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why plants can only synthesize D-glucose why not L-glucose along with D glucose. I know it very well that plants have only enzymes which can synthesize D-glucose but Why not they have enzymes which can also manufacture L-glucose. So that we could have mixture of L and D glucose molecules? What is the significance of synthesis of only D-glucose by the plants ?

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You need only one set of enzymes to make and to metabolize glucose when you choose one isoform. This saves energy for making the enzymes. –  Chris Jul 11 at 8:56
    
@Chris has explained the advantage of using one isomer - why do you think that it would be advantageous to synthesise and use both? –  Alan Boyd Jul 11 at 11:43

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D-Glucose and L-Glucose have similar names only because we humans decided that's how we should name sugars; they have as much in common as, for example, D-Glucose and D-Allose (one chiral bond is different).

So, in essence, organisms don't synthesize L-Glucose for the same reason they don't synthesize D-Allose: they have no use for it. Its similarity to Glucose is only coincidental, it is different enough that, biologically speaking, it's a totally different molecule.

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Synthesis from chemicals in organisms is fundamentally different from laboratory synthesis of chemicals. In the latter, more than one chemical species or other unwanted byproducts are usually formed, whist living organisms use enzymes, that are often stereo-specific. This means that they usually can bind, in the case, produce, only one stereo-isomer.

To produce both forms they would need double sets of enzymes.

It makes sense for animals, bacteria, protozoa, etc. to digest whatever comes, so less specific enzyme, but since plants will only access the glucose they themselves formed use of a stereo-specific enzymatic pathway probably enables a more efficient metabolism.

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