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This is a very confusing question to me that came up while I was studying photosynthesis.

Halobacterium has a photosynthetic membrane that is colored purple. Its photosynthetic action spectrum is exactly complementary (opposite to) the action spectrum for green plants. What wavelengths of light do the Halobacterium photosynthetic pigments absorb? A) red and yellow B) blue, green, and red C) green and yellow D) red and green E) blue and red

I don't understand how the answer to this is possibly "blue and red," since the pigment is purple (eliminates violet end of spectrum since purple is transmitted) and the question says the action spectrum (synonymous with the absorption spectrum) is complementary to that of green plants. Green plants absorb best in the blue and red ranges, so naturally I would think the answer is C, green and yellow. What am I missing? Am I confusing the color of the photosynthetic pigment with the absorption spectrum?

Thanks!

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    $\begingroup$ Halobacteria use bacteriorhodopsin in a photophosphorylation to synthesize ATP. Their bacteriorhodopsin pigment absorbs and uses the yellow green middle of the visible spectrum and reflects both ends, which would then be visible as purple, so you are correct, the answer to your question is C) $\endgroup$ – Ebbinghaus May 12 '16 at 6:15

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