On Learning!

AAHAHAHAAHA wow I'm so happy. um hands down, favorite part of my day (perhaps month? even year?)

So, I was studying for my metabolic biochem final and COULDN'T, for the life of me, figure out why in the world the glyoxylate (mmn hm, couldn't recognize that intense of a word could you, auto spell check??) cycle in plants is so darn important!!! So you skip 2 steps of the Krebs Cycle, but in the end you have the same malate and the same oxaloacetate! Can't they both be converted to glucose/carbohydrates? Since fatty acids can be converted to malate and oxaloacetate, why can only plants convert fatty acids to carbs and we can't??? WHYWHYWHY UGHHHHHHH I should have taken my temperature because I was seriously giving myself a migraine by pure frustration. Not to mention, I really really felt like breaking something. You have to understand: I was stuck on this conundrum for hours and couldn't do anything else. (Please refrain from telling me you knew the answer all along if you do actually know the answer because I will have a heart attack.) I wiki-ed absolutely every relevant key term but none of it could explain it to me. FINALLY, ON WIKI ANSWERS

http://wiki.answers.com/Q/Why_can%27t_fatty_acids_be_converted_directly_to_glucose
Oh my god, this has confused me for months and I finally think I get it, so I hope I can explain it decently.
When fatty acids are oxidized, the acetyl-CoA can enter the Krebs cycle, and one would think that the oxaloacetate generated by the Krebs cycle could be converted to acetyl-CoA, which could then be converted to pyruvate for gluconeogenesis. (I'm practically shaking from excitement at this point.) This can't happen, though, because even though oxaloacetate is made, there is no net increase in oxaloacetate (BUT OF COURSE! Why couldn't my prof just have said this??) (two carbons are lost in the Krebs cycle for every two in the acetyl-CoA coming in). Oxaloacetate can't be taken out of the cycle, then, because then the cycle would be depleted and the only way to replenish it is through one of the anapleoritic reactions, which involve products of glycolysis (PEP and pyruvate). If there is enough PEP or pyruvate around to replenish the oxaloacetate you're taking out to make glucose, chances are you don't need to make glucose in the first place. Pyruvate from glucose or amino acids can be used to make sugars before it is converted to acetyl-CoA, but the pyruvate dehydrogenase complex reaction is irreversible, so once pyruvate is made into acetyl-CoA it cannot be used to make glucose; it is committed to either fatty acid synthesis or the Krebs cycle.
Plants can make glucose from fatty acids, but this is only because they are able to use the glyoxlyate cycle instead of the Krebs cycle. The glyoxylate cycle bypasses the step in the Krebs cycle (the alpha-ketoglutarate dehydrogenase step) in which the two carbons are lost as CO2, so when plant acetyl-CoA enters the glyoxylate cycle there IS a net increase in oxaloacetate which can be used to make pyruvate.


YESSSSSSSSSSSSSSSSSSSSS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! AAHAHAHA I laughed out loud when I read the first sentence! I FEEL YA, PERSON-WHO-SAVED-MY-LIFE! Ah I'm so happy I'm going to stop studying ahahahahahahaahaha yesyesyes

P.S. I knew the end of my suffering was near when I typed in 'why can't fatty' and Google autocompleted 'why can't fatty acids be converted to glucose' for me.