cellular respiration help

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cellular respiration help

Post by sd215 » Thu Jul 12, 2007 5:37 am

1.Distinguish between the energy-investment and energy-harvesting steps. Why are four ATP produced during glycolysis but only two NET ATP are produced?
2.WHEN and WHY do cells need to use fermentation?
3. How does the cell utilize other food sources to produce usable energy within the cell? How are intermediates in the process of cellular respiration used to synthesize molecules needed by the cell?

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Post by blcr11 » Fri Jul 13, 2007 9:34 pm

These are rather sweeping questions about fundamental processes in almost all cells, from bacteria to plants and animals.

1. Usually, the distinction between energy “investments” vs “harvesting” means you’re talking about ATP either being used (invested) or produced (harvested; an unfortunate choice in terms; I think of energy “harvesting” as being the light harvesting systems in plants, which is a related topic, but not quite the same thing as ATP production—at least not directly). If you look at the first three steps of glycolysis you see that you need to use 2 molecules of ATP to go from glucose to 1,6-fructose-diphosphate; you’re down two molecules of ATP at this point. Aldolase then splits the six-carbon glucose (now 1,6-FDP) into two interconvertible 3-carbon fragments. Now everything doubles-up, so to speak; for each 6 carbon glucose that enters the cycle, 2 three-carbon chains are produced. Each 3-carbon fragment is processed by a subsequent step that produces a molecule of NADH/H+, and two steps that produce a molecule of ATP at each step. But you have two such 3-carbon fragments so that really means that for each molecule of glucose entering the cycle, you produce 2 molecules of NADH/H+ and 4 molecules of ATP. But you had to use 2 molecules of ATP to get started, so the net reaction from glucose to pyruvate is to produce 2 molecules of NADH/H+ and 2 molecules of ATP and 2 molecules of the 3-carbon fragment, pyruvate.

2. The ATP production in glycolysis is “enough” energy production to get by—you get 2 molecules of ATP and NADH to use, as well as 2 molecules of pyruvate, which can be used for subsequent anobolic pathways as needed. But it’s not the most efficient way to do things. If pyruvate is passed on to the TCA you produce more NADH/H+ and an FADH2, (and a GTP for good measure). Then, if the electrons produced can be passed to the electron transport/oxidative phosphorylation pathways and allowed to respire, now you get 38 molecules of ATP for each glucose molecule that entered glycolysis, or almost 20 times more ATP than glycolysis alone. (NADH/H+ and FADH2 are here acting as electron carriers connecting the metabolic pathways to electron transport. They can also be cofactors in enzyme reactions, so they have more than one function.) Fermentation is the anaerobic conversion of pyruvate produced in glycolysis to lactate, or acetate, or ethanol depending on the organism. You ferment whenever there isn’t enough oxygen to do oxidative respiration.

3. I’m not sure how to answer this one. Most of the usual carbohydrates can be used like glucose (although there is usually a strong preference for glucose when it’s available) either because they may directly interconvert, or because subsequent metabolism produces one of the same 3-carbon fragments that can be produced from glucose. Pyruvate converted to Acetyl-CoA can enter fatty acid synthesis; alpha-keto-glutarate (from the TCA) can enter amino acid catabolism as can pyruvate…and so on and so forth.

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Post by victor » Sat Jul 14, 2007 5:25 am

oh man....this seems like a homework questions.....
You actually can find the answers in your textbooks. I usually use this book: Biochemistry 2nd Edition by Mathews, Van Holde & Ahern
Q: Why are chemists great for solving problems?
A: They have all the solutions.

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