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Leaf Color Chromatography: Bite Sci-zed

4 minutes

So why does this turn into this in the fall? I live in Boston, which means that about this time each year, the leaves on the trees start to turn their colors.

(female describer) Title: "Bite Sci-zed Science" Sci-zed is spelled S-c-i hyphen z-e-d. Leaves get their color from three main compounds. The first is chlorophyll, which might sound familiar. Chlorophyll is involved in photosynthesis, which is the process by which plants turn CO2 and water into sugar and oxygen by harnessing the energy from the sun. Chlorophyll gives most plants their distinctive green color. The second group of compounds are the carotenoids, which sounds a little bit like carrot, which makes sense because carotenoids give leaves there yellow and orange hues. Carotenoids are present with chlorophyll in organelles called chloroplasts year round. And these chloroplasts are just the organelles where photosynthesis occurs. Finally, there's a group of compounds known as anthocyanins. And anthocyanins give plants reddish hues. And they're often found in things like cranberries, and blueberries, and lots of dark red leaves. These anthocyanins are actually produced in the fall when chlorophyll starts to break down, but I'm getting ahead of myself. Now, for most of the year, chlorophyll is so bright green that it blocks out some of the yellow and orange hues coming from the carotenoids. For most of the growing season, chlorophyll is constantly being broken down and reproduced. But as it starts to get colder out, the cool night air closes off the veins of the leaves. And it gets harder for water and nutrients to move into the chloroplasts and rebuild chlorophyll. So what this means is that the chlorophyll is being broken down, but it's not being reproduced. So the green color of the leaves starts to disappear. This means that the yellows and the oranges of the carotenoids can start to show through. But that's not all. Anthocyanins, by contrast, are not present year round in the leaves. Instead, they are actively produced in the fall right before the leaves drop off the trees. So why would a plant suddenly want to start producing a red pigment in a leaf that's about to drop off anyways? Scientists haven't reached a definitive conclusion yet. But they do have some pretty good ideas. One is that the red pigments act as a sort of direct sunscreen as the canopy starts to drop leaves. The leaves that remain are exposed to more light. And those leaves are losing the sunlight-absorbing powers of their chlorophyll. Another cool idea is that the red pigment might actually trick migrating insects into thinking that the tree is unhealthy and discouraging them from laying their eggs on those trees. So all of this leads us to some pretty cool science that you can do at home to visualize the pigments found in leaves where you live. So all you need for this experiment is isopropyl alcohol, some strips of filter paper-- I used coffee filters-- a couple glasses, scissors, and some leaves. And if you want to be super cool, pick a few different kinds of leaves, so that you can compare the compounds you find within them. Step one, chop each of your kinds of leaves up into lots of little tiny pieces. We really want to release all of these compounds out of the leaves. So the smaller and the finer you chop them, the better this is going to work. Next, cover the leaves in isopropyl alcohol. Now, this is just rubbing alcohol, so it's not very dangerous. But you do want to make sure you don't get this in your eyes or drink it. The next step is to grind up these leaves in the alcohol. Now, ideally, you would be doing this with a mortar and pestle. but I don't have that. And so I am using a wooden spoon. Now, at this point, you should start to see the alcohol change color as those pigments are released from the leaves. If you want to speed this reaction up a bit, you can set all of your glasses into a hot water bath and leave them for about 30 minutes. After this, you can really see the color change in the alcohol. Now, I strained the leaves out of my alcohol, so that it was a little easier to see. But you could probably leave them in there as well. Next, cut your filter paper into strips and place a strip into each glass so that one side of it is touching the alcohol and the other side is out in the air. Now, I repeated this experiment later with longer strips of filter paper and hung one side out of the edge. And this actually worked a little bit better.

(describer) Title: The key here is that the filter paper should not lean against the glass. Now, you want to let your glasses sit for about an hour. So what we're doing here is called chromatography, which just means that we are separating a mixture. Here, we are using paper chromatography to separate the different compounds within the leaves. So the alcohol, our mobile phase, is being drawn up the paper strip, our stationary phase, and it's pulling leaf pigments along with it. Depending on their chemical structure, molecules will have different polarities and will want to interact with that paper differently. Some will want to stick tightly onto the paper, while others will move more easily with the liquid. This means that some compounds will travel along the paper faster than others, separating the compounds along the strip, and giving you distinct bands of compounds--cool. So we actually got some really good separation between the yellow carotenoids on top and the green chlorophyll on the bottom. And the longer you leave these, the better that separation will get. Now, this is an experiment you can easily do at home, and you should. I want to see what the compounds from leaves from all around the world look like. So make a video response of you doing this experiment, and leave it below. And I'll check them all out. And yeah, go on. Go forth. Do science.

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Host Alex Dainis explains why leaves change colors during the fall. She examines the different colors of leaf pigments by using chromatography. In this experiment, viewers are able to see the various colors of pigments found in leaves.

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Runtime: 4 minutes

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