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Cell Respiration What is Glycolysis? • Breaking down glucose: “glyco – lysis” (splitting sugar) • • • • Most ancient form of energy capture. Starting point for all cellular respiration. Inefficient: generates only 2 ATP for every 1 glucose. Happens in the cytosol. Why does that make evolutionary sense? Evolutionary perspective • Life on Earth first evolved without free oxygen (O2) in atmosphere. • Energy had to be captured from organic molecules in absence of O2 • Organisms that evolved Glycolysis are ancestors of all modern life • All organisms still utilize glycolysis. Overview • 10 reactions • Convert 6C glucose to two 3C pyruvate • Produce 2 ATP & 2 NADH Glycolysis Summary Endergonic • Invest some ATP. Exergonic • Harvest a little ATP & a little NADH. Overall 1 Glycolysis Reaction Overview • 10 Reactions Total • 9 Different Enzymes needed. Priming reactions. Glycolysis begins with the addition of energy. Two high energy phosphates from two molecules of ATP are added to the six-carbon molecule glucose, producing a sixcarbon molecule with two phosphates. Cleavage reactions Then, the six-carbon molecule with two phosphates is split in two, forming two three-carbon sugar phosphates. Energy-harvesting reactions. Finally, in a series of reactions, each of the two three-carbon sugar phosphates is converted to pyruvate. In the process, an energyrich hydrogen is harvested as NADH, and two ATP molecules are formed. 2 Substrate-level Phosphorylation • In the last step of glycolysis, where did the P come from to make ATP? • P is transferred from PEP to ADP o kinase enzyme o ADP → ATP Is that all there is? • Not a lot of energy… • For 1 billon years+ this is how life on Earth survived • Only harvest 3.5% of energy stored in glucose • Slow growth, slow reproduction We can’t stop there… • • • Going to run out of NAD+. How is NADH recycled to NAD+? Without regenerating NAD+, energy production would stop so another molecule must accept H from NADH. 3 The answer? Fermentation. • In Bacteria & Yeast. o beer, wine, bread o at ~12% ethanol, kills yeast • In Animals & some Fungi. o cheese, yogurt o anaerobic exercise (no O2), this is why your legs burn when running. 4 Glycolysis is only the start • Glycolysis • • • • Pyruvate has more energy to yield 3 more C to strip off (to oxidize) If O2 is available, pyruvate enters mitochondria Enzymes of Krebs cycle complete oxidation of sugar to CO2 What is the point? TO MAKE ATP!!! Oxidation of pyruvate • Pyruvate enters mitochondria o 3 step oxidation process o Releases 1 CO2 (count the carbons!) o Reduces NAD → NADH (stores energy) o Produces acetyl CoA • Acetyl CoA enters Krebs cycle o Where does CO2 go? 1 Pyruvate oxidized to Acetyl CoA Yield = 2C sugar + CO2 + NADH Krebs cycle • Aka Citric Acid Cycle o in mitochondrial matrix o 8 step pathway each catalyzed by specific enzyme step-wise catabolism of 6C citrate molecule • Evolved later than glycolysis o Does that make evolutionary sense? bacteria →3.5 billion years ago (glycolysis) free O2 →2.7 billion years ago (photosynthesis) eukaryotes →1.5 billion years ago (aerobic respiration (organelles) 2 So we fully oxidized glucose: C6H12O6 ↓ CO2 & ended up with 4 ATP! 3 So why is the Citric Acid Cycle so great? Krebs cycle produces large quantities of electron carriers • NADH • FADH2 • Stored energy! • go to ETC (Electron Transport Chain) Energy accounting of Krebs Cycle Net gain = 2 ATP = 8 NADH + 2 FADH2 So why the Krebs cycle? • If the yield is only 2 ATP, then why? o Value of NADH & FADH2 Electron carriers Reduced molecules store energy! To be used in the Electron Transport Chain 4 ATP accounting so far… • Glycolysis → 2 ATP • Kreb’s cycle → 2 ATP • Life takes a lot of energy to run, need to extract more energy than 4 ATP! • There’s got to be a better way! There is a better way! • Electron Transport Chain o Series of molecules built into inner mitochondrial membrane o Mostly transport proteins o Transport of electrons down ETC linked to ATP synthesis o Yields ~34 ATP from only 1 glucose! o Only in presence of O2 (aerobic) Mitochondria (Form fits function!) Double membrane o outer membrane o inner membrane highly folded cristae* fluid-filled space between membranes = intermembrane space o matrix central fluid-filled space Electron Transport Chain 1 Remember the NADH? Glycolysis = 4 NADH Kreb’s Cycle = 8 NADH, 2 FADH2 lectron Transport Chain NADH passes electrons to ETC Electron Transport Chain NADH passes electrons to ETC o H cleaved off NADH & FADH2 o electrons stripped from H atoms → H+ (H ions) o electrons passed from one electron carrier to next in mitochondrial membrane (ETC) o transport proteins in membrane pump H+ across inner membrane to intermembrane space 2 But what “pulls” the electrons down the ETC? Electrons flow downhill Electrons move in steps from carrier to carrier downhill to O2 o each carrier more electronegative o controlled oxidation o controlled release of energy 3 Why the build up H+? ATP synthase o enzyme in inner membrane of mitochondria ADP + Pi → ATP only channel permeable to H+ H+ flow down concentration gradient provides energy for ATP synthesis o molecular power generator! o flow like water over water wheel o flowing H+ cause change in o shape of ATP synthase enzyme ATP Synthesis Chemiosmosis couples ETC to ATP synthesis build up of H+ gradient just so H+ could flow through ATP synthase enzyme to build ATP 4 Cellular respiration Summary of cellular respiration Where did the glucose come from? Where did the O2 come from? Where did the CO2 come from? Where did the H2O come from? Where did the ATP come from? What else is produced that is not listed in this equation? Why do we breathe? Taking it beyond… What is the final electron acceptor in electron transport chain? O2 So what happens if O2 unavailable? o ETC backs up o ATP production ceases o Cells run out of energy and you die! 5 Beyond glucose: Other carbohydrates • Glycolysis accepts a wide range of carbohydrates fuels o Polysaccharides → → → glucose (hydrolysis) ex. starch, glycogen o Other 6C sugars → → → glucose (modified) ex. galactose, fructose Beyond glucose: Proteins • Proteins → → → → → amino acids (Hydrolysis) • Fats → → → → → glycerol & fatty acids (Hydrolysis) o Glycerol (3C) → → PGAL → → glycolysis o Fatty acids → 2C acetyl groups → acetyl coA →Krebs cycle Carbohydrates vs. Fats • Fat generates 2x ATP vs. carbohydrate o More C in gram of fat o More O in gram of carbohydrate so it’s already partly oxidized. 1 Respond to cell’s needs • Key points of control o Phosphofructokinase Allosteric regulation of enzyme • “can’t turn back” step before splitting glucose AMP & ADP stimulate ATP inhibits citrate inhibits Why is this regulation important? • Balancing act: Availability of raw materials vs. Energy demands vs. Synthesis. 2 ATP Accounting (From Glucose to ATP) = 2 Pyruvate 4 ATP 2 NADH = ~34 ATP = 4 CO2 2 ATP 2 FADH2 6 NADH = 2 Acetyl CoA 2 CO2 2 NADH But why does the NADH from Glycolysis make less ATP per molecule than the NADH from the Kreb’s Cycle during the ETC? This yields approximately 38 Total ATP from 1 glucose molecule. (~3 ATP per NADH & ~2 ATP per FADH2) 10 NADH + 2 FADH2 ETC 2 Acetyl CoA Kreb’s Cycle 2 Pyruvate Oxidation of Pyruvate Glucose + 2 ATP Glycolysis