Chapter Overview 7.1 Energy?s Vital Role 7.2 Electrons Fall Down the Energy Hill to Drive the Uphill Production of ATP 7.3 The Three Stages of Cellular Respiration 7.4 First Stage of Respiration: Glycolysis 7.5 Second Stage of Respiration: The Krebs Cycle 7.6 Third Stage of Respiration: The Electron Transport Chain Energy?s Vital Role To live, we must gather energy from our environment Photosynthetic organisms gather energy from sunlight. Autotrophs Humans gather energy from their food. Heterotophs Electrons & ATP Glucose is a high energy molecule Electrons are removed from glucose during its breakdown Energy is removed from these electrons This energy is used to make ATP Oxygen serves as a trash can for these energy-poor electrons Oxygen is the ?final electron acceptor? Electrons & ATP Some substances attract electrons more strongly than others Molecules can gain or lose electrons Oxidation The loss of electron(s) to another molecule e.g., Rusting metal has become ?oxidized? Reduction The gain of electron(s) from another molecule Electrons & ATP Oxidation and reduction always occur together ?Redox? reactions Electrons released during a redox reaction are traveling energetically downhill Many molecules can oxidize other molecules Oxygen is not always involved Electrons & ATP Glucose is oxidized in a series of steps Not oxidized all at once In each step, electrons are removed Electrons are initially accepted by hydrogen ions H+ + e- ? H atom 7.1 Cellular Respiration (cont.) ? Oxidation, the removal of hydrogen atoms from a molecule, is a central reaction in cellular respiration. reduction 6 CO2 + 6 H2O + energy C6H12O6 + 6 O2 oxidation Electrons & ATP Nicotinamide adenine dinucleotide ?NAD? The most important electron carrier in energy transfer Exists in two forms Empty: NAD+ Oxidized (low energy) form Loaded with electrons: NADH + H+ Reduced (high energy) form Electrons & ATP Nicotinamide adenine dinucleotide NAD+ can become reduced NAD+ + 2H+ + 2e- ? NADH + H+ This occurs when it oxidizes a substance NADH can become oxidized NADH + H+ ? NAD+ + 2H+ + 2e- Like ADP & ATP, these molecules can cycle Cellular Respiration The formula for cellular respiration can be written as follows Glucose + O2 ?CO2 + H2O + energy The energy component of the reaction can be written as follows 36 ADP + 36 P ? 36 ATP Cellular Respiration Many separate steps are involved in the oxidization of glucose Three main phases Glycolysis Krebs cycle Electron transport chain Glycolysis First stage of cellular respiration Occurs in the cytosol Glucose ? 2 pyruvate molecules Pyruvate is slightly more oxidized than glucose Net production of 2 ATP 2 ATP need to be invested in the first step 4 ATP are produced in later steps 2 NADH are produced Glycolysis Glycolysis takes place in the cytosol The Krebs cycle takes place in the mitochondria A transition step connects these two phases Brings the remnants of glucose into the mitochondria Transition Step 2 pyruvate molecules were produced during glycolysis These molecules are combined with coenzyme A Formed inside the mitochondria 2 acetyl CoA molecules produced 2 CO2 molecules produced 2 NADH molecules produced Krebs Cycle Takes place in the inner compartment of the mitochondria Begins with the 2 acetyl CoA produced in the transition step These remnants of glucose are fully oxidized to form CO2 Krebs Cycle Each acetyl CoA is combined with an oxaloacetate molecule Citric acid is generated This phase can also be called ?the citric acid cycle? Through multiple steps, citric acid is converted into oxaloacetate Energy is released through this oxidation Krebs Cycle From these 2 acetyl CoA molecules, the following are produced 2 coenzyme A molecules are released 4 CO2 2 ATP 6 NADH 2 FADH2 (Another reduced coenzyme) Krebs Cycle Glycolysis 2 ATP 2 NADH Transition step 2 NADH 2 CO2 Krebs cycle 2 ATP 6 NADH 2 FADH2 High energy molecules produced Krebs Cycle Only 4 ATP are produced directly from the breakdown of glucose Most of the energy harvested from the breakdown of glucose is in the form of reduced coenzymes 10 NADH 2 FADH2 Krebs Cycle These reduced coenzymes have value to the cell High energy molecules These reduced coenzymes are not immediately useful to the cell Foreign currency has value, but it difficult to spend in this country The electron transport chain uses these reduced coenzymes to produce more ATP Electron Transport Chain Reduced coenzymes donate their electrons to an electron carrier of the electron transport chain High-energy electrons Hydrogen ions are also donated NADH and FADH2 become oxidized NAD+ and FAD are produced Donated electrons are shuttled through a series of electron acceptors Electrons take a trip down an energy hill Each time electrons are passed on, they give up a portion of their energy Electron Transport Chain Oxygen is the final electron acceptor of the electron transport chain ½ O2 + 2H+ + 2e- ? H2O General formula for respiration Glucose + O2 ? CO2 + H2O + energy Electron Transport Chain During the shuttling of electrons, energy is released High-energy electrons ? lower energy electrons The number of ATP molecules formed depends upon the electron donor. The electrons from NADH provide energy for the synthesis of three ATP molecules. The electrons from FADH2 provide energy for the synthesis of two ATP molecules. Electron Transport Chain The released energy powers the movement of hydrogen ions (H+) H+ is pumped into the outer compartment of the mitochondria Space between the mitochondria?s two membranes Hydrogen ions are pumped against their concentration and electrical gradients H+ is pumped up an energy hill Electron Transport Chain H+ move back across the membrane into the inner compartment Movement is through an enzyme called ATP synthase Movement is down their chemical and electrical gradient This movement of H+ powers the production of ATP by ATP synthase ADP + P ? ATP Electron Transport Chain Reduced coenzymes donate e- to ETC Energy removed from e- e- discarded (added to O2) H+ pumped against gradient Energy required is removed from e- H+ flows down gradient Energy is released ATP formed Energy required supplied by H+ flow Cellular Respiration 36 ATP are produced by the breakdown of a single glucose molecule The oxidation of reduced coenzymes releases enough energy to form 32 ATP 34 ATP are produced, but? 2 ATP are required to transport the NADH produced during glycolysis into the mitochondria 4 ATP are produced directly during glycolysis and the Krebs cycle Cellular Respiration Glucose + O2 ? CO2 + H2O + energy Where is each of the above used or produced during cellular respiration? Cellular Respiration We derive energy from the breakdown of glucose We can also get energy from the breakdown of other molecules Other sugars, proteins, fats, etc. These molecules are converted into glucose, pyruvate, or another intermediate in the breakdown of glucose Glycolysis, Beer, & Muscle Burn Cellular respiration requires O2 to harvest 36 ATP from a single glucose molecule Some energy can be harvested in the absence of O2 ?Fermentation? Glycolysis, Beer, & Muscle Burn Many organisms are capable of fermentation e.g., Many bacteria, yeast, some human muscles, etc. In fermentation, glycolysis is the only energy-yielding process Krebs cycle and the electron transport chain do not function Glycolysis, Beer, & Muscle Burn Glycolysis produces ATP and NADH Without the electron transport chain, the NADH cannot be used to generate ATP Can you explain why? NADH must be converted back into NAD+ NADH?s electrons are added to a molecule other than O2 Glycolysis, Beer, & Muscle Burn Alcoholic fermentation in yeast Yeast is a single-celled fungus Use O2 when it is present Undergo fermentation when O2 is absent Glycolysis produces 2 ATP 2 NADH 2 pyruvate molecules Glycolysis, Beer, & Muscle Burn Alcoholic fermentation in yeast Pyruvate is converted to acetaldehyde and CO2 Acetaldehyde is reduced by NADH to form ethanol Acetaldehyde + NADH ? ethanol + NAD+ NAD+ is regenerated, allowing glycolysis to continue Glycolysis, Beer, & Muscle Burn Alcoholic fermentation in yeast Humans make use of alcoholic fermentation in yeast Wine and beer Bread Ethanol additives for gasoline 7.4 Fermentation When fermentation occurs in muscles during vigorous exercise, the lactate builds up, as does an oxygen deficit. The increase in lactate changes the pH, creating the ?burn? associated with exercise. Microorganisms and Fermentation Bacterial fermentation produces either lactate or alcohol + CO2. Yeast are well known microorganisms that produce alcohol and CO2 during fermentation. CO2 production is what causes bread to rise. Ethanol production is critical for the making of beer and wine. Glycolysis, Beer, & Muscle Burn Alcoholic fermentation in yeast Wine production Yeast is added to grape juice, and the mixture is placed in airless wine casks Grape juice contains sugars Yeast ferment these sugars Ethanol accumulates Yeast cannot survive ethanol concentrations over about 14% Glycolysis, Beer, & Muscle Burn Alcoholic fermentation in yeast Bread rising Yeast is a component of bread dough Yeast ferment sugars CO2 is produced Bread rises Ethanol is produced Evaporates during baking Glycolysis, Beer, & Muscle Burn Lactate fermentation in animals Human skeletal muscles can be highly active High demand for glucose and O2 Remember glycogen from chapter 3? The speed of O2 delivery cannot meet the demand Glucose is fermented Glycolysis, Beer, & Muscle Burn Alcoholic fermentation in yeast Glycolysis produces 2 ATP 2 NADH 2 pyruvate molecules Pyruvate is reduced by NADH to form lactic acid Pyruvate + NADH ? lactic acid + NAD+ NAD+ is regenerated
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