Carbohydrate, Photosynthetic, and Enzymes Differences Causing Leaves to be Edible or Inedible

The Yankees

Group Members:
Chris Moore
Amanda Silic
Nikki Barton
Fallyn Stanley

Abstract

      Vegetables and green plants make up an important part of a persons diet. The leaves and sprouts that make them up contain vitamins and minerals that affect digestion and a person’s health. Other greens exist that we do not eat, but perform processes just the same as edible greens. So what are the main differences between edible and inedible plants? Through a series of tests we compared two inedible leaves, Hibiscus, and Cyclamen, to edible lettuce and spinach leaves. The experiments focused on the differences that exist in carbohydrate composition, photosynthetic quality, and quantity of proteins between the two classes of leaves. When testing for carbohydrate differences we performed the following assays: Benedict’s, Barfoed’s, Bial’s, Selivanoff’s, and Iodine tests. The edible plants tested positive for Benedict’s and Barfoed’s assays, but they rested negative for the other carbohydrate assays. The inedible plants tested negative for all of the carbohydrate tests, which may indicate they were composed of the polysaccharide cellulose. The photosynthetic qualities were examined using paper chromatography and absorption spectrum analysis. Chromatography suggested that there were no conclusive pigment differences between any of the leaves. The absorption spectrum also indicates that there are no differences in the type of light reflected. To test for the total protein concentration, we used the Bradford (Protein) Assay. A T-test confirmed a significant difference in the amount of total concentration of proteins between edible and inedible leaves. These differences may suggest the reason why humans are unable to digest and use some plants for energy.

Table 1: Results of carbohydrate tests performed on the samples of lettuce. There were three trials of each plant, as well as an indicator of a negative result (water) and an indicator of a positive result (glucose in all of the tests except for the Iodine test). The Benedict’s test is used to determine whether there is a free aldehyde or ketone. A red precipitate is formed when the carbohydrate contains a free aldehyde or ketone. Barfoed’s test is used to determine the existence of monosaccharides. Because only monosaccharides can react quickly enough to produce the red precipitate, this test is used to distinguish them from di- and polysaccharides. Selivanoff’s test is used to determine whether the carbohydrate is a ketose or an aldose. Ketoses will produce a red color in less than a minute of heating, while aldoses will take several. Bial’s test is used to test for a furanose ring. A green/olive color indicates a pentose-furanose and a muddy brown indicates a hexose (or higher)-furanose. The Iodine test is used to detect the presence of starch. If starch is present, the solution will turn blue-black.

Discussion

Written by Chris Moore
Revised by Nicole Barton and Amanda Silic

      The abundance of plant growth on the earth is astounding. Prehistoric herbivores grew to astounding heights and weights from only plants. If humans were able to grasp this valuable resource, numerous world hunger problems could be solved. So what is it that makes spinach and lettuce healthy and digestible but not maple leaves or grass? The chemical composition of these plants is the basis for this experiment. The presence or absence of a chemical (carbohydrates/enzymes) in a substance directly affects the ability of an organism to be able to digest and recover nutrients from the plant (Freeman, 2002). The experiment supports the hypothesis that there is a difference in the content of carbohydrates, the quality of photosynthesis, and the concentration of proteins between the edible (spinach and lettuce) and inedible (hibiscus and cyclamen) plants.
      The digestible plants contained reducing sugars but not starch. The non-digestible plants did not contain either of these two substances. The presence of starch and reducing sugars has a direct affect on a human’s ability to digest plants. Previous studies have been done that show the importance of nutrient balance in an organism’s diet in order to function properly. Phosphorus is an extremely important mineral when it comes to mammalian herbivore growth. Without the correct phosphorus levels, the herbivore will not be able to function correctly (Grasman, 1993 ). Another study was done supporting the phosphorus data only this time with aquatic herbivores. Phosphorus played a critical role in the development of these aquatic animals as well (Hussein, 2003). Even though phosphorus is not the basis of the tests we performed, this study demonstrates the importance of certain chemical components and their influence on digestion. The ability for humans to take advantage of, as for now, non-edible plants seems to lie within the chemical composition of the plant itself; in particular, the carbohydrates and the enzymes, which we tested for in our experiments.
      This is one perspective of a reason why humans are unable to digest certain plants. This is the molecular view. What if this is the wrong point of view to approach on why plants are non-edible for humans? Another possible view may be the anatomical or physiological function of humans. A study done revealed that a difference in gastro-intestinal tracts of meadow voles vs. prairie voles was the reason why prairie voles digested alfalfa better and meadow voles digested grass better. The meadow voles simply digested the grass better because its GI tract is longer (Owl and Batzli, 1998). Could this foreshadow that it may be a human’s physiology creating this misfortune of plant digestion? There are many views to approach the question posed in the experiment. However, the data did support our hypothesis that there were very significant molecular differences between the edible and non-edible plants.
      Our data supported our hypothesis. Lettuce and spinach tested positive for Benedict’s and Barfoed’s tests because they contain reducing sugars and monosacchrides. These are carbohydrates that can be broken down by humans. The hibiscus and the cyclamen tested negative for Benedict’s and Barfoed’s test because they do not contain reducing sugars and monosacchrides. This means that the inedible plants instead are composed of non reducing sugars along with poly- and disaccharides. These carbohydrates may not be digestible because of their complexity resulting in the leaf being indigestible. The results for Selivanoff’s, Bial’s , and the Iodine test were all similar. This suggests that the presence of a ketose or aldose, furanose rings, or starch have a minute to no effect the digestibility of the inedible versus edible leaves.
      In the photosynthesis labs, each type of leaf produced different pigments and all performed similarly in the absorption spectrum. The spinach contained all four pigments, while lettuce only contained one. Cyclamen contained three pigments while hibiscus contained two. Lettuce contained the least pigments since it’s composed mainly of water. We expected each plant to contain pigments since each performs photosynthesis, but we predicted the inedible leaves would contain more pigments. The results show that no clear ratio of pigments is visible between the inedible and edible leaves. The absorption spectrum gave a curve of absorbance of the plants at different wavelengths. The curves for all four plants were similar showing that red and blue lights were absorbed and green light was reflected. The results of the photosynthesis test showed similarities in absorbance and photosynthetic pigments, but did not suggest anything about the digestibility of the plants. Each plant performs photosynthesis , but the pigment results show the difference in the quality of photosynthesis.
      In the Bradford Assay, the inedible plants had a higher concentration of protein compared to the edible plants. After calculating the protein concentration of the four plants at each absorbance, a t-test was used to compare the edible and inedible plants. The p value of the t-test was .004 which is less than .05 and indicates a significant difference between the protein concentration of the edible and inedible plants. This difference could be the reason why some plants are edible and some are inedible. Although the inedible plants had a higher concentration of protein, the proteins present may be enzymes that are harmful to us.
      Error may be due to human mistake, instrumental calibration, laboratory technique, and equations/calculation errors. Due to time constraints, we were unable to perform the experiments in a single lab period. We stored our solutions and occasionally the leaves themselves in the refrigerator for several days. While this may have not affected our data at all, the time spent in the refrigerator may have facilitated the breakdown of the sugars. Another error we encountered were air bubbles in the micropipettes, especially with the hibiscus leaves. Due to the thickness of the solution, it was very difficult to pipette accurately. In some cases the results of the tests were hard to interpret. For example, the Bial’s test had an indicator of olive green or brown for a positive furanose test. Our solutions were already green; therefore, it was very difficult to detect a color change. To avoid errors in the future, we should allot more time and perform more trials which will ensure accuracy. We could also dilute the solutions more making it easier to detect a color change and interpret the results.