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Cell Observation Exercise

Phil Talbot



Type of entry:

  • lesson/class activity

Type of Activity:

  • hands-on activity

Target audience:

  • Biology


Background information:

This activity helps students to learn the difference between cell types, determine cell sizes, and to correctly use a microscope to so. In this exercise it is important for the teacher to let the students discover for themselves, to see how a microscope works, and require that the students record data accurately. The students are required to learn how to use a microscope, know how to determine size of objects, make a wet mount, stain cells, Identify large cellular structures, know differences between procaryotic, eukaryotic, plant and animal cells. Preparation time involves picking up an onion and Elodea the night before the exercise, and preparing hypertonic and hypotonic solutions. The exercise takes my students one forty-five minute period.


Abstract of Activity:

To avoid common problems faced by students in using microscopes, this exercise has students take measurements of the distance between the objective and the slide when in focus with each objective. Determine the diameter of the field of view at each power in order to determine the size of the cells (and their organelles) being observed. Over the years in my first year biology classes I have had to go around to most of the lab teams and help them focus their scopes on each power. They "cannot find it", even after I have shown them the procedure and what to look for using the microscope camera. This exercise has really helped the students to realize where the objective has to be in order to have it focus on the object, and to understand the relative size of objects even on different powers.


MATERIALS:

  • Compound microscope
  • prepared slide of bacteria
  • Forceps
  • Methyl blue
  • paper towel
  • ruler (clear)
  • Elodea
  • Geranium leaf
  • 2 slides (blank)
  • Iodine stain
  • Onion
  • eyedropper
  • Cover slips
  • Toothpicks


PROCEDURE:

Using lens paper, clean the lenses of the compound microscope (eye-piece and objectives) and turn on the microscope's light source. Adjust the amount of light for your eyes. Place the clear ruler on the stage and determine the diameter of the field of view while using the scanning objective (the shortest one). Adjust to medium power and again find the diameter of the field of view at that power. Adjust to high power and find the diameter of it's field of view. Now determine the total magnification of each objective by multiplying the power of the objective (the even number printed on the objective) by the ocular (the number that has a "x" behind it printed on the eye-piece).

PART A: PROCARYOTIC CELLS: BACTERIA

Bacteria are prokaryotes, and thus do not contain any membrane-bound organelles. The only organelles present are the ribosomes which are used for protein synthesis. Note also the DNA molecules and cytoplasm. Procaryotic cells are unicellular, however some do form masses because their walls do not separate completely after cell division as they are held by a mucilaginous sheath. Place the prepared slide of bacteria on the stage of the microscope. Using the scanning objective (the shortest one) find and focus on some bacteria cells. Adjust the slide so that the bacteria cells are in the center of the field of view. Using the ruler determine the distance between the bottom of the objective and the slide and record the information on the data sheet. Adjust the microscope to medium power and refocus on the cells. If necessary re-adjust the position of the cells so that they are again the in center of the field of view. Again using the ruler determine the distance between the objective and the slide and record. Now adjust the microscope to high power and again refocus and adjust the position of the cells if necessary. Once more determine the distance between the bottom of the objective and the slide. Record the information on the data sheet. Make a perfect circle in the space on the data sheet to represent your field of view at this magnification then draw three bacteria cells (labeling all the parts that you can see) in that circle, giving as much detail as you can. Indicate the power that you are using and give the size of one of the cells. Use the ruler to makeyour best estimation. Possible items that could be identified: cytoplasm, cell membrane.

Alternate Activity:

Place a dab of yogurt on a microscope slide. Mix the yogurt in a drop of water, add a coverslip, and examine it with a compound microscope. First focus the low-power objective. Then, rotate to the medium-power and focus, and last use the high-power objective to see masses of rod-shaped cells. These cells are Lactobacillus, a bacterium adapted to live on milk sugar (lactose). Lactobacillus, converts milk to yogurt. Yogurt is acidic and keeps longer than milk. Historically, Lactobacillus, was (and still is) used in many parts of the world by peoples deficient in lactase (an enzyme that breaks down lactose). Middle Eastern and African cultures use the more digestible yogurt in their diet instead of milk.

PART B: EUKARYOTIC CELLS: PLANT CELLS

Place a drop of water in the middle of a clean slide. Using the forceps, gently remove a section of the skin from the inside layer of the onion and place it on the slide in the drop of water. Put the cover slip over the top by placing the edge of the cover slip on the end of the drop of water, then gently lower the cover slip down on the drop of water using the forceps to hold it (see figure 1). Observe through the microscope (by first using low-power and working up to high-power). What internal structures do you see? Now place one drop of iodine on the slide just to the side of the cover slip. Let the slide set for 3 minutes letting the iodine stain the cells. Again observe the cells through the microscope. Do you see more details in the cell? Again in the space provided make a perfect circle, then draw one cell in that space labeling all the parts identified. Include as many structures as you can see. Indicate the power at which you are observing the cell and give the estimated size of the cell (length and width). Clean and dry the slide and cover slip when done. Possible structures that could be identified: cell wall, cell membrane, cytoplasm, nucleus, nucleolus, mitochondria, vacuole.

Place a drop of water on the slide again, and put an Elodea leaf in the water. Put the cover slip in place as you did before and observe the leaf through the microscope (again going from the scanning objective to high-power). Observe a cell. You may have to use a lower power to see all of one cell at a time. Again after making the circle in the space, make a drawing of the cell and label all of the structures that you see. Clean and dry the slide after your observations and data collection. Possible structures that could be identified: cell wall, cell membrane, cytoplasm, nucleus, nucleolus, mitochondria, vacuole, chloroplasts, grana, stroma.

ANIMAL CELLS:

Place a drop of methyl blue in the middle of the slide. With the end of the toothpick rub the inside of your cheek, then stir the toothpick in the methyl blue. Place a cover slip over the methyl blue as you have done before to avoid air bubbles, then place the slide under the microscope. Observe starting with the scanning objective and working up to the high power objective. Again after making the circle in the space, make a drawing of the cell and label all of the structures that you see. Clean and dry the slide after your observations and data collection. Possible structures that could be identified: cell membrane, cytoplasm, nucleus, nucleolus, mitochondria, vacuoles. Answer all the questions on the data sheet and turn in. Make sure that your lab station is clean and that the microscope is not left on high power, that it is unplugged and covered, that the slides have been cleaned and are dry and the station is ready for the next team.


Method of Evaluation/Assessment

Method of Evaluation/Assessment would be on a data sheet where the student would:

  • Indicate the diameter of the field of view of each power

  • The distance between the bottom of the objective and the slide when in focus with each objective.

  • Calculate the total magnification power of the microscope with each objective.

  • Give the size of the cells observed (estimated)

  • Make a drawing of the cells observed: bacteria, Elodea, onion, cheek.

  • Identify by labeling as many organelles as they see. A list is provided of the possible organelles that they may see. The drawings are to be made inside a perfect circle and should be proportional to how they appear in reference to the field of view.

  • State differences observed between procaryotic and eukaryotic cells, and between plant and animal cells.

  • Give differences observed between unstained and stained cells.

  • Estimate the number of each type of organelle observed. Indicate the rate at which it moves Do you observe any motion? If yes, what is moving? How fast is it moving? (divide the estimated distance a cell travels by the time observed).

  • Estimate the size of the organelles observed.

  • From the estimated cell size, have the student measure their height and width to determine how many cells it would take to make a body their size. Show calculations.

  • The students list all organelles they observed and, from the text, give the function of all the organelles

  • They define what tissues, and organs are.

  • The students make a composite drawing of the largest cell observed with the other cells drawn inside of it in proportion to see that many bacteria cells can fit inside a human cell. and appreciate the difference in the sizes of cells.


Extension/Reinforcement/Additional Ideas:

  • Stain living plant cells (celery) with fast green to observe mitochondria and gain evidence as to their function.

  • Observe plant cells in a fresh water medium then add a hypertonic solution as they look through the microscope to see the reaction of the cell to the change in osmotic difference. This could also be done with cheek cells.

  • Have the students build jello cell models) proportional to size with the organelles inside.

  • Have the students view media materials (laser discs, videos) on cell structure, types, and functions.

  • Assign each student to build organelle to size and appearance in order to turn the room into a cell. Some of the students could build bacteria to infect the cell as well. Several could build mitochondria, ribosomes, etc. so each student would have an assignment. The good ones could be kept for use in the future by the teacher. The teacher would go on a walking tour of the cell as each student standing by their organelle give the function, size, and number found in a cell of their organelle.

  • Have students in teams make a video using computer programs to tour the cell, or a video with the students as actors portraying the cell structures as they function. Example two students feed adding machine tape (with the letters A, G, C, T printed on it) through a student who is making a string of pop beads as the tape goes through to demonstrate the function of a ribosome.

  • Have students color and label pictures of cells (each type) and on the back give the function of each organelle.

  • Use a small piece of paper to have the students focus on to determine the distance from the bottom of the objective and the paper when in focus instead of a slide to avoid some breakage of slides.

  • Have students make a slide presentation of a type of cell assigned to them with its organelles and their functions.


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