Cell Observation Lab
Overview
Cells are the basic unit of living organisms. The structural properties of cells play a large part in the structure and function of an organ, an organ system and an organism. Although the AP Biology curriculum does not have a specific laboratory exercise on cells, it will be useful to examine a number of different cell types with the light microscope and compare their structures.
Objectives
At the completion of this laboratory you should be able to:
* Prepare free-hand samples of various cells for microscopic examination.
* Use a microscope effectively.
* Make detailed drawings and sketches of cells.
* Compare the size, shape and structure of various cells.
* Use stains to improve the visibility of cells.
* Identify cell structures that are visible with the light microscope.
Use of the Microscope - General Instructions
Many students have previous experience in the use of the microscope, therefore, detailed instructions on the use of the microscope will not be given here. However, a few suggestions or reminders are:
* Use only lens paper to clean the ocular or objective lenses. Paper towels can be used to clean coverslips and microscope slides.
* Always start viewing the specimen using the lowest power objective (the shortest objective tube). After the specimen is in focus, center the specimen in the field of view before switching to the next power objective. The red, yellow and blue objectives are permissible for this lab. Do not use the white or oil immersion objective lens.
* All wet or live specimens must be covered with a cover slip to protect the objective lenses.
* Remember to adjust the light levels as needed. More details are typically seen on lower light settings than on bright light.
* Hand-mounted specimens are typically too thick to give a clear view. Examine the edges of
the specimen or tease the specimen apart to isolate individual cells for viewing.
* Stains will not only make the cells darker and easier to observe, but they will also stain your clothing. Handle with care!
* Scan the slide under low power to find a good viewing area, then switch to higher magnifications.
Making Drawings
A drawing of a representative cell should be made for all material examined in lab. You may wish to use a cup to make a circle (similar to your field of view in the microscope) and make your drawing in the circle. Special attention should be given to the size of the cell (note the magnification used when making the observations), the cell's shape, and any cell structures that are visible. Label all structures. You may wish to make the drawings in color for better detail. Be sure that each sketch is labeled as to the specimen being examined.
Onion Epidermis
A thin sheet of epidermis tissue will be peeled from an onion bulb. Mount the tissue in water and add a coverslip. Examine the tissue and try to identify the parts of an individual cell.
Make a second mount of onion epidermis, but place the specimen in a drop of iodine solution instead of water. Examine the tissue and see if the iodine stains any cellular structure as compared to the water mounted specimen.
Elodea Leaf
Remove one leaf from the aquatic plant Elodea. Place the leaf in a drop of water and add a coverslip. Examine the cell and identify the structures.
What structure makes the leaf appear "green"? Approximately how many of these structures are there per cell?
Examine several cells and watch for the green structures to move or circle inside the cell. This movement is known as cyclosis or cytoplasmic streaming. Describe the pathway the green structures take inside the cell.
Human Cheek Cells
Use a toothpick to gently scrape the inside lining of your mouth. Hold the toothpick on the slide and wash the cells off the toothpick with a drop of methylene blue stain. Add a coverslip and look for the cells.
What structures are visible in the cell? What structure appears to react with the stain?
Tomato Pulp
Remove a small portion of the red pulp of a tomato. Add a drop of water and a coverslip. Scan the specimen and locate a region where individual cells can be seen clearly. Identify the cell parts. What cell structure appears to have the "red" pigment that makes the tomato pulp look red?
Potato Slice
Use a razor blade to cut a small triangular wedge of potato. Add a drop of water and a coverslip. Scan the edge of the wedge and locate an area where individual cells can be seen clearly.
What is the probable identity of the "ice-chip" structures that are inside the cells?
Add a drop of iodine solution to a small piece of potato and note the color. This color is characteristic of a chemical reaction between iodine and starch.
Add a drop of iodine to one side of your potato specimen on the microscope slide. While watching under the microscope, touch a piece of paper to the other side of the coverslip. The paper should draw away the water, allowing the iodine solution to touch the potato cells. Watch for a color change in the cells.
Given the evidence of a color change, now what do you think is the probable identity of the "ice-chip" structures inside the potato cells? Explain.
Zebrina Stem
Cut a small piece of the stem of the Zebrina plant and place it in a drop of water. Crush the stem and tease apart the tissues. Add a coverslip and look for evidence of small crystals that should now be spilled over the microscope slide.
What is the function of the crystals?
Zebrina Flowers
If available, examine the hairs on the stamens of the Zebrina flowers. Place a stamen (the pollen structure) in a drop of water and add a coverslip. Examine the chains of cells and look for evidence of cyclosis.
Lab Report
Your lab report will consist of your drawings of the observed cells. The drawings will be graded on the accuracy of the shape and size of the cells and the identification of the structures observed.