CONCEPTUAL UFE SCIENCE Cellular Respiration Introduction Cellular respiration is the oxidative, chemical attack on energy-rich molecules to provide useful energy for the cell. Enzymes catalyze the oxidation reactions. These reactions are known as catabolic reactions because they break molecules down to release energy. Anaerobic respiration The first part ofrespiratory pathways in the cell is anaerobic. This term means thatoxygenisnotinvolved. Evencellsoforganismsthatutilizeoxygen,suchashumans, have an anaerobic component. The anaerobic component is known as glycolysis and the Embden-Meyerhof pathway carries it out. This pathway produces ATP and NADli as energy-storage molecules. NADH is similar to the NADPH produced in photosynthesis. In anaerobic respiration, this NADH needs to be recycled. The"total anaerobic pathway iric1uding the recycling process is calledfermentation. We will consider two kinds of fennentation reactions. The first will be the laclic acidfermentation as it occurs in muscles. The second will be the alcoholfermentation as it'occurs in yeast. Glycolysis Glycolysisisapathwaythatthecellusestooxidizeglucose. Someofthe steps in this pathway are the reverseofstepsfound in thedarkreactionsofphotosynthesis. . Glycolysis is part of the fennentation process found in muscle and yeast cells. It is important to avoid confusion between photosynthesis and cellular respiration. Photosynthesis is reductive and anabolic. It produces glucose and releases oxygen as a waste product. In contrast, cellular respiration is .oxidative and catabolic. It uses glucose and oxidizes it to produce C<h as the waste product. Fermentation Fermentation is used by muscle cells to provide quick energy. Yeast cells use it to produce alcohoJ and C<h. This is why yeast is used for brewing and baking. Lactic acid bacteria produce lactic acid, which is used to make fermented foods like pickles, sauerkraut and yogurt. Fermentation is required under anaerobic conditions because ofthe need to recycle NADH. The Embden-Meyerhofpathway produces two molecules ofNADH. These cannot be used to make ATP under anaerobic conditions so they are recycled and oxidized using the fermentation process. This recycling pennits the Embden-Meyerhof pathway to continue functioning." -I 7-2 Glycolysis is the conversion ofglucose to pyruvate. Glucose 7" ~ 2 'GAL 7" ~ 2'GA 7"~ 21\fnm11a ZAT' ZAD, Z HAD. ZHADH • AD' • AT'•• • 1 Figure 7-1. Glycolysis. Energy is made available all the time. Two ATP are used to startandthepathwaymakes4ATP,sothereisanetyieldoftwoATP. ThePOALto PGAstepisthereverseofa similarstepintheCalvin-Bensoncycle. Theresultingtwo NADH molecules must be reoxidized (recycled) to permit the pathway to continue working. Fermentationisthereductionofpyruvate(usingtheNADH)toproduce 8reduced product such as lactic acid or alcohol. Net reactions ofanaerobic respiration. A. Lactic acid fermentation in muscle Glucose + 2 Pi + 2 ADP -+ 2 lactate + 2 ATP B. Alcohol fermentation in yeast Glucose + 2 Pi + 2 ADP -+ 2 ethanol + 2 C<h + 2 ATP , Each ofthe rea~ons beginswithglucose and producesa netyieldoftwo ATP. .ThislownumberofATPresultsfromthelackofaerobic(oxidative)conditioDS. Muscles use fennentation under conditions when oxygen is low or when energy is needed quickly. Yeast cells are used in the processes ofbrewins and baking. In brewing, the , product, such as wine, contains alcohol. Ii1 the case of'beer, the product contains both" ,'.-alcohol and e<h. In baking, the product ofinterest is the e<h. The carbon dioxide helps the baked products rise. That is why yeast is used in making bJad, cakes and t)ther baked products. Recycling ofNADH The NADH produced in the glycolysisreactioDS is in its reduced fonn. It must become oxidized again in order for the pathway to function. The final fermentation reactions are used to regenerate or recycle NAD+ for use in the Embden-Meyerhof pathway. '7-3 a. Reoyelng of NADH In IDUsoIe 2 pyruvate r ~ , 2 ..c.... 2NADH 2NAif b. RecycUng of NADH in yeIISt 2 pyruvate 7'""""\) 2 ethanol + 2 CO2 2 NADH 2 NAD· Figure 7-2. Production ofNAD+ by recycling. The centralreactionofthe Embden-Meyerhofpathwayrequirestheoxidizedfonn ofNAD+. The pathway converts it to the reduced fonn ofNADH. In order for the pathway to continue, the NADH must be recycled. In oxidative conditions, the NADH is not recycled but is sent to the Electron TransPOrtlChemiosmosis pathway instead. Under oxidative conditions, fennentation does not take place. The pyruvate resulting from the Embden-Meyerhofpathway is scrit to the Krebs Citric Acid Cycle, instead. Aerobic respiration Aerobic respiration consists ofthree parts. • Glycolysis • Krebs Ci1ric Acid Cycle • Electron TransportIChemiosmosis Overall reaction ofaerobic respiration Aerobicrespirationisthereverseofphotosynthesis. Theglucosemadebytheplantis oxidized in the cell to produce the waste products, Cen and H20, and energy. Cellular respiration is the opposite ofphotosynthesis. The energy stored by the plant is released in our cells for our use. Glycolysis Glycolysis produces two pyruvates, two ATP and two NADH. Unlike the situation in fennentation, the two NADH molecules are not recycled. The process occurs in the cytoplasm ofthe cell.I I 7-4 Krebs Citric Acid Cycle TheKrebs Citric Acid Cycle isaseriesofeightreactionsthatgo ar01md in a circle. It is found in mitochondria. Each pyruvate molecule resulting from glycolysis enters the cycle and is oxidized to Produce three C02, and the following energy molecules: 4 NADH, I FADH2 and I ATP. Electron TransportlChemiosmosis This is the final pathway of aerobic respiration. It occurS in the mitochondria. The pathway removes electrons and protons from the NADH and FADH2 molecules. FAD is Flavin Adenine Dinucleotide. It is an electron carrier like NADH. FADH2 is the reduced fonn that carries the electrons and protons. . Electron Transport and ChemioSJDosis occur in the mitochondria The electrons are passed through the electron transport chain, aseries ofenzymesthat transport electroDS. The electrons eventually end up on oxygen, which is the terminal electron acceptor of the chain. The protODS are pumped out of the mitQChondria through the mitochondrial membrane. The pumping mechanism is called chemiosmosis. Energy. from chemioSJDOsis Produces 3 ATP for each NADH and 2 ATP for each FADH2. So, the result ofall ofthese pathways is to make useful energy for the cell in the fonn of ATP. Final