GENERAL BIOLOGY LAB 1 BSC1010L Lab 6 Cellular Respiration OBJECTIVES Understand the major events of glucose catabolism cellular respiration glycolysis the citric acid cycle and oxidative phosphorylation Compare and contrast the processes involved in aerobic and anaerobic respiration Demonstrate carbon dioxide production during anaerobic respiration Determine oxygen consumption during aerobic respiration Measure the relative production of carbon dioxide by plants and animals INTRODUCTION All living organisms have evolved mechanisms to obtain the energy needed to fuel basic biological functions including growth metabolism and maintenance These mechanisms include a series of biochemical reactions collectively referred to as cellular respiration During this process organic molecules e g glucose are enzymatically broken down releasing energy that is stored in the negatively charged bonds of adenosine triphosphate or ATP which is used by cells to perform essential metabolic functions Energy flow through biological systems occurs through oxidation reduction or redox reactions where electrons are transferred from one molecule to another Recall from the Biologically Important Molecules lab Lab 4 that reduction is defined as the gain of electrons or hydrogen atoms while oxidation involves the loss of electrons or hydrogen atoms During cellular respiration electrons are removed from glucose i e glucose is oxidized and some of the released energy is stored in the form of ATP The dozens of redox reactions that take place during this process use electron acceptors for energy transfer Two of the most important electron acceptors are nicotinamide adenine dinucleotide NAD and flavin adenine dinucleotide FAD derived from niacin Vitamin B3 and riboflavin Vitamin B2 respectively These molecules are reduced to NADH and FADH2 when they acquire electrons which they then transfer to other molecules to generate ATP Depending on which molecule serves as the final electron acceptor the entire process is considered aerobic or anaerobic In aerobic respiration the final electron acceptor is oxygen while in anaerobic respiration the final electron acceptor can be inorganic compounds other than oxygen such as nitrates and sulfates or organic molecules such as ethanol or lactate Fig 1 1 Figure 1 Comparison of redox reactions in aerobic and anaerobic respiration Cellular respiration Fig 2 can be divided into four stages 1 Glycolysis 2 Pyruvate oxidation 3 Kreb s cycle 4 Electron Transport Chain and Chemiosmosis Figure 2 An Overview of Aerobic Respiration 2 Glucose catabolism begins with glycolysis in the cytoplasm Fig 3 Glycolysis includes a series of reactions where each entering glucose 6 carbons molecule is split into 2 molecules of pyruvate 3 carbons In total glycolysis yields 4 ATP molecules however 2 ATPs are used for the priming reactions that initiate glycolysis Thus a net of 2 ATPs are generated for the entire process In addition 2 NADH molecules are reduced from NAD during this stage If oxygen is present then the processes of aerobic respiration will begin with pyruvate oxidation During this phase each pyruvate molecule generated from gylcolysis enters the mitochondria and is converted into carbon dioxide CO2 which is released as a side product and acetyl a 2 carbon sugar that joins with coenzyme A to form acetyl CoA More importantly this process also reduces NAD to NADH which can be used to generate ATP During aerobic respiration acetyl CoA enters the Krebs cycle also known as the citric acid cycle or tricarboxilic acid TCA cycle For every turn of the Krebs cycle one ATP molecule is produced and multiple NAD and FAD molecules are reduced to NADH and FADH2 respectively The final products of the Krebs cycle per glucose molecule include 2 ATP 2 FADH2 6 NADH and 4 CO2 In the final stage of cellular respiration Fig 3 the electrons carried by FADH2 and NADH are transferred through a series of transmembrane proteins known as the electron transport chain ETC creating a proton gradient that is used to drive ATP synthesis chemiosmosis Each molecule of NADH yields 3 ATPs while each FADH2 generates 2 ATPs resulting in an overall production of 32 ATPs in this stage alone Intermembrane space Pyruvate from cytoplasm H H NADH e H 1 Electrons are harvested and carried to the transport system Acetyl CoA NADH Krebs cycle 2 Electrons provide energy to pump protons across the membrane e H2 O FADH2 3 Oxygen joins with protons to form water 1 2 O2 O2 2H CO2 32 2 ATP H ATP Mitochondrial matrix 4 Protons diffuse back in driving the synthesis of ATP ATP synthase Figure 3 Overview of ETC and Chemiosmosis Conversely in the absence of oxygen anaerobic respiration the pyruvate molecules produced during glycolysis do not enter the Kreb s cycle but undergo fermentation instead Without oxygen pyruvate cannot enter the remaining steps of aerobic respiration and must be utilized differently In the process either ethanol or lactic acid is produced Along the way NADH that was created in glucose catabolism is oxidized back to NAD pyruvate is reduced and broken down and a small quantity of ATP is produced There are two main types of fermentation reactions 1 ethanol fermentation and 2 lactic acid fermentation Ethanol 3 fermentation occurs in organisms such as yeast Fig 4a which have been utilized industrially for food and alcoholic beverage production Lactic acid fermentation on the other hand occurs in animal cells For example when oxygen is not readily available to muscle tissue the muscle cells use lactic acid fermentation to produce ATP Fig 4b Build up of lactic acid is the primary cause of muscle fatigue often experienced during strenuous exercise Overall the anaerobic process yields a net of 2ATP an 18 fold decrease in ATP production per glucose molecule compared to aerobic respiration a b Figure 4 Anaerobic respiration a alcohol fermentation and b lactic acid fermentation Figure 5 summarizes the basic differences between the products of aerobic and anaerobic cellular respiration While all three types of cellular respiration aerobic respiration alcohol and lactic acid fermentation produce carbon dioxide CO2 water H2O energy ATP and heat they do so at different efficiencies Both types of anaerobic fermentation produce a net total of 2 ATP since they only undergo glycolysis In contrast during aerobic respiration up to 38 ATP molecules are produced through the continuous redox reactions of glycolysis pyruvate oxidation Krebs cycle and