Chapter 6 : How Cells Harvest Energy (B) Dr. Chris Doumen 10/28/14 CITRIC ACID CYCLE. Acetyl CoA CoA CoA CO 2 NAD + FADH 2 NADH FAD + 3 H + ADP + ATP

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1 Chapter 6 : How Cells Harvest Energy (B) Dr. Chris Doumen Acetyl CoA CoA CoA Oxaloacetate Citrate CITRIC ACID CYCLE CO FADH 3 NAD + FAD 3 NADH ADP + P + 3 1

2 Pyruvate oxida.on and Citric Acid Cycle Thus For each turn of the cycle Two CO molecules are released Energy is released yielding one molecule Additional oxidative exergonic reactions transfer hydrogens and electrons, generating three NADH, and one FADH molecule The cycle can continue as long as new Acetyl- CoA is available. Pyruvate oxida.on and Citric Acid Cycle Glucose The availability of AcetylCoA obviously depends on the funneling of pyruvate into the mitochondria, which in turn depends on availability and breakdown of glucose. Pyruvate FADH FAD OxaloAcetate Acetyl CoA CoA CITRIC ACID CYCLE ADP + CoA Citric Acid P CO 3 NAD + 3 NADH + 3

3 Sum of events this far? In Glycolysis Glucose is broken down into pyruvates The energy yield is s, NADHs, Pyruvate enters mitochondria. Keep in mind that glucose produces two pyruvates Pyruvates are converted to AcetylCoA molecules CO released and NADH are formed When AcetylCoA enter into the Citric Acid cycle For each turn, 4 CO are produced ( per AcetylCoA) are formed, as well as 6 NADH and FADH Sum of events this far? In other words, at the end of a the Citric Acid Cycle, Glucose is completely broken down into 6 CO and the energy released is captured in 4 s, 10 NADH and FADH Thus all the CO is formed by mitochondrial processes and complete breakdown of pyruvate occurs here Energy in the form of hydrogen and electrons has been passed on to NADHs and FADH. 3

4 Last step in cellular respira.on : ETC and Oxida.ve Phosphoryla.on The energy captured in the form of NADHs and FADH will now be released via an electron transfer chain and will be coupled to the formation of. Oxidative phopshorylation indicates the formation of with the use of oxygen within the mitochondria Most production in the complete process of cellular respiration occurs by oxidative phosphorylation Electron Transport Chain NAD + + NADH e Controlled release of energy for synthesis of Electrons from NADH and FADH will travel via a sequence of redox (reduction oxidation) reactions down the electron transport chain to oxygen Electron transport chain e 1 O H O 4

5 Electron Transport Chain (ETC) ETC Energy is released at each step and used to pump hydrogen ions into the space between the mitochondrial membranes. The final electron acceptor in these redox reactions is oxygen which becomes reduced to HO! Electron Transport Chain High proton gradient This creates a gradient of protons across the inner mitochondrial membrane, high outside and low inside! From what we have seen, gradients are like potential energy systems. The flow of hydrogen ions back inside will release energy. 5

6 Electron Transport Chain Protons will diffuse back inside via an enzyme called synthase. The energy released during inflow will drive this enzyme to perform an endergonic reaction: the making of from ADP and inorganic phosphate. Oxida.ve Phosphoryla.on The flow of hydrogen ions inwards via the synthase to create is referred to as Chemiosmosis. Thus, oxidative phosphorylation includes Electron transport chain Chemiosmosis with synthase And requires an adequate supply of oxygen as the final electron acceptor 6

7 Oxida.ve Phosphoryla.on The bottom line is that breakdown of glucose into pyruvate, and full oxidation with the mitochondria generates energy in the form of a gradient across the mitochondrial membrane which drives the formation of. When an NADH releases the hydrogens and electrons down the ETC, enough energy is released to make 3 s When an FADH releases the hydrogens and electrons down the ETC, enough energy is released to make s Oxida.ve Phosphoryla.on Intermembrane space Protein complex of electron carriers Electron carrier synthase Inner mitochondrial membrane Mitochondrial matrix Electron FADH FAD flow NADH NAD + 1 H O O + ADP + P Electron Transport Chain Chemiosmosis OXIDATIVE PHOSPHORYLATION 7

8 yield in Glucose oxida.on So how much is produced per glucose? during glycolysis during the Citric Acid Cycle Each NADH creates enough energy via the ETC to eventually generate 3 s There were a total of 10 NADHs formed : this makes thus 30 s Each FADH generates enough energy via the ETC to generate s There were a total of FADHs produced : this makes thus 4 s This totals : = 38 s yield in Glucose oxida.on There are actually not 38 s formed, since of the NADH are formed in the cytoplasm during glycolysis. These have to be shuttled into the mitochondria in order to be play a part in the ETC This transport of NADH into the mitochondria comes at a small cost of about 6. So the current estimates on production is 38-6 = 3 s formed per glucose broken down That is 34% of the energy in glucose is converted to 8

9 yield in Glucose oxida.on Cytoplasm Electron shuttle across membrane Mitochondrion NADH NADH NADH (or FADH ) 6 NADH FADH GLYCOLYSIS Glucose Pyruvate Acetyl CoA CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION (Electron Transport and Chemiosmosis) + by substrate- level phosphorylation + by substrate- level phosphorylation + About 8 by oxidative phosphorylation Maximum per glucose: About 3 Poisons that interrupt cri.cal events in cellular respira.on There are three different categories of cellular poisons that affect cellular respiration The first category blocks the electron transport chain (for example, rotenone, cyanide, and carbon monoxide) The second inhibits synthase (for example, oligomycin) Finally, the third makes the membrane leaky to hydrogen ions (for example, dinitrophenol) 9

10 Rotenone Cyanide, carbon monoxide Oligomycin synthase DNP FADH FAD NADH NAD + 1 O + H O ADP + P Electron Transport Chain Chemiosmosis Fermenta.on Fermentation is an anaerobic (without oxygen) energy- generating process Since there is no oxygen, the complete mitochondrial processes back up and come to a halt Pyruvate stops entering the mitochondria and builds up in the cytoplasm and now becomes processed in the cytoplasm instead of the mitochondria. 10

11 Glucose Fermenta.on All vertebrate skeletal muscle cells and certain bacteria process pyruvate with NADH through lactic acid fermentation The result is the formation of LACTIC ACID (also called lactate). This pathway thus only generates s per glucose broken down to lactate.a drastic reduction in energy production. ADP NAD + + P NADH Pyruvate NADH GLYCOLYSIS NAD + Lactate Lactic acid fermentation Glucose Fermenta.on Yeasts are single- celled fungi that use respiration for energy but can also ferment under anaerobic conditions! ADP + P GLYCOLYSIS NAD + NADH They convert pyruvate to CO and ethanol (using alcohol dehydrogenase) while oxidizing NADH back to NAD + The baking and winemaking industry have used alcohol fermentation for thousands of years CO released Pyruvate Ethanol NADH Alcohol fermentation NAD + 11

12 Glucose Fermenta.on Goldfish have ADH in their skeletal muscles, where it catalyzes alcoholic fermentation during periods of anoxia ADP + P GLYCOLYSIS NAD + NADH Pyruvate CO released NADH NAD + Ethanol Alcohol fermentation Glycolysis was an early inven.on Fermentation is just one additional step to glycolysis. The metabolic machinery of glycolysis is almost identical in bacteria (prokaryotes) and eukaryotes. This provides strong evidence that this process was the original universal energy harvesting process in early cells. This also clears things up if we take into account that early condition on planet earth occurred without oxygen. 1

13 Food sources for cellular energy Although glucose is considered to be the primary source of sugar for respiration and fermentation, there are actually three sources of molecules for generation of Carbohydrates: become mostly converted to glucose and are funneled via a normal glycolysis Fats are broken down and part enters into glycolysis and parts enter as AcetylCoEnzyme A into the Citric Acid Cycle. Amino acids from proteins can be converted into intermediates of the citric acid cycle and then used for energy purposes. Food, such as peanuts Carbohydrates Fats Proteins Sugars Glycerol Fatty acids Amino acids Amino groups Glucose G3P Pyruvate GLYCOLYSIS Acetyl CoA CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION (Electron Transport and Chemiosmosis) 13

14 Food sources for biosynthesis Not all food is used for energy production. Many building blocks from the breakdown of food are used to make new material needed for the cell (= biosynthesis) These biosynthetic processes do require energy in the form of. needed to drive biosynthesis CITRIC ACID CYCLE Acetyl CoA GLUCOSE SYNTHESIS Pyruvate G3P Glucose Amino groups Amino acids Fatty acids Glycerol Sugars Proteins Fats Carbohydrates Cells, tissues, organisms 14