A student prepares a slide of fresh plant tissue (such as potato parenchyma) to observe under a light microscope. Before viewing, they add a few drops of iodine-potassium iodide (I2KII_2KII2KI) solution. The diagram below shows one of the observed cells containing several dark-stained organelles labeled Structure Y.
Which statement correctly identifies Structure Y and explains the biochemical basis for its stained appearance?
Structure Y is a chloroplast; the iodine stain reacts selectively with the hydrophobic tails of chlorophyll molecules in the thylakoid membrane, causing them to precipitate as a black complex.
Structure Y is the nucleus; the iodine ions bind directly to the negatively-charged phosphate backbone of DNA, causing chromatin to condense and appear black.
Structure Y is a mitochondrion; its outer membrane actively accumulates iodide ions, reducing them to elemental iodine (I2I_2I2) which appears dark purple.
Structure Y is an amyloplast; it contains amylose which forms a deep blue-black coordination complex with triiodide ions (I3−I_3^-I3−) from the stain.