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Transcript
Objective # 28 Module 2E – How Cells Harvest Energy Identify the following terms and explain how each is involved in the transfer of energy within a cell: a) oxidationoxidation-reduction reactions b) electron carriers c) electron transport chain d) ATP All cells can harvest energy from organic molecules. To do this, they catabolize (break down) organic molecules and use the energy that is released to make ATP from ADP and phosphate. In this module, we will look at several ways cells can catabolize glucose and use the energy that is released to make ATP. 1 Objective 28a 2 Objective 28a Redox reactions play a key role in the flow of energy within cells because electrons have energy. Therefore, when electrons are transferred, energy is transferred along with them. The amount of energy an electron has depends on the energy level it occupies. Light and other forms of energy can boost electrons to a higher energy level. High energy electrons can be used to make ATP. Reactions that involve a transfer of electrons from one atom or molecule to another are called oxidationoxidation-reduction (redox) reactions. reactions. The substance that loses electrons is oxidized and the substance that gains electrons is reduced reduced.. 3 4 Objective 28a During Objective 28a H+ redox reactions, ions are often transferred along with electrons. Since a hydrogen atom consists of one H+ ion (proton) and one electron, a substance that loses hydrogen atoms is oxidized (loses electrons) and a substance that gains hydrogen atoms is reduced. Cells use reduced organic molecules (molecules rich in hydrogen) as fuel. Cells extract energy from these molecules by oxidizingg them i.e. removing hydrogen atoms. Energy stored in the electrons of removed hydrogen atoms can be used to make molecules of ATP. 5 6 1 Objective 28b Objective 28b Electron carriers are molecules that help transfer electrons from one substance to another within the cell. They pick up electrons from substances being oxidized and donate them to substances being reduced. As mentioned previously, H+ ions are often transferred along with the electrons. For example, during the breakdown of glucose : enzymes remove 2 H atoms (2 H+ and 2e-) from gglucose both electrons and one H+ are picked up by NAD+ to form NADH the other H+ is released as a free ion 7 8 Objective 28b Some Important Electron Carriers Oxidized Form Add → Reduced Form ← Remove NAD+ 2e- and 2H+ NADH + H+ FAD 2e- and 2H+ FADH2 NADP+ 2e- and 2H+ NADPH + H+ 9 Objective 28c 10 Objective 28c Electron Transport Chain An electron transport chain (ETC) is a series of electron carriers that are embedded in a membrane and that p pass electrons from one carrier to the next in a specific sequence: Mitochondrial matrix Cytochrome oxidase complex NADH dehydrogenase NADH + H+ NAD+ FADH2 bc1 complex 2H+ + 1/2O2 H2O 2 e– FAD 2 e– 2 e– Q C Inner mitochondrial H+ membrane H+ H+ Intermembrane space Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 11 12 2 Objective 28c An Electron Transport Chain Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2e– Energy released from electrons as they move along the ETC is used for ATP synthesis Electrons from food As electrons are passed from one carrier to the next in the ETC, some of their energy is released. This energy can be used to make ATP ATP. In the case of aerobic respiration, the final electron acceptor is oxygen, which joins with 2H+ to form water. High energy The final electron acceptor is oxygen, which joins with 2H+ to form water Low energy 2H+ 13 H2 O Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Objective 28d 1/ O 2 2 Structure of ATP ATP: Adenine ¾ plays a key role in short short--term storage and release of energy within the cell ¾ is sometimes called the “energy currency” of the cell because it is the immediate source of energy for most cell activities ¾ is made by adding a phosphate group (phosphorylating) ADP NH2 Triphosphate group O O P O- O O P O- N O O P N O CH2 N O- Electrostatic repulsion between phosphates N O OH OH Sugar 15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Objective # 29 16 Objective 29a Describe the following methods that are used by cells to make ATP: a) substrate substrate--level phosphorylation b) electron transport phosphorylation (also called chemiosmotic phosphorylation or chemiosmosis) During substratesubstrate-level phosphorylation, a high high--energy phosphate group is enzymatically transferred from an organic molecule (such as the sugar PEP) directly to ADP to form ATP: 17 18 3 Objective 29a Objective 29b Enzymatic transfer of a phosphate group from PEP to ADP, forming pyruvate and ATP: Chemiosmotic phosphorylation occurs in 2 stages: 1) An ETC uses energy from electrons to pump H+ across a membrane against their concentration gradient. This builds up a store of potential energy. PEP P Enzyme Enzyme P ADP P ATP Adenosine Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 19 20 21 22 Objective 29b 2) As the H+ ions move back through the membrane down their concentration gradient, through g special p p protein complexes, the stored energy is released and used to make ATP from ADP and phosphate. Objective # 30 Objective 30 Chemiosmotic Explain why chemiosmotic phosphorylation is sometimes called oxidative phosphorylation. phosphorylation phosphorylation is called oxidative phosphorylation if the electrons are accepted by oxygen when they reach the end of the ETC. 23 24 4 Objective # 31 Objective 31 Aerobic respiration, anaerobic respiration, and fermentation refer to 3 different ways of breaking down glucose ((glucose glucose catabolism). catabolism). The chemical bond energy of glucose (and all other molecules) is stored in the electrons that make up the covalent bonds. Distinguish between aerobic respiration, anaerobic respiration, i i and d ffermentation. i 25 26 Objective 31 glucose catabolism, high energy elecrons are stripped from glucose and picked up by the electron carriers NAD+ and FAD. In most cases, the electrons are passed to an ETC where their energy is used to make ATP by chemiosmosis. Eventually, the electrons, along with H+, are passed to a final acceptor. Objective 31 During If 27 Objective 31 molecular oxygen (O2) is the final electron acceptor, the process is called aerobic respiration respiration.. If some other inorganic g molecule is the final electron acceptor, the process is called anaerobic respiration respiration.. If an organic molecule is the final electron acceptor, the process is called fermentation.. fermentation 28 Objective # 32 First we will examine the process of aerobic respiration, then we will take a brief look at anaerobic respiration respiration, and finally we will discuss the process of fermentation. Write a summary chemical equation for aerobic respiration andd describe d ib the h origin i i and d ffate of each substance involved. 29 30 5 Objective 32 Aerobic Objective # 33 respiration: Identify the 4 main stages of aerobic respiration . For each stage discuss: a) b) c) d) the cellular location the main events that take place starting substances and end products the major enzymes involved and the type of reaction each catalyzes e) the number of ATPs, NADHs, and FADH2s, produced (or used up) per glucose molecule C6H12O6 + 6O2 → 6CO2 + 6H2O + eenergy e gy some of the released energy is captured and stored in molecules of ATP. 31 Objective 33 32 1. Glycolysis NADH The 4 main stages of aerobic respiration are: 1) Glycolysis 2) Pyruvate P t Oxidation O id ti 3) The Krebs Cycle (Citric Acid Cycle) 4) Oxidative Phosphorylation (ETC) Stages of Aerobic Respiration Glucose Pyruvate 2. Pyruvate Oxidation NADH CO2 Acetyl-CoA NADH e– CO2 3. Krebs Cycle H+ 32 ATP FADH2 2 ATP e– 2H+ H2O 1 + /2O2 e– 4. Oxidative Phosphorylation Q C H+ H+ H+ 33 34 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1st Stage - Glycolysis: Glucose (6C) is broken down into 2 molecules of pyruvate (3C each). 2 ATP are used for activation energy and 4 ATP are produced by substrate level phosphorylation resulting in a net gain of 2 ATP. The oxidation of glucose releases 4 high energy electrons which are used to reduce 2 NAD+ to form 2 NADH. 35 36 6 2nd Stage - Pyruvate Oxidation: 3rd Stage - Krebs Cycle: Each acetyl group (2C) from pyruvate oxidation is broken down into 2 molecules of carbon dioxide (1C) The breakdown of each acetyl group releases enough energy to form 1 ATP by substrate level phosphorylation The oxidation of each acetyl group releases enough high energy electrons to form 3 NADH and 1 FADH2 Each pyruvate (3C) from glycolysis is broken down into 1 CO2 (1C) and 1 acetyl group (2C) The acetyl g group p jjoins with coenzyme A to form acetyl acetyl--CoA The oxidation of each pyruvate releases 2 high energy electrons which are used to reduce 1 NAD+ to form 1 NADH. 37 38 4th Stage - Oxidative Phosphorylation: NADH and FADH2 (produced during glycolysis, pyruvate oxidation, and the Krebs cycle) donate high energy electrons to an electron transport chain As the electrons are passed along the ETC, their energy is used to make ATP by chemiosmosis At the end of the ETC, electrons join with oxygen and 2H+ to form water 39 40 Objective 33 The theoretical yield of ATP from the breakdown of one molecule of glucose by aerobic respiration is 38. In eukaryotes, this is reduced to 36 ATP because NADH generated by glycolysis in the cytoplasm has to be actively transported into the mitochondria. This costs the cell 1 ATP per NADH transported. 41 42 7 Objective 33 Theoretical ATP Yield from Aerobic Respiration Glucose 2 ATP Glycolysis Pyruvate 2 ATP 2 NADH 6 ATP 2 NADH 6 ATP 2 ATP Chemiosmosis Pyruvate oxidation Krebs Cycle 6 NADH 18 ATP 2 FADH2 4 Chemiosmosis ATP Total net ATP yield = 38 (36 in eukaryotes) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Objective 33 The actual yield for eukaryotes is reduced to approximately 30 ATP per glucose molecule for 2 reasons: ¾ The inner mitochondrial membrane is y allowingg some protons p to pass p “leaky” through without passing through ATP synthase. ¾ Energy stored in the proton gradient is used for other purposes besides generation of ATP 43 44 Objective # 34 Objective 34 During aerobic respiration, cells use O2 as the final electron acceptor at the end of the electron transport chain. Many bacteria can respire p without O2, using CO2, SO4, nitrates, or other inorganic compounds as the final electron acceptor in place of O2. This process is called anaerobic respiration. Describe the process of p anaerobic respiration. 45 46 Objective # 35 Objective 35 In some cases, the high energy electrons picked up by NAD+ during glycolysis are not donated to an ETC. Instead, NADH donates its extra electrons and H+ directly to an organic molecule, which serves as the final electron acceptor. This process is called fermentation. fermentation. Describe the process of fermentation and distinguish b between alcoholic l h li ffermentation i and lactic acid fermentation. 47 48 8 Objective 35 Objective 35 Alcoholic fermentation: ¾ occurs in singlesingle-celled fungi called yeast ¾ a terminal CO2 is removed from the pyruvic acid (3C) produced during glycolysis, producing acetaldehyde (2C) ¾ acetaldehyde accepts 2 ee- and a H+ from NADH, producing ethanol and NAD+ Although bacteria can carry out more than a dozen kinds of f fermentation, i eukaryotes k are capable of only a few: 49 50 Objective 35 Alcohol Fermentation in Yeast H Glucose H 2 ADP G L Y C O L Y S I S 2 ATP O– 2 NAD+ Lactic Acid Fermentation: ¾ used by most animal cells when O2 is not available ¾ NADH N DH do donates ates 2 ee- aand d a H+ ddirectly ect y to the pyruvate (3C) produced during glycolysis, producing lactate (3C) and NAD+ OH CH3 2 Ethanol 2 NADH H C O C O CO2 C O CH3 C 2 Pyruvate CH3 2 Acetaldehyde 51 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Objective 35 Lactic Acid Fermentation in Muscle Cells 2 ADP 2 ATP Glucose O– G L Y C O L Y S I S C O C OH H 2 NAD+ Alcoholic fermentation and lactic acid fermentation each generate 2 ATP per glucose molecule (generated by substrate level phosphorylation during glycolysis)) compared to the theoretical glycolysis maximum of 36 ATP per glucose during aerobic respiration in eukaryotes. CH3 2 Lactate O– 2 NADH C O C O CH3 52 2 Pyruvate 53 54 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 9