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Glycolysis RESPIRATION (2009) Glycolysis Objective Explain the stepwise breakdown of glucose in cellular Respiration Function: • Glycolysis is a partial breakdown of a six-carbon glucose molecule into two, three-carbon molecules of pyruvate, 2NADH +, 2H+, and 2 net ATP as a result of substrate-level phosphorylation • Glycolysis occurs in the cytoplasm of the cell. The overall reaction is: glucose (6C) + 2 NAD+ 2 ADP +2 inorganic phosphates (Pi) yields 2 pyruvate (3C) + 2 NADH + 2 H+ + 2 net ATP • Glycolysis also produces a number of key precursor metabolites Precursor metabolites produced by glucose • Glycolysis does not require oxygen and can occur under aerobic and anaerobic conditions. • During aerobic respiration, two NADH molecules transfer protons and electrons to the electron transport chain to generate additional ATPs by way of oxidative phosphorylation. • The glycolysis pathway involves 9 distinct steps, each catalyzed by a unique enzyme. glucose is phosphorylated • Glycolysis does not require oxygen and can occur under aerobic and anaerobic conditions. • During aerobic respiration, two NADH molecules transfer protons and electrons to the electron transport chain to generate additional ATPs by way of oxidative phosphorylation. • The glycolysis pathway involves 9 distinct steps, each catalyzed by a unique enzyme. glucose is phosphorylated 3. A second phosphate provided by the hydrolysis of a second molecule of ATP is added to the fructose 6-phosphate to form fructose 1,6-bisphosphate. 4. The 6-carbon fructose 1,6 bisphosphate is split to form two, 3-carbon molecules: gleraldehyde 3-phosphate and dihydroxyacetone phosphate. The dihydroxyacetone phosphate is then converted into a second molecule of glyceraldehyde 3-phosphate. Two molecules of glyceraldehyde 3-phosphate will now go through each of the remaining steps in glycolysis producing two molecules of each product. 5. As each of the two molecules of glyceraldehyde 3-phosphate is oxidized energy is released, this energy is used to add an inorganic phosphate (Pi) group to form two molecules of 1,3-biphosphoglycerate each containing a high-energy phosphate bond. • During these oxidations, two molecules of NAD+ are reduced to form two NADH + 2H+. 6. As each of the two molecules of 1,3-biphosphoglycerate are converted to 3phosphoglycerate, the high-energy phosphate group is added to ADP producing 2 ATP by substrate-level phosphorylation 7. The two molecules of 3-phosphoglycerate are rearranged to form two molecules of 2-phosphoglycerate 8. Water is removed from each of the two molecules of 2-phosphoglycerate converting the phosphate bonds to a high-energy phosphate bonds as two molecules of phosphoenolpyruvate are produced.. 9. Two molecules of phosphoenolpyruvate are converted to two molecules of pyruvate, the high-energy phosphate groups are added to ADP producing 2 ATP by substrate-level phosphorylation KREBS CYCLE / CITRIC ACID CYCLE OBJECTIVES Outline the Krebs cycle Explain the significance of the Krebs cycle in ATP formation The overall reaction for the transition / link reaction is: 2 pyruvate + 2 NAD+ + 2 coenzyme A yields 2 acetyl-CoA-SH + 2 NADH + 2 H+ + 2 CO2 FORMATION OF ACETYL-CoA-SH THROUGH THE TRANSITION REACTION OR LINK REACTION • The Link reaction connects glycolysis to the citric acid (Krebs) cycle. Through a process called oxidative decarboxylation • The link reaction converts the two molecules of the pyruvate (3C) from glycolysis into two molecules of the acetyl Coenzyme A (acetyl-CoA-SH), [2C] and 2 molecules of carbon dioxide. • First, a carboxyl group of each pyruvate is removed as carbon dioxide and then the remaining acetyl group combines with coenzyme A (CoA-SH) to form acetyl-CoA-SH. • As the two pyruvates undergo oxidative decarboxylation, two molecules of NAD+ become reduced to 2 NADH + 2H+ • The 2 NADH + 2H+ carry protons and electrons to the electron transport chain to generate additional ATP by oxidative phosphorylation • Before the pyruvates from glycolysis can feed into the citric acid cycle, they must undergo a transition reaction. The pyruvate is converted into a 2-carbon acetyl group as the third carbon is lost as CO2. The acetyl group is attached to coenzyme A to form acetyl-CoA. • The 2-carbon acetyl-CoA combines with the 4-carbon oxaloacetate of the citric acid cycle to form 6-carbon citrate. • Citrate is converted (isomerisation) to isocitrate. • The 6-carbon isocitrate is oxidized by NAD+ and decarboxylated to produce reduced NADH and 5-carbon alpha-ketoglutarate. (One carbon is lost as CO2.) Krebs Cycle . The 5-carbon alpha-ketoglutarate is oxidized by NAD+ and decarboylated to produce reduced NADH and 4-carbon succinyl-CoA. (One carbon is lost as CO2.) 6. Oxidation of succinyl-CoA produces succinate and one GTP that is converted to ATP. 7. Oxidation of succinate by FAD produces reduced FADH2 and fumarate. 8. Fumarate is converted into malate. 9. Oxidation of malate by NAD+ produces reduced NADH and oxaloacetate. Two molecules of acetyl-CoA from the link reaction enter the citric acid cycle. This results in the formation of • 6 molecules of NADH • 2 molecules of FADH2 • 2 molecules of ATP • 4 molecules of CO2 The NADH and FADH2 molecules then carry electrons to the electron transport system for production of ATPs by oxidative phosphorylation RESPIRATION NOTES 3 –ELECTRON TRANSPORT CHAIN ECT. OXIDATIVE PHOSPHORYLATION OBJECTIVE • Explain the process of oxidative phosphorylation with reference to the electron transport chain Include the roles of hydrogen and electron carriers; the synthesis of ATP and the role of oxygen. No details of the carriers are required. A summary of ATP production should be known • The process of respiration has so far been geared to the production of NADH + H+ and FADH2 • During the electron transfer chain (ETC) NADH + H+ and FADH2 donate high energy electrons which are passed along a series of carrier molecules before combining with O to form water • The movement of these electrons results of pumping of protons and synthesis of ATP. • The carriers are components of the inner mitochondrial membrane and include four large multi-enzyme protein complexes • NADH dehydrogenase, • Succinate Dehydrogenase • cytochrome reductase • cytochrome oxidase In addition to shuttles • Ubiquinone or Cytochrome Q • Cytochrome C The NAD molecules then returns to the Krebs Cycle and Glycolysis to collect more hydrogen. • FADH binds to complex II, succinate dehydrogenase rather than complex I NADH dehydrogenase, to release its hydrogen. • The electrons are passed down the chain of proteins complexes from I to IV, each complex binding electrons more tightly than the previous one. • In complexes I, III and IV the electrons give up some of their energy, which is used to pump protons across the inner mitochondrial membrane by active transport through the complexes. • During the oxidation of NADH, electrons enter the electron transport chain at Complex I NADH dehydrogenase or NADH-Q reductase) • As the electrons move through Complex I via a series of redox reactions, protons are pumped from the mitochondrial matrix into the intermembrane space. • The electrons are transferred to the first shuttle molecule, ubiquinone. • Ubiquinone or Cytochrome Q is reduced to ubiquinol which diffuses to Complex III. • As the electrons move through Complex III - Cytochrome c reductase), protons are again pumped from the mitochondrial matrix to the intermembrane space. • The electrons are transferred to the second shuttle molecule, cytochrome c. • The reduced cytochrome c diffuses to Complex IV - cytochrome oxidase. • Again protons are pumped from the mitochondrial matrix to the intermembrane space. • The ultimate electron acceptor is molecular oxygen which is reduced to water. • Altogether 10 protons are pumped across the membrane for every hydrogen from NADH (or 6 protons for FADH). • In complex IV the electrons are combined with protons and molecular oxygen to form water, the final end-product of respiration. The process is catalyzed by cytochrome oxidase • The oxygen diffused in from the tissue fluid, crossing the cell and mitochondrial membranes • Oxygen is only involved at the very last stage of respiration as the final electron acceptor, without it the respiratory chain does not function. ATP Synthase • The movement of protons from the mitochondrial matrix to the intermembrane space generates an electrochemical potential across the inner mitochondrial membrane. • The proton gradient that is generated across the inner membrane is required by the enzyme ATP synthase, which contains a proton pump. • Following the phosphorylation of ADP, the enzyme must release the ATP. • This release is dependent on the presence of a proton gradient. • It takes 4 protons to synthesize 1 ATP molecule. This process is referred to as the chemiosmotic theory • Some poisons act by making proton channels in mitochondrial membranes, so giving an alternative route for protons and stopping the synthesis of ATP. • This also happens naturally in the brown fat tissue of new-born babies and hibernating mammals: • Respiration takes place, but no ATP is made, with the energy being turned into heat instead Comparison ATP production in respiration and photosynthesis Summary of Respiration • We can now see how much ATP is made from each glucose molecule. ATP is made in two different ways: • Some ATP molecules are made directly by the enzymes in glycolysis or the Krebs cycle. This is called substrate level phosphorylation (ADP is being phosphorylated to form ATP). • Most of the ATP molecules are made by the ATP synthase enzyme in the respiratory chain. This requires oxygen it is called oxidative phosphorylation. Summary of Products of Respiration Stage Mols. produced per glucose glycolysis 2 ATP used 4 ATP (2 per triose phosphate) 2 NADH (1 per triose phosphate) 2 4 6 Link reaction 2 NADH (1per pyruvate) 6 5 Krebs Cycle 2 ATP (1 per Acetyl CoA-SH) 6 NADH (3 per Acetyl CoA-SH) 2 FADH (1 per Acetyl CoA-SH) 2 18 4 2 15 3 Total Final ATP yield Old method Final ATP yield New method 2 4 5 38 Other substances used to make ATP • Triglycerides are broken down to fatty acids and glycerol, both enter the Krebs Cycle. 32 • A typical triglyceride might make 50 acetyl CoA molecules, yielding 500 ATP molecules • Fats are a very good energy store, yielding 2.5 times as much ATP per g dry mass as carbohydrates. • Proteins are not normally used to make ATP, but in times of starvation they can be broken down and used in respiration. • They are first broken down to amino acids, which are converted into pyruvate and Krebs Cycle metabolites and then used to make ATP. Anaerobic Respiration Goal: to reduce pyruvate, thus generating NAD+ Where: the cytoplasm Why: in the absence of oxygen, it is the only way to generate NAD+ and ADP Alcohol Fermentation - occurs in yeasts in many bacteria o The product of fermentation, alcohol, is toxic to the organism Lactic Acid Fermentation - occurs in humans and other mammals o The product of Lactic Acid fermentation, lactic acid, is toxic to mammals o This is the "burn" felt when undergoing strenuous activity The only goal of fermentation reactions is to convert NADH to NAD+ (to use in glycolysis). No energy is gained Note differences - anaerobic respiration - 2 ATP's produced (from glycolysis), aerobic respiration - 32 ATP's produced (from glycolysis, Krebs cycle, and Oxidative Phosphorylation) Thus, the evolution of an oxygen-rich atmosphere, which facilitated the evolution of aerobic respiration, was crucial in the diversification of life Respiratory Substrates and Respiratory Quotient (RQ) • It is sometimes useful to deduce which substrate is being used in a person’s metabolism at a specific time. • This can be done if the volume of oxygen taken in, and the volume of carbon dioxide given out are measured. • From this data the respiratory quotient (RQ) can be calculated: RQ = Volume of carbon dioxide given off Volume of oxygen taken in The values of RQ to be expected vary depending of which substances are broken down by respiration. • Carbohydrates (glucose) 1.0 • protein 0.9 • fat (lipids) 0.7 • Under normal conditions the human RQ is in the range of 0.8-0.9, indicating that some fats and proteins, as well as carbohydrates, are used for respiration. • Values greater than 1.0 are obtained when anaerobic respiration is in progress. Measuring respiratory rate can be done by using a respirometer • The potassium hydroxide solution removes carbon dioxide from the surrounding air. • Therefore any carbon dioxide, which is produced by respiration, is immediately absorbed and it does not affect the volume of remaining air. • Any changes in volume, which take place, must be due to oxygen uptake. • A manometer and the calibrated scale measure these changes. • Tube B acts as a control.