The diagram shows the respiration process in the cytoplasm of a yeast cell or an animal cell when O_2 is limiting:
Which option correctly identifies substances X, Y and Z?
A. X = pyruvate; Y = ethanol; Z = lactate
B. X = pyruvate; Y = lactate; Z = ethanol
C. X = lactate; Y = ethanol; Z = pyruvate
D. X = ethanol; Y = pyruvate; Z = lactate
E. X = lactate; Y = pyruvate; Z = ethanol
Before we rationalize the correct answer, let us establish what the main goal is during respiration in times where O2 is at a limited supply.
ATP is essential for the different biological processes that our cells perform. During times where there is decreased O2 for cells to use, these cells will then be forced to undergo fermentation to continue producing ATP through glycolysis, which is the anaerobic breakdown of glucose. Why is this relevant? Remember, that in glycolysis, one of the reactants needed to make ATP is NAD+. Under abundant O2 concentrations, NAD+ is regenerated by the electron transport chain, where the electron in NADH is dropped off to create NAD+. Now, under decreased oxygen supply, the main goal is to now regenerate NAD+ without using oxygen, to continue creating ATP for the cells to use. This is the goal of both O2 limiting respiration in animal cells and yeast cells.
With that established, let us then rationalize why choice E is the correct choice here.
Y: The common breakdown product in glycolysis is pyruvate, which under normal conditions, will continue on to the TCA or Kreb’s cycle. However, the scenario given in this question assumes that there is NOT MUCH oxygen, thus, pyruvate will now be forced to undergo fermentation. X: In an animal cell, pyruvate will undergo lactic acid fermentation to regenerate the NAD+ needed. Pyruvate will be acted upon by the enzyme lactate dehydrogenase to produce lactate from pyruvate, and at the same time, regenerating the NAD+ reactant needed to make ATP. A good example of this process happening within our bodies is when you tire out your muscles and starting cramping. The “cramp” is due to the build up of lactic acid that was forced to be produced in order to maintain the demands that is being put on the muscle. Z: In cells like the yeast cell, instead of lactic acid fermentation, alcohol fermentation is the fermentation process used to regenerate ATP. Pyruvate to ethanol is a 2-step reaction. First, a carboxyl group is removed from pyruvate and released as CO2 to form the product acetaldehyde. In the second reaction, NADH donates its electron to acetaldehyde to regenerate NAD+ and produce the product ethanol. The NAD+ is then used to make ATP through glycolysis.
The the only correct choice is choice E, where X= Lactic Acid, Y = Pyruvate, and Z = ethanol.
