Cell Respiration - 12/17/24 PDF

12/17/24 How Cells Harvest Energy √ 1.Cellular Energy Harvest 2.Cellular Respiration √ – Glycolysis – Oxidation of Pyruvate – Krebs Cycle – Electron Transport Chain 3.Catabolism of Protein and Fat 4.Fermentation In Glycolysis,...

12/17/24 How Cells Harvest Energy √ 1.Cellular Energy Harvest 2.Cellular Respiration √ – Glycolysis – Oxidation of Pyruvate – Krebs Cycle – Electron Transport Chain 3.Catabolism of Protein and Fat 4.Fermentation In Glycolysis, 2 ATP used in the investment phase 4 ATP produced in oxidation phase Net gain of ATP = 4 – 2 = 2 4 H added to NAD+ = 2NADH + 2H+ The breakdown of glucose to pyruvate can be summarized as follows: Glucose (6C) + 2 Pi + 2 ADP + 2 NAD+ → 2 Pyruvate (3C) + 2 ATP + 2 NADH + 2 H+ + 2 H2O 1  12/17/24 The possible fates of pyruvate in glycolysis Aerobic condition (presence of O2) § Pyruvate à Acetyl-CoA § Pyruvate is oxidized with loss of the carboxyl group as CO2. The remaining two-carbon unit becomes the acetyl group of acetyl-CoA § Catalyzed by pyruvate dehydrogenase (PDH) § Metabolized in Tricarboxylic acid cycle (TCA cycle) in mitochondria for 3 complete oxidation to CO2 and H2O The possible fates of pyruvate in glycolysis Anaerobic condition (absence of O2) § Pyruvate à Lactate (in some microorganisms and animals) § Pyruvate can be reduced to lactate through oxidation of NADH to NAD + § Catalyzed by lactate dehydrogenase (LDH) § Lactic acid fermentation § E.g. anaerobic glycolysis in contracting muscle / muscle exercise In yeast (anaerobic condition) § Pyruvate à Acetaldehyde à Ethanol § Pyruvate can be reduced to ethanol, with oxidation of NADH to NAD+ § Catalyzed by pyruvate decarboxylase and ADH (alcohol dehydrogenase) § Alcoholic fermentation: § Brewing for beers; fermentation of grape sugar in making wine; 4 yogurt; cheese; kim chi 2  12/17/24 Stage II : Oxidation of Pyruvate (Link Reaction) Link reaction (linking glycolysis to Krebs cycle) Pyruvate enter mitochondria – decarboxylated (CO2 removed) and oxidised ( H removed) to become acetate (2C) Acetate combines with coenzyme A to become acetyl CoA Equation, 2 pyruvates + 2 CoA + 2NAD+ 2 acetyl CoA + 2CO2 + 2NADH + 2H+ enzyme system called ‘pyruvate dehydrogenase complex (PDC)’ Pyruvate dehydrogenase complex (PDC) The size of PDC is enormous. It is several times bigger than a ribosome. In bacteria, PDC are located in the cytosol; in eukaryotic cells, PDC are located in the mitochondrial matrix. PDC are also present in chloroplasts. Five enzymes make up PDC: i. Pyruvate dehydrogenase (PDH) Involved in the conversion of ii. Dihydrolipoyl transacetylase pyruvate to acetyl-CoA iii. Dihydrolipoyl dehydrogenase iv. Pyruvate dehydrogenase kinase Used in the control of PDH v. Pyruvate dehydrogenase phosphatase The eukaryotic PDC is the largest multienzyme complex known: § The core of the complex is formed from 60 E2 subunits. § The outer shell has 60 E1 subunits. § The E3 enzyme lies in the center of the pentagon formed by the core E2 enzymes. 6 § There are 12 E3 enzymes 3  12/17/24 CYTOSOL MITOCHONDRION NAD+ NADH + H+ O– S CoA 2 C O C O C O CH3 1 3 CH3 Acetyle CoA Pyruvate CO2 Coenzyme A Transport protein Because pyruvate is a charged molecule, it must enter the mitochondrion via active transport, with the help of a transport protein. Upon entering the mitochondrion via active transport, pyruvate is first converted to a compound called acetyl coenzyme A, or acetyl CoA. A complex of several enzymes or multi-enzyme complex (the pyruvate dehydrogenase complex) involved in these reactions. The acetyl group of acetyl CoA will enter the Krebs cycle. The CO2 molecule will diffuse out of the cell. CYTOSOL MITOCHONDRION NAD+ NADH + H+ O– S CoA 2 C O C O C O CH3 1 3 CH3 Acetyle CoA Pyruvate CO2 Coenzyme A Transport protein Three reactions: 1. Pyruvate’s carboxyl group (--COO-), which is already fully oxidized and thus has little chemical energy, is removed and given off as a molecule of CO2. (This is the first step in which CO2 is released during respiration). 4  12/17/24 CYTOSOL MITOCHONDRION NAD+ NADH + H+ O– S CoA 2 C O C O C O CH3 1 3 CH3 Acetyle CoA Pyruvate CO2 Coenzyme A Transport protein 2. The remaining two-carbon fragment is oxidized, forming a compound named acetate (the ionized form of acetic acid). An enzyme transfers the extracted electrons to NAD+, storing energy in the form of NADH. CYTOSOL MITOCHONDRION NAD+ NADH + H+ O– S CoA 2 C O C O C O CH3 1 3 CH3 Acetyle CoA Pyruvate CO2 Coenzyme A Transport protein 3. Finally, coenzyme A, a sulfur-containing compound derived from a B vitamin, is attached to the acetate by unstable bond that makes the acetyl group (the attached acetate) very reactive. The product of this chemical grooming, acetyl CoA, is now ready to feed its acetyl group into the Krebs cycle for further oxidation. 5  12/17/24 How Cells Harvest Energy √ 1.Cellular Energy Harvest 2.Cellular Respiration √ – Glycolysis √ – Oxidation of Pyruvate – Krebs Cycle (Citric Acid Cycle) – Electron Transport Chain 3.Catabolism of Protein and Fat 4.Fermentation Stage III - The citric acid cycle (CAC) Lipid catabolism Protein catabolism (carbon skeleton) Glycolysis (Carbohydrate) [Oxidative phosphorylation] CITRIC ACID CYCLE (CAC) Sir Hans Adolf Krebs was awarded the 1953 Nobel Prize in Physiology or Medicine for his discovery of Krebs cycle Gluconeogenesis Amino acid anabolism Lipogenesis - CAC plays a central role in both catabolism and anabolism - CAC / Krebs cycle / Tricarboxylic acid cycle - Some of the compounds are acids with 3 carboxyl groups 12 6  12/17/24 Stage III : Krebs cycle Hans Krebs, 1930s Citric acid cycle (CAC) Tricarboxylic acid cycle (TCA) Very important in all living cells A series of 8 enzyme-catalysed chemical reactions Use oxygen All glucose are totally broken apart/ oxidized Stage II : Krebs cycle (citric acid cycle) Acetyl CoA (2C) combines with oxaloacetate (4C) citrate (6C) – a condensation reaction 2 decarboxylation and 4 dehydrogenation occurred Product:- 3NADH + 3H+ + 1FADH2 + 1ATP Since 2 pyruvates enter Krebs cycle, ⇒ 3 NADH + 3H+ X 2 = 6 NADH + 6H+ 1 FADH2 X 2 = 2FADH2 1ATP X 2 = 2 ATP 7  12/17/24 Stages Product Glycolysis 2 ATP + 2 NADH + 2H+ Link reaction 2 NADH + 2H+ Krebs cycle 2ATP + 6NADH + 6H+ + 2FADH2 Total = 4 ATP + 10 NADH + 10 H+ + 2FADH2 All 10 NADH, 10 H+ and 2FADH2 will proceed to the Electron Transport Chain Figure 9.11 An overview of the citric acid cycle Pyruvate Glycolysis Citric acid Oxidative (from glycolysis, cycle phosphorylation 2 molecules per glucose) ATP ATP ATP CO2 NAD+ CoA *To calculate the NADH inputs and + H+ Acetyle CoA CoA outputs on a per- glucose basis, CoA multiply by 2, because each glucose molecule Citric is split during acid 2 CO2 glycolysis into cycle pyruvate FADH2 3 NAD+ molecules. FAD 3 NADH + 3 H+ ADP + P i ATP 8  12/17/24 Figure 9.12 Closer look at the citric acid cycle Citirc Glycolysis acid Oxidative cycle phosphorylation ATP ATP ATP S CoA C O CH3 7. Addition of water Acetyl CoA molecule rearranges CoA SH bonds in the substrate. NADH O C COO– + H+ CH2 1 COO– H 2O COO– CH2 COO– NAD+ 8 Oxaloacetate HO C COO– CH2 CH2 2 HC COO– COO– CH COO– HO CH 8. The substrate HO Malate CH2 Citrate COO– oxidized, reducing Isocitrate COO– NAD+ to NADH and Citric CO2 acid 3 regenerating 7 NAD+ H 2O cycle oxaloacetate COO– COO– NADH CH Fumarate + H+ CoA SH CH2 HC CH2 α-Ketoglutarate COO– C O 6 4 COO– CoA SH COO– COO– CH2 5 CH2 FADH2 CH2 CH2 CO2 FAD NAD+ COO– C O Succinate Pi S CoA NADH GTP GDP Succinyl + H+ CoA ADP ATP Figure 9.12 Closer look at the citric acid cycle S CoA 1. Acetyl CoA adds its two-carbon acetyl group C O to oxaloacetate, CH3 producing citrate Acetyl CoA CoA SH Citrate synthase O C COO– CH2 1 COO– COO– CH2 Oxaloacetate HO C COO– CH2 COO– Citrate 9  12/17/24 Figure 9.12 Closer look at the citric acid cycle COO– H 2O 2. Citrate is converted to its CH2 COO– isomer, isocitrate, by removal of one HO C COO– CH2 water molecule 2 and addition of CH2 HC COO– another Aconitase COO– HO CH Citrate COO– Isocitrate Figure 9.12 Closer look at the citric acid cycle COO– 3. Citrate loses a CH2 CO2 molecule, and HC COO– the resulting HO CH compound is oxidized, reducing COO– Isocitrate NAD+ to NADH CO2 3 Isocitrate NAD+ dehydrogenase NADH COO– + H+ CH2 α-Ketoglutarate CH2 C O COO– 10  12/17/24 Figure 9.12 Closer look at the citric acid cycle COO– 3. Citrate loses a CH2 CO2 molecule, and HC COO– the resulting HO CH compound is oxidized, reducing COO– Isocitrate NAD+ to NADH CO2 3 Isocitrate NAD+ dehydrogenase NADH COO– + H+ CH2 α-Ketoglutarate CH2 C O COO– Figure 9.12 Closer look at the citric acid cycle COO– CoA SH CH2 α-Ketoglutarate CH2 α-ketoglutarate dehydrogenase 4 C O 4. Another CO2 is lost, and COO– COO– the resulting compound is oxidized, reducing NAD+ to CH2 NADH. The remaining CH2 CO2 molecule is then attached to NAD+ coenzyme A by an unstable C O bond S CoA NADH + H+ SUCCINYL CoA 11  12/17/24 Figure 9.12 Closer look at the citric acid cycle COO– CoA SH CH2 α-Ketoglutarate CH2 α-ketoglutarate dehydrogenase 4 C O 4. Another CO2 is lost, and COO– COO– the resulting compound is oxidized, reducing NAD+ to CH2 NADH. The remaining CH2 CO2 molecule is then attached to NAD+ coenzyme A by an unstable C O bond S CoA NADH + H+ SUCCINYL CoA Figure 9.12 Closer look at the citric acid cycle Succinyl-CoA synthase CoA SH 5. CoA is displaced COO– COO– by a phosphate CH2 group, which is CH2 5 transferred to GDP, CH2 CH2 forming GTP, and then to ADP, COO– C O forming ATP Succinate Pi S CoA (substrate level phosphorylation) GTP GDP Succinyl CoA ADP ATP 12  12/17/24 Figure 9.12 Closer look at the citric acid cycle Succinyl-CoA synthase CoA SH 5. CoA is displaced COO– COO– by a phosphate CH2 group, which is CH2 5 transferred to GDP, CH2 CH2 forming GTP, and then to ADP, COO– C O forming ATP Succinate Pi S CoA (substrate level phosphorylation) GTP GDP Succinyl CoA ADP ATP Figure 9.12 Closer look at the citric acid cycle 6. Two hydrogens COO– are transferred to FAD, forming FADH2 CH Fumarate and oxidizing HC succinate COO– Succinic dehydrogenase 6 COO– CH2 FADH2 CH2 FAD COO– Succinate 13  12/17/24 Figure 9.12 Closer look at the citric acid cycle 8 COO– 7. Addition of water HO CH Malate molecule rearranges CH2 bonds in the substrate. COO– Fumarase 7 H 2O COO– CH Fumarate HC COO– 6 Figure 9.12 Closer look at the citric acid cycle C 8. The substrate O COO– NADH oxidized, reducing CH2 1 NAD+ to NADH and + H+ regenerating COO– oxaloacetate NAD+ Oxaloacetate 8 Malate dehydrogenase COO– HO CH Malate CH2 COO– 14  12/17/24 Figure 9.12 Closer look at the citric acid cycle Citirc Glycolysis acid Oxidative cycle phosphorylation ATP ATP ATP S CoA C O CH3 Acetyl CoA CoA SH NADH O C COO– + H+ CH2 1 COO– H 2O COO– CH2 COO– NAD+ 8 Oxaloacetate HO C COO– CH2 CH2 2 COO– HC COO– COO– HO CH HO CH Malate Citrate COO– CH2 Isocitrate COO– Citric CO2 3 7 acid H 2O NAD+ cycle COO– COO– NADH CH Fumarate + H+ CoA SH CH2 HC CH2 α-Ketoglutarate COO– C O 6 4 COO– CoA SH COO– COO– CH2 5 CH2 FADH2 CH2 CH2 CO2 FAD NAD+ COO– C O Succinate Pi S CoA NADH GTP GDP Succinyl + H+ CoA ADP ATP Summary: The reactions of the CAC 1. Formation of Citrate by citrate synthase – Condensation of acetyl-CoA and oxaloacetate 2. Isomerization of citrate to isocitrate by aconitase – The reaction proceeds by removal of a H2O molecule from the citrate (dehydration) to produce cis-aconitate, and then H2O is added back to the cis-aconitate to give isocitrate (rehydration) 3. Formation of a-ketoglutarate and CO2 by isocitrate dehydrogenase 1st oxidation – Oxidative decarboxylation of isocitrate to α-ketoglutarate and CO2 – 1 NADH is produced 4. Formation of succinyl-CoA and CO2 by α-ketoglutarate dehydrogenase – 2nd oxidation – Oxidative decarboxylation of α-ketoglutarate and CoA to form succinyl- CoA and CO2 – 1 NADH is produced 30 15  12/17/24 Summary: The reactions of the CAC 5. Formation of succinate by succinyl-CoA synthase – Hydrolysis of thioester bond of succinyl-CoA to produce succinate and CoA-SH – Accompanied by phosphorylation of GDP + Pi à GTP 6. Formation of fumarate- FAD-linked oxidation by succinate dehydrogenase – Succinate is oxidized to fumarate and FAD is reduced to FADH2 7. Formation of malate by fumarase – Hydration of fumarate to produce malate 8. Regeneration of oxaloacetate – final oxidation by malate dehydrogenase – Malate is oxidized to oxaloacetate and NAD+ is reduced to NADH 31 Summary: Enzymes 32 16  12/17/24 Overall Stoichometry: Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + H2O → 2 CO2 + 3 NADH + FADH2 + GTP + 2H+ + CoA NADH and FADH2 are then oxidized via the electron transport chain (also called the respiratory chain) in the inner membrane (IM) of the mitochondria (cristae) to generate ATP (oxidative phosphorylation) 33 Figure 9.11 An overview of the citric acid cycle Pyruvate Glycolysis Citric acid Oxidative (from glycolysis, cycle phosphorylation 2 molecules per glucose) ATP ATP ATP CO2 NAD+ CoA *To calculate the NADH inputs and + H+ Acetyle CoA CoA outputs on a per- glucose basis, CoA multiply by 2, because each glucose molecule Citric is split during acid 2 CO2 glycolysis into cycle pyruvate FADH2 3 NAD+ molecules. FAD 3 NADH + 3 H+ ADP + P i ATP 17  12/17/24 How Cells Harvest Energy √ 1.Cellular Energy Harvest 2.Cellular Respiration √ – Glycolysis √ – Oxidation of Pyruvate √ – Krebs Cycle (Citric Acid Cycle) – Electron Transport Chain √ 3.Catabolism of Protein and Fat √ 4.Fermentation Stage IV : Electron Transport Chain Figure 9.5 An introduction to electron transport chains H2 + 1/2 O2 2H + 1 /2 O2 (from food via NADH) Controlled release of 2 H+ + 2 e– energy for Elec synthesis of ATP Free energy, G Free energy, G Explosive ATP tron release of ATP heat and trans light ATP energy port chai 2 e– 1/ n 2 2 H+ O2 H 2O H 2O (a) Uncontrolled reaction (b) Cellular respiration 18  12/17/24 Stage III : Electron Transport Chain Occurs on the inner membrane of mitochondria (cristea) Consists of 4 protein complexes - NADH dehydrogenase complex (I) - Succinate dehydrogenase complex (II) - cytochrome bc1 complex (III) - cytochrome oxidase complex (IV) and 2 mobile carriers - ubiquinone (Q) between I and II - cytochrome c (C) between III and IV 19  12/17/24 Protein complexes and mobile carriers transport hydrogen and electron. (H/e‾ are passed from one carrier to the next) When a carrier accept H & e‾ → reduced When a carrier release H & e‾ → oxidised H & e‾ move along ETC from high energy level to low energy levels Energy released during the transfer is used to synthesize ATP (oxidative phosphorylation) i.e. through chemiosmosis The chain ends with oxygen (final acceptor) 2H+ + ½ O2 + 2e‾ → H2O 20  12/17/24 Figure 9.13 Free-energy change during electron transport Citirc Glycolysis acid Oxidative cycle phosphorylation ATP ATP ATP NADH 50 FADH2 Multiprotei Free energy (G) relative to O2 (kcl/mol) 40 FMN I FAD n Fe S II Fe S O complexes Cyt b III 30 Fe S Cyt c1 Cyt c IV Cyt a Cyt a3 20 10 0 2H++ 1 ⁄2 O2 H 2O - Electron receive high energy from glucose - Complex I (NADH coenzyme Q reductase or NADH dehydrogenase) accepts electrons from the NADH - passes them to coenzyme Q (UQ) - which also receives electrons from complex II, (succinate dehydrogenase) 21  12/17/24 - UQ passes electrons to complex III (cytochrome bc1 complex) - to cytochrome c (mobile carrier) - to Complex IV (cytochrome c oxidase) - Every electron transport, one H+ ion will be pumped into intermembrane space - The ETC ends with oxygen as final electron acceptor that form water. 2H+ + ½ O2 + 2e- → H2O - Oxygen accept e- (bind with hydrogen) water. Final step of e- in oxidation of glucose. - Formation of ATP is phosphorilation - ETC = Oxidative phosphorilation 22  12/17/24 In the electron transport chain:- 1 NADH + H+ release H to the ETC → 3 ATP 1 FADH2 release H to the ETC → 2 ATP Since 10 NADH + 10H+ and 2 FADH2 produced in Glycolysis, link reaction and Krebs cycle, (10 NADH + 10H+) x 3 → 30 ATP ( 2 FADH2 ) x 2 → 4 ATP Total of 34 ATP produced in ETC. Chemiosmosis Theory The gradient forces H+ to diffuse through ATP synthase complex Potential energy released during diffusion of H+ is used to synthesize ATP from ADP + Pi. The process is called chemiosmosis. 23  12/17/24 Chemiosmosis Theory As electrons flow down the ETC, energy is released The energy released is used to pump H+ ions (proton) from the matrix to the intermembrane space → leads to a high concentration of H+ ions in the intermembrane space - H+ enter the intermembrane space through electron transport protein comp (Comp. I, III and IV) High concentration of H+ ions in intermembrane space Creating an electrochemical proton gradient (energy) in intermembrane space The gradient forces H+ to diffuse through ATP synthase complex The inner membrane is impermeable to H+ H+ can only pass thru the protein – ATP synthase complex 24  12/17/24 - Potential energy released during transfer of H+ from intermembrane space to matrix - Used to synthesize ATP from ADP and Pi. - The process is called chemiosmosis. - Movement of H+ from high concentration to low concentration gradient (like osmosis) Figure 9.14 ATP synthase, a molecular mill INTERMEMBRANE SPACE A rotor within the H+ membrane spins H+ H+ clockwise when H+ H+ flows past H+ it down the H+ H+ gradient. H+ A stator anchored in the membrane holds the knob stationary. A rod (or “stalk”) extending into the knob also spins, activating catalytic sites in H+ the knob. Three catalytic sites in the ADP stationary knob + join inorganic Pi ATP Phosphate to ADP MITOCHONDRIAL to make ATP. MATRIX 25  12/17/24 Figure 9.15 Chemiosmosis couples the electron transport chain to ATP synthesis Inner Mitochondrial Glycolysis Citirc acid Oxidative phosphorylation membrane cycle electron transport and chemiosmosis ATP ATP ATP H+ H+ H+ H+ Cyt c Protein complex Intermembrane of electron space carners Q IV I III ATP Inner II synthase mitochondrial FADH2 2 H+ + 1/2 O2 H 2O membrane FAD+ NADH+ NAD+ ADP + Pi ATP (Carrying electrons from food) H+ Mitochondrial Chemiosmosis Electron transport chain matrix Electron transport and pumping of protons (H+), ATP synthesis powered by the flow which create an H gradient across the membrane Of H back across the membrane + + Oxidative phosphorylation Summary of aerobic respiration Glycolysis - 1 glucose 2 pyruvates - 2 ATP used (debt) - 2 (NADH + H+) and 4 ATP produced (ATP – substrate level phosphorylation) - Net gain = 2 ATP 26  12/17/24 Link reaction 2 pyruvates 2 acetylCoA - 2 (NADH + H+) and 2 CO2 produced Krebs cycle - 6NADH + 6H+ + 2FADH2 + 2ATP + 4CO2 produced Electron Transport Chain - Electrons flow through ETC, ATP is produced through chemiosmosis process (oxidative phosphorilation). 1 NADH can generate 3 ATP 1 FADH2 can generate 2 ATP Total from stage I and II, 10 NADH + 10H+ and 2 FADH2 → 10 NADH + H+ = (10 X 3) = 30 ATP → 2 FADH2 = ( 2 X 2) = 4 ATP 27  12/17/24 Total ATP produced in aerobic respiration:- Glycolysis = 2 ATP Krebs cycle = 2 ATP ETC 10 NADH + H+ = 30 ATP 2 FADH2 = 4 ATP _____________________ Total = 38 ATP The equation, (1) Combining glycolysis and Krebs cycle C6H12O6 + 6H2O 6CO2 + 12H2 + 4ATP (2) Electron Transport Chain 12H2 + 6O2 12 H2O + 34 ATP Add (1) and (2) C6H12O6 + 6O2 6CO2 + 6H2O + 38ATP Thus 38 ATP molecules produced for every glucose molecule oxidized in aerobic respiration. 28  12/17/24 Number of ATP produced varies slightly in different cells Depends on the type of shuttle used to transport electrons from the cytosol into the mitochondrion Mitochondrial inner membrane is impermeable to NADH NADH from glycolysis pass the H/e- either to NAD+ or to FAD (shuttle) 29  12/17/24 For example: - brain cells – electrons are passed to FAD → 2 ATP results from NADH of glycolysis → 36 ATP - liver cells and heart cells – electrons are passed to NAD+ → 3 ATP results from NADH of glycolysis → 38 ATP Figure 9.16 ATP yield per molecule of glucose at each stage of cellular respiration Electron shuttles MITOCHONDRION CYTOSOL 2 NADH span membrane or 2 FADH2 2 NADH 2 NADH 6 NADH 2 FADH2 Glycolysis Oxidative 2 Citric phosphorylation: 2 Acetyl acid Glucose Pyruvate electron transport CoA cycle and chemiosmosis + 2 ATP + 2 ATP + about 32 or 34 ATP by substrate-level by substrate-level by oxidative phosphorylation, depending phosphorylation phosphorylation on which shuttle transports electrons from NADH in cytosol About Maximum per glucose: 36 or 38 ATP 30  12/17/24 Other respiratory substrates Besides carbohydrates, proteins and fats can also be used as respiratory substrates Fats (energy store) are hydrolysed to produce glycerol and fatty acids. In severe starvation or on a high protein diet, proteins are hydrolysed into amino acids and their amino groups removed (deamination) 31  12/17/24 ANAEROBIC RESPIRATION Organisms which can respire aerobically or carry out anaerobic respiration when oxygen is limiting = facultative anaerobes e.g. intestinal parasite worms Organisms which can only live in places where there is low or no oxygen and only respire anaerobically = obligate anaerobes e.g. certain bacteria FERMENTATION i) Alcoholic fermentation Carried out by yeast Pyruvate decarboxylated to become 2C ethanal NADH reduce ethanal to ethanol NAD+ regenerated to be reused Glucose 2 ethanol + 2 CO2 + 2 ATP e.g. baking industry, CO2 release helps dough rise 32  12/17/24 ii) Lactic acid fermentation Occurs in muscles cells during strenuous exercise and in some bacteria When oxygen level in muscle limited due to rate of consumption exceeds rate of supply Pyruvate reduced to lactate (ionized form of lactic acid) by NADH and NAD+ is regenerated, with no release of CO2 Glucose 2 lactate + 2 ATP Lactic acid fermentation by certain fungi and bacteria is used in the dairy industry to make cheese and yogurt In human, muscle cells make ATP by lactic acid fermentation when oxygen is scarce. This occurs during strenuous exercise, when sugar catabolism for ATP production outspaces the muscle’s supply of oxygen from the blood. Under these conditions, the cells switch from aerobic respiration to fermentation. The lactate that accumulate was previously thought to cause muscle fatique and pain, but research suggests because of increased level of K+ 33  12/17/24 While lactate appears to enhanced muscle performance The excess lactate is gradually carried away by the blood to the liver, where it is converted back to pyruvate by liver cells Because oxygen is available, this pyruvate can then enter the mito. in liver cells and complete cellular respiration. 34  12/17/24 Regulation of Cellular Respiration Cell does not waste energy making more of a particular substance than it needs. Control through feedback mechanisms In cellular respiration, phosphofructokinase is an allosteric enzyme with receptor sites for inhibitors and activators ATP = inhibitor and AMP = activator As ATP accumulates, ATP inhibits the enzyme and slows down glycolysis. Enzyme becomes active again when rate of ATP being converted to ADP and AMP is faster than ATP being produced. AMP activates the enzyme and rate of ATP production is increased. 35  12/17/24 END 36