I remember a question from my own MCAT which was basically to identify which of the answer choices was a pigment. Pigments have extended conjugated systems. In pigments, conjugation decreases the energy difference between the highest occupied molecular orbital (HOMO) of the ground state π system and the lowest unoccupied molecular orbital (LUMO). This change enables pigments to absorb light in the visible range. Visible photons are of lower energy than would be typically absorbed by organic molecules. Important pigments in biology include chlorophylls a and b, the cytochromes of the mitochondrial matrix, retinal in vision, and from the laboratory perspective, many of the stains used in histology.
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