Cellular Biology

How Photosynthesis Works



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An Experimental Demonstration of Photosynthesis

Several experiments were conducted in order to attain a better understanding of the process of photosynthesis. First, the process of plasmolysis was examined to look at the effects of salt concentrations on a cell. Next, a decolorized leaf was examined using iodine in order to determine the sites at which starch is stored and whether or not photosynthesis occurs at these sites. Then, paper chromatography was used to separate the pigments contained in a leaf in order to examine the wavelengths of light with which each pigment is associated. Finally, A demonstration was conducted to show that oxygen gas is produced through photosynthesis.

Introduction: Photosynthesis refers to the process by which most plants, algae, and some bacteria produce oxygen and sugar through reactions involving sunlight, water and carbon dioxide (Freeman, pgs 215-216). Organisms such as these are known as autotrophs due to their ability to produce their own food.
The process of photosynthesis occurs in two separate stages: the light-dependent phase and the light-independent phase or Calvin cycle. During the light-dependent phase, water is broken down to produce oxygen while the release of electrons leads to the formation of NADPH and the production of ATP. The NADPH is then used in the Calvin cycle to aid in the reduction of carbon dioxide while using ATP to produce sugars. The process of photosynthesis takes place in the chloroplasts of green plant cells (V.E. Wamback, VE, 2007).

Plants have several types of pigments. The main pigment type is known as chlorophyll. Chlorophyll is dominant in plants and is responsible for the green color of plants. The other pigments, known as accessory pigments, include carotenoids, which include carotenes and xanthophylls, and flavonoids. These accessory pigments are responsible mostly for protecting the chlorophyll from degradation (Freeman, 2005).
The following series of experiments set out to examine parts of plants in order to gain a better understanding of the processes by which photosynthesis occurs.

Likewise, the extent to which certain parts of a plant are affected by photosynthesis and the storage and transportation of sugar throughout the plant was also studied in order to determine if the process of photosynthesis occurs only in regions of the plant where specific pigments are present and does not occur where these pigments are not present.

Methods:

Plasmolysis - In this experiment, an elodea leaf was examined under a microscope. After examination, one drop of 30% sodium chloride solution was added to the slide and the leaf was re-examined in order to determine the extent to which a hypertonic solution would affect the cells of the leaf.

Green and Non-green Leaf Tissues - In this experiment, a leaf from a coleus plant was decolorized to examine the distribution of starch storage throughout the leaf. The leaf was first boiled in water and then in alcohol in order to remove as much pigment as possible from the leaf. Then, a drop of iodine was dropped on the leaf. After allowing the leaf to absorb the iodine, it was examined in order to determine the portions of the leaf containing starch based on whether or not they turned dark blue/black.

Paper Chromatography - A sample of a type of lettuce leaf was examined via paper chromatography in order to determine which bands of light are absorbed by the various pigments contained in the leaf. The leaf was smeared on to a piece of chromatography paper and developed in a test tube for 35 minutes using acetone as a solvent.
Elodea Leaf Demonstration

In this experimental demonstration, an elodea leaf was grown in an enclosed system under full light and under water. The system in which the leaf was grown was enclosed so as to collect the gas produced through photosynthesis. A match was then lit in the enclosed system in order to determine what type of gas was collected, either oxygen or carbon dioxide.
Results:

Plasmolysis - In experiment 1, the addition of sodium chloride to the elodea leaf resulted in a change in the distribution of the contents of the cells as seen microscopically. The cytoplasm of the cells appeared to shrink away from the cell wall.

Green and Non-green Leaf Tissues - In experiment 2, the addition of iodine to the color-drained leaf resulted in the appearance of several blocks of dark blue/black starch located near to the vascular structures of the leaf.

Paper Chromatography - From the chromatograph produced, it appears that the pigment that traveled the farthest thru the mobile/solvent phase was the yellow pigment known as carotene. The next fastest moving pigment appeared to be lighter yellow than the carotene and is known as xanthophyll. The last section of pigment includes chlorophyll A and B and appears to be a bright Kelly-green color followed by a lighter green color.

Elodea Leaf Demonstration - The demonstration with the elodea leaf did not work due to a leak in the enclosed system that allowed the gas produced through photosynthesis to escape.

Discussion: The examination of different plants in this series of experiments allowed us to gain a greater understanding of the process of photosynthesis. In experiment 1, the addition of sodium chloride to the elodea leaf resulted in the outward osmotic flow of water, thus causing the contents of the cell to appear to shrink away from the cell wall. It could be argued that if a single-celled freshwater organism was submitted to such a hypertonic solution, it too would experience an outward flow of water from its cell in order to attempt to create a condition of equilibrium with regard to the concentration of salt inside and outside on the cell. Conversely, placing a single-celled salt water organism into freshwater would result in an inward flow of water for the same purpose.

In experiment 2, the presence of starch in the leaf was indicated by dark patches on the pigment-drained leaf with the addition of iodine to the leaf's surface. The sites of these starch patches had been green on the original leaf. Thus, it can be argued that the pigments associated with starch production are chlorophyll A and B. The starch in the leaf was measured because it was not broken down in the process of pigment extraction and thus was the only aspect of the leaf that remained preserved. Similarly, based on the present findings, it can be argued that photosynthesis only occurs in regions where starch is present, thus causing areas where no starch is present to receive food through the vascular structures of the plant leaf.

In experiment 3, paper chromatography was used to separate the different pigments of a plant leaf. Different pigments move up the chromatogram at different speeds due to their chemical affinity for the solvent. Thus, pigments that are most chemically-like acetone will travel the furthest/fastest up the chromatogram.

Based on information about the various pigments of a leaf, it can be shown that the pigment carotene serves to absorb wavelengths of light that are not absorbed by the chlorophylls in order to extend the range of wavelengths that can be used in photosynthesis (Freeman, 2005). This is why leaves turn orange in the fall. Because the accessory pigments of a plant are able to absorb wavelengths of light that the chlorophylls cannot, they serve to increase the ability of the plant to continue to undergo photosynthesis even as light waves that are normally absorbed by chlorophylls, which are red and blue, can no longer be absorbed.

In experiment 4, the gas that should have collected in the enclosed system is oxygen. Because oxygen is the by product of photosynthesis, it should have been the only gas contained in the enclosed system. Carbon dioxide is a product of respiration and is one of the reactants of the process of photosynthesis.



Literature Cited:

[Companion website for Biological Science, Second Edition.]
.



Freeman, Scott. (2005). Biological Science, Second Edition. Prentice Hall, Upper Saddle,
NJ



SUNY Fredonia Biology Department. (2005). Photosynthesis Lab. SUNY Fredonia
College, Fredonia, NY



V.E. Wamback, VE, (2007). Biologist. An Experimental Demonstration of Photosynthesis. State University of New York College at Fredonia, Fredonia, New York. Department of Biology. May 2007

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