Question

6. The cells of some organisms other than animals and yeast (such as bacteria) carry out fermentation using molecules other than pyruvate, which are similarly reduced as pyruvate is. What is the goal of fermentation and why is the reduction of some molecule (whether it be pyruvate or some other molecule) important to achieve this goal? What would happen if the organism is unable to carry out this fermentation?

7. You want to study the enzymes that function in glycolysis, but do not want any interference from the enzymes that are involved in the citric acid cycle. Based on your knowledge of the cell (and perhaps related to a laboratory exercise that we performed), how could you separate the glycolytic enzymes from those of the citric acid cycle?

8. What is the source(s) of the oxygen that is contained within the CO₂ molecules that are released during the citric acid cycle? In which steps of the citric acid cycle do these sources come in to play? (careful – tricky question!)

9.   What type of reaction does a dehydrogenase catalyze? Why are all of the reactions of the citric acid cycle that are catalyzed by dehydrogenases able to produce NADH or FADH₂?

10. Look at the first box in panel 13-2 of your text to answer the following: In the citrate that is formed to “start” the cycle, the carbon in the COO^- group at the top of citrate that is shown in orange comes from the C=O of acelyl coA. How many turns of the citric acid cycle must occur before that carbon is removed as CO₂?    Harder optional question: The CH₂ carbon in citrate that is shown in orange comes from the CH₃ group of acetyl coA. How many turns of the citric acid cycle must occur before that carbon is removed as CO₂?

Answer 1

6. Some microbes are obligate anaerobe which cannot tolerate oxygen. They always prefer anaerobic respiration instead of aerobic respiration. so they undergo fermentation to produce ATP even though they harvest lesser amount of ATP compares to aerobic respiration. They cannot survive without doing fermentation because they produce ATP out of it.

7. All the glycolytic pathway enzymes located in the cytoplasm and all citric acid cycle enzymes located in the mitochondria. If you remove the mitochondria from cytoplasm you can study the glycolytic enzymes without disturbing the citric acid cycle enzymes.

8. The oxygen eliminated with carbondioxide in citric acid cycle is from the carboxylic groups of citric acid( first and second carbon atom). Isocitrate dehydrogenase and alpha- ketoglutarate dehydrogenase are the two steps where each carbondioxide is released from the TCA cycle.

9. Dehydrogenases catalyse redox recations. They either oxidise or reduce the substrate according to the reaction cataysed by this enzyme. In all the steps of TCA cycle catalysed by dehydrogenases, substrates are oxidised (removal of hydrogen from the substrates) and the hydrogen is transferred to coenzymes like NAD+ and FAD+.

10. Two cycles of citric acid cycle has to be finished to remove the carbons of acetyl CoA as CO₂ which is transferred into oxaloacetate to form citrate in the first step. This is because the two CO₂ removed in a TCA cycle is from first and second carbon atom of citrate. Acetyl CoA attached with the third carbon atom. So in the next successive cycle they will be removed.