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???

plastids make for great de-differentiation and re-differentiation February 4, 2016 10:15

Because all the plastids within a plant differentiate from a proplastid in the zygote, they are all genetically identical, even if they express different proteins.  But differentiation into one type of plastid does not have to be final.  Consider the cells within a potato.  Potato cells are filled with amyloplasts, the starch-storers.  Yet, when a potato is allowed to root and grow, green shoots emerge from the potato.  The shoots get their green color from the chloroplasts inside them.  So, as amyloplast-filled cells divide, cells with chloroplasts are the daughter cells.  That tells us that a plastid remains multipotent even after differentiation.  Pretty awesome, huh?

Plastids in general November 14, 2015 19:38

Our Dynamic Cell Models kit includes chloroplasts, chromoplasts, and amyloplasts.  So I find that we end up explaining about the different types of plastids at meetings when we show our Dynamic Cell Models.  Most teachers never specifically studied plants, so they never had a way to learn about plastids.  Usually they find them pretty cool.  I was a neuroscience major, so I only learned about plastids after I started teaching.  Most biology books don't even mention plastids other than chloroplasts, so it can be difficult to get introductory information about them.

The plant zygote has a proplastid that it gets from the female gamete.  As the plant embryo divides and cells differentiate, the proplastid differentiates into whichever plastid is needed in the cell it is found in.  So, if the cell is a green leaf cell, the plastid differentiates into a chloroplast.  But if the cell is in a root, for example, there's no point in the proplastid becoming a chloroplast because no sunlight reaches it.  Some cells in this situation develop amyloplasts, to store starch.  Meanwhile, if the cell ends up in a part of a plant that should have a color other than green, like a fruit or a flower, the proplastid can differentiate into a chromoplast, to make pigment.

With all this differentiation information, I hope you can see that plastids are a great differentiation lesson in a high school classroom (NGSS puts it in high school, but it could be at any level).

Future blog posts:

• plant cells are the ultimate stem cells
• plastids make for great de-differentiation and re-differentiation
• some pigments are inside plastids while others are not...