New Labs


Border

Home • 2008 Workshop Agenda • Past Participants • Lab Descriptions
Pictures • Email Us • Links to Other Sites


Photosynthetic Adaptations of Euglena

Euglena, a common inhabitant of pond water, is a complex single cell that moves by whipping its flagellum through water. A simple light sensing organelle called the eyespot, is located at the base of the flagellum. Using information from this photoreceptor the flagellum propels the protist toward light levels appropriate for photosynthesis.
In a light rich environment Euglena use chloroplasts to gather solar energy for use in the manufacture of glucose from carbon dioxide and water. Chlorophyll, the chief photosynthetic pigment, is a family of light-excitable molecules that occur in slightly different forms in the Monerans, Protistans and Plants. Many photosynthetic organisms possess light sensitive "accessory pigments," often of colors other than green, by which they can absorb additional solar energy which is then passed to chlorophyll. Chlorophyll absorbs light primarily in the red and blue-violet portions of the spectrum. Because green light is mostly not absorbed but rather reflected by chlorophyll, chlorophyll-containing tissues appear green.

The aim of this exercise is to investigate light related behavior in Euglena and to determine:
1. whether Euglena has "color vision" - an ability to discriminate between different wavelengths of incoming light, and
2. if it does, whether Euglena favors those wavelengths most valuable to it in photosynthesis.

Procedures

Each group of students will be responsible for preparing and monitoring one experimental Euglena culture, measuring and graphing the transmission spectrum of various colored cellophanes, and measuring the absorption spectrum of Euglena pigments.

1. Color sensitivity experiment with Euglena

Each table will be supplied with a test tube, black paper, and cellophane of various colors. Measure the amount of black paper needed to construct a light-proof sleeve to the tube and cut to size. Then draw a straight line down the paper, and with a paper punch make three-five holes along this line. No hole should be nearer than 1/2 inch to the top of the tube. Cover each hole with a different colored piece of cellophane. Secure the sleeve around the tube, being careful to prevent light leaks. Fill the tube completely with a suspension of Euglena, and cap it firmly with a rubber stopper that has been covered with a small piece of saran wrap. Make sure that when the stopper is inserted it does not block any of the cellophane-covered "windows". Check again for extraneous light leaks, then place the tube beneath a light source with the windows facing the light. After 1-1/2 - 2 hours, decant the Euglena suspension back into the culture flask, while holding the tube with the window facing up. Remove the sleeve and observe where the organisms are aggregated, comparing the densities of the aggregations beneath each window. "Score" these densities, 0 (no Euglena) to +++ (considerable concentration of Euglena). If Euglena proves to be color sensitive, it might be expected to aggregate in different amounts beneath the different windows. After you determine the absorption characteristics of its pigments (next step) you should be able to predict which windows might be most "popular."

2. Absorption spectrum of Euglena pigments

Filter 15-20 ml of Euglena through filter paper, then put the paper into a beaker containing about 20 ml of acetone. The acetone will turn green as it dissolves the chlorophyll out of the Euglena cells, and you will use this solution, plus an acetone "blank" (i.e., pure acetone in another tube) in the absorption measurements using the spectrophotometer.
Record the absorption at 20 nm intervals from 380 to 720 nm. If you encounter any regions of very rapid change in absorption, back up and make readings at 10 nm intervals in this region. Graph the results.

3. Transmission spectra of the colored cellophanes

Euglena will only utilize wavelengths of light that its pigments will absorb. But do the different windows of cellophane transmit light belonging to these parts of the spectrum? To determine which windows should show the most aggregation of Euglena, you must find out if the cellophanes are transmitting the proper wavelengths. Cut a rectangle of cellophane that, when curled as a cylinder inside a Spec tube, will just go around once without appreciable overlap. Fill the tube with water, and use another tube of water (containing no cellophane) as the blank. Measure and graph the % transmission at 20nm increments from 380-720nm for each of the colors you used.


Compare the absorption spectrum of Euglena with the transmission spectra of the various cellophanes, together with the results of the Euglena aggregation study, and interpret the data in your discussion.

Home • 2005 Workshop Agenda • Past Participants • Lab Descriptions
Pictures • Email Us • Links to Other Sites

Rainbow Spiral

Please send questions, comments, and suggestions to Pam Bryer .

HHMI Logo

Howard Hughes Medical Institute and Bowdoin College

 College Sun