Question

Which of the following is NOT expected when a eukaryotic cell is deprived from Oxygen? Select one: a. All listed events are expected in a eukaryotic cell deprived of oxygen b. The cell continues to oxidize glucose to pyruvate through glycolysis c. The formation of CO2 from cell respiration stops d. Reduction of the production of ATP by substrate level phosphorylation in the mitochondria e. The cell reduces pyruvate (fermentation) to allow the ETC to continue working

Answer 1

ANSWER - OPTION E - The cell reduces pyruvate(fermentation)to allow the ETC to continue working.

EXPLANATION

• For an aerobic respiration to occur, it needs Oxygen (O2) as the final electron acceptor for the electron transport chain (ETC).
• When aerobic respiration happens, approximately 30 – 31 molecules of ATP will be produced with the help of ETC and chemiosmosis utilizing the energy of the high-energy electrons carried by molecules NADH or FADH₂ to the electron transport chain.
• When the high energy molecules; NADH or FADH₂ give their high energy electrons to the ETC, NAD⁺ and FAD are regenerated. These low energy molecules created again cycles back to glycolysis and/or the citric acid cycle, where they pick up more high energy electrons and allow the process to continue, making it a cyclic process.
• Glycolysis & the TCA cycle (citric acid cycle) will not occur if there is no NAD⁺ present to pick up electrons as the reactions proceeds. When oxygen is present, this is not at all a problem – all of the NADH & FADH₂ that are produced during glycolysis and the TCA cycle are converted back into NAD⁺ and FAD respectively after the ETC.
• But in case of absence of oxygen, the electron transport chain can’t run because there is no oxygen to act as the final electron acceptor. This means that the electron transport chain or ETC will not be accepting electrons from NADH as its source of power, so NAD⁺ will not be regenerated.
• Both glycolysis and the TCA cycle require NAD⁺ to accept electrons during their chemical reactions. So for the cell to continue to generate any ATP, NADH have to be converted back to NAD⁺ for use as an electron carrier. Anaerobic processes use different mechanisms, but all function with an aim to convert NAD⁺ back into NADH.

How this is done?

• Processes that use an organic molecule for the regeneration of NAD⁺ from NADH are collectively referred to as fermentation.
• In contrast, some living systems use an inorganic molecule such as nitrate or sulphur for the regeneration of NAD^+.

Both of these methods are known as anaerobic cellular respiration. They do not require oxygen to achieve NAD⁺ regeneration & enable organisms to convert energy for their use in the absence of oxygen.

During anaerobic respiration, only glycolysis occurs. The 2 molecules of NADH generated during glycolysis are then converted back into NAD+ during anaerobic respiration so that glycolysis can continue.

Since glycolysis only produces 2 ATP, anaerobic respiration is much less efficient compared to that of aerobic respiration (2 ATP molecules compared to 36 ATP molecules in aerobic respiration). However, 2 ATP molecules are much better for a cell than 0 ATP molecules.

In anaerobic situations, the cells have to continue performing glycolysis for generating 2 ATP per glucose; because if a cell is not generating any ATP, it will die.

So from the above explanations we can understand that the only part of aerobic respiration that physically uses oxygen is the electron transport chain or the ETC. So ETC will stop working if oxygen is absent (hence option E - cannot be expected as per the question).

(However, the TCA cycle cannot occur in the absence of oxygen because there is no way to regenerate the NAD+ used during this process.)