Bio 312- 02 Cell Biology
Microscopy: Cavendish Banana Cells Treated with Lugol’s Iodine
A photograph was taken of mesocarp and endocarp of the banana fruit. The mesocarp and endocarp are the layers of the flesh which surround the ovules of the banana. First, background information of the banana will be discussed; focusing on the fruit. Next, the materials and methods used to take the photograph will be listed. Finally, future directions of the project if given the opportunity will conclude the report.
Bananas are an extremely important source of starch for many developing countries. In addition, bananas can provide reasonable amounts of vitamin A and potassium. The banana is the fruit of the banana tree, which is an herb with a woody trunk, not a tree. Specifically, a banana is a berry of the tree. The banana herb is very diverse, home to many species, but the focus of this project was the Cavendish banana. The most common banana eaten in the U.S. and many countries is the Cavendish banana. The Cavendish banana herb is triploid; therefore, it cannot undergo meiosis and produce viable gametes. So, the Cavendish banana is termed “seedless”, because instead they have the remnants of ovules (the brown objects). The Cavendish banana herb is parthenocarpic, meaning it does not need to be pollinated to produce fruit (Ram, 665). The development of the fruit starts with the elongation of the pericarp, the central part of the fruit containing seeds, then the placenta of the fruit grows to surround it (Ram, 666). Lastly, the ovarian cavity is destroyed to be immediately filled with the edible flesh (Ram 666). The ovules would turn into seeds at that point if it was a seeded banana. The edible flesh is filled with starch which can be easily converted into simple sugars to feed the seeds, if there were seeds to feed.
The conversion from starch to the soluble sugars occurs during ripening (Soares, 6672). The fruit contains around 20% starch which is converted to the sugars, mostly sucrose, in the matter of about four days (Soares, 6672). Ripening is done by the release of ethylene. Removing ethylene will slow the ripening process and adding ethylene will speed it up. Starch granules can vary in size and shape, even within the family of bananas. This conversion is done by the amylase enzyme. Starch structure resembles a coil which is made of amylose and amylopectin.
To differentiate the starch from the soluble sugars, Lugol’s iodine was used. Iodine stains the starches a dark purple color. Iodine does this be placing itself within the lumen of the coil. Iodine is usually insoluble and would not be able to do this. Potassium iodide makes a soluble triiodide ion which can go into the coil. The reaction between the coil and the iodide ion produces the dark purple coloring. The iodine test is only qualitative, a quantitative test is possible but is difficult (Woo-Hyeun, 161). In order to more accurately quantify the starch content using the iodine method, the stained starch must be isolated from the specimen.
Materials and Methods:
Materials. A Cavendish banana from the Dining Commons of Keene state college was used. The banana was unripe, green. A disposable scalpel was used to take samples from the banana. A glass slide and coverslip were used to mount the specimen. To stain the starch of the banana, Lugol’s Iodine was used. Lugol’s Iodine is potassium iodide in distilled water. A compound microscope was used to take the photograph, 10x magnification. Lastly, a toothpick was used to smear the sample on the slide.
Isolating the Sample. To isolate the sample, a disposable scalpel was used to cut the skin of the banana. Next, the cut was opened by hand and by other cuts to the skin leaving an opening to the flesh of the banana. From there, the scalpel was used to cut into the flesh of the banana to reveal the inner layers of the flesh close to the ovules (i.e. the mesocarp and endocarp). A small sample of that layer was taken, about 2 millimeters on diameter, which was then applied to the glass slide. Then, using the toothpick, the sample was smeared on the slide. 5 microliters of Lugol’s iodine was dropped directly on the sample. Lastly, a glass coverslip was placed onto the sample and pressure was applied to the slip. If any solution had leaked out a kimwipe was used to clean it.
Taking the Photograph. The sample slide was placed under a compound microscope. Under the 10x magnification the cells were as seen in the photograph. Three other photos were taken, but the first one taken showed what was stained and looked better than the rest. The photograph was then edited. A filter was used to more clearly define the cell shape and show more of the purple coloring of the stain. On the editing software, the Napa filter was used. Then, the less clearly defined and messy parts of the picture were cropped out to create the photograph submitted.
If I was to continue the project I would like to do something similar, but looking at the potassium content in bananas and other foods. I personally eat bananas to prevent cramps, and they are effective. As long as I eat them regularly I do not cramp up. Then I would like to be able to quantify the amount of potassium in different bananas. I could determine what would be the best option. If I wanted to do a more important project later in life, I could find a way to make the Cavendish banana more resistant to panama disease. Panama disease eliminated another banana that was the most common banana eaten a long time ago called the “Gros Michael”. Panama disease completed obliterated the Gros Michael business and a new strain is able to infect the Cavendish banana. Every Cavendish banana is a clone, there is very little genetic diversity. The only diversity would come from different banana herb lines. This makes the Cavendish banana extremely vulnerable to disease such as panama disease. Genetic modification might be the only way to save the Cavendish banana industry, without changing the way they are produced. If the farms used polyculture instead of monoculture infestations and disease would not be as much of a problem, but it could be expensive.
1. Ram, HY Mohan, Manasi Ram, and F. C. Steward. “Growth and Development of the Banana Plant 3. A. The Origin of the Inflorescence and the Development of the Flowers: B. The Structure and Development of the Fruit.” Annals of Botany 26.4 (1962): 657-673.
2. Soares, Claudineia Aparecida, et al. “Plantain and Banana Starches: Granule Structural Characteristics Explain the Differences in Their Starch Degradation Patterns.” Journal of Agricultural and Food Chemistry, vol. 59, no. 12, n.d., pp. 6672-6681. EBSCOhost, search.ebscohost.com/login.aspx?direct=true&db=bxh&AN=BACD201100306707&site=ehost-live&scope=site.
3. Woo-Hyeun, Jeong, et al. “Establishment of New Method for Analysis of Starch Contents and Varietal Differences in Soybean Seeds.” Breeding Science, vol. 60, no. 2, June 2010, pp. 160-163. EBSCOhost, search.ebscohost.com/login.aspx?direct=true&db=a9h&AN=52884992&site=ehost-live&scope=site.