Carbohydrate Metabolism: Citric Acid Cycle and Electron Transport

Carbohydrate Metabolism: Citric Acid Cycle and Electron Transport

The respiratory pathways in the plant mitochondria include the citric acid cycle, the electron transport chain and ADP phosphorylation. Pyruvate and malate from glycolysis are imported into the mitochondrion.

The protein responsible for import of pyruvate into mitochondria has only recently been discovered. Herzig et al. Identification and Functional Expression of the Mitochondrial Pyruvate Carrier, Science 337:93-96 (6 July 2012)

Major products of the CAC are NADH, FADH 2, and ATP. Per each pyruvate, 4 NADH, 1 FADH 2, 1 ATP

NADH and FADH 2 are oxidized in the electron transport chain. Notice the 10 H + exported across the membrane for each NADH oxidized.

Notice the uncoupling protein : what would be the effect of activating this protein?

Alternative oxidase (AOX) Pathway: Cyanide Resistant Respiration Physiological roles: Thermogenesis--oxidation releases energy without coupling it to formation of ATP. The energy appears as heat. Occurs in certain family members of the Araceae, where tissues of the spadix heat and release odors. Prevention of over-oxidation--reduces the probability of superoxide formation by reduced complex IV when ATP consumption is slowed by low temperature or other stresses.

Amorophophallis titanum

Here is a true mystery: one fall, a silo filled with wheat exploded and was completely destroyed. The company that handled the silo s insurance looked for accelerants and other signs of sabotage, but without success. The weather was cool, and it had rained for two days before. What was the cause of the explosion?

Here is a true mystery: one fall, a silo filled with wheat exploded and was completely destroyed. The company that handled the silo s insurance looked for accelerants and other signs of sabotage, but without success. The weather was cool, and it had rained for two days before. What was the cause of the explosion? An investigation by the claims adjuster found that a roof leak was reported two weeks before. Is this significant?

Here is a true mystery: one fall, a silo filled with wheat exploded and was completely destroyed. The company that handled the silo s insurance looked for accelerants and other signs of sabotage, but without success. The weather was cool, and it had rained for two days before. What was the cause of the explosion? An investigation by the claims adjuster found that a roof leak was reported two weeks before. Is this significant? A plant physiologist explained that the moisture had most likely stimulated germination of the wheat. Germinating seeds respire, and the respiration releases heat: ΔG o = -2840 kj/mol. The heat from the complete respiration of just 1 kg of starch is (1000/168)(2840) = 17,000 kj. The heat build-up was sufficient to explode the wheat and the structure.

The reactions of the citric acid cycle are under allosteric control. NADH inhibits the three dehydrogenases in the cycle. ATP inhibits the electron transport chain. ADP and Pi stimulate the ETC and pyruvate oxidation.

The citric acid cycle, like glycolysis and the PPP, provides intermediates for many essential cell components.

Summary Pyruvate from glycolysis is imported into mitochondria, oxidized by pyruvate oxidase and the citric acid cycle. The NADH produced is oxidized by the electron transport chain, forming an e- gradient (like that across thylakoid membranes); the e- gradient is used to power ATP synthesis. The maximum yield of ATP/sucrose is about 60. Uncoupling protein and the alternative oxidase reduce the yield of ATP/NADH and release heat. Reactions in the citric acid cycle, as in glycolysis, are under allosteric control. Glycolysis and the CAC produce intermediates for several biochemical pathways.