4: In yeast pyruvate is normally metabolised to ethanol. Why is this metabolic o

Question

4: In yeast pyruvate is normally metabolised to ethanol. Why is this metabolic option more beneficial for the yeast cells than conversion of pyruvate to lactate? Conversion to ethanol releases more NAD* per mole than the conversion to lactate. The carbon atoms are more oxidized in ethanol than in lactate. Increased levels of lactate increase the cellular osmotic pressure, which disrupts the oxygen gradient and flow of oxygen into cells and their mitochondria. Ethanol has a neutral pH, but lactate (lactic acid) production is accompanied by a detrimental decrease in pH. Ethanol production is not more beneficial. Yeast normally produces ethanol and lactate in equimolar amounts a) b) c) d) e) 5: Yeasts are unicellular eukaryotic microorganisms and many species of yeast can be classified as facultative anaerobes. Which option below provides a correct definition for the term facultative anaerobe? a) An organism that can grow with or without oxygen, because it can make ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation or anaerobic respiration if oxygen is absent An organism that lives and grows in the absence of molecular oxygen and, hence will die in the presence of oxygen. An organism that needs oxygen because it cannot make ATP in the absence of oxygen. Hence, it cannot ferment or respire anaerobically An organism that does not require oxygen as it metabolises energy anaerobically An organism that needs oxygen because it cannot ferment or respire anaerobically However, it will be poisoned by high concentrations of oxygen. b) c) d) e) 6: Which of the following options provides a correct definition for the Pasteur effect? a) b) c) It is the inhibition of oxygen consumption on the addition of glucose to tissues or microorganisms having a high rate of aerobic glycolysis It is the phenomenon that occurs in facultatively anaerobic cells whereby oxygen inhibits glycolysis or fermentation. It is the process that involves heat-treating milk or similar fluids to improve storage qualities and destroy pathogenic bacteria, without radically altering taste and nutritional characteristics If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change to reestablish an equilibrium. d) e) It describes the metabolism of cancer cells (and many cells grown in vitro) that involves high levels of glucose fermentation to lactate even when an adequate level of oxygen is present for aerobic respiration.

Explanation / Answer

4.d. Ethanol has a neutral pH, but lactate(lactic acid) production is accompanied by a detrimental decrease in pH.

5.a. An organism that can grow with or without oxygen, because it can make ATP by aerobic respiration, if oxygen is present but is capable of switching to fermentation or anaerobic respiration if oxygen is absent.

6. b. It is the phenomenon that occurs in facultatively anaerobic cells, whereby oxygen inhibits glycolysis or fermentation.

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