Life through the lens
Classroom Explorations: Elodea Explorations
> Link to Student Pages
Materials & Equipment
Going Further (optional)
  • Elodea plants for slide preparations
  • compound microscopes (one per pair)
  • microscope slides and cover slips
  • water and droppers for wet mounts
Group Size
  • whole class & pairs
Preparation
  • Preview the Web page.
  • Print out color copies of the large image of Elodea cells.
  • Print and duplicate the student pages.
  • Set up the microscope activity (optional)
Objectives
  • To learn about the structure and function of a plant cell and its parts.
  • To calculate the actual size of a cell using a highly magnified image and mathematical equations.
Getting Started
  • Introduce students to the aquatic plant Elodea by showing them the image of Elodea in an aquarium . Or, if you have acquired Elodea plants for the Going Further microscope activity, display the plants.
  • Review with students the major differences between plant and animal cells.
  • Show students the video of Elodea leaf cells . Which cellular structures can they observe? In discussing the moving chloroplasts, ask students if there are areas that the chloroplasts seem to avoid and what might account for this. (Replay the video if students would like to watch the chloroplasts more carefully.)
  • Play the video of Elodea leaf cells with structures labeled so students can check their understanding.
Procedure
  • Pass out the Elodea images , rulers, and student pages, then lead students in the explorations on the student pages.
  • Note that student answers will vary depending on the cell they choose and their measurement technique. For the bonus question, the answer should be similar to this example: If the area of a cell is 0.00125 mm 2 , then the approximate number of cells in a leaf that is 55 mm 2 and two layers thick is 88,000 cells:

    55 mm 2 per leaf layer  = 44,000 cells per leaf layer
    0.00125 mm 2 per cell

     

    44,000 cells per leaf layer x 2 layers = 88,000 cells per leaf

What’s Going On?
Two characteristics that distinguish plant cells from animal cells are the presence of a cell wall and chloroplasts—structures that can be seen in the videos.

Views from the Videos
In the videos, the chloroplasts are seen moving through the cell because of a phenomenon called cytoplasmic streaming : the cytoplasm flows around the nucleus. It also flows around the vacuole (although the vacuole itself can’t be seen in the videos). Visible in the projected videos, in addition to the cytoplasm, chloroplasts, and nuclei, are the cell wall and mitochondria. The plasma membrane is too thin to see at this magnification. In the printed image the students work with, the mitochondria aren’t visible.

What’s the Size?
A “typical” Elodea cell is approximately 0.05 millimeters long (50 micrometers long) and 0.025 millimeters wide (25 micrometers wide). As you can see in the image, the shapes of the cells vary to some degree, so taking an average of three cells’ dimensions, or even the results from the entire class, gives a more accurate determination of “typical” Elodea cell size.
Going Further
Students can determine the typical Elodea cell size with an actual Elodea leaf and a classroom microscope.

Under the Microscope
  • Have students determine the field diameter of the compound microscope objectives. (See the activity What’s the Size of What You See? )
  • Have them prepare wet mounts of Elodea leaves by peeling a thin section from the surface of a single leaf, mounting it in a drop of water on a microscope slide, and covering the leaf with a cover slip.
  • Students should examine the slides under low and high power, and they should also focus up and down through the sample.
  • They should use the field diameter measurements to calculate the length and width of three different cells, and take the average dimensions (as they did with their earlier measurements). How close are these dimensions to the dimensions determined by using the Elodea image?
Counting Chloroplasts
  • Ask students to choose one cell in the image of Elodea cells and to count the chloroplasts they see in that cell.
  • Call on several students to tell how many chloroplasts they counted, write their answers on the board, then find the average number of chloroplasts that students could see.
  • Ask students if they think that this is the total number of chloroplasts in a typical Elodea cell. Why or why not? (The Elodea cells are highly magnified, so the depth of field is quite limited. Students’ experience in focusing up and down through a sample should tell them that only a portion of each cell was in focus when the photomicrograph was made.)
Related Activity
> What’s the Size of What You See?
Acknowledgement
Many thanks to Trish Mihalek, life sciences teacher in the San Francisco Unified School District, for developing a prototype of this activity.