Glycolysis and Cellular Respiration Questions

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Bring on the tough stuff

1. You are asked to be part of a team looking for life on the Moon. If there were organisms living on the Moon, what type of metabolism might they have?
    
2. The inner membrane of a mitochondrion has lots of folds, or cristae, giving it lots of surface area for electron transport chains. Suppose the inner membrane were smooth instead. How would having less surface area affect energy production in a cell?
    
3. What are the differences between lactic acid fermentation and alcohol fermentation?
    
4. How is fermentation different than anaerobic respiration?
    
5. Why is the citric acid cycle called a cycle?

Cell Respiration Answers

1. Answer: The metabolism "of choice" would probably be anaerobic respiration. Since the Moon does not have an oxygen-rich atmosphere, organisms living there would not use oxygen as their terminal electron acceptor in cellular respiration. Instead, we can hypothesize that they must have evolved the ability to use some other compound as an electron acceptor. It is also possible that they would survive only using fermentation. Or, a completely different type of metabolism may have evolved, but it would still somehow need to oxidize "food" into chemical energy.
    
2. Answer: Fewer ATP molecules would be produced if the inner mitochondrial membrane were smooth. The cristae allow many electron transport chains to work all at once in a mitochondrion. Since the electron transport chain and chemiosmosis depend on the inner mitochondrial membrane, there would be much less space to pump protons across the membrane.
    
3. Answer: In alcohol fermentation, pyruvate is reduced to ethanol. In lactic acid fermentation, pyruvate is reduced to lactic acid. Alcohol fermentation produces two carbon dioxide molecules. Remember: beer has bubbles, but yogurt does not.
    
4. Answer: Anaerobic respiration is similar to aerobic respiration in that it uses an electron transport chain with something other than oxygen as the terminal electron receptor. Anaerobic respiration is the common metabolism in organisms that live in anoxic environments because they have evolved advanced ways of harvesting energy without oxygen. Fermentation is basically glycolysis on steroids. Fermentation adds a reaction to glycolysis that replenishes NAD⁺ so that glycolysis can keep happening over, and over, and over again. There is no citric acid cycle and no electron transport chain in glycolysis.
    
5. Answer: The first step of the citric acid cycle combines acetyl-CoA with oxaloacetate. The following steps produce ATP, NADH, and FADH₂ by oxidizing various compounds, which regenerates oxaloacetate. The beginning compound is the same as the one at the end, coming full circle. Or cycle. Har-har.