Bananas glow. It’s true and it’s all because of one molecule.
Source: Wiley InterScience (Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Chlorophyll absorbs energy in the form of light and converts this energy into oxygen gas, providing us mammals with fresh air to breathe. The chlorophyll molecule is a large ring, with carbon, oxygen, hydrogen and nitrogen atoms (the four elements of life) bonding together in a variety of ways. Some of these atoms are hanging off the side, while others form the core of the ring. A magnesium atom plops itself right in the center.
Because chlorophyll absorbs either red light or blue light (the corresponding wavelengths are 665 nanometers and 465 nanometers), the pigment itself is green. Plants, which are chock full of the molecule, are thus green.
As plants age, their color fades. The green color disappears as the chlorophyll breaks down. This process of breaking down is called catabolism. The chlorophyll, because it is so large, forms smaller and smaller molecules as it breaks down. Scientists call these molecules chlorophyll catabolites.
The final products, called non-fluorescent chlorophyll catabolites, are found in aging plants, and scientists have recently discovered that they also exist in aging fruit. Dying plants and ripening fruits break down chlorophyll in the same way.
Why mention that these molecules are not fluorescent? Turns out that during chlorophyll catabolism, intermediate molecules are formed that are, in fact, fluorescent. As the fruit ripens, or ages, these fluorescent chlorophyll catabolites are released by the skins or peels. Normally, we look at our fruit under white light, within the visible spectrum, so we don’t see anything out of the ordinary. But shine UV light on a pile of ripe bananas and the banana peels glow blue.
A group of scientists in Austria studying aging and catabolism published this discovery last year. They exposed bananas of varying ripeness to UV light (wavelength of 350 nanometers) and observed the light that the bananas emitted. The underripe green bananas were barely observable, the ripe yellow bananas were blue, and the overripe brown bananas were, again, hard to observe.
These results showed the researchers that the fluorescent chlorophyll catabolite was an intermediate product. Moreover, it seems to be in bananas of just the right degree of ripeness. Here is a good summary of the article.
Two weeks ago a PNAS study by this research team showed that they can track these fluorescent chlorophyll catabolites as they appear and then disappear. These molecules serve as markers to track cell aging and death. The researchers state, “Thus, they [the fluorescent chlorophyll catabolites] allow for in vivo studies, which provide insights into critical stages preceding cell death.”
Here is a good summary of the article. Watching the fluorescence as it grows in and out reports on the aging process of the plant or fruit.
Take home message of these studies? Grab your black light next time you go to the grocery store and you’ll have a cart full of perfectly ripe fruit.