Chapter 9 Cellular Respiration: Harvesting Chemical Energy

Chemical Equation ox C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + Energy red (ATP) Cellular Respiration (overview) Glycolysis (cytosol) Krebs cycle (mitochondria) Electron transport chain (mitochondria)

Vocabulary phosphorylation – adding a phosphate substrate-level phosphorylation – adding PO 4 group to an intermediate compound oxidative phosphorylation – adding a PO 4 to a compound (ADP) using oxygen’s electronegativity to drive the process

Vocabulary chemiosmosis – coupling of chemical reactions with transport across the membrane ATP synthase (synthetase) – protein complexes inserted into membranes oxidation – oxidation is losing (e-) reduction – reduction is gaining (e-)

Glycolysis – breakdown of glucose in the cytosol *Energy investment phase Step 1: glucose is phosphorylated to glucose-6 *ATP consumed phosphate Step 2: isomerization of glucose-6 phosphate Step 3: another phosphate is added to produce *ATP consumed fructose1,6 diphosphate Step 4: molecule is cleaved into 2 isomers Step 5: enzyme catalyzes only 1 of the isomers (glyceraldehyde 3-phosphate)

Glycolysis *Energy payoff phase Step 6: enzyme catalyzes 2 sequential reactions: oxidation of sugar & formation of NADH Step 7: 2 ATP’s produced when phosphate is transferred to ADP’s; PGA results Step 8: enzyme rearranges phosphate on 3-PGA to 2 PGA Step 9: enzyme removes H 2 O to form PEP resulting in instability of phosphate bond (high energy) Step 10: transfer of PO 4 to ADP results in 2 ATP’s; 2 pyruvic acid molecules are the end products

Intermediate step (between glycolysis and Krebs cycle) acetyl coA formed by the removal of – CO 2 & H + from pyruvic acid

Krebs cycle also called TCA (tricarboxylic acid) cycle or Citric Acid cycle Step 1: Acetyl CoA passes its compound to oxaloacetic acid to form citric acid Step 2: H 2 O removed, another added back to isomerize citric acid Step 3: CO 2 released, 5-C compound results, is oxidized, reducing NAD + to NADH Step 4: CO 2 lost; 4-C cmpd. oxidized by NAD + to form NADH & attached to CoA

Krebs Cycle Step 5: phosphorylation of cmpd. to displace CoA Step 6: 2 FADH 2 form when cmpd. oxidized Step 7: cmpd. rearranges by addition of H 2 O Step 8: cmpd. oxidized to produce NADH & results in oxaloacetic acid

Electron Transport Chain drives production of ATP by oxidative phosphorylation (makes no ATP directly) produces most of the ATP in cellular respiration molecules (mostly proteins) embedded in folds of cristae in multiple copies electrons removed from food during glycolysis & Krebs cycle are transferred by NADH (or FADH 2 ) to the electron transport chain

Electron Transport Chain The 1 st electron acceptor in the chain is FMN – (flavin mononucleotide) – gets reduced, then oxidized & passes its e- on to the next compound in the chain many of these proteins are cytochromes the final electron acceptor in the chain is oxygen which picks up a pair of H + from the aqueous solution to form H 2 O

Electron Transport Chain For every 2 NADH molecules, one O 2 is reduced to 2 molecules of water FADH 2 also transports e- to the chain, but at a lower energy level than NADH, therefore, providing 1/3 less energy for ATP synthesis the purpose is to ease the fall of e- from food to O 2 managing the energy release in small steps

Fermentation provides a mechanism by which some cells can oxidize organic fuel & generate ATP without the help of oxygen (anaerobic) there are many types differing in their waste products formed from pyruvate

Alcohol fermentation pyruvate is converted to ethanol (ethyl alcohol) in 2 steps step 1 releases CO 2 from pyruvate, which is converted to acetaldehyde step 2 acetaldehyde is reduced by NADH to ethanol (regenerating NAD+ for glycolysis) performed only by yeast and many bacteria

Lactic acid fermentation pyruvate is reduced directly by NADH to form lactate as a waste product with no release of CO 2 most organisms perform this type of fermentation including human muscle cells which produce lactic acid when O 2 is in short supply; causes muscle fatigue (cramping) & failure