A eukaryotic cell that can carry out only fermentation instead of complete aerobic respiration of glucose is most likely in which condition?

Introduction / Context:
Fermentation is an anaerobic strategy to regenerate NAD+ so glycolysis can proceed when oxygen is unavailable or when the respiratory chain cannot be used. This question examines what situation forces a eukaryotic cell to rely only on fermentation rather than complete aerobic respiration of glucose.

Given Data / Assumptions:

• Fermentation occurs in the cytosol and does not use mitochondria.
• Aerobic respiration requires O2 as the terminal electron acceptor in mitochondria.
• Some eukaryotes can select fermentation even with mitochondria present if O2 is absent.

Concept / Approach:
When O2 is not available, the electron transport chain cannot accept electrons from NADH/FADH2, preventing oxidative phosphorylation. To keep glycolysis running, cells reduce pyruvate to lactate (animals) or to ethanol and CO2 (yeast), regenerating NAD+ without using the mitochondrion. Therefore, oxygen lack is the key condition leading to “only fermentation.”

Step-by-Step Solution:

Verification / Alternative check:
In yeast, air-lock fermentation releases modest CO2 (from pyruvate → acetaldehyde → ethanol) compared with the 6 CO2 per glucose in aerobic respiration. In muscle, lactate fermentation yields no CO2 at all, showing that total CO2 is indeed lower without the TCA cycle fully engaged.

Why Other Options Are Wrong:

• Produces less CO2: often true, but this is a consequence, not the defining reason for using fermentation.
• Has mitochondria present and fully competent: mitochondria may be present, but “only fermentation” occurs because O2 is lacking; mitochondrial competence does not force fermentation.
• All of these: incorrect because one choice (mitochondria fully competent) does not explain the reliance on fermentation.
• Accelerated Krebs cycle: cannot proceed without O2; NADH reoxidation is blocked.

Common Pitfalls:
Confusing the presence of mitochondria with their ability to operate; without O2, the respiratory chain cannot run, regardless of organelle presence.

Final Answer:
is lacking in O2 (oxygen is unavailable).