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Why can't fatty acids be converted directly to glucose??
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. This can't happen, though, because even though oxaloacetate is made, there is no net increase in oxaloacetate (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.
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