Chapter 9.2 Cellular Respiration: Pyruvate Oxidation & Citric Acid Cycle

OVERVIEW OF GLYCOLYSIS 123 (Starting material) 6-carbon sugar diphosphate 6-carbon glucose 2 PP 6-carbon sugar diphosphate PP 3-carbon sugar phosphate PPPP Priming reactions. 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 six- carbon molecule with two phosphates. 3-carbon pyruvate 2 NADH ATP 2 NADH ATP 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 energy-rich hydrogen is harvested as NADH, and two ATP molecules are formed. 3-carbon sugar phosphate 3-carbon sugar phosphate 3-carbon sugar phosphate 3-carbon pyruvate

 Glycolysis  but pyruvate has more energy to yield!  3 more C to strip off (to oxidize)  if O 2 is not available, pyruvate is reduced to regenerate NAD+ (meaning that NADH is oxidized to NAD+) Glycolysis is only the start 2x2x 6C 3C glucose      pyruvate

 Glycolysis  but pyruvate has more energy to yield!  3 more C to strip off (to oxidize)  if O 2 is available, pyruvate enters mitochondria  enzymes of Krebs cycle complete oxidation of sugar to CO 2 Glycolysis is only the start 2x2x 6C 3C glucose      pyruvate 1C pyruvate       CO 2 3C

Cellular Respiration

Oxidation of Pyruvate  Pyruvate enters mitochondria  3 step oxidation process (on next slide)  releases 1 CO 2 (count the carbons!)  reduces NAD  NADH (stores energy—endergonic)  2C combines with CoA, producing acetyl CoA  Acetyl CoA enters Krebs cycle  where does CO 2 go? NADNADH 3C 2C 1C pyruvate    acetyl CoA + CO 2 [ 2x ] Waiting to exhale?

Pyruvate oxidized to Acetyl CoA reduction oxidation Yield = 2C compound + CO 2 + NADH x2

Krebs cycle  a.k.a. Citric Acid Cycle  in mitochondrial matrix  8 step pathway  each catalyzed by specific enzyme  step-wise catabolism of 6C citrate molecule  Evolved AFTER glycolysis  does that make evolutionary sense?  bacteria  3.5 billion years ago (glycolysis)  free O 2  2.7 billion years ago (photosynthesis)  eukaryotes  1.5 billion years ago (aerobic respiration) Hans Krebs

4C6C4C 2C6C5C4C CO 2 citrate acetyl CoA Count the carbons! 3C pyruvate x2x2 oxidation of sugars This happens twice for each glucose molecule!

4C6C4C 2C6C5C4C CO 2 citrate acetyl CoA Count the electron carriers! x2x2 3C pyruvate reduction of electron carriers NADH FADH 2 NADH This happens twice for each glucose molecule! ATPADP

So we fully oxidized glucose C 6 H 12 O 6  CO 2 & ended up with a net gain of 4 ATP! What was the point? Is 4 ATP all we get?

 Krebs cycle produces large quantities of electron carriers  NADH  FADH 2  stored energy!  they go to ETC NADH & FADH 2 What’s so important about NADH and FADH 2 ?

Energy stored from Pyruvate Oxidation and Krebs Cycle 4 NAD + 1 FAD4 NADH + 1 FADH 2 1 ADP1 ATP 3x3x pyruvate          CO 2 ][ 3C 1C 2x Net gain = 2 ATP = 8 NADH = 2 FADH 2 Net gain = 2 ATP = 8 NADH = 2 FADH 2 Stored energy? What good is that?

So why the Krebs cycle?  If the yield is only 2 ATP, then why?  value of NADH & FADH 2  electron carriers  reduced molecules store energy!  to be used in the Electron Transport Chain Ohhhh! I see. We just are slowly releasing the chemical energy from the bonds of glucose! Not so bad, right? RIGHT?!?

Cellular Respiration

The Point is to Make ATP! What’s the point? ATP Any Questions??