4. Maximal production of ATP from glucose involves the reactions of glycolysis,

Question

4. Maximal production of ATP from glucose involves the reactions of glycolysis, the citric acid cycle, and the electron- transport chain. Which of these reactions requires O2, and why? Which, in certain organisms or physiological condi- tions, can proceed in the absence of O2 5. Fermentation permits the continued extraction of energy from glucose in the absence of oxygen. If glucose catabolism is anaerobic, why is fermentation necessary for glycolysis to continue? 6. Describe the step-by-step process by which electrons from glucose catabolism in the cytoplasm are transferred to the electron-transport chain in the inner mitochondrial mem- brane. In your answer, note whether the electron transfer at each step is direct or indirect.

Explanation / Answer

4. Aerobic respiration pathway involves four steps.

i. Glycolysis: Independent of presence or absence of oxygen

ii. Link reaction

iii. TCA cycle

iv. Electron transport chain

Steps II and III are not directly dependent on oxygen, but they can not occur in the absence of oxygen.

Oxygen is one of teh substrate molecules in ETC. It is the terminal electron transporter in the ETC. It accepts electrons from the reducing equivalents and forms H2O.

5. In the absence of oxygen, pyruvate is converted to lactate/ethanol via fermentation pathway. Conversion of pyruvate to lactate/ethanol does not produce any ATP. However, it is essential to regenerate NAD+ that supports continued glycolysis under anaerobic conditions. In aerobic conditions, NADH is oxidized to NAD+ in the ETC.

Pyruvate + NADH ----------> Ethanol + NAD+ + CO2

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