1. The Krebs Cycle 9-2

2.  At the end of glycolysis, about 90% of the chemical energy available in glucose is still unused  To extract the rest, cells need oxygen Powerful electron acceptor Required for final steps of cellular respiration b/c they require oxygen - aerobic

3.  Cellular respiration should not be confused with respiration Cellular respiration is the releasing of energy stored in food Respiration gives our cells oxygen  The oxygen needed to power cellular respiration

4. The Krebs Cycle  In the presence of oxygen, pyruvic acid from glycolysis is passed to the Krebs cycle  Second stage of cellular respiration  Named after Hans Krebs

5.  During the Krebs cycle - pyruvic acid is broken down into carbon dioxide  Also known as citric acid cycle Citric acid is the first compound formed

7. Step A  Begins with pyruvic acid produced by glycolysis entering the mitochondria  1 carbon atom from pyruvic acid becomes part of a CO 2 molecule Eventually released into the air

8.  Remaining 2 carbon atoms from pyruvic acid are joined to a compound called coenzyme A Forms acetyl-CoA Acetyl is made of 2 C atoms, 1 O atom, and 3 H atoms  Acetyl-CoA adds the 2 carbon acetyl group to a 4-carbon molecule Produces 6-carbon citric acid

9. Step B  Citric acid is broken down into a 4- carbon molecule  More CO 2 is released  Electrons are transferred to electron carriers NAD+ and FAD

10.  At each turn of the cycle, a molecule similar to ADP is converted to a molecule similar to ATP  At 4 places in the cycle, a pair of high- energy electrons is accepted by electron carries Changes NAD+ to NADH and FAD to FADH 2

12.  What happens to the products? CO 2 released to atmosphere ATP produced can be used for cellular activities High-energy electrons can be used to generate large amounts of ATP in electron transport chain