Plastids and cell color July 14, 2016 21:21
Some cells, like red pepper cells or green leaf cells, are colored because they contain colorful plastids. In red pepper cells, red chromoplasts abound as red spots throughout the cytosol. In green leaf cells, like in Elodea, chloroplasts are the green spots found throughout the cytosol. In both of these examples, the cytosol remains clear-- totally uncolored-- and the color is only within the plastids. When one zooms out in magnification, like going from 100X to 40X magnification, or to the naked eye, the tiny plastids are no longer individually visible so the entire cell and tissue appears colored.
However, there are other plant cells that differ in the way they are colored. Red onion cells, for example, are pigmented a purplish-red color and that pigment is spread throughout the cytosol. There are no colorful dots visible with the light microscope in these cells.
So the question is, "why are some plant cells colored by colorful plastids while others are not?" I posed this question to some botanists, and the most likely answer we could all come up with related to the hydrophobicity of the pigment. A hydrophobic pigment couldn't dissolve in the cytosol and would need to remain embedded in membrane. A hydrophilic pigment could dissolve in the cytosol and would not remain embedded in membrane.
After coming up with this idea, my students and I set off to test if there were plastids that were not visible within red onion cells. We were not successful, probably due to methodological error. We attempted to stain sectioned red onion cells with a histological stain that causes chromatin to appear contrasted. Unfortunately, none of our cells had any intracellular detail we could make out... not even nuclei. Maybe we should have avoided sectioning the material and just peeled it off before staining. I'll try with my students next year again... or maybe someone else has some interest in investigating this???