Preparing a Wet Mount
Introduction
In science, and especially in the field of biology, microscopes are very important tools. Without them, an entire world of microscopic organisms and cells would be a mystery to us. Microscopes use slides that contain material for viewing. In our class we will be viewing slides under our microscopes. However, we will go one step further and will be making the slides ourselves. To do this we must prepare a wet mount. Now what does this mean? A mount is the arrangement of a specimen for examination (usually on a microscope slide). A wet mount is prepared by adding water to the specimen which increases its translucency and makes it easier to stain. It also has a tendency to cause the specimen to flatten which makes the specimen easier to view.
Safety Considerations
The only safety consideration in regards to the actual preparing of the wet mount is being careful with the handling of the glass microscope slide. If broken, or even chipped, the slide can cause a deep cut. So before you grab the slide, inspect it for any sharp edges and show caution with handling it. Obtaining a specimen can present another safety consideration. In certain cases, a razor blade may be used to slice off a sample. In this situation, it is extremely important that you show caution. In some cases, the teacher will have already prepared specimens, and it is up to you to simply prepare the wet mount.
Curriculum Objectives
This activity fits in to the Biology 30S curriculum under Cluster 0: Skills and Attitudes. Specifically it addresses the following specific learning outcome:
S3B-0-S4 Select and use scientific equipment appropriately and safely.
Examples: microscopes, dissection equipment, prepared slides
Materials
Slide (x 2) Water Dropper
Cover Slip (x 2) Tweezers
Water Specimens (paper with a printed letter, onion skin)
Skill Procedures
The skill that you will be learning today is how to prepare a wet mount. As you follow through the steps, the teacher will also be going through the procedures with you to make sure there is no confusion.
First of all, you must obtain a clean, dry slide. Then, using the water dropper, place a single drop of water in the center of the slide.
Why is it important to make sure our slide is clean and dry before using it?
_______________________________________________________________________________________________________________________________________________________________________________________
Now we must obtain our specimen. For this practice run, we will be using a small piece of paper with a letter printed on it. Cut out a single letter from the scrap documents the teacher has provided. Once you have your letter, using the tweezers, place the letter into the drop of water.
Why is it important to have the slide (with a drop of water) ready before obtaining the specimen? (Imagine we were using an organic specimen instead of a piece of paper)
_______________________________________________________________________________________________________________________________________________________________________________________
At this time, we would add a drop of dye (eg. iodine, methylene blue) to help increase visibility if needed. Now it is time to add our cover slip. Carefully place the cover slip upright next to the edge of the water. Slowly, lower the top part of the cover slip down so that the cover slip is covering the water drop containing the specimen. See Figure 1 below to see how the cover slip should be lowered.
FIGURE #1
Why do you think this method of placing the cover slip in the slide is important to do as opposed to just dropping the cover slip straight down on top of the water drop? (The teacher will do an example using the proper technique, and an example using the improper technique. Use these examples to find your answer to this question).
_______________________________________________________________________________________________________________________________________________________________________________________
Congratulations! You have just prepared your first wet mount. Now our slide is ready to be viewed. You should all remember the proper procedures for using a microscope as we covered this last class. Turn on the microscope and place the slide on the stage. Using the low power objective lens (4X), focus on the letter. Draw what you see in the eyepiece in the space provided below. Repeat this for the 10X objective and the 100X objective.
4X Objective 10X Objective
100X Objective
Skill Application
Now that you have gained the skill of how to prepare a wet mount, we are ready to apply this skill to a biological investigation. We will be investigating the differences between plant and animal cells.
To carry out this investigation, we will prepare wet mounts of a plant cell and an animal cell separately. Once this is done, we will examine each slide through a microscope and analyze each type of cell to locate similarities and differences.
What are some methods that you can think of that we could implement to obtain a plant cell for our wet mount? (Try to come up with at least three)
_________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
What are some methods that you can think of that we could implement to obtain an animal cell for our wet mount? (Try to come up with at least three)
_________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
For the plant cell wet mount, we will be using an onion. Using tweezers, peel a small piece of the delicate, transparent, inner surface of the onion. The piece only needs to be half a centimeter across. Once you have obtained a piece of the onion skin, prepare a wet mount of the specimen using the procedures we have already gone over for preparing a wet mount (make sure to add a drop of iodine before placing the cover slip on the slide). Using a microscope, observe the specimen under low power (4X). Draw what you see in the space provided on the next page. Repeat this procedure with medium power (10X), and then high power (100X). Make sure to focus the eyepiece before moving to each power level.
4X Objective 10X Objective
100X Objective
Describe the appearance of the cells you see.
_______________________________________________________________________________________________________________________________________________________________________________________
What cell structures and organelles can you see?
_______________________________________________________________________________________________________________________________________________________________________________________
For the animal cell mount, we will be obtaining animal cells from the inside of our cheeks! Using the blunt end of a clean toothpick, gently scrape the inside of your cheek (you do not need to press very hard). You should get some cells on the end of the toothpick. Prepare a wet mount (using the guidelines already covered) of the cells from your cheek (make sure to add a drop of methylene blue before placing the cover slip on the slide). Using a microscope, observe the specimen under low power (4X). Draw what you see in the space provided below. Repeat this procedure with medium power (10X), and then high power (100X).
4X Objective 10X Objective
100X Objective
Describe the appearance of the cells you see.
_______________________________________________________________________________________________________________________________________________________________________________________
What cell structures and organelles can you see?
_______________________________________________________________________________________________________________________________________________________________________________________
Follow up Questions
1) The onion skin we viewed lacked chloroplasts which are found in many plant cells. Why is this?
_______________________________________________________________________________________________________________________________________________________________________________________
2) Many of the cells you observed in the animal cell wet mount may have appeared folded or wrinkled. Why is this?
_______________________________________________________________________________________________________________________________________________________________________________________
3) What are some similarities between plant and animal cells?
_______________________________________________________________________________________________________________________________________________________________________________________
4) What are some differences? Construct a chart in the space below that contrasts the differences you’ve observed.
References
GreatScopes Activity: Wet Mounting a Specimen. (n.d.) Retrieved December 15, 2006 from http://www.greatscopes.com/act005.htm
Lab Activities. (n.d.) Retrieved December 15, 2006 from http://www.edu.pe.ca/gray/class_pages/rcfleming/cells/lab.htm
Jarrett Seidler