IMAT 2013 Q39 [Respiration]

In which of the following stages of mammalian respiration is CO_2 released?

  1. Glycolysis
  2. Anaerobic respiration
  3. Krebs cycle

A. 3 only
B. 1 and 2 only
C. 1 only
D. 2 and 3 only
E. 2 only

The answer to this question is choice A, where only Krebs cycle is the only one out of the choices that includes the release of CO2

Let us rationalize each of the choices to further understand the rationale:

  1. Glycolysis: This is a part of cellular metabolism that uses glucose to create the product pyruvate. Glycolysis in short, uses glucose to make ATP, which is vital for cells to perform their necessary functions. Now, does glycolysis involved the release of carbon dioxide? The answer is no. This part of cellular metabolism releases products like ADP, ATP, NADH, and even water, but not carbon dioxide, thus making this an incorrect consideration.

  2. Anaerobic respiration: This kind of respiration ensues when the oxygen supply for the cell to use to create ATP is at a low level, insufficient to sustain the functions of the cell. Thus, the cell will be forced to forego the Krebs cycle, and use the pyruvate that has been produced by glycolysis to regenerate the NAD+ needed as a reactant to make ATP in glycolysis. An example of this type of respiration would be lactic acid fermentation, where pyruvate acid is acted upon by enzymes to produce NAD+. The release of carbon dioxide within these reactions are not included, which makes this an incorrect consideration. Please do not confuse this with alcohol fermentation wherein pyruvate is acted upon by pyruvate decarboxylase to produce acetaldehyde and carbon dioxide. The question is asking for mammalian respiration, thus do not include alcohol fermentation which is most commonly seen in yeast cells.

  3. Krebs cycle: In contrast with the description of anaerobic respiration above, Krebs cycle is the next step after glycolysis if there is abundant oxygen to sustain the actions of the electron transport chain, which uses oxygen. If you recall, after pyruvate is acted upon by pyruvate dehydrogenase into acetyl CoA, it proceeds to undergo the Krebs cycle, where products such as NADH, FADH2 and GTP produce ATP yields using the electron transport chain. Concomitant with the release of the mentioned products, there is also the release of carbon dioxide, which makes this the only correct consideration.

Thus, the correct answer is choice A.