molecules are produced in glycolysis Glycolysis proceeds in the same way under aerobic with oxygen or anaerobic without oxygen conditions Glycolysis occurs in the cytoplasm An overview of glucose breakdown continued The second stage of glucose breakdown is cellular respiration and occurs when oxygen is available In this stage 2 pyruvate molecules produced by glycolysis are broken down into 6 carbon dioxide molecules and 6 water molecules For every two pyruvate molecules an additional 34 or 36 ATP molecules are generated Occurs in mitochondria organelles specialized for the aerobic breakdown of pyruvate An overview of glucose breakdown continued If oxygen is not available the second stage of glucose break down is fermentation Does NOT produce any ATP Instead pyruvate remains in the cytoplasm and is converted into lactate or ethanol CO2 Author Animation Fate of Pyruvate An overview of glucose breakdown continued The overall equation for the complete breakdown of glucose is C6H12O6 6 O2 6 CO2 6 H2O ATP heatheat A Summary of Glucose Breakdown cytoplasmic fluid glucose glycolysis 2 ATP 2 pyruvate fermentation If O2 is available If no O2 is available lactate ethanol CO2 6 O2 cellular respiration 34 or 36 ATP 6 CO2 6 H2O mitochondrion Fig 8 2 Efficiency of Aerobic Respiration Efficiency is only 39 solar cells Sounds really low but really it is not Most energy is lost as heat 8 2 What Happens During Glycolysis Glycolysis has two parts each with several steps 1 Glucose activation these are energy investment steps required to invest a small amount of energy Energy extraction these are the energy harvesting steps releasing enery 2 Summary of glycolysis Each molecule of glucose is broken down to two molecules of pyruvate A net of two ATP molecules and two NADH high energy electron carriers are formed 2 ATP 2 ADP 4 ADP 4 ATP CC CC CC glucose CC CC CC P fructose P bisphosphate CC G3P 2 C P 2 C CC pyruvate 2 2 NAD NADH Energy extraction 2 Glucose activation 1 Fig 8 3 Author Animation Glycolysis 8 3 What Happens During Cellular Respiration Cellular respiration in eukaryotic cells occurs in mitochondria in three stages 1 Pyruvate is broken down in the mitochondrial matrix releasing energy and CO2 2 High energy electrons travel through the electron transport chain 3 ATP is generated by chemiosmosis First pyruvate is broken down in the mitochondrial matrix releasing energy and CO2 In eukaryotes cellular respiration occurs within the mitochondria organelles with two membranes matrix fluid inner membrane intermembrane space outer membrane Fig 8 4 First pyruvate is broken down in the mitochondrial matrix releasing energy and CO2 continued Glucose is first broken down into pyruvate through glycolysis in the cell cytoplasm Pyruvate is next transported into the mitochondrion matrix where further breakdown occurs in 2 stages The formation of acetyl coenzyme A acetyl CoA The Krebs cycle During the mitochondrial reactions CO2 is generated as a waste product CO2 diffuses out of cells and into the blood which carries it into the lungs where it is exhaled Author Animation Acetyl CoA and the Krebs Cycle Reactions in the Mitochondrial Matrix 1 Formation of acetyl CoA 3 NADH 3 NAD coenzyme A C CO2 coenzyme A FAD FADH2 CCC pyruvate NAD NADH CC CoA acetyl CoA 2 Krebs cycle 2 C CO2 ADP ATP Fig 8 5 In the second stage of cellular respiration high energy electrons travel through the electron transport chain These high energy electrons jump from molecules to molecule in the ETC losing small amounts of energy This energy is harnessed to pump H from the matrix and into the intermembrane space producing a concentration gradient of H In the second stage of cellular respiration high energy electrons travel through the electron transport chain continued The buildup of H in the intermembrane space is used to generate ATP during chemiosmosis At the end of the ETC the energy depleted electrons are transferred to oxygen which acts as an electron acceptor Energy depleted electrons oxygen and hydrogen ions combine to form water ATP generation continues only when there is a steady supply of oxygen The Electron Transport Chain NADH e 2
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