Chapter 8 Harvesting Energy Glycolysis and Cellular Respiration ATP is the Universal Energy Source Photosynthesizes get energy from the sun Animals get energy 2nd or 3rd hand from plants or other organisms Regardless the energy is converted to chemical bonds energy of ATP Two Main Types of Energy Releasing Pathways 1 Anaerobic pathways 2 o Evolved first o Don t require oxygen o Start with glycolysis in cytoplasm o Completed in cytoplasm Aerobic pathways o Evolved later o Require oxygen o Start with glycolysis in cytoplasm o Completed in mitochondria Overview of Aerobic Respiration 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 2 stages 1 Glycolysis liberates a small quantity of ATP 2 Cellular respiration produces far more ATP 36 38 ATP Photosynthesis Provides the Energy Released by Glycolysis and Cellular Respiration The Role of Coenzymes for Cellular Respiration Small molecules associated with an enzyme that participates in enzymatic catalysis NAD FAD NADH FADH2 are examples 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 o When a molecule becomes oxidized it loses an electrons and becomes more positively o Na Na e Reduction refers to the gain of electrons o When a molecule becomes reduced it gains an electrons and becomes more negatively charged charged o F e F LEO the lion goes GER Glucose C6H12O6 is a key energy storage molecule o All cells metabolize glucose for energy o In humans energy is stored as long chains of glucose called glycogen or as fat o These storage molecules are converted to glucose to produce ATP for energy harvesting An overview of glucose breakdown o 1st stage glycolysis o Begins by splitting glucose a 6 carbon sugar into two molecules of pyruvate a 3 carbon sugar o 2 ATP molecules are produced in glycolysis o Glycolysis proceeds in the same way under aerobic with oxygen or anaerobic without oxygen conditions o Glycolysis occurs in cytoplasm o 2nd stage cellular respiration o Occurs when oxygen is available oxygen is needed for cellular respiration o In this stage 2 pyruvate molecules produced by glycolysis are broken down into 6 carbon dioxide molecules and 6 water molecules o For every two pyruvate molecules an additional 34 or 36 ATP molecules are generated o Occurs in mitochondria organelles specialized for the aerobic breakdown of pyruvate o If oxygen is not available the 2nd stage of glucose breakdown is fermentation o Does NOT produce any ATP o Producing ATP but burning them at the same time so there is no net production o Instead pyruvate remains in the cytoplasm and is converted into lactate or ethanol CO2 o Pyruvate and NADH are produced in glycolysis and used in fermentation o The overall equation for the complete breakdown of glucose is o C6H12O6 6 O2 6 CO2 6 H2O ATP heat A Summary of Glucose Breakdown Efficiency of Aerobic Respiration o Efficiency is only 39 o Sounds really low but really it is not o Most energy is lost as heat 8 2 What Happens During Glycolysis Glycolysis has 2 parts each with several steps Glucose activation these are the energy investment steps required to invest a small amount of energy Energy extraction these are the energy harvesting steps releasing energy Summary of glycolysis o Each molecule of glucose is broken down to two molecules of pyruvate o A net of two ATP molecules and two NADH high energy electron carriers are formed 8 3 What Happens During Cellular Respiration Cellular respiration in eukaryotic cells occurs in mitochondria in 3 stages energy and CO2 releasing o In eukaryotes cellular respiration occurs within mitochondria organelles with two 1 Pyruvate is broken down in the mitochondrial matrix membranes o Glucose is first broken down into pyruvate through glycolysis in the cell cytoplasm o Pyruvate is next transported into the mitochondrion matrix where further breakdown occurs in 2 stages 1 The formation of acetyl coenzyme A acetyl CoA 2 The Krebs cycle o During the mitochondrial reactions CO2 is generated as a wasted product o CO2 diffuses out of cells and into the blood which carries it to the lungs where it is o The carbon atoms in the CO2 molecules in this equation come from C6H12O6 during exhaled reactions of the Krebs cycle 2 High energy electrons travel through the o These high energy electrons jump from molecule to molecule in the ETC losing small electron transport chains amounts of energy at each step o This energy is used to pump H from the matrix and into the intermembrane space producing a concentration gradient of H across a membrane o The buildup of H in the intermembrane space is used to generate ATP during o At the end of the ETC the energy depleted electrons are transferred to oxygen which chemiosmosis acts as electron acceptors o Energy depleted electrons oxygen and hydrogen ions combine to form water o ATP generation continues only when there is a steady supply of oxygen generated 3 ATP is chemiosmosis by intermembrane to matrix thru an ATP synthase o Chemiosmosis is the process by which energy is first used to generate a gradient of H and then capture in the bonds of ATP as H flows down its gradient o The H ions flow across the membrane through ATP synthase channels generating ATP from ADP phosphate o The flow of H through the synthase channel provides the energy to synthesize 32 or 34 molecules of ATP for each molecule of glucose o The newly formed ATP leaves the mitochondrion and enters the cytoplasm where it provides the energy needed by the cell 8 4 What Happens During Fermentation Why is fermentation necessary o For glycolysis to continue the NAD used to generate NADH must constantly be regenerated o If NAD was depleted glycolysis would stop energy production would cease and organism would o There are 2 types of fermentation to regenerate NAD o Lactic acid fermentation produces lactic acid from pyruvate o Alcohol fermentation generates alcohol and CO2 from pyruvate o Both pathways consume far more sugar to generate the same amount of ATP than do those relying on cellular respiration not nearly as efficient Some cells ferment pyruvate to form lactate o Muscles that are working hard enough to use up all the available oxygen ferment pyruvate to die lactate o To regenerate NAD muscle cells ferment pyruvate to
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